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Assessment of soil erosion in olive orchards (Olea europaea L.) under cover crops management systems in the tropical region of Brazil

ABSTRACT

In the tropics, water erosion is one of the most important factors leading to the degradation and deterioration of agricultural land. Olive orchards have a low canopy coverage, especially during the first years after planting, due to the low density of olive trees. Given the fast expansion of olive orchards in Brazil, this study aimed to evaluate the effect of cover vegetation on soil and water losses under natural rainfall. In addition, it was assessed the crop performance and the vegetation cover index in different management systems in olive orchards. The study was carried out in soil erosion plots, where water and sediment were sampled and measured over two crops season, under the following treatments: in the first season, bare soil with olive cultivation (OBS); olive trees intercropped with spontaneous vegetation (OSV); olive trees intercropped with jack beans (OJB); olive trees intercropped with millet (OM) and, as a control, only bare soil (BS). In the second season, the OM treatment was replaced by olive trees intercropped with sunn hemp (OSH). On bare soils, soil loss was the highest reaching 303.9 Mg ha-1 yr-1 and where the surface runoff amounted to 484.8 mm yr-1. However, in the absence of competition for resources with other crops, olive trees performed best under this system. The olive orchards planted in shallow and sloping soils without cover crops showed unsustainable soil loss, crusting, and sealing in the superficial soil layer, which can progress quickly for soil degradation in the future. The efficiency in the reduction of loss in relation to bare soil was 4.11 and 12.93 % for the soil loss and 12.15 and 25.17 % for water loss, respectively, for olive with spontaneous vegetation and olive with jack beans. Cover crops combined with olive trees, and reconciled with the crop performance aspects of cultivation in tropical regions, is of great relevance for improving sustainability, especially regarding the reduction of soil and water losses due to water erosion.

soil conservation; soil loss; land degradation; runoff; olive cultivation

INTRODUCTION

Soil is a finite natural resource, where more than one generation is necessary for natural recovery of soil, depending on the relationship between the rate of soil genesis and soil erosion (Lal, 2009Lal R. Laws of sustainable soil management. In: Lichtfouse E, Navarrete M, Debaeke P, Véronique S, Alberola C, editors. Sustainable agriculture. Dordrecht: Springer; 2009. p. 9-12.). The current increase in land degradation (García-Ruiz et al., 2017García-Ruiz JM, Beguería S, Lana-Renault N, Nadal-Romero E, Cerdà A. Ongoing and emerging questions in water erosion studies. Land Degrad Dev. 2017;28:5-21. https://doi.org/10.1002/ldr.2641
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; Taguas et al., 2017Taguas EV, Vanderlinden K, Pedrera-Parrilla A, Giráldez JV, Gómez JA. Spatial and temporal variability of spontaneous grass cover and its influence on sediment losses in an extensive olive orchard catchment. Catena. 2017;157:58-66. https://doi.org/10.1016/j.catena.2017.05.017
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) has been a global concern, considering the decline in soil functions (McBratney et al., 2014McBratney A, Field DJ, Koch A. The dimensions of soil security. Geoderma. 2014;213:203-13. https://doi.org/10.1016/j.geoderma.2013.08.013
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) and the decrease in the growth of world agricultural production (FAO/ITPS, 2015Food and Agriculture Organization of the United Nations - FAO / Intergovernmental Technical Panel on Soils - ITPS. Status of the World’s Soil Resources (SWSR) - Main Report. Rome: FAO / ITPS; 2015. Available from: http://www.fao.org/3/i5199e/i5199e.pdf
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). Keesstra et al. (2018)Keesstra SD, Mol G, Leeuw J, Okx J, Molenaar C, Cleen M, Visser S. Soil-related sustainable development goals: four concepts to make land degradation neutrality and restoration work. Land. 2018;7:133. https://doi.org/10.3390/land7040133
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emphasized that the soil has a key component in Land Degradation Neutrality (LDN) to achieve its ecosystem services. The LDN is part of the Sustainable Development Goals adopted by the United Nations for sustainable exploitation of the planet’s resources to enhance food security, in a holistic approach aimed at reducing soil degradation and rehabilitation of degraded areas (Keesstra et al., 2016Keesstra SD, Bouma J, Wallinga J, Tittonell P, Smith P, Cerdà A, Montanarella L, Quinton JN, Pachepsky Y, van der Putten WH, Bardgett RD, Moolenaar S, Mol G, Jansen B, Fresco LO. The significance of soils and soil science towards realization of the United Nations Sustainable Development Goals. Soil. 2016;2:111-28. https://doi.org/10.5194/soil-2-111-2016
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).

Soil resources are threatened by various degradation factors, such as water and wind erosion, compaction, leaching, and pollution. Worldwide, water erosion is one of the major causes of land degradation. Water erosion affects soil quality and induces soil deterioration due to the loss of its superface layer, which is usually the most fertile layer where organic matter and nutrients necessary for plant development are concentrated (Cerdà et al., 2018Cerdà A, Rodrigo-Comino J, Giménez-Morera A, Keesstra SD. Hydrological and erosional impact and farmer’s perception on catch crops and weeds in citrus organic farming in Canyoles river watershed, Eastern Spain. Agr Ecosyst Environ. 2018;258:49-58. https://doi.org/10.1016/j.agee.2018.02.015
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; Rodrigo-Comino et al., 2018Rodrigo-Comino J, Keesstra S, Cerdà A. Soil erosion as an environmental concern in vineyards: the case study of Celler del Roure, Eastern Spain, by means of rainfall simulation experiments. Beverages. 2018;4:31. http://hdl.handle.net/10550/65731
http://hdl.handle.net/10550/65731...
; Keesstra et al., 2019Keesstra SD, Rodrigo-Comino J, Novara A, Giménez-Morera A, Pulido M, Di Prima S, Cerdà A. Straw mulch as a sustainable solution to decrease runoff and erosion in glyphosate-treated clementine plantations in Eastern Spain. An assessment using rainfall simulation experiments. Catena. 2019;174:95-103. https://doi.org/10.1016/j.catena.2018.11.007
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).

