Biomass and nutrient cycling by winter cover crops

Submitted on November 28 , 2014 and accepted on June 02 , 016. 1 Universidade de Cruz Alta, Centro de Ciências da Saúde e Agrárias, Cruz Alta, Rio Grande do Sul, Brazil. jkoefender@unicruz.edu.br; candidamanfio@gmail.com; diego.golle@gmail.com 2 Universidade Federal de Santa Maria, Departamento de Fitotecnia, Santa Maria, Rio Grande do Sul, Brazil. andre-schoffel@hotmail.com *Corresponding author: jkoefender@unicruz.edu.br Biomass and nutrient cycling by winter cover crops


INTRODUCTION
In southern Brazil, as the so-called "modern" technologies have expanded in agriculture, cover crops and green manures had been left out.Although studies on this subjected have resumptined, there is still much to do in terms of studies and dissemination of those practices such as the search for more efficient and adapted species to those production systems.Soil protection and cycling and/or biological fixation of nitrogen, both in winter and in summer or between harvest seasons will allow the construction of a production system that prioritizes the concepts of sustainability, minimizing production costs, characterized as no-tillage cropping system.
Among the plants used as green manure, legumes present the advantage of symbiotically associating themselves with the nitrogen fixing micro-organisms, thus increasing levels of that macronutrient in the straw (Perin et al., 2004).Furthermore, the grasses are able to accumulate large amounts of green matter (Barradas, 2010) with a high C/ N ratio and slower decomposition rate in relation to the legumes, favoring the maintenance of straw on the soil surface for a longer period of time, acting as a regulator of temperature and soil moisture, as well as reducing the risk of erosion.Regarding grasses intercropped with legumes, nutrient uptake and mineralization rates are higher due to the presence of legumes, thus the straw of these plants is more interesting for the composition of a crop rotation system or as contribution to nutrition in agro-ecological systems.The capacity of those species to cycle the available nitrogen in soil and/or to fix atmospheric nitrogen, the high nitrogen demand of grasses and the high cost of nitrogen fertilizers contribute to the inclusion of these species in rotation with corn (Giacomini et al., 2004).Thus, green manures are of great importance to agriculture because they promote a more rapid cycling of nutrients promoting their use by the following crop, particularly of those elements with leaching potential such as nitrogen and exchangeable cations, or of those that may be relatively easy retained in weathered soils, such as phosphorus (Rodrigues et al., 2012).
When evaluating the effect of cover crops on maize grown in no-tillage system, Bonjorno et al. (2010) demonstrated the economic viability of agro-ecological management of no-till corn following different winter cover crops.Because of its ability to fix atmospheric nitrogen and the high rate of decomposition of crop residues, vetch is able to provide significant amounts of nitrogen to corn in succession (Heinrichs et al., 2001;Aita et al., 2001).However, black oat affect the supply of nutrients and productivity of grains (Aita et al., 2001) since it accumulates lower amount of nitrogen in the green matter, and releases it slowly after its management.Considering the role played by soil cover crops plants in agriculture, particularly during the winter period, it is highlighted the importance of carrying out studies that quantify the production and the supply of nutrients in dry matter.The objective of this study was to evaluate the production of green matter, dry matter, the content and the accumulation of nutrients in the aboveground part of plants used in winter cover crops.
The experimental design was a randomized block with six treatments and four replications, where treatments consisted of different winter cover crop plants in single or intercropped systems, as follows: oilseed radish (Raphanus sativus var oleiformis), Vetch (Vicia sativa), black oat (Avena strigosa), vetch + black oat, vetch + oilseed radish and fallow (spontaneous vegetation).During fallow, the following weeds were more frequent: Conyza spp, Bidens pilosa, Ipomoea spp.and Lolium multiflorum.Plots consisted of a 24 m 2 area and sowing was carried in the broadcast system in May, with no additional fertilization.
Fresh matter production was determined by weighing the aboveground part of the plants after harvest and dry matter was quantified by drying the material in an oven at 65 °C until constant mass, expressing the results in kg ha - 1 .Samples of the dry matter of the plants were ground in a Wiley mill, then packed in containers with 100 g and sent to the Soil and Plant Tissue Analysis Laboratory at the Universidade Federal do Rio Grande do Sul to determine the nutrient content, according to the methodology described by Tedesco et al. (1995).Accumulations of macro and micronutrients were obtained by multiplying the production of dry mass by the nutrient content of each treatment.Statistical analyzes were performed with the aid of the statistical program Sisvar (Ferreira, 2011).

