Department of Biology, West Virginia University, Morgantown, WV, USA.
Department of Ecology, Evolution, and Behavior, University of Minnesota, St Paul, MN, USA.
Glob Chang Biol. 2018 Jun;24(6):2721-2734. doi: 10.1111/gcb.14081. Epub 2018 Feb 28.
Atmospheric nitrogen (N) deposition has enhanced soil carbon (C) stocks in temperate forests. Most research has posited that these soil C gains are driven primarily by shifts in fungal community composition with elevated N leading to declines in lignin degrading Basidiomycetes. Recent research, however, suggests that plants and soil microbes are dynamically intertwined, whereby plants send C subsidies to rhizosphere microbes to enhance enzyme production and the mobilization of N. Thus, under elevated N, trees may reduce belowground C allocation leading to cascading impacts on the ability of microbes to degrade soil organic matter through a shift in microbial species and/or a change in plant-microbe interactions. The objective of this study was to determine the extent to which couplings among plant, fungal, and bacterial responses to N fertilization alter the activity of enzymes that are the primary agents of soil decomposition. We measured fungal and bacterial community composition, root-microbial interactions, and extracellular enzyme activity in the rhizosphere, bulk, and organic horizon of soils sampled from a long-term (>25 years), whole-watershed, N fertilization experiment at the Fernow Experimental Forest in West Virginia, USA. We observed significant declines in plant C investment to fine root biomass (24.7%), root morphology, and arbuscular mycorrhizal (AM) colonization (55.9%). Moreover, we found that declines in extracellular enzyme activity were significantly correlated with a shift in bacterial community composition, but not fungal community composition. This bacterial community shift was also correlated with reduced AM fungal colonization indicating that declines in plant investment belowground drive the response of bacterial community structure and function to N fertilization. Collectively, we find that enzyme activity responses to N fertilization are not solely driven by fungi, but instead reflect a whole ecosystem response, whereby declines in the strength of belowground C investment to gain N cascade through the soil environment.
大气氮（N）沉降增加了温带森林的土壤碳（C）储量。大多数研究认为，这些土壤 C 的增加主要是由于真菌群落组成的变化所致，随着 N 的升高，木质素降解的担子菌减少。然而，最近的研究表明，植物和土壤微生物是动态交织在一起的，植物向根际微生物输送 C 来增强酶的产生和 N 的动员。因此，在 N 升高的情况下，树木可能会减少地下 C 分配，从而对微生物降解土壤有机质的能力产生级联影响，这是通过微生物物种的变化和/或植物-微生物相互作用的变化来实现的。本研究的目的是确定植物、真菌和细菌对 N 施肥的反应之间的耦合程度如何改变是土壤分解的主要因素的酶的活性。我们测量了真菌和细菌群落组成、根-微生物相互作用以及美国西弗吉尼亚州弗诺实验林长期（>25 年）全流域 N 施肥实验中土壤根际、体相和有机相中的胞外酶活性。我们观察到植物对细根生物量（24.7%）、根系形态和丛枝菌根（AM）定殖（55.9%）的 C 投资显著减少。此外，我们发现胞外酶活性的下降与细菌群落组成的变化显著相关，但与真菌群落组成无关。这种细菌群落的变化也与 AM 真菌定殖的减少有关，这表明地下植物投资的减少驱动了细菌群落结构和功能对 N 施肥的响应。总的来说，我们发现，对 N 施肥的酶活性响应不仅受真菌驱动，而且反映了整个生态系统的响应，即地下 C 投资强度的下降通过土壤环境产生级联效应。