Olives are predominantly cultivated in the Mediterranean region that accounts for 97 % of the total area of the world olive trees (FAO, 2019Food and Agriculture Organization of the United Nations - FAO. FAOSTAT. Rome: FAO; 2019 [cited 2019 April 4]. Available from: http://www.fao.org/faostat/en/.
http://www.fao.org/faostat/en/...
). Reducing soil erosion in olive orchards is also a major challenge in the Mediterranean region (Keesstra et al., 2019Keesstra SD, Rodrigo-Comino J, Novara A, Giménez-Morera A, Pulido M, Di Prima S, Cerdà A. Straw mulch as a sustainable solution to decrease runoff and erosion in glyphosate-treated clementine plantations in Eastern Spain. An assessment using rainfall simulation experiments. Catena. 2019;174:95-103. https://doi.org/10.1016/j.catena.2018.11.007
https://doi.org/10.1016/j.catena.2018.11...
). Given the low density of olive trees and the intensive weeding, particularly during the first years after planting, soils are prone to water erosion (Repullo-Ruibérriz et al., 2018Repullo-Ruibérriz MAT, Ordóñez-Fernández R, Giráldez JV, Márquez-García J, Laguna A, Carbonell-Bojollo R. Efficiency of four different seeded plants and native vegetation as cover crops in the control of soil and carbon losses by water erosion in olive orchards. Land Degrad Dev. 2018;29:2278-90. https://doi.org/10.1002/ldr.3023
https://doi.org/10.1002/ldr.3023...
). Moreover, as olive trees can grow in poor environmental conditions, it is commonly grown in poor soils with pronounced slopes (Espejo-Pérez et al., 2013Espejo-Pérez AJ, Rodríguez-Lizana A, Ordóñez R, Giráldez JV. Soil loss and runoff reduction in olive-tree dry-farming with cover crops. Soil Sci Soc Am J. 2013;77:2140-8. https://doi.org/10.2136/sssaj2013.06.0250
https://doi.org/10.2136/sssaj2013.06.025...
). Studies conducted by García-Orenes et al. (2012)García-Orenes F, Roldán A, Mataix-Solera J, Cerdà A, Campoy M, Arcenegui V, Caravaca F. Soil structural stability and erosion rates influenced by agricultural management practices in a semi-arid Mediterranean agro-ecosystem. Soil Use Manage. 2012;28:571-9. https://doi.org/10.1111/j.1475-2743.2012.00451.x
https://doi.org/10.1111/j.1475-2743.2012...
consider that the main cause of erosion in olive orchards, besides natural factors, is the inadequate management systems. Espejo-Pérez et al. (2013)Espejo-Pérez AJ, Rodríguez-Lizana A, Ordóñez R, Giráldez JV. Soil loss and runoff reduction in olive-tree dry-farming with cover crops. Soil Sci Soc Am J. 2013;77:2140-8. https://doi.org/10.2136/sssaj2013.06.0250
https://doi.org/10.2136/sssaj2013.06.025...
reported that the practice of removing spontaneous vegetation between the olive trees, which aims to reduce the competition for light, water, and nutrients between the weeds and the olive trees, renders the soil susceptible to erosion. However, Taguas and Gómez (2015)Taguas EV, Gómez JA. Vulnerability of olive orchards under the current CAP (Common Agricultural Policy) regulations on soil erosion: a study case in Southern Spain. Land use Policy. 2015;42:683-94. https://doi.org/10.1016/j.landusepol.2014.09.001
https://doi.org/10.1016/j.landusepol.201...
emphasized that the efficient use of the soil conservation strategies like the use of cover crops adapted for each region is the unique intervention that can reduce the unsustainable soil losses in olive orchards.

In tropical regions, the problem is increased by the high rainfall erosivity (Aquino et al., 2012Aquino RF, Silva MLN, Freitas DAF, Curi N, Mello CR, Avanzi JC. Spatial variability of the rainfall erosivity in southern region of Minas Gerais state, Brazil. Cienc Agrotec. 2012;36:533-42. https://doi.org/10.1590/S1413-70542012000500006
https://doi.org/10.1590/S1413-7054201200...
). Southern Minas Gerais in Brazil, is a tropical region where agricultural lands have been impacted by water erosion due not only to high rainfall erosivity, but also due to the high altitudes and steep slopes that make up the regional landscape (Oliveira et al., 2012Oliveira PTS, Wendland E, Nearing MA. Rainfall erosivity in Brazil: a review. Catena. 2012;100:139-47. https://doi.org/10.1016/j.catena.2012.08.006
https://doi.org/10.1016/j.catena.2012.08...
; Anache et al., 2017Anache JAA, Wendland EC, Oliveira PTS, Flanagan DC, Nearing MA. Runoff and soil erosion plot-scale studies under natural rainfall: a meta-analysis of the Brazilian experience. Catena. 2017;152:29-39. https://doi.org/10.1016/j.catena.2017.01.003
https://doi.org/10.1016/j.catena.2017.01...
; Pinto et al., 2018Pinto LC, Mello CR, Norton LD, Poggere GC, Owens PR, Curi N. A hydropedological approach to a mountainous Clayey Humic Dystrudept in the Mantiqueira Range, southeastern Brazil. Sci Agric. 2018;75:60-9. https://doi.org/10.1590/1678-992x-2016-0144
https://doi.org/10.1590/1678-992x-2016-0...
). In this region, Silva et al. (2005)Silva AM, Silva MLN, Curi N, Lima JM, Avanzi JC, Ferreira MM. Perdas de solo, água, nutrientes e carbono orgânico em Cambissolo e Latossolo sob chuva natural. Pesq Agropec Bras. 2005;40:1223-30. https://doi.org/10.1590/S0100-204X2005001200010
https://doi.org/10.1590/S0100-204X200500...
reported soil erosion values of 205.65 Mg ha-1 yr-1 in Cambisols (Eutrudepts) kept uncovered.

Considering the vast expansion of olive orchards in the Southern Minas Gerais in Brazil, there is a high need for assessing the potential and constraints of including cover crops on water erosion in olive orchards. In this context, the following hypotheses were formulated: in olive orchard, the water erosion can be significantly reduced by cover crops; the high soil erosion rates in the region can be caused by low soil infiltration, low vegetation cover index, and high erosivity; there is a relationship between soil and water erosion and crop performance of olive orchard.

This study aimed to evaluate the relationship between soil and water losses by water erosion and the crop performance in an olive orchard managed with or without cover crops in southern Minas Gerais, Brazil.

MATERIALS AND METHODS

Study area

The experiment was conducted at the experimental farm of the Federal University of Lavras (UFLA), Lavras, Minas Gerais, Brazil (21° 13’ 20” S and 44° 58’ 17” W) (Figure 1), during two hydrological years, between November 2015 and October 2017.

Figure 1
Location of the study area (Lavras) on the Southern of Minas Gerais, Brazil (a). Plots used in the study of erosion in hydrological years (b) 2015/2016 (March 23, 2016) and (c) 2016/2017 (February 15, 2017), under the following treatments: olive cultivation on bare soil (OBS); olive cultivation intercropped with spontaneous vegetation (OSV); olive cultivation intercropped with jack beans (OJB); olive cultivation intercropped with millet (OM); olive cultivation intercropped with sunn hemp (OSH) and bare soil (BS). Side view of erosion plot (d). Bare soil plot (e).

The experimental site is at an altitude of 918 m a.s.l., with a subtropical humid climate classified as Cwa according to the Köppen Climate Classification System. The mean annual rainfall is 1,530 mm and the mean annual temperature is 19.4 °C. The winter months are dry and cool, while the summer months are rainy and with daily average temperatures exceeding 22 °C (Dantas et al., 2007Dantas AAA, Carvalho LG, Ferreira E. Classificação e tendências climáticas em Lavras, MG. Cienc Agrotec. 2007;31:1862-6. https://doi.org/10.1590/S1413-70542007000600039
https://doi.org/10.1590/S1413-7054200700...
).

The soil in the study area was classified as Cambissolo Háplico Tb Distrófico according to the Brazilian Soil Classification System (Santos et al., 2018Santos HG, Jacomine PKT, Anjos LHC, Oliveira VA, Lumbreras JF, Coelho MR, Almeida JA, Araújo Filho JC, Oliveira JB, Cunha TJF. Sistema Brasileiro de Classificação de Solos. 5ª ed. revisada e ampliada. Brasília: Embrapa;2018), which corresponds to Dystrustepts in Soil Taxonomy (Soil Survey Staff, 2014Soil Survey Staff. Keys to soil taxonomy. 12th ed. Washington, DC: United States Department of Agriculture, Natural Resources Conservation Service; 2014.), and to Dystric Cambisols in WRB (IUSS Working Group WRB, 2015IUSS Working Group WRB. World reference base for soil resources 2014, update 2015: International soil classification system for naming soils and creating legends for soil maps. Rome: Food and Agriculture Organization of the United Nations; 2015. (World Soil Resources Reports, 106).). The soil properties of the 0.00-0.05 and 0.05-0.10 m layers are presented in table 1.

Table 1
Soil physical and chemical properties of the Cambissolo Háplico Tb Distrófico in the experimental area

Design of the experimental area

During the first year (2015/2016), each treatment consisted of the following management practices: olive trees (Olea europaea L.) on bare soil (OBS); olive trees intercropped with spontaneous vegetation (OSV); olive trees intercropped with jack beans (Canavalia ensiformis L.) (OJB); olive trees intercropped with millet (Pennisetum glaucum L.) (OM) and, as a control, bare soil without any olive trees (BS). The spontaneous vegetation was composed of grasses with Brachiaria decumbens Stapf, as the dominant/most common species, followed by Digitaria sanguinalis L., Melinis minutiflora P. Beauv., and Eleusine indica L. Some broadleaf species included Ipomoea acuminata Roem., Bidens pilosa L., Oxalis corniculata L., Emilia fosbergii Nicolson., Conyza bonariensis L., Euphorbia heterophylla L., and Amaranthus viridis L.

