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

Abstract:

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.