Harnessing biological nitrification inhibition to reduce soil nitrogen losses – Systematic quantification of plant and soil factors to maximise field-scale benefits

Nitrogen fertilisation in cropping can be ineffective in conversion to production outputs due to nitrogen losses. These occur via nitrate leaching, ammonia volatilisation or as gaseous emissions, including as nitrous oxide, a potent greenhouse gas. Biological nitrification inhibition (BNI), the exudation of compounds through plant roots that limit conversion of ammonium into nitrate, may reduce such losses. Here we discuss and quantify the key plant and soil factors that affect BNI activity. This systematic approach provides a basis for understanding, and ultimately maximizing, field-scale benefits of BNI. To date, Brachiaria humidicola and sorghum have received most research attention, including characterisation of the BNI activity of their root exudates. Limited information is available on composition of BNI compound release in rice and maize, with even fewer published data on composition and efficacy of individual BNI compounds in other major grain crops. Plant BNI compound release is mainly affected by soil pH, soil ammonium concentration and plant growth stage. Specifically, understanding release over time in annual crops is critical for assessing BNI benefits. Turnover of plant tissue containing BNI compounds could also cause potential legacy effects in subsequent seasons, requiring quantification. We further review mobility, longevity, physical/chemical inactivation and inhibition potential of BNI compounds in soil and propose a conceptualisation of BNI quantification based on plant and soil parameters. Finally, we highlight the need for more systematic bio-physical studies to accurately assess the impact of BNI on soil nitrogen losses.