Assessing the real nitrogen benefits to subtropical cane from soybean break crops: final report 2016/404
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There is concern among subtropical cane farmers that much of the N in the residues of summer soybean crops is lost over the winter fallow period prior to cane planting. Nitrogen fixed in 10 soybean paddocks over summer 2016-17 was assessed using Nitrogen - 15 natural abundance methodology. Owing to wet autumn conditions, four of these crops were not harvested and were sprayed out or ploughed in. Nitrogen - 15 labelled soybeans (i.e. isotopically labelled plants) were grown in the glasshouse and residues were applied to three replicate boxes in three fields in the subtropics to track the fate of N over winter. In spring, prior to land being ploughed in preparation for cane planting, soil samples were taken to a depth of 90cm in each box to quantify the amount of soybean residue-N remaining in the soil profile. Losses of 32-45% of soybean residue-N occurred over winter, with the 45% losses occurring on a sandier soil. Notably, around 20% of the remaining Nitrogen - 15 from the soybean residues in the sandy soil was located at 300-600 mm depth in early spring, and given that there was further heavy rainfall between the soil sampling in September prior to soil cultivation and sowing of plant cane in October it is possible that much of this N may have moved deeper into the soil beyond the root zone. The use of winter cover crops should be investigated as a means of retaining the N in the systems and potentially adding further species diversity into subtropical cane systems.