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

• Published in: Applied soil ecology : a section of Agriculture, ecosystems & environment Vol. 176; p. 104499
• Main Authors: Nuñez, Jonathan, Orwin, Kate H., Moinet, Gabriel Y.K., Graham, Scott L., Rogers, Graeme N.D., Turnbull, Matthew H., Clough, Timothy J., Dopheide, Andrew, Davis, Carina, Grelet, Gwen-Aëlle, Whitehead, David
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.08.2022
• Subjects: Alfalfa; Microbial communities; Nitrate leaching; Nitrification; Nitrogen mineralization; Structural equation modelling; Nitrogen mineralization; Nitrate leaching; Alfalfa; Microbial communities; Structural equation modelling; Nitrification
• ISSN: 0929-1393; 1873-0272
• DOI: 10.1016/j.apsoil.2022.104499

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− concentrations in 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 (Nm) 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 Nm, 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− 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 Nm. We found that bacterial richness was also associated with Nm 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. •Subsoil NO3− concentrations were strongly associated with microbial properties.•The subsoil mineralizable N (Nm) pool was positively associated with NO3−.•Subsoil denitrification enzyme activity (De) was negatively associated with NO3−.•Topsoil NO3− concentrations were not associated with Nm or De.•Increasing denitrification in subsoil may contribute to reducing NO3− leaching.