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Potassium Fertilization of Eucalyptus in an Entisol in Low-Elevation Cerrado

ABSTRACT

Potassium (K) is one of the most highly accumulating nutrients in Eucalyptus and, consequently, is heavily exported by the harvesting of wood. Moreover, its availability in the soil in most Brazilian plantation areas is very low, especially in the regions of the Cerrado biome, which has soils with low natural fertility and marked water deficits, implying a lack of nutrient supply and, consequently, a less efficient water use. Our objective was to evaluate the effects of K fertilization on Eucalyptus biomass yield, the addition of nutrients to the soil by leaf deposition, nutrient use efficiency, and soil K availability. The experiment was conducted with clone I144 (Eucalyptus urophylla) in the municipality of Três Lagoas/MS, in a Neossolo Quartzarênico Órtico (Entisol). The experimental design was a randomized block with four treatments and five replicates. The treatments consisted of four K doses (0, 90, 135, and 180 kg ha-1 of K2O) as KCl. Plant biomass production (leaves, branches, trunk, and bark), senescent leaf deposition, leaf nutrient concentrations, nutrient accumulation in the different plant compartments, nutrient use efficiency, addition of nutrients to the soil by leaf deposition, and soil K availability were evaluated. Potassium fertilization increased the biomass yield of Eucalyptus plants, senescent leaf K content, the transfer of K to the soil, the accumulation of K in the aerial plant parts, and the K content in the soil. However, it did not influence senescent leaf deposition yield or plant K use efficiency.

senescent leaf nutrients; fertilization; biomass; macronutrients; nutritional efficiency

INTRODUCTION

Areas of forestation with Eucalyptus species have occupied new regions of Brazil in addition to the traditional ones, such as the south and southeast (Santana et al., 2008Santana RC, Barros NF, Leite HG, Comerford NB, Novais RF. Estimativa de biomassa de plantios de eucalipto no Brasil. Rev Arvore. 2008;32:697-706. https://doi.org/10.1590/S0100-67622008000400011
https://doi.org/10.1590/S0100-6762200800...
). These new regions present soils of low natural fertility, in addition to high and frequent water deficits, such as in the Cerrado biome (Gava et al., 1997Gava JL, Gonçalves JLM, Shibata FY, Corradini L. Eficiência relativa de fertilizantes fosfatados no crescimento inicial de eucalipto cultivado em solos do cerrado. Rev Bras Cienc Solo. 1997;21:497-504. https://doi.org/10.1590/S0100-06831997000300020
https://doi.org/10.1590/S0100-0683199700...
; Oliveira Neto et al., 2010; Alves, 2011Alves FF. Seca de ponteiros e crescimento de clones de eucalipto em diferentes doses de adubação. [dissertação]. Viçosa, MG: Universidade Federal de Viçosa; 2011.). Moreover, anthropogenic activities such as extensive livestock farming have resulted in the long-term impoverishment of these soils, with decreased nutrient amounts and, consequently, negative impacts on the newly forested areas (Gazola et al., 2015Gazola RN, Buzetti S, Teixeira Filho MCM, Dinalli RP, Moraes MLT, Celestrino TS, Silva PHM, Dupas E. Doses of N, P and K in the cultivation of eucalyptus in soil originally under Cerrado vegetation. Semin: Cienc Agrar. 2015;36:1895-912. https://doi.org/10.5433/1679-0359.2015v36n3Supl1p1895
https://doi.org/10.5433/1679-0359.2015v3...
).

Potassium (K) is one of the nutrients that most limits the Eucalyptus yield in Brazil, and its application is relevant to successive rotations (Gava, 1997Gava JL. Efeito da adubação potássica em plantios de E. grandis conduzidos em segunda rotação em solos com diferentes teores de potássio trocável. Série Técnica IPEF. 1997;11:84-94.; Silveira and Malavolta, 2000Silveira RLVA, Malavolta E. Nutrição e adubação potássica em Eucalyptus. Piracicaba: Potafos; 2000. (Informações Agronômicas, 91). p. 1-10.). Among the nutrients demanded by different species of Eucalyptus, K plays a major role, and K fertilization has allowed significant increases in yield in most of the planted areas due to the low levels of K available in soils (Almeida et al., 2007Almeida JCR, Laclau JP, Gonçalves JLM, Moreira RM, Rojas JSD. Índice de área foliar de Eucalyptus grandis em resposta à adubação com potássio e sódio. In: Anais do I Seminário de recursos hídricos da bacia hidrográfica do Paraíba do Sul; 2007; Taubaté. Taubaté: Universidade de Taubaté; 2007. p. 1-7.).

Several studies report responses of Eucalyptus in soil with K contents below 1.0 mmolc dm-3. Gava (1997)Gava JL. Efeito da adubação potássica em plantios de E. grandis conduzidos em segunda rotação em solos com diferentes teores de potássio trocável. Série Técnica IPEF. 1997;11:84-94., in plantations of Eucalyptus grandis with K contents of 0.4 mmolc dm-3, Almeida et al. (2010)Almeida JCR, Laclau J-P, Gonçalves JLM, Ranger J, Saint-André L. A positive growth response to NaCl applications in Eucalyptus plantations established on K-deficient soils. Forest Ecol Manag. 2010;259:1786-95. https://doi.org/10.1016/j.foreco.2009.08.032
https://doi.org/10.1016/j.foreco.2009.08...
, in E. grandis with K levels of 0.2 mmolc dm-3, Melo (2014)Melo EASC. Nutrição e fertilização de plantações clonais de eucalipto sob diferentes condições edafoclimáticas [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2014., in clonal eucalypt plantations under different soil and climatic conditions with K contents between 0.3 and 0.4 mmolc dm-3, and Gazola et al. (2015)Gazola RN, Buzetti S, Teixeira Filho MCM, Dinalli RP, Moraes MLT, Celestrino TS, Silva PHM, Dupas E. Doses of N, P and K in the cultivation of eucalyptus in soil originally under Cerrado vegetation. Semin: Cienc Agrar. 2015;36:1895-912. https://doi.org/10.5433/1679-0359.2015v36n3Supl1p1895
https://doi.org/10.5433/1679-0359.2015v3...
, in Eucalyptus urophylla stands in Neossolo Quartzarênico (Entisol) with initial K contents of 0.2 mmolc dm-3. Thus, the responses of Eucalyptus to K application are not consistent when K contents of the soil are higher; positive responses were obtained only in soils whose K contents did not exceed 1.0 mmolc dm-3 (Silveira and Malavolta, 2000Silveira RLVA, Malavolta E. Nutrição e adubação potássica em Eucalyptus. Piracicaba: Potafos; 2000. (Informações Agronômicas, 91). p. 1-10.).