RESULTS AND DISCUSSION
No difference was found in the dry and fresh matter production of cover crops (Table 1), and the fresh matter production ranged from 5,507 kg ha -1 to 9,248 kg ha -1 .
According to Lima et al. (2001), fresh matter demonstrates high variability, probably due to the oscillation that occurs in the water content of the plants in the interval between sampling and weighing, which may cause variable underestimation of the results.Fallow showed the lowest production of fresh matter, significantly differing from the cover species in single and intercropping cultivation systems.
A statistically significant difference was found for the nutrient content in the dry matter of cover crops, except for iron (Tables 2 and 3).The content of nitrogen was higher in the vetch grown in single cultivation, a higher result than that reported by (Bettiol et al., 2015) in a study with Brachiaria (Urochloa sp.) and its intercropping with gray velvet bean, Canavalia ensiformis, Crotolaria juncea and Cajanus cajan.Increases in the content of phosphorus were observed in fallow, in the single cultivation of vetch and in vetch intercropped with oilseed radish.That accumulation of phosphorus is higher than that found in the first year of Mucuna aterrima (3.15 g kg -1 ) and lower than that found in the second year (4.80 g kg -1 ) by Silva et al. (2014).In relation to potassium, the highest values for the nutrient content were obtained in the single cultivation of vetch, oilseed radish and in black oats + vetch and vetch + oilseed radish intercropping, which did not differ statistically from the fallow.This result is higher than that found by (Bettiol et al., 2015) for Brachiaria decumbens (18.31g kg -1 ) and its intercropping with: Mucuna cinereum (19.51 g kg -1 ), Canavalia ensiformis (20.06 g kg -1 ), Crotalaria juncea (20.08g kg -1 ) and Cajanus cajan (18.44 g kg -1 ).
The highest magnesium content was provided by oilseed radish and was higher than that found for Crotalaria juncea (4.0 g kg -1 ) by Silva et al. (2014).Oilseed radish grown in single cultivation also responded for the highest contents of sulfur and calcium.These results differ from those reported by Silva et al. (2010), who found higher sulfur and calcium contents in pearl millet (2.5 g kg -1 ) and in Crotalaria juncea (8.9 g kg -1 ).As for the copper content, single cultivation of vetch and fallow showed the highest levels, while for zinc and boron, the highest levels were found in the cultivation of vetch, oilseed radish and its intercropping; however, for zinc, it did not differ statistically from fallow.According to Faquin (2001), dicotyledonous absorb more amounts of boron than monocotyledonous.The manganese content was high in single cultivation of black oat and black oat intercropped with vetch.These results differ from those observed by Teixeira et al. (2008), who found no difference in the content of manganese in the single cultivation of Canavalia ensiformis, Cajanus cajan, pearl millet and their intercrops.As for sodium, the highest values were found in the intercropping.
Higher accumulations of nitrogen and sodium were observed in vetch intercropped with oilseed radish.Although it did not present higher nitrogen content, the greater accumulation of nutrients in the vetch + oilseed radish intercropping may be attributed to a higher production of dry matter in relation to single cultivation of vetch.Nitrogen accumulation was higher than that observed by Silva et al. (2007) in vetch + oilseed radish intercropping, which was 50 kg ha -1 .In addition, the nitrogen accumulation in this study was higher than that found by Silva et al. (2014) for pearl millet, Crotalaria juncea and Cajanus cajan grown in 2010 and 2011, and lower than the accumulation of Mucuna aterrima (361 kg ha -1 and 357 kg ha -1 ) in two years of cultivation, respectively.Although vetch presents higher nitrogen content in the present study in relation to the nutrient content in Mucuna aterrima (30.4 g kg -1 and 29.5 g kg -1 ) of the previous mentioned study, the largest production of dry matter provided the high nitrogen accumulation in Mucuna aterrima.Giacomini et al. (2003) found higher nitrogen accumulation in the first year of oilseed radish crop (101 kg ha -1 ), which decreased in the following years (67 kg ha -1 and 63 kg ha -1 ).The highest

Cover Crop
accumulation in the first year was because of the higher availability of nitrogen in the soil, where oilseed radish succeeded soybean cultivation and in the following years, the decrease occurred because oilseed radish was sown after corn cultivation.
The highest accumulations of phosphorus and iron were found in single crops and intercropping of cover crops plants, which differed only from fallow.Pittelkow et al. (2012) found higher values for phosphorus accumulation (44.40 kg ha -1 ) in the cultivation of Brachiaria (Urochloa sp.).Giacomini et al. (2003) found higher phosphorus accumulation in the single cultivation of oilseed radish and vetch, differing from black oats, and, according to the authors, this result is due to the greater accumulation of this nutrient in legumes in comparison to grasses.The accumulated amounts of phosphorus in the cover crops were higher than that recorded by Barros et al. (2013) in Canavalia ensiformis, Mucuna aterrima,

Cover crops
Cover crops

Table 1 :
Production of aboveground dry and fresh matter of winter cover crops Groups of means not followed by the same letter in the column are different by the Scott Knott's test at 5% of probability.

Table 2 :
Macronutrient content and accumulation of the aerial part of winter cover crops

Table 3 :
Micronutrient content and accumulation of the aerial part of winter cover crops