Impacts of fertilisers and legumes on N2O and CO2 emissions from soils in subtropical agricultural systems: A simulation study

There is increasing focus on greenhouse gas emissions from agricultural systems. One suggested method for increasing the sequestration of carbon (C) within agricultural soils is to increase crop productivity and therefore C input into the soil. However, if enhanced production is achieved via nitrogenous fertilisers, there is a potential tradeoff between decreased C emissions and increased nitrous oxide (N2O) emissions due to the increased soil C and nitrogen (N). An alternative is to incorporate leguminous crops into cereal cropping rotations to provide a biological source of N. However, the likely production of N2O from N released during the decomposition of leguminous residues is unknown as is the impact on C input into the soil when some cereal crops are replaced with grain legumes. Consequently, an analysis of the likely impacts has been undertaken for a subtropical dryland cropping system in Queensland, Australia where soil, climate and management are conducive to denitrification losses. A series of scenarios embracing a range of cropping rotations, N fertilisers and leguminous crops was tested using the Agricultural Production Systems Simulator (APSIM). The model configuration was tested using long term data from the Brigalow Catchment Study site near Theodore, Queensland, Australia (24.81 degrees S, 149.80 degrees E). A wide range of data was used in testing the model for the major terms in the C, N and water balances. Scenario analyses of alternative management systems including the use of fertiliser or legume grain or forage crops within cereal rotations demonstrated that soil C can be managed to some degree via simple changes in agronomic practice. The use of legumes within cereal rotations was not always as effective in reducing N2O emissions as improved fertiliser practice. For example, replacing wheat with chickpea did not reduce N2O emission relative to fertilised systems and did not assist in increasing soil C due to impacts on stubble cover over the important summer months. The fact that some interventions proved counterproductive due to complex feedback mechanisms highlights the need for detailed models which capture the links between water, C, N and management. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.