In addition to soil conditions, it is considered that the crop presents a high accumulation of K in the trunk and in the total aerial parts (N > Ca > K > Mg > P) (Silveira et al., 2005Silveira RLVA, Gava JL, Malavolta E. O potássio na cultura do eucalipto. In: Yamada T, Roberts TL, editores. Potássio na agricultura brasileira. Piracicaba. Potafos; 2005. p. 523-90.; Andrade et al., 2006Andrade GC, Bellote AFJ, Silva HD, Rizzi NE, Gava JL. Acúmulo de nutrientes na biomassa e na serapilheira de Eucalyptus grandis em função da aplicação de lixo urbano e de nutrientes minerais. Bol Pesq Fl. 2006;53:109-36.; Faria et al., 2008Faria GE, Barros NF, Cunha VLP, Martins IS, Martins RCC. Avaliação da produtividade, conteúdo e eficiência de utilização de nutrientes em genótipos de Eucalyptus spp. no Vale do Jequitinhonha, MG. Cienc Florest. 2008;18:363-73. https://doi.org/10.5902/19805098448
https://doi.org/10.5902/19805098448...
; Benatti, 2013)Benatti BP. Compartimentalização de biomassa e de nutrientes em estruturas de plantas de eucalipto cultivadas em solos distintos [dissertação]. Lavras: Universidade Federal de Lavras; 2013., which demonstrates the great requirement of Eucalyptus for this nutrient. This explains the high demand for this nutrient in later cycles of the crop due to its removal from the soil by extraction and, consequently, its export by wood harvesting. This demand varies between genetic materials (species clones or Eucalyptus hybrids); cultivars with a higher nutritional efficiency require lower amounts of nutrient to produce the same amount of biomass in relation to the less efficient ones.

The clone I144 (Eucalyptus urophylla), used in this study, is more productive and more efficient in the uptake and use of N, P, K (Faria et al., 2008Faria GE, Barros NF, Cunha VLP, Martins IS, Martins RCC. Avaliação da produtividade, conteúdo e eficiência de utilização de nutrientes em genótipos de Eucalyptus spp. no Vale do Jequitinhonha, MG. Cienc Florest. 2008;18:363-73. https://doi.org/10.5902/19805098448
https://doi.org/10.5902/19805098448...
; Pinto et al., 2011Pinto SIC, Furtini Neto AE, Neves JCL, Faquin V, Moretti BS. Eficiência nutricional de clones de eucalipto na fase de mudas cultivados em solução nutritiva. Rev Bras Cienc Solo. 2011;35:523-33. https://doi.org/10.1590/S0100-06832011000200021
https://doi.org/10.1590/S0100-0683201100...
). This characteristic is desirable because these clones can be established in soils with less availability of these nutrients without compromising biomass production. This finding is of great importance because it is desirable to establish certain materials (species clones or eucalyptus hybrids) that are compatible with soil conditions, which are among the most limiting factors for crop development.

Regarding the crop response to K fertilization throughout the crop cycle, Gazola et al. (2015)Gazola RN, Buzetti S, Teixeira Filho MCM, Dinalli RP, Moraes MLT, Celestrino TS, Silva PHM, Dupas E. Doses of N, P and K in the cultivation of eucalyptus in soil originally under Cerrado vegetation. Semin: Cienc Agrar. 2015;36:1895-912. https://doi.org/10.5433/1679-0359.2015v36n3Supl1p1895
https://doi.org/10.5433/1679-0359.2015v3...
found that the response obtained with a higher dose at 21 months in relation to previous evaluations was associated with a higher requirement for this nutrient during the crop cycle. The research conducted by Melo et al. (2016)Melo EASC, Gonçalves JLM, Rocha JHT, Hakamada RE, Bazani JH, Wenzel AVA, Arthur Junior JC, Borges JS, Malheiros R, Lemos CCZ, Ferreira EVO, Ferraz AV. Responses of clonal Eucalypt plantations to N, P and K fertilizer application in different edaphoclimatic conditions. Forests. 2016;7:0002. https://doi.org/10.3390/f7010002
https://doi.org/10.3390/f7010002...
, studied the nutrition and fertilization of clonal Eucalyptus plantations under different edaphoclimatic conditions, the authors verified that the K requirement increased with age in all evaluated locations. Similarly, Gonçalves et al. (2008)Gonçalves JLM, Stape JL, Laclau J-P, Bouillet J-P, Ranger J. Assessing the effects of early silvicultural management on long-term site productivity of fast-growing eucalypt plantations: the Brazilian experience. Southern Forests. 2008;70:105-18. https://doi.org/10.2989/SOUTH.FOR.2008.70.2.6.534
https://doi.org/10.2989/SOUTH.FOR.2008.7...
found that the application of K led to increases in the responses with aging of the culture, as opposed to the applications of N and P, where diminished responses were observed with age. These findings, according to Barros et al. (1990)Barros NF, Novais RF, Neves JCL. Fertilização e correção do solo para o plantio de eucalipto. In: Barros NF, Novais RF. Relação solo-eucalipto. Viçosa, MG: Editora Folha de Viçosa; 1990. p. 127-86., were due to the increase in the critical level of K in the soil over time.

