Mineralizable nitrogen and denitrification enzyme activity drive nitrate concentrations in well-drained stony subsoil under lucerne (Medicago sativa L.)
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2022-08
Publication Type
Journal Article
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yes
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Abstract
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 (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.
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published
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Journal / series
Volume
176
Pages / Article No.
104499
Publisher
Elsevier
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Subject
Alfalfa; Microbial communities; Nitrification; Nitrogen mineralization; Structural equation modelling; Nitrate leaching
Organisational unit
02350 - Dep. Umweltsystemwissenschaften / Dep. of Environmental Systems Science