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 (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⁻ 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.

农业系统的氮 (N) 输入对土壤硝酸盐 (NO ₃ ^- ) 浓度有很大影响，这会增加 NO ₃ ^-浸出和地下水污染。土壤微生物对调节表层土壤中 NO ₃ ^-浓度的影响已得到充分研究，但通常认为微生物对底土中 NO ₃ ^-浓度的调节可以忽略不计。本研究的目的是通过确定下层土壤和表层土壤中微生物特性与 NO ₃ ^{-浓度之间的关系来检验这一假设。}我们测量了可矿化 N ( N _m) 池、微生物特性（微生物生物量、细菌丰富度）、硝化菌基因丰度（amoA基因拷贝数）、反硝化菌基因丰度（nirK和nirS基因拷贝数）、反硝化菌酶活性和表层土壤和底土中的NO ₃ ^-浓度成熟的苜蓿作物下排水良好的石质土壤。我们使用结构方程模型 (SEM) 来识别和比较微生物特性与每个深度的 NO ₃ ^-浓度之间的联系。在表土中，我们发现更高的N _m、基因丰度、反硝化酶活性、细菌丰富度和微生物量均高于底土，但这些变量与表层土壤中 NO ₃ ^{-浓度之间没有关系（SEM 模型解释了 NO} ₃ ^-中 0.06% 的变异性）浓度）。相反，在底土中，NO ₃ ^-浓度与细菌amoA丰度和反硝化酶活性密切相关，这两个变量都与N _m显着相关。我们发现细菌丰富度也与N _m有关在底土中。我们的研究结果强调，微生物特性与底土中的 NO ₃ ^{-浓度相关（SEM 模型解释了 NO} ₃ ^-浓度变化的 82% ），这表明硝化和反硝化可能有助于调节底土中的 NO ₃ ^-浓度。我们的研究结果还表明，反硝化有助于降低底土中的 NO ₃ ^-浓度。我们得出结论，解决 NO ₃ ^-浸出驱动因素的研究需要考虑微生物介导的 NO ₃ ^{-衰减（或增加）的可能性。}整个土壤剖面的浓度。