Ideally, Eucalyptus crops are fertilized before canopy closure, and after this period, there is practically no response from the crop to fertilization, due to the beginning of the biogeochemical cycle. This cycle is responsible for the maintenance and supply of nutrients to the plants through the deposition of leaves, branches, and other parts of the plant on the soil, which form the litter layer, providing nutrients through decomposition (Laclau et al., 2003Laclau J-P, Deleporte F, Ranger J, Bouillet J-P, Kazzotti G. Nutrient dynamics throughout the rotation of Eucalyptus clonal stands in Congo. Ann Bot. 2003;91:879-92. https://doi.org/10.1093/aob/mcg093
https://doi.org/10.1093/aob/mcg093...
; Silva, 2011Silva PHM. Impactos das doses e do parcelamento da fertilização na produtividade, lixiviação e ciclagem de nutrientes em plantações de eucalipto [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2011.; Benatti, 2013Benatti BP. Compartimentalização de biomassa e de nutrientes em estruturas de plantas de eucalipto cultivadas em solos distintos [dissertação]. Lavras: Universidade Federal de Lavras; 2013.).

The increase in the fertilizer dose led to higher yields and nutrient cycling in a Eucalyptus crop (Silva et al., 2013Silva PHM, Poggiani F, Libardi PL, Gonçalves AN. Fertilizer management of eucalypt plantations on sandy soil in Brazil: initial growth and nutrient cycling. Forest Ecol Manag. 2013;301:67-78. https://doi.org/10.1016/j.foreco.2012.10.033
https://doi.org/10.1016/j.foreco.2012.10...
). The understanding of nutrient removal from the soil by its extraction and, consequently, the export of nutrients upon wood harvesting is of great importance to the understanding of the inputs and outputs of the nutrients during the crop cycle (nutrient balance). To maintain and improve the soil chemical properties in Eucalyptus plantations, the nutrient balance should be adequately monitored, mainly by fertilization and the correct management of crop residues (Silva, 2011Silva PHM. Impactos das doses e do parcelamento da fertilização na produtividade, lixiviação e ciclagem de nutrientes em plantações de eucalipto [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2011.).

Our objective was to evaluate the effects of K fertilization on biomass yield, nutrient accumulation in aerial part compartments, the transfer of nutrients to the soil by leaf deposition, and the efficiency of nutrient use by a Eucalyptus crop in the Cerrado, as well as the availability of soil K at different ages and depths.

MATERIALS AND METHODS

The experiment was conducted from September 2011 to July 2017 at the Renascença Farm (Fazenda Renascença), an agricultural fund managed by Cargill Agrícola S/A, located in the municipality of Três Lagoas, state of Mato Grosso do Sul, Brazil. According to the Köppen system, the climate of the region is classified as Aw, defined as humid tropical with a rainy season in the summer and a dry season in the winter. The rainfall data recorded during the execution of the experiment are presented in figure 1.

Figure 1
Monthly accumulations of rainfall registered at the Três Lagoas Automatic Meteorological Station, 2012/17.

The experimental area was occupied by natural vegetation (plants of the Cerrado biome); prior to the implementation of the experiment, the area consisted of degraded pasture, with a slope of 4 %. The soil presented a sandy texture with only 8 % clay, classified as Neossolo Quartzarênico Órtico (Entisol) (Santos et al., 2013Santos HG, Jacomine PKT, Anjos LHC, Oliveira VA, Oliveira JB, Coelho MR, Lumbreras JF, Cunha TJF. Sistema brasileiro de classificação de solos. 3. ed. rev. ampl. Rio de Janeiro: Embrapa Solos; 2013.).

Before the experiment, soil samples were collected at layers of 0.00 to 0.20 and 0.20 to 0.40 m to determine the soil chemical properties, according to the method described by van Raij et al. (2001)van Raij B, Andrade JC, Cantarella H, Quaggio JA. Análise química para avaliação da fertilidade de solos tropicais. Campinas: Instituto Agronômico de Campinas; 2001.. The chemical properties at the layer of 0.00-0.20 m were as follows: pH(CaCl2) 4.2; 7.4 g dm-3 of organic matter (OM); 1 mg dm-3 P in resin; K+, Ca2+, Mg2+, H+Al, and Al3+ contents of 0.2, 4.2, 1.9, 17.0, and 4.3 mmolc dm-3, respectively; and a base saturation (V) of 27 %. At the layer of 0.20-0.40 m, we measured the following parameters: pH(CaCl2) 4.2; 6.8 g dm-3 of OM; 1 mg dm-3 P in resin; K+, Ca2+, Mg2+, H+Al, and Al3+ contents of 0.3, 1.6, 1.1, 18.0, and 4.5 mmolc dm-3, respectively; with a V level of 14 %.

The experimental design was a randomized block with four treatments and five repetitions, where four doses of K (0, 90, 135, and 180 kg ha-1 of K2O) were applied in the planting groove and in the cover. The study of these doses was based on the following recommendations for Eucalyptus fertilization under the soil conditions of this research. According to Gonçalves et al. (1997)Gonçalves JLM, van Raij B, Gonçalves JC. Florestais. In: van Raij B, Cantarella H. Quaggio JA, Furlani AMC. Recomendações de adubação e calagem para o estado de São Paulo. 2. ed. rev. atual. Campinas: Instituto Agronômico de Campinas; 1997. p. 245-60. (Boletim técnico, 100)., the dose of 50 kg ha-1 of K2O is ideal for soils with a clay content <15 % and a K content <0.7 mmolc dm-3. However, based on economical considerations, the adequate dose in soils with K contents between 0 and 1.0 mmolc dm-3 is in the range of 120 to 180 kg ha-1 of K2O (Silveira and Malavolta, 2000)Silveira RLVA, Malavolta E. Nutrição e adubação potássica em Eucalyptus. Piracicaba: Potafos; 2000. (Informações Agronômicas, 91). p. 1-10.. This range was selected in our study because it is in accordance with the practices adopted by forestry companies.

Seedlings of Eucalyptus urophylla hybrids (clone I144) were planted in January 2012 with a spacing of 3.0 × 2.5 m. Each plot contained 56 plants, distributed in seven rows of eight plants each, totaling 420 m2. Only 30 plants in the center of the plots were considered, resulting in a total effective sampling area of 225 m2. In September 2011, 1,500 kg ha-1 of limestone of PRNT 80 % and 500 kg ha-1 of gypsum were applied by broadcasting over the total area.