In the second year (2016/2017), the OM treatment was replaced by olive trees intercropped with sunn hemp (Crotalaria juncea L.) (OSH), maintaining all other treatments from the first period. During both years, all treatments were done in three replicates (Figure 1).

The olive trees treatments were planted in March 2015 following the direction of the slope. A total of 4 olive plants per plot were used with a spacing of 4 m in the line and 5 m between lines. The chosen cultivar was Arberquina (Olea europaea L.), the most cultivated in Brazil (Borges et al., 2017Borges TH, López LC, Pereira JA, Cabrera–Vique C, Seiquer I. Comparative analysis of minor bioactive constituents (CoQ10, tocopherols and phenolic compounds) in Arbequina extra virgin olive oils from Brazil and Spain. J Food Compos Anal. 2017;63:47–54. https://doi.org/10.1016/j.jfca.2017.07.036
https://doi.org/10.1016/j.jfca.2017.07.0...
).

The cover crops (jack beans, millet, and sunn hemp) were manually seeded at the beginning of November, which is the beginning of the rainy season of each hydrological year. In the treatment with jack beans as a cover crop, soil furrows were spaced at every 0.5 m in a density of 8 seeds m-1. Regarding millet and sunn hemp, the spacing used was 0.25 m with densities of 90 seeds m-1 and 40 seeds m-1, respectively. Table 2 presents more details about crop management during experimentation (Neto Vieira et al., 2008Neto Vieira J, Oliveira AF, Oliveira NC, Duarte H da SS, Gonçalves ED. Aspectos técnicos da cultura da oliveira. Belo Horizonte: Empresa de Pesquisa Agropecuária de Minas Gerais. 2008. (Boletim técnico).).

Table 2
Description of the management conducted in olive cultivation intercropped with cover plants during the experiment period from March 2015 to October 2017

Erosivity determination

Erosivity was determined by calculating the index EI30 (MJ mm ha-1 h-1 yr-1) developed by Aquino et al. (2012)Aquino RF, Silva MLN, Freitas DAF, Curi N, Mello CR, Avanzi JC. Spatial variability of the rainfall erosivity in southern region of Minas Gerais state, Brazil. Cienc Agrotec. 2012;36:533-42. https://doi.org/10.1590/S1413-70542012000500006
https://doi.org/10.1590/S1413-7054201200...
for the southern region of Minas Gerais, Brazil (Equation 1):

EI 30 = 85 . 672 x p 2 P 0 . 6557 Eq. 1

in which p is the mean monthly rainfall (mm) and P the mean annual rainfall (mm) over 30 years.

In analogy, we determined the actual monthly erosivity index as EIa (MJ mm ha-1 h-1 month-1) by equation 2:

EI a = 85 . 672 x p a 2 P a 0 . 6557 Eq. 2

In which pa is the actual monthly rainfall, Pa is the actual annual rainfall, and the EI30 (MJ mm ha-1 h-1 month-1) was calculated by equation 3:

EI 30 1 n + 1 i = 1 n = 30 EI a , i Eq. 3

We also calculated an actual annual erosivity (EIa12) as the total of the 12 months EIa per hydrologic years. Erosive rainfall events were considered when the rainfall was over 10 mm with 0.2 mm tolerance (Lima et al., 2018Lima PLT, Silva MLN, Quinton JN, Batista PVG, Cândido BM, Curi N. Relationship among crop systems, soil cover, and water erosion on a typic Hapludox. Rev Bras Cienc do Solo. 2018;42:e0170081. https://doi.org/10.1590/18069657rbcs20170081
https://doi.org/10.1590/18069657rbcs2017...
).

Soil chemical and physical properties

Soil pH(H2O) was determined at a ratio of soil:solution equal to 1:2.5, exchangeable Ca2, Mg2, and Al3 were determined according to the methodology described by McLean et al. (1958)McLean EO, Heddleson MR, Bartlett RJ, Holowaychuk N. Aluminum in soils: I. Extraction methods and magnitudes in clays and ohio soils. Soil Sci Soc Am J. 1958;22:382-7. https://doi.org/10.2136/sssaj1958.03615995002200050005x
https://doi.org/10.2136/sssaj1958.036159...
. The available P and K extracted with Mehlich-1. Soil organic matter (SOM) was determined according to Walkley and Black (1934)Walkley A, Black IA. An examination of the degtjareff method for determining soil organic matter, and a proposed modification of the chromic acid titration method. Soil Sci. 1934;37:29-38. https://doi.org/10.1097/00010694-193401000-00003
https://doi.org/10.1097/00010694-1934010...
. The soil texture was determined by the pipette method according to Day (1965)Day PR. Particle fractionation and particle-size analysis. In: Black CA. Methods of soil analysis: Physical and mineralogical properties, including statistics of measurement and sampling. Madison: American Society of Agronomy; 1965. Part 1. p. 545-66.. The variation of the soil organic matter (%) for both seasons was calculated by equation 4:

Variation SOM ( Treatment i ) = SOM ( season 2 ) SOM ( season 1 ) SOM ( season 1 ) × 100 Eq. 4

In which SOMseason2 is the soil organic matter content in the second season, and SOMseason1 is the soil organic matter content in the first season.

Soil water infiltration was determined using the Mini Disk Infiltrometer, following the methodology proposed by Robichaud et al. (2008)Robichaud PR, Lewis SA, Ashmun LE. New procedure for sampling infiltration to assess post-fire soil water repellency. Fort Collins: USDA Forest Service Research; 2008. (Research Note 33).. Measurements were obtained from four points in each plot, with the suction rate of two cm. The infiltration was measured for ten times in the field, each 30 seconds. The infiltration calculation was determined by using Spreadsheet Macro available in the Decagon website Decagon (2016).

Evaluation of soil and water losses

Soil loss was measured following the methodology proposed by Wischmeier and Smith (1978)Wischmeier WH, Smith DD. Predicting rainfall erosion losses: a guide to conservation planning. Washington, DC: USDA; 1978. (Agricultural handbook, 537)., with plots size iqual to 12.0 m long by 4.0 m wide (Figure 1). The mean slope of the plots was 0.23 m m-1.

Runoff and sediment collection was performed according to Cogo et al. (2003)Cogo NP, Levien R, Schwarz RA. Perdas de solo e água por erosão hídrica influenciadas por métodos de preparo, classes de declive e níveis de fertilidade do solo. Rev Bras Cienc Solo. 2003;27:743-53. https://doi.org/10.1590/S0100-06832003000400019
https://doi.org/10.1590/S0100-0683200300...
at each erosive event, using two tanks with 250 L installed at the bottom of each plot (Figure 1). The first tank was connected to the second by a Geib divisor type, with nine entrance windows. When the first tank was completely filled, 1/9 of the runoff volume was conducted to the second tank.

The evaluation of the effect of each treatment over the Loss Reduction Efficiency (LRE) was calculated using the equation 5 proposed by Amaral et al. (2008)Amaral AJ, Bertol I, Cogo NP, Barbosa FT. Redução da erosão hídrica em três sistemas de manejo do solo em um Cambissolo Húmico da região do Planalto Sul-Catarinense. Rev Bras Cienc Solo. 2008;32:2145-55. https://doi.org/10.1590/S0100-06832008000500035
https://doi.org/10.1590/S0100-0683200800...
:

LRE = Loss of the cultivated treatment Loss of the bare soil treatment × 100 Eq. 5

The Surface Runoff Coefficient (SRC) was also determined in relation to the total rainfall during the studied period for the different management systems by using the equation 6 (Şen and Altunkaynak, 2005Şen Z, Altunkaynak A. A comparative fuzzy logic approach to runoff coefficient. Hydrol Process. 2005;20:1993-2009. https://doi.org/10.1002/hyp.5992
https://doi.org/10.1002/hyp.5992...
).

