Improving management practices of legume crop residues to maximize economic and environmental benefits
Final report 2015/074
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Growing a soybean break crop during the fallow period between two cane crop cycles could add 70-280 kg nitrogen/ha into the cropping systems. About 74-88% of the soybean biomass nitrogen derived from biological N2 fixation and 22-26% from soil nitrogen recovery. Mineral nitrogen accumulated in bare soil during the fallow period could be lost substantially following high rainfall events. Legume crop rotation provided an effective means to retain the soil mineral nitrogen in plant biomass. Compared to bare fallow, legume cropping significantly increased abundances of total archaeal and bacterial microbes, but decreased abundances of the nitrifying microbes and the amoA gene related to nitrification. Soybean crop residues contained two to three times more nitrogen than peanut crop residues. However, with grain or pod harvest, farming profit was markedly higher for growing peanut than soybean thanks to higher market values of peanut pods. Rapid release of legume residue nitrogen occurred in the first 2-3 months after harvest or spray out. Nitrogen release was significantly faster from the legume residues incorporated into soil than those retained on the soil surface. If sugarcane is not planted for a prolonged period after the legume cropping, no-till could help slow down nitrogen release thus reduce the risk of nitrogen loss after rainfall. Allowing volunteer soybean to re-grow after grain harvest or growing a nitrogen catch crop significantly decreased mineral nitrate accumulation in soil thus could potentially reduce nitrogen loss. The effects of legume residue management practices on sugar yield and crop nitrogen uptake varied with sites.