The plantation fertilization involved 70 kg ha-1 of P2O5 (triple superphosphate) and 15 kg ha-1 of N (urea), with 37.5, 37.5, and 50 kg ha-1 of N in the cover (at 2, 9, and 14 months, respectively) as ammonium nitrate. The treatments of K fertilization were 0 kg ha-1 K2O (control); 90 kg ha-1 K2O (15.0, 22.5, 22.5, and 30.0 kg ha-1 K2O); 135 kg ha-1 K2O (15.0, 36.0, 36.0, and 48.0 kg ha-1 K2O), and 180 kg ha-1 of K2O (15.0, 49.5, 49.5, and 66.0 kg ha-1 of K2O), respectively, at planting and at 2, 9, and 14 months after planting. The following micronutrients were applied at planting in all treatments: 1 kg ha-1 of B (boric acid), 1 kg ha-1 of Zn (zinc sulphate), and 1 kg ha-1 of Cu (copper sulphate). In the cover fertilizations with N and K, carried out at 9 and 14 months after planting, 1 kg ha-1 of B (boric acid) was applied to all treatments.

To quantify senescent leaf deposition, sombrite-type screens were fixed onto four Eucalyptus plants per row and between rows, with three collectors per plot distributed within the useful plot area. The collections were carried out quarterly from 30 to 66 months after planting. The data presented correspond to the total amount deposited in this period (accumulated over 36 months).

Samples of fallen leaves were taken to form the composite sample, oven-dried (65 °C for 72 h), and milled in a Willey mill for the chemical nutrient analysis according to the method described by Malavolta et al. (1997)Malavolta E, Vitti GC, Oliveira SA. Avaliação do estado nutricional das plantas: princípios e aplicações. 2. ed. Piracicaba: Potafos; 1997.. The data presented correspond to the mean nutrient contents in the 12 collection periods. The addition of nutrients to the soil by leaf deposition in the period from 30 to 66 months was estimated by multiplying the senescent leaf deposition by the nutrient content in the leaf. At 66 months after planting, four trees in the useful area of each plot of one block were cut, where one tree was representative of the small-diameter class, two represented the intermediate-diameter class, and one was representative of the large-diameter class, according to the method described by Silva (2011)Silva PHM. Impactos das doses e do parcelamento da fertilização na produtividade, lixiviação e ciclagem de nutrientes em plantações de eucalipto [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2011.. Samples of all components of the trees (leaves, branches, and trunk) were collected manually for the determination of the moisture in the laboratory and the subsequent quantification of the dry biomass. Prior to drying, the samples obtained from the trunk were separated into bark and wood.

The plant samples of the different tree components were oven-dried (65 °C for 72 h), weighed, and ground in a Willey mill. Chemical analysis was performed to determine the macronutrient content according to the method described by Malavolta et al. (1997)Malavolta E, Vitti GC, Oliveira SA. Avaliação do estado nutricional das plantas: princípios e aplicações. 2. ed. Piracicaba: Potafos; 1997.. To measure the nutrients in the wood and the bark, the average content obtained in discs collected at the base and the diameter at breast height (DBH) and at 10, 30, 50, and 70 % of the total height were considered. Using the values of the contents in each compartment, the contents in the different components of the trees (mineral mass) were estimated. Nutrient use efficiency was evaluated according to the method presented by Barros et al. (1986)Barros NF, Novais RF, Carmo DN, Neves JCL. Classificação nutricional de sítios florestais - descrição de uma metodologia. Rev Arvore. 1986;10:112-20. as the ratio between the dry biomass of the trunk and the mineral mass of the nutrient accumulated in the trunk.

At 24, 36, 48, 60, and 66 months after planting, four soil samples were collected per plot in the planting line at layers of 0.00-0.20 and 0.20-0.40 m and in the interline (plant canopy projection - perpendicular to the planting line at a distance of 0.50 m from the plant, where K cover fertilization was carried out) at layers of 0.00-0.20 and 0.20-0.40 m (or layers of 0.40-0.60 and 0.60-0.80 m only in the last evaluation), using an auger. The samples were homogenized and an aliquot of each sample was used to form a composite sample; available K levels in the soil were determined according to the method described by van Raij et al. (2001)van Raij B, Andrade JC, Cantarella H, Quaggio JA. Análise química para avaliação da fertilidade de solos tropicais. Campinas: Instituto Agronômico de Campinas; 2001..

Data were analyzed using a regression analysis for the doses of K2O and an analysis of variance (test F) and Tukey’s test at 5 % probability to compare the soil K availability levels among the five evaluation periods (at 24, 36, 48, 60, and 66 months after planting). The SISVAR software (Ferreira, 2008Ferreira DF. SISVAR: um programa para análises e ensino de estatística. Rev Symposium. 2008;6:36-41.) was used for statistical tests.

RESULTS AND DISCUSSION

Potassium fertilization positively influenced leaf and wood biomass production as well as the total biomass of the aerial parts after 66 months (Figures 2a and 2b). The dose of 180 kg ha-1 of K2O provided the highest leaf and wood biomass values and the highest total biomass values, with an increase of 31.5, 17.8, and 18.2 %, respectively, in relation to the control (0 kg ha-1 of K2O). Silva (2011)Silva PHM. Impactos das doses e do parcelamento da fertilização na produtividade, lixiviação e ciclagem de nutrientes em plantações de eucalipto [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2011. evaluated the impacts of fertilization doses and rates in Eucalyptus plantations in the initial growth phase in Neossolo Quartzarênico (Entisol) and found that the biomass production increased with increasing fertilizer doses, at all ages evaluated. At 24 months, the group receiving the highest fertilizer dose had 50 % more biomass compared to the other groups.