SRC = Loss of the cultivated treatment Total precipitation × 100 Eq. 6

Vegetation cover index and crop performance

The vegetation cover index in each plot for cover crop and olive tree was determined by using images from an RGB digital camera with a 1/2.3 “CMOS” sensor and resolution of 12 megapixels, carried in Unmanned Aerial Vehicle (UAV), model professional DJI Phantom 3. The photographic parameters were: f/2.8aperture, shutter speed of 1/290 s, ISO 100, white balance of 4500 K, and focal length of 3.6 mm (DJI, 2018DJI. DJI Phantom 3. Shenzhen; DJI; 2018. [cited 2019 June 16]. Available from: https://www.dji.com/phantom-3-pro/info#downloads.
https://www.dji.com/phantom-3-pro/info#d...
). The flights were automatically managed by an iPad (model A1489- ME279KH / A), every 15 days, with 20 minutes duration, at a height of twenty meters, by georeferencing using 36 control points to geotag.

A total of 200 photos were recorded per flight, in JPEG format, with a 60 % overlap. PhotoScan Pro 1.2.6 was used for image processing, alignment, georeferencing, and orthophoto generation (Agisoft, 2016Agisoft. Tutorial (Intermediate level): 3D Model Reconstruction with Agisoft PhotoScan 1.1. Petersburg: Agisoft; 2016.). The images for calculating the vegetation cover index (VCI) were classified by equation 7, according to the methodology proposed by Beniaich et al. (2019)Beniaich A, Silva MLN, Avalos FAP, Menezes MD, Cândido BM. Determination of vegetation cover index under different soil management systems of cover plants by using an unmanned aerial vehicle with an onboard digital photographic camera. Semin - Cienc Agrar. 2019;40:49-66. https://doi.org/10.5433/1679-0359.2019v40n1p49
https://doi.org/10.5433/1679-0359.2019v4...
.

VCI = Number of pixels classified as vegetation Total number of pixels ( per plot ) × 100 Eq. 7

Crop performances of the olive trees were monitored by measuring diameter at breast height, plant heights, and crown radius in May 2016 and May 2017.

Experimental and statistical design

The experimental design was a partial completely randonized block (Figure 1) due to a technical limitation of randonizing the bare soil treatment for each season in a perennial crop, so we replicate such treatment in the same order, aiming to reduce the effect of the previous crop. Differences between the treatment in terms of soil loss and water runoff were tested with analysis of variance and the means were compared by the Tukey test at 5 % probability.

RESULTS

Rainfall erosivity

From November to March, there was a high occurrence of rainfall, with 92.4 and 70.8 % of annual erosivity (Table 3), for the periods of 2015/2016 and 2016/2017, respectively. December and January, represented 55.0 and 30.0 % of the total annual erosivity for the periods of 2015/2016 and 2016/2017, respectively, which is close to the half of the total erosivity for the period of 2015/2017.

Table 3
Precipitation and actual monthly erosivity (EIa12) and actual annual erosivity (EIay) for the two hydrologic years

Soil water infiltration

There was no statistical difference for soil water infiltration between the treatments in both periods (Table 4). The water infiltration values presented a high variability for the treatments, the variation coefficient ranged from 19.70 to 86.28 %. The highest soil water infiltration was always registered in treatment with cultivated or spontaneous cover crops. In the first period, the treatments OJB and OM presented the highest values of infiltration. In the second period, the highest value occurred for the treatment of OSV.

Table 4
Soil water infiltration for each period and in different soil cover management systems in olive cultivation

Vegetation cover index

In the first period (2015/2016), the OJB treatment presented the highest mean vegetation cover index, with a value of 81 %, followed by OM, with a value of 70 % (Table 5). Regarding the second period (2016/2017), the OJB treatment presented the highest mean vegetation cover index, with a value of 60 %, followed by OSH, with a value of 46 %.

Table 5
Vegetation cover index for each period and in different soil cover management systems in olive cultivation

Olive cultivation on bare soil in the first two years presented a very low coverage, with mean values of 2 and 6 % in the first and second years, respectively (Table 5). The direct exposure of the soil surface inevitably means a high risk of erosion in the early years of cultivation.

The treatment with spontaneous vegetation presented mean vegetation cover index values of 58 % in the first period and 42 % in the second period. Similar findings were observed in the treatment using jack beans (OJB), with the vegetation cover index in the first period presenting higher mean values than the second does.

Figure 2 illustrates the temporal variation in the vegetation cover index in each treatment. By the visual observation, the OBS treatment showed a constant linear behavior along the time due to the slow growth of the olive plants when compared to the other treatments. The vegetation cover index of the OSV treatment presented a “saw teeth” behavior, also observed in a study conducted by Sastre et al. (2017)Sastre B, Barbero-Sierra C, Bienes R, Marques MJ, García-Díaz A. Soil loss in an olive grove in Central Spain under cover crops and tillage treatments, and farmer perceptions. J Soils Sediments. 2017;17:873-88. https://doi.org/10.1007/s11368-016-1589-9
https://doi.org/10.1007/s11368-016-1589-...
.

Figure 2
Vegetation cover index for the periods: 2015/2016 (a) and 2016/2017 (b) of different soil covers under young olive trees. Olive cultivation on bare soil (OBS); olive cultivation intercropped with jack beans (OJB); olive cultivation intercropped with millet (OM); olive cultivation intercropped with sunn hemp (OSH); and olive cultivation intercropped with spontaneous vegetation (OSV).

Variation of soil organic matter

In the layer of 0.00-0.05 m, the variation of Soil Organic Matter (SOM) showed a statistically significant difference between the treatments (Table 6). The SOM variation presented a high variability with standard deviation ranging from 15.07 to 69.68 %. The SOM variation in the BS treatment was -23.77±69.68 (mean ± standard deviation). The highest SOM variation was observed in the treatment with spontaneous vegetation followed by the treatment constituted by Jack Beans. On the other hand, in the soil layer of 0.05-0.10 m, no differences were observed in SOM variation between the treatments (Table 6).

Table 6
Soil organic matter variation between both periods of experimentation

Soil loss assessment

Table 7 summarizes the mean values of soil loss for both periods studied. The first period showed high soil loss, which can be explained by the number of erosive events, 52 compared to 37 events of the second period, and by the number of events from November to January, 33 compared to 19 in the second period. Moreover, the values of erosivity should also be considered (Table 3).

Table 7
Mean annual values of soil loss and efficiency in the reduction of loss in relation to bare soil in different vegetation cover management systems in olive cultivation

The BS treatment presented higher values for both studied periods, with losses of 311.55 and 296.28 Mg ha-1 yr-1 in the first and second periods, respectively (Table 7). These values corroborates with the study performed by Silva et al. (2005)Silva AM, Silva MLN, Curi N, Lima JM, Avanzi JC, Ferreira MM. Perdas de solo, água, nutrientes e carbono orgânico em Cambissolo e Latossolo sob chuva natural. Pesq Agropec Bras. 2005;40:1223-30. https://doi.org/10.1590/S0100-204X2005001200010
https://doi.org/10.1590/S0100-204X200500...
in which they found high variability of soil loss during the five years evaluating water erosion in a Cambisol with bare soil plots treatments, with a mean annual soil loss of 205.65 Mg ha-1 yr-1, with values ranging from 98.47 to 374.10 Mg ha-1 yr-1.

