Effects of Soil Management on N Mineralization and Nitrification Rates in Soybean Fields of the Amazon, Brazil

Tuesday, 16 December 2014
Michela Figueira, Miyuki Mitsuya and Jose Mauro Sousa Moura, Federal University of Western Para, Santarem, Brazil
The biological nitrogen fixation (BNF) promoted by the cultivation of soybean [Glycine max. (L.) Merrill], consists in a significant input of reactive nitrogen (N) in the system. The availability of this nutrient to plants occurs through the mineralization of organic-N of the soil, conducted by soil organic matter and / or crop residues. Thus, understanding the fate of this increased supply, as well as changes in the dynamics of N in the system is essential in the context of the recent expansion of soybean in the Amazon. This work aims to understand the dynamics of N in the soil cultivated with soybean, through the determination of ammonium and nitrate concentrations, as well as, mineralization and nitrification rates in soybean fields under till and no-till systems in the municipality of Santarém, Brazil. We collected 12 soil samples (0-10cm depth) in each crop and the extractions were done in 2M KCl solution using 10 g of soil. The samples were incubated for 7 days when new extractions were done to determine the rates of mineralization and nitrification. The till system showed higher concentrations of nitrate (0.82 ± 0.13 mg kg-1) and ammonium (8.15 ± 0.54 mg kg-1) compared to the results found in no-till system (0.59 ± 0.09 mg kg-1 and 5.25 ± 0.28 mg kg-1 for nitrate and ammonium respectively). The mineralization rates were negative in the till system, indicating possible immobilization of N in the soil during that period. In contrast, no-till system showed higher rates of mineralization in the soil. Although there is a great removal of N by grain harvest, it is possible to infer that no-till system has higher nitrogen availability to soybean and that this management could contribute to an increased productivity mainly due to deposition of crop residues enriched in N on the soil.