In a study carried out in Cerrado soil, Almeida (2009)Almeida JCR. Nutrição, crescimento, eficiência de uso de água e de nutrientes em povoamentos de Eucalyptus grandis fertilizados com potássio e sódio [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2009. concluded that Eucalyptus grandis responds to K fertilization in its leaf area index. Christina et al. (2015)Christina M, Le Maire G, Battie-Laclau P, Nouvellon Y, Bouillet J-P, Jourdan C, Gonçalves JLM, Laclau J-P. Measured and modeled interactive effects of potassium deficiency and water deficit on gross primary productivity and light-use efficiency in Eucalyptus grandis plantations. Glob Change Biol. 2015;21:2022-39. https://doi.org/10.1111/gcb.12817
https://doi.org/10.1111/gcb.12817...
evaluated the effects of K deficiency and water deficit on yield and light use efficiency in E. grandis plantations and found that the plants that did not receive K application had a 64 % lower leaf area index than the plants fertilized with K. In this sense, K fertilization favors an increase in leaf area, which results in a greater intensity of the interception of sunlight and the photosynthetic processes, which in turn results in greater growth. This effect is also due to the functions performed by K in the plant, including the regulation of the osmotic potential of plant cells, thus controlling the opening and closing of the stomata, resulting in a higher water use efficiency (Siddiqui et al., 2008Siddiqui MY, Shah AH, Tariq MA. Effects of fertilization and water stress on Eucalyptus camaldulenses seedlings. J Trop For Sci. 2008;20:205-10.; Taiz and Zeiger, 2013Taiz L, Zeiger E. Fisiología Vegetal. 5. ed. Porto Alegre: Artmed; 2013.). Therefore, plants spend less energy during periods of water deficit, which are common in the study region (Figure 1).

On average, the biomass distribution in the different components of the aerial part of the Eucalyptus plants at 66 months after planting presented the following magnitudes: wood (86 %) > bark (6 %) > leaf (4 %) = branch (4 %). Verão et al. (2016)Verão DS, Bleich ME, Martins NP, Bassotto JM, Mortat AF, Santos AFA. Concentração de nutrientes em Eucalyptus urograndis (Eucalyptus grandis W. Hill ex Maiden x Eucalyptus urophylla S. T Blake) com sete anos de idade na borda sul da Amazônia. Biodiversidade. 2016;15:35-44. evaluated the nutrient content in Eucalyptus urophylla × Eucalyptus grandis at 84 months in an Ultisol from low to medium fertility and found that wood (89.4 %) and bark (8.20 %) are the most representative aerial part biomass fractions. Faria et al. (2008)Faria GE, Barros NF, Cunha VLP, Martins IS, Martins RCC. Avaliação da produtividade, conteúdo e eficiência de utilização de nutrientes em genótipos de Eucalyptus spp. no Vale do Jequitinhonha, MG. Cienc Florest. 2008;18:363-73. https://doi.org/10.5902/19805098448
https://doi.org/10.5902/19805098448...
, investigating the same hybrid used in this study at 57 months in an Oxisol, found that under the conditions of dominance of the closed Cerrado ecosystem, the plants tended to allocate, on average, a larger percentage of trunk biomass (92.3 %) in relation to the plant canopy (7.7 %).

Total senescent leaf deposition yield was not influenced by K fertilization in the period from 30 to 66 months after planting, with a mean yield of 13.2 Mg ha-1 during that period (Figure 2a). Silva (2011)Silva PHM. Impactos das doses e do parcelamento da fertilização na produtividade, lixiviação e ciclagem de nutrientes em plantações de eucalipto [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2011. found that more than 3.5 Mg ha-1 of senescent leaf deposition were produced from 12 to 24 months by clones of the hybrid Eucalyptus urophylla x grandis without NPK fertilization, while 7.1 Mg ha-1 were produced with the highest fertilizer dose. This higher production in the initial phase of the Eucalyptus growth is due to the reduced residence time of the leaves at the eucalyptus branches due to the addition of fertilizers, which accelerates the growth of the trees after anticipating the intra-specific competition for light and water due to the mutual shading of the plant canopy. This process accelerates leaf senescence. At the later stages of the crop cycle, this competition decreases, resulting in lower senescent leaf deposition.

Figure 2
Biomass of leaves, branches, bark, senescent leaf deposition (a), wood and total of the aerial part (b) of E. urophylla hybrid, evaluated at 66 months after planting, as a function of doses of K2O.

Eucalyptus is not a deciduous genus; however, in the dry season, it loses leaves to reduce water consumption. This mechanism is caused by drought to increase ethylene synthesis, which results in leaf abscission. As a stress response, abscisic acid levels are increased; this plant hormone regulates the opening and closing of the stomata, especially when the plant is under environmental stress, reducing the loss of water by transpiration (Florence, 2004Florence RG. Ecology and silviculture of eucalypt forest. Collingwood: Csiro Publishing; 2004.; Taiz and Zeiger, 2013Taiz L, Zeiger E. Fisiología Vegetal. 5. ed. Porto Alegre: Artmed; 2013.). Therefore, plants that are well supplied with K present greater water use efficiency, which would result in an increase in the lifetime of leaves (Silva et al., 2002Silva SR, Barros NF, Novais RF, Pereira PRG. Eficiência nutricional de potássio e crescimento de eucalipto influenciados pela compactação do solo. Rev Bras Cienc Solo. 2002;26:1001-10. http://dx.doi.org/10.1590/S0100-06832002000400018
http://dx.doi.org/10.1590/S0100-06832002...
; Laclau et al., 2009Laclau J-P, Almeida JCR, Gonçalves JLM, Saint-André L, Ventura M, Ranger J, Moreira RM, Nouvellon Y. Influence of nitrogen and potassium fertilization on leaf lifespan and allocation of above-ground growth in Eucalyptus plantations. Tree Physiol. 2009;29:111-24. https://doi.org/10.1093/treephys/tpn010
https://doi.org/10.1093/treephys/tpn010...
). This explains the finding that K fertilization increased the production of leaf biomass and did not influence senescent leaf deposition.

The increase in K2O led to an increase in K content and a decrease in Mg in the leaves (Figure 3a), and this behavior was verified for the K and Mg contents in the deposited senescent leaves (Figure 3e). The probable cause of the decrease in Mg content is the “dilution effect” of these nutrients on the leaf because with increasing K doses, there was a higher leaf biomass production (Figure 2a), resulting in a lower content of this nutrient. In addition, these macronutrients compete for the same absorption site; that is, the competitive inhibition of Mg may have occurred in parts (Malavolta, 2006Malavolta E. Manual de nutrição mineral de plantas. São Paulo: Editora Agronómica Ceres; 2006.; Marschner, 2012Marschner H. Mineral nutrition of higher plants. 3rd. ed. London: Elsevier; 2012.).