The OSV treatment presented a low soil loss in the first period. However, all treatments with intercropping cover crops (OSH, OJB, and OSV) presented no significant differences between them (Table 7).

The pattern of soil loss follows the erosivity evaluated in the same period (Figure 3), with higher soil loss values from November to April, most notably in December and January. We note that the OJB treatment presented a high soil loss in November (2015/2016), with a value equal to 49.80 Mg ha-1 yr-1, which can be explained by the high rainfall erosivity (Table 3) at the cycle crop beginning, when there is a low soil cover index and along with there is a greater soil instability resultant from sowing operations.

Figure 3
Soil loss in different vegetation cover management systems in olive cultivation during the studied periods of (a) 2015/2016 and (b) 2016/2017. Bare soil (BS); olive cultivation on bare soil (OBS); olive cultivation intercropped with jack beans (OJB); olive cultivation intercropped with millet (OM); olive cultivation intercropped with sunn hemp (OSH); and olive cultivation intercropped with spontaneous vegetation (OSV).

Water loss assessment

The water loss had no significant difference between treatments BS and OBS for both studied periods (Table 8). Studies developed by Silva et al. (2005)Silva AM, Silva MLN, Curi N, Lima JM, Avanzi JC, Ferreira MM. Perdas de solo, água, nutrientes e carbono orgânico em Cambissolo e Latossolo sob chuva natural. Pesq Agropec Bras. 2005;40:1223-30. https://doi.org/10.1590/S0100-204X2005001200010
https://doi.org/10.1590/S0100-204X200500...
with data from a 5-year study of soil and water losses in a bare plot (Cambisol), showed great variability of water loss, with a mean annual of 369 mm yr-1, corroborating with the results found in the present study. We also verified a significant difference between the treatments with and without cover crops, for both studied periods.

Table 8
Mean annual values of water loss, efficiency in reducing soil loss in relation to bare soil and surface runoff in relation to the total precipitation in different olive cultivation management systems

Figure 4 shows the distribution of water losses during the studied periods, highlighting the variability caused by the irregular distribution of rain during the both periods evaluated (Silva et al., 2005Silva AM, Silva MLN, Curi N, Lima JM, Avanzi JC, Ferreira MM. Perdas de solo, água, nutrientes e carbono orgânico em Cambissolo e Latossolo sob chuva natural. Pesq Agropec Bras. 2005;40:1223-30. https://doi.org/10.1590/S0100-204X2005001200010
https://doi.org/10.1590/S0100-204X200500...
).

Figure 4
Average and monthly water loss in different olive cultivation management systems during the studied periods of (a) 2015/2016 and (b) 2016/2017. Bare soil (BS); olive cultivation on bare soil (OBS); olive cultivation intercropped with jack beans (OJB); olive cultivation intercropped with millet (OM); olive cultivation intercropped with sunn hemp (OSH); and olive cultivation intercropped with spontaneous vegetation (OSV).

Crop performance of olive cultivation

The results showed a negative effect of cover crops on the crop performance of olive plants and this effect was more pronounced in OM treatment (Table 9). Thus, the evaluation of plant hight showed a significant difference when comparing OM to the other treatments, and when comparing OSH to the OBS and OJB treatments. The highest values were obtained in the treatments OBS and OSV, in the period of 2015/2016, and OBS, OJB, and OSV in the period of 2016/2017.

Table 9
Values of mean, standard deviation, and coefficient of variation (CV) of the olive plantation performance for each period and in different soil cover management systems in olive cultivation

Regarding olive tree heights (Table 9), the lower performance was obtained for olive cultivation intercropped with millet (OM), presenting a mean height of 70.08 cm compared to 173.00 cm in the OBS treatment. The OBS treatment presented the highest values for the median crown radius (Table 9), with an average value of 62.92 cm in the first period and 94.83 cm in the second. Concerning the trunk diameter (Table 9), in 2015/2016, there was a significant difference between the OM treatment and the other treatments. In 2016/2017, the highest trunk diameter values were obtained for the OBS, OVE and, OJB treatments.

DISCUSSION

Erosivity

The rainfall erosivity is the driving force of erosion and has a direct impact by the falling raindrop on the detachment of soil particles, the breakdown of aggregates, and the transport of eroded particles (Panagos et al., 2015Panagos P, Ballabio C, Borrelli P, Meusburger K, Klik A, Rousseva S, Tadić MP, Michaelides S, Hrabalíková M, Olsen P, Aalto J, Lakatos M, Rymszewicz A, Dumitrescu A, Beguería S, Alewell C. Rainfall erosivity in Europe. Sci Total Environ. 2015;511:801-14. https://doi.org/10.1016/j.scitotenv.2015.01.008
https://doi.org/10.1016/j.scitotenv.2015...
). Mello et al. (2007)Mello CR, Sá MAC, Curi N, Mello JM, Viola MR, Silva AM. Erosividade mensal e anual da chuva no Estado de Minas Gerais. Pesq Agropec Bras. 2007;42:537-45. https://doi.org/10.1590/S0100-204X2007000400012
https://doi.org/10.1590/S0100-204X200700...
classified the erosivity in the studied region as a high erosivity. Results of annual erosivity showed the value of 5,437 mm ha-1 yr-1 in the first season and 4,277 MJ mm ha-1 yr-1 in the second season (Table 3). In the southern region of the state of Minas Gerais, Aquino et al. (2012)Aquino RF, Silva MLN, Freitas DAF, Curi N, Mello CR, Avanzi JC. Spatial variability of the rainfall erosivity in southern region of Minas Gerais state, Brazil. Cienc Agrotec. 2012;36:533-42. https://doi.org/10.1590/S1413-70542012000500006
https://doi.org/10.1590/S1413-7054201200...
observed that the annual erosivity ranged from 5,145 to 7,776 MJ mm ha-1 h-1 yr-1 in a historical series of 40 years of meteorological data. The erosivity in the region was influenced by the orographic effect and local weather characteristics (Mello et al., 2007Mello CR, Sá MAC, Curi N, Mello JM, Viola MR, Silva AM. Erosividade mensal e anual da chuva no Estado de Minas Gerais. Pesq Agropec Bras. 2007;42:537-45. https://doi.org/10.1590/S0100-204X2007000400012
https://doi.org/10.1590/S0100-204X200700...
). From November to March, there was a high occurrence of rainfall, with 92.4 and 70.8 % of annual erosivity (Table 3), for the periods of 2015/2016 and 2016/2017, respectively. The high rainfall erosivity between November and March draws attention to the high risk of water erosion for the studied region.

The rainfall distribution has an important effect over soil saturation and runoff coefficient, what means that a large number of erosive events in a short time can lead to soil saturation and increase the water loss through runoff (Guimarães et al., 2017Guimarães DV, Silva MLN, Bispo DFA, Martins SG, Melo Neto JO, Martins RP, Curi N. Water erosion associated with rainfall patterns in the extreme South of Bahia in eucalyptus post-planting. Semin - Cienc Agrar. 2017;38:2463-78. https://doi.org/10.5433/1679-0359.2017v38n4Supl1p2463
https://doi.org/10.5433/1679-0359.2017v3...
).

Soil infiltration

In many regions, it was demonstrated that cover crop in olive orchards increase soil porosity, water infiltration, and reduce runoff (Zuazo et al., 2009Zuazo VHD, Pleguezuelo CRR, Panadero LA, Raya AM, Martínez JRF, Rodríguez BC. Soil conservation measures in rainfed olive orchards in South-Eastern Spain: impacts of plant strips on soil water dynamics. Pedosphere. 2009;19:453-64. https://doi.org/10.1016/S1002-0160(09)60138-7
https://doi.org/10.1016/S1002-0160(09)60...
; Vicente-Vicente et al., 2017Vicente-Vicente JL, Gómez-Muñoz B, Hinojosa-Centeno MB, Smith P, Garcia-Ruiz R. Carbon saturation and assessment of soil organic carbon fractions in Mediterranean rainfed olive orchards under plant cover management. Agr Ecosyst Environ. 2017;245:135-46. https://doi.org/10.1016/j.agee.2017.05.020
https://doi.org/10.1016/j.agee.2017.05.0...
). However, the use of inadequate management practices reduced the soil protection by vegetation and caused the crusting and surface sealing, which are a result of the direct impact of raindrops on the soil.

