Microbial driven variation in carbon cycling during root litter decomposition

Soils store more carbon than plants and the atmosphere combined, and the ultimate fate of this carbon is tied to subsurface microbial communities. Yet, our fragmented understanding of how subsurface communities influence carbon cycling prohibits accurate estimates of the fate of carbon produced on the surface and future carbon storage. The conditions in the subsurface differ from the surface conditions in key factors such as temperature, humidity, nutrient availability and consequently microbial community composition. These all might vary somewhat independently for the surface because of the buffering effects of soil, but they are also influenced by surface processes such as precipitation, litter decomposition and nitrogen deposition. Because of the tight link between nitrogen and carbon cycles, nitrogen deposition has potential to drastically alter subsurface microbial composition and function. In this study, we used a common garden microcosm experiment to assess the range of subsurface microbial driven variation in carbon and nitrogen pools during root litter decomposition under fertilized (NH4NO3) and unfertilized conditions. We demonstrate that subsurface microbial community composition can create large variation in dissolved organic carbon (DOC, ~ 6x) and total nitrogen (TN, ~7x), and two fold variation in CO2. Nitrogen addition altered the balance of DOC, TN, and CO2 in ways that were specific to the origin of microbial inoculum. These communities differed drastically in their ability to use the added nitrogen, which when consumed, was related to higher levels of CO2 and lower levels of DOC compared to communities from the same origin without added N. Thus, we conclude that differences in subsurface microbial community composition and their response to nitrogen deposition could have far-reaching impacts on ecosystem function and feedbacks to global nutrient cycles.

土壤储存的碳总量超过植物与大气的碳储量之和，而该部分碳的最终归趋与地下微生物群落紧密相关。然而，我们对于地下群落如何影响碳循环的认知仍较为零散，这使得我们无法准确估算地表产生的碳的归趋以及未来的碳储量。地下环境在温度、湿度、养分有效性等关键因素上均与地表环境存在差异，进而导致微生物群落组成有所不同。由于土壤的缓冲作用，地表的上述因素可能会在一定程度上独立变化，但同时它们也会受到降水、枯落物分解、氮沉降等地表过程的影响。鉴于氮循环与碳循环之间存在紧密关联，氮沉降有可能会显著改变地下微生物的群落组成与功能。本研究采用共同花园微宇宙实验（common garden microcosm experiment），在施加硝酸铵（NH4NO3）与未施加氮肥的条件下，评估了根枯落物分解过程中由地下微生物驱动的碳库与氮库的变异范围。研究结果表明，地下微生物群落组成可导致溶解性有机碳（dissolved organic carbon, DOC，约6倍差异）与总氮（total nitrogen, TN，约7倍差异）产生大幅变异，同时使二氧化碳（CO2）产生两倍的变异量。氮素添加对溶解性有机碳、总氮与二氧化碳的平衡的改变效果，因微生物接种物的来源不同而存在特异性差异。不同来源的微生物群落对添加氮素的利用能力存在显著差异；与未施加氮素的同源群落相比，利用了添加氮素的群落会产生更高浓度的二氧化碳与更低浓度的溶解性有机碳。据此我们得出结论：地下微生物群落组成的差异及其对氮沉降的响应，可能会对生态系统功能以及全球养分循环的反馈效应产生深远影响。