What Are the Key Drivers of Nitrification Inhibitor Efficacy in Australian Agricultural Soils?.

The reactive nature of nitrogen (N) can lead to large inefficiencies from the application of fertiliser, causing both economic and environmental impacts. Nitrification inhibitors, which inhibit the oxidation of ammonia, have been shown to effectively reduce the rate of nitrification and nitrous oxide (N₂O) emissions from applied fertilisers across a broad range of soils and environmental conditions. They therefore can have positive efficiency and environmental outcomes. However it is still difficult to determine in what situations these products will be effective and this is one reason for low adoption rates of these products. It is clear that edaphic and climatic factors impact on the performance of inhibitors, but it is not clear whether any particular soil property or properties are the key driver(s) of this performance.  Average nitrification rates (over 14 days) across a broad range (30) of Australian soils were, in our laboratory incubation experiments, to vary widely, from essentially none in an acidic loam sugarcane soil to 25.5 ug NO₃^-N produced /g soil/day in an organic acidic sandy cropping soil. Use of the inhibitors, including 3,4-Dimethyl-pyrazole phosphate (DMPP), dicyandiamide (DCD), Nitrapyrin (N-serve) and 3MP+Triazole (3MP+TZ), reduced nitrate formation by an average 39%, with complete inhibition in some soils. N₂O emissions varied widely across the soils tested (cumulative over 28 days, from 19 g N₂O ha^-1 to 2.0 kg N₂O ha^-1). Use of the inhibitors reduced net N₂O (less control) emissions by on average 60% and to below control levels in some soils. pH (P<0.05), organic C (P<0.1) and manganese (P<0.05) all affected the inhibition achieved.  Investigation of the soil microbial responses to the use of inhibitors, currently on-going, will provide a clearer picture of the reasons for variations in efficiencies.