The soil of the area had a low infiltration rate, and the lowest value was found in the bare soil treatment (Table 4). The crusting can explain this low infiltration in Cambisols due to the high silt/clay ratio (Pinto et al., 2018Pinto LC, Mello CR, Norton LD, Poggere GC, Owens PR, Curi N. A hydropedological approach to a mountainous Clayey Humic Dystrudept in the Mantiqueira Range, southeastern Brazil. Sci Agric. 2018;75:60-9. https://doi.org/10.1590/1678-992x-2016-0144
https://doi.org/10.1590/1678-992x-2016-0...
) and the soil compaction caused by the exposition of the soil to raindrops, as it was observed in the treatments BS and OBS. The OSV treatment is constituted mainly by Brachiaria decumbens, this grass that can provide greater soil structure, increase the soil aggregation and permeability (Bono et al., 2012Bono JAM, Macedo MCM, Tormena CA, Nanni MR, Gomes EP, Müller MML. Infiltração de água no solo em um Latossolo Vermelho da região sudoeste dos cerrados com diferentes sistemas de uso e manejo. Rev Bras Cienc Solo. 2012;36:1845-53. https://doi.org/10.1590/S0100-06832012000600019
https://doi.org/10.1590/S0100-0683201200...
).

The variation in the infiltration between BS and OBS can be explained by the fact that olive plantation was very young to have an effect in soil properties. In addition, the BS treatment is managed through plowing and harrowing in full area, allowing a larger area of water infiltration, whereas in OBS treatment, these practices are performed only between the rows. However, there is a tendency for this behavior to reverse over the time.

The variation in soil infiltration between the treatments with cover crops can be explained by the system root morphology of each cover crops, besides of the spatial variability that characterized the soil infiltration in Cambisols (Cardoso et al., 2013Cardoso DP, Carvalho GJ, Silva MLN, Freitas DAF, Avanzi JC. Atributos fitotécnicos de plantas de cobertura para a proteção do solo Attributes phytotechnical of cover plants for soil protection. Revista Verde. 2013;8:19-24.). According to Krstić et al. (2018)Krstić Đ, Vujić S, Jaćimović G, D’Ottavio P, Radanović Z, Erić P, Ćupina B. The effect of cover crops on soil water balance in rain-fed conditions. Atmosphere. 2018;9:492. https://doi.org/10.3390/atmos9120492
https://doi.org/10.3390/atmos9120492...
, the choice of which cover crop depends on the farmer’s objective. The legume cover crops (jack beans and sunn hemp) are chosen for the fast-growing and the ability to carry out the biological nitrogen fixation. On the other side, the cereals cover crops that are chosen because of increasing the content of soil organic matter and stabilizing the soil aggregates by their dense root system (Cardoso et al., 2013Cardoso DP, Carvalho GJ, Silva MLN, Freitas DAF, Avanzi JC. Atributos fitotécnicos de plantas de cobertura para a proteção do solo Attributes phytotechnical of cover plants for soil protection. Revista Verde. 2013;8:19-24.; Wittwer et al., 2017Wittwer RA, Dorn B, Jossi W, van der Heijden MGA. Cover crops support ecological intensification of arable cropping systems. Sci Rep. 2017;7:41911. https://doi.org/10.1038/srep41911
https://doi.org/10.1038/srep41911...
).

Vegetation cover index

Regarding the relation between water erosion and cover vegetation, it is crucial to have a good soil cover in periods with greater erosivity. Nevertheless, in periods with low rainfall, the vegetation cover greatly contributes to temperature regulation and water availability in the soil, which favors plant growth and development (Souza et al., 2010Souza FS, Silva MLN, Curi N, Avanzi JC, Von Pinho RG, Lima GC. Índice de cobertura vegetal pela cultura do milho no período de chuvas intensas no sul de Minas Gerais. Cienc Agrotec. 2010;34:345-51. https://doi.org/10.1590/S1413-70542010000200011
https://doi.org/10.1590/S1413-7054201000...
). Espejo-Pérez et al. (2013)Espejo-Pérez AJ, Rodríguez-Lizana A, Ordóñez R, Giráldez JV. Soil loss and runoff reduction in olive-tree dry-farming with cover crops. Soil Sci Soc Am J. 2013;77:2140-8. https://doi.org/10.2136/sssaj2013.06.0250
https://doi.org/10.2136/sssaj2013.06.025...
demonstrated that the use of cover crops associated with olive trees is more appropriate when cover rates remain between 30 and 87 %.

The vegetation cover index had a difference in both periods. It can be explained by the higher precipitation in the period of 2015/2016 (Table 5), which favored crop development. In the treatment OSV, the low values of the vegetation cover index corresponded to the dates of vegetation weeding, as detailed in table 2. The curves of the cover crops (jack bean, millet, and sunn hemp) intercropped with olive cultivation presented a bell-type curve, also observed by Cardoso et al. (2012)Cardoso DP, Silva MLN, Carvalho GJ, Freitas DAF, Avanzi JC. Plantas de cobertura no controle das perdas de solo, água e nutrientes por erosão hídrica. Rev Bras Eng Agr Amb. 2012;16:632-8. https://doi.org/10.1590/S1415-43662012000600007
https://doi.org/10.1590/S1415-4366201200...
. In April, vegetation cover index decreases due to low rainfall and senescence of the cover crop leaves, because of the end of the crop cycle.

The spontaneous vegetation presented high growth variability in relation to time and space, which caused differences between the values of the cover indices between both studied periods. The same behavior was observed by Taguas et al. (2017)Taguas EV, Vanderlinden K, Pedrera-Parrilla A, Giráldez JV, Gómez JA. Spatial and temporal variability of spontaneous grass cover and its influence on sediment losses in an extensive olive orchard catchment. Catena. 2017;157:58-66. https://doi.org/10.1016/j.catena.2017.05.017
https://doi.org/10.1016/j.catena.2017.05...
when they were studying the spatial and temporal variability of the cover plants (grasses) and their effects over erosion in olive cultivation.

Comparing the OSV treatment with the treatments with cover crops, we verify that OSV showed a high initial growth rate during some critical periods, from December to January, providing a cover vegetation index greater than 50 %. The good development of the spontaneous vegetation was favored by the climatic conditions and the seed bank present in the experimental area, along with the history of the study area and the adopted management (Nichols et al., 2015Nichols V, Verhulst N, Cox R, Govaerts B. Weed dynamics and conservation agriculture principles: a review. Field Crop Res. 2015;183:56-68. https://doi.org/10.1016/j.fcr.2015.07.012
https://doi.org/10.1016/j.fcr.2015.07.01...
).

Comparing the cover plants with each other, we verify that jack beans stood out with the highest vegetation cover indexes, 81 and 60 %, in the first and second periods, respectively. Cardoso et al. (2012)Cardoso DP, Silva MLN, Carvalho GJ, Freitas DAF, Avanzi JC. Plantas de cobertura no controle das perdas de solo, água e nutrientes por erosão hídrica. Rev Bras Eng Agr Amb. 2012;16:632-8. https://doi.org/10.1590/S1415-43662012000600007
https://doi.org/10.1590/S1415-4366201200...
also identified a higher vegetation cover index for jack beans when compared to sunn hemp and millet. The tropical climate revealed the importance of maintaining a continuous vegetation cover, especially during the summer season, when the rainfall erosivity is high, presenting a great risk of water erosion.

