Mineralizable nitrogen and denitrification enzyme activity drive nitrate concentrations in well-drained stony subsoil under lucerne (Medicago sativa L.)

Nitrogen (N) inputs to agricultural systems contribute substantially to soil nitrate (NO3-) concentrations, which increase NO3- leaching and contamination of groundwater. The influence of soil microbes in regulating NO3- concentrations in the topsoil are well studied but it is often assumed that microbial regulation of NO3- con centrationsin the subsoil is negligible. The aim of this study was to test this assumption by determining the relationships between microbial properties and NO3- concentrations in both the subsoil and the topsoil. We measured the size of the mineralizable N (N-m) pool, microbial properties (microbial biomass, bacterial richness), nitrifier gene abundance (amoA gene copy number), denitrifier gene abundance (nirK and nirS gene copy number), denitrifier enzyme activity and NO3- concentrations in the topsoil and the subsoil in a well-drained stony soil under an established lucerne crop. We used structural equation modelling (SEM) to identify and compare the linkages of microbial properties with NO3- concentrations at each depth. In the topsoil, we found higher N-m, gene abundance, denitrification enzyme activity, bacterial richness, and microbial biomass than those in the subsoil, but there were no relationships between these variables and NO3-& nbsp;concentrations in the topsoil (the SEM model explained 0.06% of the variability in NO3- concentrations). In contrast, in the subsoil, NO3- concentrations were strongly correlated with bacterial amoA abundance and denitrification enzyme activity, with both variables associated significantly with N-m. We found that bacterial richness was also associated with N-m in the subsoil. Our findings highlight that microbial properties are associated with NO3- concentrations in the subsoil (the SEM model explained 82% the variability in NO3- concentrations) and this suggest that nitrification and denitrification may contribute to regulating NO3- concentrations in the subsoil. Our findings also suggest that denitrification contributes to reducing NO3- concentrations in the subsoil. We conclude that studies addressing drivers of NO3- leaching need to consider the potential for microbially-mediated attenuation (or an increase) in NO3- concentrations throughout the soil profile.ISSN:0929-1393ISSN:1873-027