Figure 3
Contents of nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur in the different compartments of Eucalyptus plants (leaves, branches, wood, and bark, respectively, a, b, c, and d) and senescent leaf deposition mean (e), as a function of doses of K2O.

Silva (2011)Silva PHM. Impactos das doses e do parcelamento da fertilização na produtividade, lixiviação e ciclagem de nutrientes em plantações de eucalipto [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2011. evaluated the effects of fertilization levels and fertilization rates (NPK) on Eucalyptus plantations in the initial phase of growth and found that the differences in K concentration occurred in all compartments of the trees; further, the treatment without fertilization presented the lowest content. The decrease in foliar content of Mg as a function of the increase in K was also reported by Silveira (2000)Silveira RLVA. Efeito do potássio no crescimento, nas concentrações dos nutrientes e nas características da madeira juvenil de progênies de Eucalyptus grandis W. Hill ex Maiden cultivadas em solução nutritiva [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2000. in E. grandis progenies grown in nutrient solution.

The lowest contents of nutrients were observed in the wood compartment (Figure 3c), while the highest contents of N, P, K, and S were observed in the leaves (Figure 3a). The higher Ca content was found in the bark (Figure 3d) and the Mg content in the leaves and bark which were very close values (Figures 3a and 3d). The highest contents of N, P, K, and Mg in the leaves are due to the high mobility of these nutrients in the plant and to the presence of Ca in the bark due to its low mobility or immobility in the plant (Malavolta, 2006Malavolta E. Manual de nutrição mineral de plantas. São Paulo: Editora Agronómica Ceres; 2006.; Marschner, 2012Marschner H. Mineral nutrition of higher plants. 3rd. ed. London: Elsevier; 2012.). In addition, the main metabolic activities occur in the leaves, and consequently, leaves show a greater nutrient demand (Viera et al., 2013Viera M, Schumacher MV, Trüby P, Araújo EF. Biomassa e nutrientes em um povoamento de Eucalyptus urophylla x Eucalyptus globulus, em Eldorado do Sul - RS. Ecologia e Nutrição Florestal. 2013;1:1-13. https://doi.org/10.13086/2316-980x.v01n01a01
https://doi.org/10.13086/2316-980x.v01n0...
). Verão et al. (2016)Verão DS, Bleich ME, Martins NP, Bassotto JM, Mortat AF, Santos AFA. Concentração de nutrientes em Eucalyptus urograndis (Eucalyptus grandis W. Hill ex Maiden x Eucalyptus urophylla S. T Blake) com sete anos de idade na borda sul da Amazônia. Biodiversidade. 2016;15:35-44. also found higher N, P, K, Mg, and S contents in the leaves and lower contents in the trunk in E. urophylla x E. grandis at 84 months.

Potassium fertilization positively and linearly influenced the accumulation of K (leaves, bark, and total), Ca (wood, bark, and total), Mg, and S (total) (Figures 4a, 4c, 4d, and 4e). Higher doses of fertilizers had a positive effect on mineral mass production (nutrients accumulated in biomass) for E. urophylla x E. grandis at 12 and 24 months of age (Silva, 2011Silva PHM. Impactos das doses e do parcelamento da fertilização na produtividade, lixiviação e ciclagem de nutrientes em plantações de eucalipto [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2011.). This result corroborates those obtained in the present research. With increasing K2O levels, the content of K increased linearly in leaves, bark, and the total plant. There was also an increase in the stock of Ca in the wood and bark compartments and in the total aerial part. In turn, the increases in Mg and S in the aerial part were 21.2 and 23.4 %, respectively (of the highest dose of K in relation to the control).

Regarding the compartments, in the wood, the nutrient stocks of the Eucalyptus plants at 66 months after planting followed the order K > N > Ca > Mg > S > P. In the aerial part of the Eucalyptus plants, the order was Ca > K > N > Mg > S > P. Of the total nutrients stored in the aerial part, approximately 66 % of Ca were accumulated in the bark and 89 % of K in the trunk, being 18 % in bark and 81 % in wood. According to Santana et al. (2002)Santana RC, Barros NF, Neves JCL. Eficiência de utilização de nutrientes e sustentabilidade da produção em procedências de Eucalyptus grandis e Eucalyptus saligna em sítios florestais do estado de São Paulo. Rev Arvore. 2002;26:447-57. https://doi.org/10.1590/S0100-67622002000400007
https://doi.org/10.1590/S0100-6762200200...
, Ca is the most abundant nutrient in the bark, whereas K is the most abundant nutrient in the wood; these are the most exported nutrients by harvesting the trunk (wood + bark). In this sense, debarking the trunk is a practice that reduces the export of nutrients, mainly of Ca, and minimizes the export of other nutrients contained in this compartment.

Similar to the K and Mg contents in senescent leaf depositions, the amounts of these nutrients transferred to the soil increased and decreased, respectively (Figure 4f). With increasing doses of K2O, the amounts of K transferred to the soil increased by 66 % (57.5 kg ha-1 of K), while those of Mg decreased by 23 % when comparing the highest dose of K with the control. This finding is associated with the increased supply of K to the plant, which, as reported, increased the K contents in leaf tissues and decreased Mg (Figure 3e), since the senescent leaf deposition yield was not influenced by the doses of K2O (Figure 2a). The amount of K transferred via senescent leaf deposition in the control was 34.6 kg ha-1 of K (Figure 4f), and the difference between the highest potassium dose and the control was 22.9 kg ha-1 of K (or 27.5 kg ha-1 of K2O), which means that 15.3 % of K applied by mineral fertilization were recycled by fallen leaves during the period from 36 to 66 months.

Figure 4
Accumulation of nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur in the different compartments of Eucalyptus plants (leaves, branches, wood, bark, and total, respectively, a, b, c, d, and e) and addition of nutrients by leaf deposition (f), as a function of doses of K2O.

