Modelling plausible impacts of Biological Nitrification Inhibition in Australian farming systems

Australia can report world-leading efficiencies in grain production; high water and nutrient use efficiencies and low greenhouse gas (GHG) emissions intensity (kg CO2-eq/t grain). However, the GRDC-commissioned Baseline Report (Sevenster et al., 2022) concluded that Australian grain is not near zero absolute emissions and current known practices cannot mitigate emissions without vegetation offsets. In fact, the report suggested that Australia’s continued productivity in grain production will likely be driven by higher Nitrogen (N) fertilizer rates and, unfortunately, higher absolute nitrous oxide (N2O) emissions. Lowering absolute and net emissions without taking arable land out of production (with offsets) is a priority for the GRDC 2023-28 RDE Plan.

Currently, plant-accessible N derived from most fertilizers, or through breakdown of crop residues and soil organic matter, is available first in the form of ammonium-N (NH4) but converts relatively quickly to nitrate-N (NO3) through the actions of nitrifying microbes. This process (nitrification) can result in the release of N2O. Under certain conditions (e.g., anaerobic soils), denitrifying soil microbes will further convert NO3 to dinitrogen (N2) and nitrous oxide (N2O) – N2 is inert and harmless, however N2O is heavily detrimental to the environment as a greenhouse gas 300 times the impact of CO2. Denitrification depletes the amount of available N in soils, negatively impacts plant N uptake and contributes over 30% of greenhouse gas emissions from grain production (Sevenster et al., 2022).

Retaining N in soils in the form of NH4 by inhibiting nitrification processes has been long hypothesised as a solution to N loss from denitrification (and leaching of NO3 below the root zone). Enhanced efficiency fertilisers (EEF) that inhibit nitrification offer a partial solution, although they are ineffective on soil and residue derived NO3. More recently, nitrification reactions have been found to be inhibited by root exudates from some crops (e.g., wheat, rice, sorghum, maize, canola) and these biological nitrification inhibitors (BNIs) are hypothesised as potential contributors to maximising N uptake, improving Nitrogen Use Efficiency (NUE) and minimising environmental nitrogen pollution in global agriculture (Subbarao et al., 2013). This prospect is being actively progressed by a BNI-Consortium of international researchers who are researching the processes involved in BNI, including breeding for enhanced BNI expression in crops. In Australia, O’Sullivan et al. (2016) reported that the wheat cultivar Wyalkatchem has moderate BNI activity resulting in 11% reduction in nitrification rate.

The research to date on BNI proffers a potential opportunity for the breeding programs of Australia’s main crops. However, the research of O’Sullivan et al. (2016) reported discrepancies between BNI production, BNI exudation and nitrification efficacy, likely dependent on genotype and environmental conditions. This investment proposes a modelling systems approach using the current body of literature on BNI to assess the prospects of potential impacts from BNI for the wheat, canola and sorghum crops in Australian farming systems.