Bacterial traits, N deposition and mycorrhiza. qSIP Aspartic Acid

Nitrogen (N) deposition increases soil carbon (C) storage by reducing
microbial activity.  These effects vary in soil beneath trees
that associate with arbuscular (AM) and ectomycorrhizal (ECM) fungi.
Variation in carbon C and N uptake traits among microbes may explain
differences in soil nutrient cycling between mycorrhizal associations in
response to high N loads, a mechanism not previously examined due to
methodological limitations. Here, we used quantitative Stable Isotope
Probing (qSIP) to measure bacterial C and N assimilation rates from an
added organic compound, which we conceptualize as functional
traits.  As such, we applied a trait-based approach to explore
whether variation in assimilation rates of bacterial taxa can inform
shifts in soil function under chronic N deposition. We show taxon-specific
and community-wide declines of bacterial C and N uptake under chronic N
deposition in both AM and ECM soils. N deposition-induced reductions in
microbial activity were mirrored by declines in soil organic matter
mineralization rates in AM but not ECM soils. Our findings suggest C and N
uptake traits of bacterial communities can predict C cycling feedbacks to
N deposition in AM soils but additional data, for instance on the traits
of fungi, may be needed to connect microbial traits with soil C and N
cycling in ECM systems. Our study also highlights the potential of
employing qSIP in conjunction with trait–based approaches to inform how
ecological processes of microbial communities influence soil functioning

氮（N）沉降可通过抑制微生物活性提升土壤碳（C）储量。这类效应在与丛枝菌根（AM）真菌、外生菌根（ECM）真菌共生的树木林下土壤中存在显著差异。微生物类群间碳、氮吸收功能性状的差异，或可解释不同菌根共生体系下土壤养分循环对高氮负荷响应的分化——这一机制此前因方法学局限未被系统探究。本研究采用定量稳定性同位素探针（qSIP）技术，通过添加有机化合物测定细菌的碳、氮同化速率，并将其界定为功能性状。据此，我们运用基于功能性状的研究方法，探讨细菌类群的同化速率差异能否揭示长期氮沉降下土壤功能的转变方向。研究结果表明，在丛枝菌根和外生菌根林下土壤中，长期氮沉降均会导致细菌类群特异性及群落水平的碳、氮吸收能力下降。氮沉降引发的微生物活性降低，在丛枝菌根林下土壤中伴随土壤有机质矿化速率的显著下降，但在外生菌根林下土壤中未观察到此对应变化。本研究结果显示，细菌群落的碳、氮吸收功能性状可预测丛枝菌根土壤中碳循环对氮沉降的反馈效应；而在外生菌根体系中，若要建立微生物性状与土壤碳、氮循环的关联，还需补充真菌功能性状等相关数据。此外，本研究还凸显了将定量稳定性同位素探针技术与基于功能性状的研究方法相结合，以解析微生物群落生态过程如何影响土壤功能的应用潜力。