Gross N transformation rates in soil system with contrasting Urochloa genotypes do not confirm the relevance of BNI as previously assessed in-vitro

The capacity of several plant species or landraces to inhibit nitrification in soil (biological nitrification inhibition, BNI) has been assessed in certain tropical pastures. These assessments are commonly based on potential net nitrification rates, which do not differentiate between gross nitrification and other processes that may reduce the amount of nitrate in soil. In a greenhouse experiment using two genotypes of Urochloa humidicola with contrasting BNI capacity in vitro, we evaluated gross N transformation rates before and after (7 and 21 days) N fertilization, while periodically measuring N2O emissions. Gross nitrification rates (in fact gross nitrate production assessed by pool dilution technique) were comparable in both genotypes and were low in comparison to strong microbial NH4+ immobilization. The N2O emissions were higher in pots with low-BNI plants. The discrepancy between the potential net nitrification rates assessed in laboratory assays (higher in low-BNI plants) and gross nitrification in pot or field experiments (no differences between genotypes) can be attributed to the out-competition of ammonia oxidizers by plant N uptake and ammonia immobilizing heterotrophic microbes, resulting in low nitrification under conditions where growing plants are present. This study confirmed the capacity of certain U. humidicola genotypes to reduce N2O emissions but warrants further investigation of the underlying mechanisms. It also questions the relevance of BNI in the rhizosphere of this plant species as other mechanisms (rather than the inhibition of gross nitrification) seem to be more important in maintaining low-nitrate soil environments in soil–plant systems of U. humidicola