Prospects for Putting "Biological Brakes" on Nitrification.

Global inputs of reactive nitrogen are increasing, but with only about half of all fertilizer N taken up by crops, agricultural N use inefficiency imposes severe costs to water quality and environmental health. One explanation for poor N use efficiency has been that more than 90% of agricultural N flows through the nitrification pathway, despite extensive use of nitrification inhibitors like dicyandiamide and nitrapyrin. The resulting higher mobility of nitrate-N, compared to ammonium and organic N, leads to rapid N losses from agricultural systems. In contrast, the very low levels of net nitrification in climax ecosystems suggest there are biological mechanisms that prevent or slow nitrification. More efficient recycling of oxidized forms of N to reduced states may also occur in native soils. In this presentation, three areas of opportunity for putting “biological brakes” on nitrification in agricultural soils will be reviewed. The first approach is to deprive autotrophic nitrifiers of ammonium by enhancing its uptake by plant roots and heterotrophic microbes. Possible biological mechanisms to reduce nitrification include intensified ammonium uptake by cover crops and niche sequestration of nitrifiers by enhancing soil aggregation. A second approach is the use of crops and/or their compounds that inhibit nitrifier activity. Using a bioluminescent Nitrosomonas bioassay, G.V. Subbarao and colleagues have demonstrated biological nitrification inhibition (BNI) by root-derived compounds including isothiocyanates (cruciferous plants), cyclic diterpenes and free fatty acids (Brachiaria spp.), and quinones and phenyl propanoids (Sorghum spp.). For this approach, questions include determining effective concentrations of BNI compounds, their stability and persistence in soils, and the potential for differential effects on ammonia-oxidizing bacteria and archaea. A third approach is to understand soil conditions that promote nitrate ammonification. This little-studied process, also called dissimilatory nitrate reduction to ammonium, can be carried out by phylogenetically diverse bacteria. Little is known about the extent to which this process occurs in agricultural soils, much less how it might be influenced by soil management promoting low-disturbance, high-carbon soil conditions. Elucidation of biological N-cycling processes that occur in native soils could therefore help achieve better N use efficiency in agricultural soils.