Variation of soil organic matter

The variation of SOM showed a negative value in bare soil in the topsoil (Table 6), this can be explained by the high impact of the erosion in the exposed soil, the soil particles transport and the acceleration of the organic matter decomposition (Almagro and Martínez-Mena, 2014Almagro M, Martínez-Mena M. Litter decomposition rates of green manure as affected by soil erosion, transport and deposition processes, and the implications for the soil carbon balance of a rainfed olive grove under a dry Mediterranean climate. Agr Ecosyst Environ. 2014;196:167-77. https://doi.org/10.1016/j.agee.2014.06.027
https://doi.org/10.1016/j.agee.2014.06.0...
). The behaviors of the variation of SOM in different treatments are consistent with the mean soil losses in the two periods (Tables 6 and 7). Gómez et al. (2009)Gómez JA, Sobrinho TA, Giráldez JV, Fereres E. Soil management effects on runoff, erosion and soil properties in an olive grove of Southern Spain. Soil Till Res. 2009;102:5-13. https://doi.org/10.1016/j.still.2008.05.005
https://doi.org/10.1016/j.still.2008.05....
demonstrated that the SOM in olive orchards has a positive correlation with infiltration and soil aggregation, which proves the important role of cover crops in improving soil conditions in olive orchards (Gómez et al., 2009Gómez JA, Sobrinho TA, Giráldez JV, Fereres E. Soil management effects on runoff, erosion and soil properties in an olive grove of Southern Spain. Soil Till Res. 2009;102:5-13. https://doi.org/10.1016/j.still.2008.05.005
https://doi.org/10.1016/j.still.2008.05....
; Soriano et al., 2014Soriano M-A, Álvarez S, Landa BB, Gómez JA. Soil properties in organic olive orchards following different weed management in a rolling landscape of Andalusia, Spain. Renew Agr Food Syst. 2014;29:83-91. https://doi.org/10.1017/S1742170512000361
https://doi.org/10.1017/S174217051200036...
). However, the OM treatment showed a low SOM variation; contrary to that, it was expected by cereal crop crops.

Soil loss assessment

There was no significant difference between BS and OBS, confirming the predisposition of olive cultivation to water erosion (Table 7). The high values of soil loss in the BS and OBS can be explained by the high Cambisols erodibility. According to Silva et al. (2005)Silva AM, Silva MLN, Curi N, Lima JM, Avanzi JC, Ferreira MM. Perdas de solo, água, nutrientes e carbono orgânico em Cambissolo e Latossolo sob chuva natural. Pesq Agropec Bras. 2005;40:1223-30. https://doi.org/10.1590/S0100-204X2005001200010
https://doi.org/10.1590/S0100-204X200500...
, these soils are considered shallow and reach saturation levels more quickly, thus, reducing the infiltration rate and increasing the surface runoff, especially in the absence of ground vegetal cover (Figure 2). Moreover, the crusting caused by the raindrop impact contributes to the formation of an impermeable layer, which contributes to the increase of water erosion. Nevertheless, the high values of loss that correspond to the first experimental period are due to the impact of installing the standard plots and planting the olive trees, providing great soil management.

The differences between different treatments and periods can be explained by the similar value of the vegetation cover index (Table 5) and by the soil protection by organic matter, resultant from periodic cutting operations (Sastre et al., 2017Sastre B, Barbero-Sierra C, Bienes R, Marques MJ, García-Díaz A. Soil loss in an olive grove in Central Spain under cover crops and tillage treatments, and farmer perceptions. J Soils Sediments. 2017;17:873-88. https://doi.org/10.1007/s11368-016-1589-9
https://doi.org/10.1007/s11368-016-1589-...
). According to Zuazo and Pleguezuelo (2008)Zuazo VHD, Pleguezuelo CRR. Soil-erosion and runoff prevention by plant covers. A review. Agron Sustain Dev. 2008;28:65-86. https://doi.org/10.1051/agro:2007062
https://doi.org/10.1051/agro:2007062...
, the cover crops control the soil erosion in two ways. Firstly, in the short term, the cover crops intercept the rainfall and protect the soil against the impact of rainfall drops. Secondly, in the long term, the vegetation influences the fluxes of water and sediments by improving the soil aggregation, increasing water infiltration, and soil organic matter.

The high soil losses observed in the period of 2015/2016 in the plots with cover crops (millet and jack beans) were due to soil preparation and planting practices, which are explained by the lower soil loss in the plots with spontaneous vegetation (OSV). In this treatment, manual weeding was performed, with a preparation of the planting furrows in the direction of the slope. During the furrows planting, preferential paths may be formed, where water can concentrate its flow and increase its disintegrating and transporting power. Studies in olive orchards in the Mediterranean region indicated that cover crops which are sown or spontaneous can reduce soil erosion in the olive orchard by more than 92 % compared to management based in tillage (Repullo-Ruibérriz et al., 2018Repullo-Ruibérriz MAT, Ordóñez-Fernández R, Giráldez JV, Márquez-García J, Laguna A, Carbonell-Bojollo R. Efficiency of four different seeded plants and native vegetation as cover crops in the control of soil and carbon losses by water erosion in olive orchards. Land Degrad Dev. 2018;29:2278-90. https://doi.org/10.1002/ldr.3023
https://doi.org/10.1002/ldr.3023...
). However, the capacity of cover crops to reduce erosion depends on the characteristics of the plant species (De Baets et al., 2009De Baets S, Poesen J, Reubens B, Muys B, De Baerdemaeker J, Meersmans J. Methodological framework to select plant species for controlling rill and gully erosion: application to a Mediterranean ecosystem. Earth Surf Processes Landforms. 2009;34:1374-92. https://doi.org/10.1002/esp.1826
https://doi.org/10.1002/esp.1826...
).

Water loss assessment

The results of water loss illustrate the importance of cover crops in cultivated areas due to the increase in water infiltration rate, as observed by Almeida et al. (2018)Almeida WS, Panachuki E, Oliveira PTS, Menezes RS, Sobrinho TA, Carvalho DF. Effect of soil tillage and vegetal cover on soil water infiltration. Soil Till Res. 2018;175:130-8. https://doi.org/10.1016/j.still.2017.07.009
https://doi.org/10.1016/j.still.2017.07....
. High water loss in cultivated soils is critical for crops of agricultural species, notably in shallow and declining soils, and it can be aggravated during periods of higher water deficit, considering that, along with water, nutrients and organic matter, important components used by plants for growth and development, can be lost.

The first period presented the highest values for water loss in relation to the same treatments in the second period (Table 8), demonstrating the effect of management and the greater soil change in the first period in relation to the second, and the distinct precipitation between the studied periods (Table 3). In addition, cover crops presented a different performance in reducing water loss. Both treatments, BS and OBS, have uncovered soil, high silt content and high slope, these conditions promote soil sealing. The sealing makes the soil surface less permeable and increases the runoff (Assouline and Ben-Hur, 2006Assouline S, Ben-Hur M. Effects of rainfall intensity and slope gradient on the dynamics of interrill erosion during soil surface sealing. Catena. 2006;66:211-20. https://doi.org/10.1016/j.catena.2006.02.005
https://doi.org/10.1016/j.catena.2006.02...
; Carvalho et al., 2015Carvalho DF, Eduardo EN, Almeida WS, Santos LAF, Sobrinho TA. Water erosion and soil water infiltration in different stages of corn development and tillage systems. Rev Bras Eng Agr Amb. 2015;19:1072-8. https://doi.org/10.1590/1807-1929/agriambi.v19n11p1072-1078
https://doi.org/10.1590/1807-1929/agriam...
).