Silva (2011)Silva PHM. Impactos das doses e do parcelamento da fertilização na produtividade, lixiviação e ciclagem de nutrientes em plantações de eucalipto [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2011. found that from 12 to 24 months, the amounts of N, K, Ca, Mg, and S transferred via senescent leaf deposition by Eucalyptus urophylla x grandis were higher in the treatments with fertilizer application, mainly due to the higher senescent leaf deposition yield of these treatments.

The addition of macronutrients by senescent leaf deposition between 30 and 66 months followed the order Ca > N > K > Mg > S > P, with the following quantities: Ca = 113.9 kg ha-1; N = 107.5 kg ha-1; K = 47.8 kg ha-1; Mg = 27.3 kg ha-1; S = 17.4 kg ha-1, and P = 9.2 kg ha-1. The same sequence was also found in other studies with different species of Eucalyptus at different ages (Silva, 2011Silva PHM. Impactos das doses e do parcelamento da fertilização na produtividade, lixiviação e ciclagem de nutrientes em plantações de eucalipto [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2011.; Salvador et al., 2014)Salvador SM, Consensa CB, Araújo EF. Produção de serapilheira e devolução de macronutrientes em um povoamento de Eucalyptus saligna (F. Muell). Ecologia e Nutrição Florestal. 2014;2:52-62. https://doi.org/10.5902/2316980X15426
https://doi.org/10.5902/2316980X15426...
. Potassium is the third-most transferred nutrient by senescent leaf deposition; however, because it is not part of any plant structure or organic molecule and rather occurs as a free cation (monovalent), it becomes available for plants more quickly. Moreover, the leaf fraction is the most notable litter component in forest soil in terms of deposited K, and this nutrient presents the highest intensities of biochemical and biogeochemistry cycling (Cunha et al., 2005Cunha GM, Gama-Rodrigues AC, Costa GS. Ciclagem de nutrientes em Eucalyptus grandis W. Hill ex Maiden no Norte Fluminense. Rev Arvore. 2005;29:353-63. https://doi.org/10.1590/S0100-67622005000300002
https://doi.org/10.1590/S0100-6762200500...
; Meurer, 2006Meurer EJ. Potássio. In: Fernandes MS, editor. Nutrição mineral de plantas. Viçosa, MG: Sociedade Brasileira de Ciência do Solo; 2006. p. 281-98.; Caldeira et al., 2008Caldeira MVW, Vitorino MD, Schaadt SS, Moraes E, Balbinot R. Quantificação de serapilheira e de nutrientes em uma Floresta Ombrófila Densa. Semin: Cienc Agrar. 2008;29:53-68. https://doi.org/10.5433/1679-0359
https://doi.org/10.5433/1679-0359...
; Salvador et al., 2014)Salvador SM, Consensa CB, Araújo EF. Produção de serapilheira e devolução de macronutrientes em um povoamento de Eucalyptus saligna (F. Muell). Ecologia e Nutrição Florestal. 2014;2:52-62. https://doi.org/10.5902/2316980X15426
https://doi.org/10.5902/2316980X15426...
.

Potassium fertilization influenced the efficiency of nitrogen use by Eucalyptus plants (Figure 5a). The efficiency of nutrient use in genotypes of Eucalyptus spp. was evaluated by Faria et al. (2008)Faria GE, Barros NF, Cunha VLP, Martins IS, Martins RCC. Avaliação da produtividade, conteúdo e eficiência de utilização de nutrientes em genótipos de Eucalyptus spp. no Vale do Jequitinhonha, MG. Cienc Florest. 2008;18:363-73. https://doi.org/10.5902/19805098448
https://doi.org/10.5902/19805098448...
, the authors found that clone I144 (Eucalyptus urophylla), at 57 months of cultivation, showed a higher N use efficiency. In the present study, K fertilization increased the efficiency of N use as the doses of K2O increased. Such efficiency in the increase of biomass production per kilogram of N stored can be explained by the role of K in controlling the opening and closing of the stomata (Taiz and Zeiger, 2013Taiz L, Zeiger E. Fisiología Vegetal. 5. ed. Porto Alegre: Artmed; 2013.). Therefore, with a higher K2O dose, plants spend less energy during periods of water deficit (a typical condition of the study region) (Figure 1), which is best used for N metabolism (Xu et al., 2002Xu G, Wolf S, Kafkafi U. Ammonium on potassium interaction in sweet pepper. J Plant Nutr. 2002;25:719-34. https://doi.org/10.1081/PLN-120002954
https://doi.org/10.1081/PLN-120002954...
), for example, in the assimilatory reduction of nitrate, by the participation of K in nitrate reductase activation (Marschner, 2012Marschner H. Mineral nutrition of higher plants. 3rd. ed. London: Elsevier; 2012.).

Figure 5
Efficiency of use of nitrogen (EUN), potassium (EUK) (a), and phosphorus (EUP) (b) of E. urophylla hybrid, evaluated at 66 months after planting, as a function of doses of K2O.

The non-efficiency of the use of K as a function of K fertilization was probably due to the increase in wood biomass production (Figure 2b) and K accumulation in the bark (Figure 4d) because nutrient use efficiency is calculated by the relationship between the dry trunk biomass (wood + bark) and the nutrient accumulated in the trunk. Therefore, an increase in biomass and the accumulation of K in the trunk as a function of the increase in K2O dose did not alter this relationship.

The soil K levels in the planting row at the superficial layer (0.00-0.20 m) at 24, 36, and 66 months and from 0.20 to 0.40 m at 24 months were influenced by the doses of K2O, which was also observed for the nutrient contents in the inter-row area in the layer of 0.0 to 0.20 m at 24 months and in the subsurface layer at all evaluation times (Table 1). These contents increased linearly with increasing K2O doses (Figures 6a, 6b, 6c, and 6d). However, only in the inter-row area at the two depths in the second year, we observed contents above 1 mmolc dm-3 K at doses of 135 and 180 mmolc dm-3 of K2O. These contents would be classified as low (0.8-1.5 mmolc dm-3), according to the interpretation limits established by van Raij et al. (1997)van Raij B, Cantarella H, Quaggio JA, Furlani AMC. Recomendações de adubação e calagem para o estado de São Paulo. 2. ed. Campinas: Instituto Agronômico de Campinas; 1997. (Boletim técnico, 100)..

