Advanced process level understanding of factors controlling gaseous nitrogen partitioning to reduce N2O losses from Australian agricultural soils

The microbial mediated production of nitrous oxide (N2O) and its reduction to dinitrogen (N2) via denitrification represents a loss of nitrogen (N) from fertilised agro ecosystems to the atmosphere. Although denitrification remains a major uncertainty in estimating N losses from soils, the magnitude of N2 losses and specifically the variation in the N2O/N2 ratio during emissions events remain a major area of uncertainty due to methodical constraints. To date, the magnitude of denitrification losses and N2 emissions are virtually unknown for the majority of Australian agricultural systems and popular models (DayCent and DNDC) show vastly different N2O/N2 partitioning when simulating identical treatments. This limited process understanding is a major constraint when identifying management strategies that provide both abatement and productivity benefits (through reduced gaseous N losses). This study aimed to directly quantify N2 emissions and the partitioning of N2O and N2 from different cropping soils in Australia by soil core incubations and direct field measurements using the 15N gas flux method. The results confirmed that denitrification can be a major pathway of N loss from Australian agricultural systems and that the amount of nitrogen lost as N2 is mainly influenced by soil moisture, carbon and nitrogen availability. The 15N gas flux method proved an effective tool assessing N losses from fertilised soils. However, analytical constraints need to be solved and new approaches are required in order to account for various source pools of different enrichment and different pathways of N2 and N2O production