The OSV treatment proved to be the most efficient in reducing water loss, with an LRE of 8.04 % and an SRC of 7.87 % in 2015/2016. In 2016/2017, treatments OSV, OJB, and OSH presented similar values for the evaluation parameters of water loss. In a study conducted by Gómez et al. (2004)Gómez JA, Romero P, Giráldez JV, Fereres E. Experimental assessment of runoff and soil erosion in an olive grove on a Vertic soil in southern Spain as affected by soil management. Soil Use Manage. 2004;20:426-31. https://doi.org/10.1079/SUM2004275
https://doi.org/10.1079/SUM2004275...
comparing the SRC in different management systems in olive cultivation in the region of Córdoba in Spain, an SRC value of 2.55 % was obtained for olive system associated with spontaneous vegetation, with low losses of soil and water.

Crop performance of olive cultivation

The vegetation cover index presented an inverse relation of the crop performance. The plants with the highest performance occurred in the plots with low vegetation cover rates and with higher soil and water losses (Tables 5 and 9).

By comparing the treatments with each other, we observed that the olive cultivation on bare soil (OBS) presented the highest soil and water losses and low vegetation cover index. The other treatments presented similar behavior. Olive cultivation intercropped with jack beans (OJB) and with millet (OCM) showed greater variability due to the high soil and water losses in the first period (Tables 7 and 8).

The results observed in the field illustrate the interference of the associated treatment over plant development, given that millet plants presented fast growth in January and February, shading the olive plants and interfering in their development. Despite this, the crowning in the millet was carried out in January but was not sufficient to attenuate the effect of the cover crop shade.

In this study, the olive plants of the experiment are still very young, so much more subject to cover crops competition. It is therefore still impossible to separate the effects of the cover crops protection to erosion from their competition with olive plants. Moreover, according to the results of soil loss in olive with bare soil treatment, the average loss is 300.48 Mg yr-1ha-1 (2.32 cm yr-1). In the future, with ten years of exploitation with the same management in the bare soil, it may cause a cumulative loss of 23.2 cm of soil. Considering that the depth of this soil is 0.60 m, as it was observed in the field, the soil will disappear in a few decades.

These results might support farmer’s decisions to remove vegetation cover using chemical or manual methods, which is a common practice in olive orchards in Brazil and Mediterranean countries, as reported by several authors (Gómez et al., 2014Gómez JA, Infante-Amate J, Molina MG, Vanwalleghem T, Taguas EV, Lorite I. Olive cultivation, its impact on soil erosion and its progression into yield impacts in Southern Spain in the past as a key to a future of increasing climate uncertainty. Agriculture. 2014;4:170-98. https://doi.org/10.3390/agriculture4020170
https://doi.org/10.3390/agriculture40201...
; Ibáñez et al., 2014Ibáñez J, Martínez-Valderrama J, Taguas EV, Gómez JA. Long-term implications of water erosion in olive-growing areas in southern Spain arising from a model-based integrated assessment at hillside scale. Agr Syst. 2014;127:70-80. https://doi.org/10.1016/j.agsy.2014.01.006
https://doi.org/10.1016/j.agsy.2014.01.0...
; Taguas et al., 2015Taguas EV, Arroyo C, Lora A, Guzmán G, Vanderlinden K, Gómez JA. Exploring the linkage between spontaneous grass cover biodiversity and soil degradation in two olive orchard microcatchments with contrasting environmental and management conditions. Soil. 2015;1:651-64. https://doi.org/10.5194/soil-1-651-2015
https://doi.org/10.5194/soil-1-651-2015...
). It is common for farmers to consider difficulties in managing olive cultivation with intercropped cover plants due to the additional management operations and, consequently, additional costs (Posthumus et al., 2015Posthumus H, Deeks LK, Rickson RJ, Quinton JN. Costs and benefits of erosion control measures in the UK. Soil Use Manage. 2015;31:16-33. https://doi.org/10.1111/sum.12057
https://doi.org/10.1111/sum.12057...
).

Conversely, Sastre et al. (2016)Sastre B, Pérez-Jiménez MA, Bienes R, García-Díaz A, Lorenzo C. The effect of soil management on olive yield and VOO quality in a rainfed olive grove of Central Spain. J Chem. 2016;2016:4974609. https://doi.org/10.1155/2016/4974609
https://doi.org/10.1155/2016/4974609...
found no effect of cover crops over fruit yield or olive oil quality, highlighting the importance of cover crops in olive orchard. Also, cover crops reduce water erosion, improve water recharge and water retention (Bombino et al., 2019Bombino G, Denisi P, Gómez JA, Zema DA. Water infiltration and surface runoff in steep clayey soils of olive groves under different management practices. Water. 2019;11:240. https://doi.org/10.3390/w11020240
https://doi.org/10.3390/w11020240...
), increase soil organic matter and carbon accumulation (Cerdà et al., 2018Cerdà A, Rodrigo-Comino J, Giménez-Morera A, Keesstra SD. Hydrological and erosional impact and farmer’s perception on catch crops and weeds in citrus organic farming in Canyoles river watershed, Eastern Spain. Agr Ecosyst Environ. 2018;258:49-58. https://doi.org/10.1016/j.agee.2018.02.015
https://doi.org/10.1016/j.agee.2018.02.0...
; Guimarães et al., 2018Guimarães DV, Silva MLN, Beiniach A, Bispo DFA, Contins JGP, Curi N. Relationship between soil organic matter fractions and cover plants in Olive post planting. Rev Bras Frutic. 2018;40:e-027. https://doi.org/10.1590/0100-29452018027
https://doi.org/10.1590/0100-29452018027...
; Novara et al., 2019Novara A, Pulido M, Rodrigo-Comino J, Di Prima S, Smith P, Gristina L, Giménez-Morera A, Terol E, Salesa D, Keesstra S. Long-term organic farming on a citrus plantation results in soil organic carbon recovery. Cuad Investig Geográfica. 2019;45:271-86. https://doi.org/10.18172/cig.3794
https://doi.org/10.18172/cig.3794...
), and there is a large reserve of nutrients coming from cover crops biomass decomposition (Gómez-Muñoz et al., 2014Gómez-Muñoz B, Hatch DJ, Bol R, García-Ruiz R. Nutrient dynamics during decomposition of the residues from a sown legume or ruderal plant cover in an olive oil orchard. Agr Ecosyst Environ. 2014;184:115-23. https://doi.org/10.1016/j.agee.2013.11.020
https://doi.org/10.1016/j.agee.2013.11.0...
). Thus, an adequate olive orchard management should conciliate crop performance and environmental aspects of the crop, especially in Cambisols. It is necessary to have more studies that evaluate the erosion behavior in olive orchards in tropical conditions with different management practices and cover crops.

CONCLUSION

The olive orchards planted in shallow and sloping soils without cover crops showed unsustainable soil loss, crusting, and sealing the superficial soil layer, which can progress quickly for soil degradation.

The mitigation of soil erosion in olive cultivation is associated with the adoption of cover crops allowing the improvement of soil conditions by soil organic matter accumulation, increasing soil infiltration, and reducing runoff.

Spontaneous vegetation in between planting rows increases the performance of olive trees, improves soil conditions, and reduces environmental impact. However, the use of cover crops in the first year after planting the olive orchards needs some special care to conciliate olive plantation growth with soil protection.

ACKNOWLEDGMENTS

The authors thank the World Academy of Sciences (TWAS), the National Council for Scientific and Technological Development (CNPq) (processes 190119/2013-6, 306511/2017-7, 150.689/2017-8, and 202938/2018-2), the Foundation for Research Support of the State of Minas Gerais (FAPEMIG) (processes APQ-00802-18 and CAG-APQ-01053-15), and the Coordination for the Improvement of Personnel (CAPES) for the financial support (CAPES/PROEX AUXPE 593/2018). We are also grateful for the guidance of Professors John Quinton and Mike James of Lancaster University.

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Publication Dates

  • Publication in this collection
    22 Apr 2020
  • Date of issue
    2020

History

  • Received
    29 July 2019
  • Accepted
    09 Dec 2019
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