The turnover of nitrogen (N) in soils is primarily regulated by microbial action. With increasing pressures on Australian agriculture to increase production whilst reducing its impact on the environment, quantification of N loss via leaching and N2O emissions is required. However, due to high spatial and temporal variation in many N cycling processes, it is not feasible to adopt monitoring approaches such as those used for soil carbon sequestration. Rather, greater accuracy of predictive capacity via farming system models such as the Agricultural Production SIMulator (APSIM) is required. This has necessitated a comprehensive study into coarse and fine-scale drivers of microbial N turnover and stubble decomposition in Australian soils to enhance APSIM’s predictive capacity for key N cycling processes.
Incorporating soils from 12 distinct grain-growing sites stretching from Wongan Hills (WA) to Kingsthorpe (Qld) in a series of microcosm incubation experiments, we are systematically quantifying the impacts of soil type, crop residue type and particle size, temperature, moisture, and carbon and nutrient availability on stubble decomposition and key N cycling processes including immobilisation, mineralisation, and nitrification. Nitrogen is being tracked through soil pools using 15N , and N-cycling functional genes quantified by qPCR. On a subset of samples, and also on crop residues from field experiments, microbial community structure is being examined to provide further insights into the key process of stubble decomposition. Results will be tested against APSIM simulations of the incubation experiments, and modifications to APSIM will then be verified against datasets of N balance and crop production from experiments established at the 12 sites as part of a wider, collaborative project. These improvements will facilitate a step-change improvement in the predictive capacity of N dynamics and losses in Australian cropping soils