Table 1
Potassium levels in the soil in the planting line and in the interline of E. urophylla hybrid at layer of 0.00-0.20 and 0.20-0.40 m, evaluated at 24, 36, 48, 60, and 66 months after planting, as a function of doses of K2O

Figure 6
Potassium levels in the soil in the line (0.00-0.20 and 0.20-0.40 m) and interline (0.00-0.20 and 0.20-0.40 m) of the E. urophylla hybrid, evaluated at 24, 36, 48, 60, and 66 months after planting (respectively, a, b, c, and d), as a function of doses of K2O.

In subsequent evaluations, the highest levels of K in the soil were obtained with the highest dose of K2O; however, they were similar to those found in the initial soil analysis (0.2 and 0.3 mmolc dm-3 of K); these contents would be classified as very low, which indicates the high absorption of this nutrient by Eucalyptus due to the large accumulation of K in the biomass (Figure 4e). In this sense, higher K levels in the soil [values of K classified from medium to high according to van Raij et al. (1997)van Raij B, Cantarella H, Quaggio JA, Furlani AMC. Recomendações de adubação e calagem para o estado de São Paulo. 2. ed. Campinas: Instituto Agronômico de Campinas; 1997. (Boletim técnico, 100).] would be observed in the initial phases of the development of the culture, in the period of K application.

Regarding the availability of K between 24 and 66 months after planting, the highest level occurred in the first evaluation (Table 1). Between 24 and 36 months, K levels were greatly reduced by 80 to 70 % in the inter-row area at depths of 0.00-0.20 and 0.20-0.40 m, respectively, and by 58.3 and 39.1 % in the planting row at layers of 0.00-0.20 and 0.20-0.40 m, respectively. These findings evidence the high K absorption by Eucalyptus as early as in the first few years of cultivation.

In addition, the sandy soil presents a favorable condition for nutrient leaching. Soil K levels in the surface layer of the last evaluation were higher than those in the layers of 0.20-0.40, 0.40-0.60, and 0.60-0.80 m (Table 2), demonstrating that K leaching was probably reduced by the instalment of K fertilization, which shows that the main process responsible for the decrease in K availability was the high uptake of this nutrient by the crop. Studies evaluating potassium fertilization in Eucalyptus plantations on Neossolo Quarztzarênico (Entisol) show that K splitting reduces its leaching in the soil, with the fast K absorption by the crop being the main factor responsible for its decrease in the soil (Silva et al., 2013Silva PHM, Poggiani F, Libardi PL, Gonçalves AN. Fertilizer management of eucalypt plantations on sandy soil in Brazil: initial growth and nutrient cycling. Forest Ecol Manag. 2013;301:67-78. https://doi.org/10.1016/j.foreco.2012.10.033
https://doi.org/10.1016/j.foreco.2012.10...
; Melo, 2014Melo EASC. Nutrição e fertilização de plantações clonais de eucalipto sob diferentes condições edafoclimáticas [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2014.).

Table 2
Potassium levels in the soil in the interline of E. urophylla hybrid at depths of 0.00-0.20, 0.20-0.40, 0.40-0.60, and 0.60-0.80 m, evaluated at 66 months after planting, as a function of doses of K2O

The K contents in the surface layer of the planting row increased by 75 % between 36 and 66 months. Such an increase can be explained by biogeochemical cycling process, mainly because K was the third-most transferred nutrient to the soil via senescent leaf deposition (Figure 4f). Melo (2014)Melo EASC. Nutrição e fertilização de plantações clonais de eucalipto sob diferentes condições edafoclimáticas [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2014. found that K increased between 24 and 48 months in one forest site and attributed this to the nutrient cycling process arising from the deposition of branches and leaves. According to Gonçalves et al. (2008)Gonçalves JLM, Stape JL, Laclau J-P, Bouillet J-P, Ranger J. Assessing the effects of early silvicultural management on long-term site productivity of fast-growing eucalypt plantations: the Brazilian experience. Southern Forests. 2008;70:105-18. https://doi.org/10.2989/SOUTH.FOR.2008.70.2.6.534
https://doi.org/10.2989/SOUTH.FOR.2008.7...
, this process starts in the second year of the Eucalyptus crop cycle, when the closure of the forest canopy occurs, and is responsible for the mineralization of the nutrients contained in the litter. However, the phase that precedes the harvest is characterized by the intense recycling of nutrients, either through internal retranslocation (biochemical cycle) or through the biogeochemical cycle responsible for the maintenance and supply of nutrients to the plants (Laclau et al., 2003Laclau J-P, Deleporte F, Ranger J, Bouillet J-P, Kazzotti G. Nutrient dynamics throughout the rotation of Eucalyptus clonal stands in Congo. Ann Bot. 2003;91:879-92. https://doi.org/10.1093/aob/mcg093
https://doi.org/10.1093/aob/mcg093...
; Silva, 2011Silva PHM. Impactos das doses e do parcelamento da fertilização na produtividade, lixiviação e ciclagem de nutrientes em plantações de eucalipto [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2011.; Benatti, 2013Benatti BP. Compartimentalização de biomassa e de nutrientes em estruturas de plantas de eucalipto cultivadas em solos distintos [dissertação]. Lavras: Universidade Federal de Lavras; 2013.).

CONCLUSIONS

Potassium fertilization linearly increased the biomass yield of Eucalyptus plants at 66 months, the K content in the deposition of senescent leaves, K transfer to the soil, the accumulation of K, Ca, Mg, and S in the aerial parts of the plant, the N use efficiency, and the K concentration in the soil.

Between the second and the third year of cultivation, soil K was greatly reduced, with levels being lower compared to those at harvest time. Such an increase in soil K availability at later stages was facilitated by the K transfer via senescent leaf deposition, independent of the K dose applied. The largest stocks of N, P, K, Mg, and S were found in the wood, while Ca accumulated more in the bark.

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Publication Dates

  • Publication in this collection
    14 Feb 2019
  • Date of issue
    2019

History

  • Received
    6 Apr 2018
  • Accepted
    19 Sept 2018
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