Root Exudation Simulation Experiment in a Red Oak Stand at Harvard Forest 2011

Root exudation is thought to increase the activity of microbes and the exoenzymes they synthesize, leading to accelerated rates of carbon (C) mineralization and nutrient cycling in rhizosphere soils relative to bulk soils. The nitrogen (N) content of microbial biomass and exoenzymes may introduce a stoichiometric constraint on the ability of microbes to effectively utilize the root exudates, particularly if the exudates are rich in C but low in N. We combined a theoretical model of microbial activity with an exudation experiment to test the hypothesis that the ability of soil microbes to utilize root exudates for the synthesis of additional biomass and exoenzymes is constrained by N availability. The field experiment simulated exudation by automatically pumping solutions of chemicals often found in root exudates (“exudate mimics”) containing C alone or C in combination with N (C:N ratio of 10) through microlysimeter “root simulators” into intact forest soils in two 50-day experiments. The delivery of C-only exudate mimics increased microbial respiration but had no effect on microbial biomass or exoenzyme activities. By contrast, experimental delivery of exudate mimics containing both C and N significantly increased microbial respiration, microbial biomass, and the activity of exoenzymes that decompose low molecular weight components of soil organic matter (SOM, e.g., cellulose, amino sugars), while decreasing the activity of exoenzymes that degrade high molecular weight SOM (e.g., polyphenols, lignin). The modeling results were consistent with the experiments; simulated delivery of C-only exudates induced microbial N-limitation, which constrained the synthesis of microbial biomass and exoenzymes. Exuding N as well as C alleviated this stoichiometric constraint in the model, allowing for increased exoenzyme production, the priming of decomposition, and a net release of N from SOM (i.e., mineralization). The quantity of N released from SOM in the model simulations was, under most circumstances, in excess of the N in the exudate pulse, suggesting that the exudation of N-containing compounds can be a viable strategy for plant-N acquisition via a priming effect. The experimental and modeling results were consistent with our hypothesis that N-containing compounds in root exudates affect rhizosphere processes by providing substrates for the synthesis of N-rich microbial biomass and exoenzymes.

学界普遍认为，根系分泌物（root exudation）可提升微生物及其合成的胞外酶（exoenzyme）活性，进而使根际土壤（rhizosphere soil）相较于非根际土壤（bulk soil）具备更快的碳（C）矿化速率与养分循环效率。微生物生物量与胞外酶的氮（N）含量，可能会对微生物有效利用根系分泌物的能力形成化学计量约束，尤其当分泌物碳含量高、氮含量低时，该约束效应更为显著。本研究将微生物活性理论模型与分泌物添加实验相结合，旨在验证如下假说：土壤微生物利用根系分泌物合成额外生物量与胞外酶的能力，会受到氮素有效性的制约。本研究开展两项为期50天的野外实验，通过微型渗漏计（microlysimeter）搭建的根系模拟装置（root simulators），向原状森林土壤中自动泵入模拟根系分泌物的化学试剂溶液（exudate mimics），该溶液仅含碳，或同时添加碳与氮（碳氮比为10），以此模拟根系分泌物的释放过程。仅添加含碳模拟分泌物的处理组，其微生物呼吸速率显著提升，但对微生物生物量与胞外酶活性无显著影响。与之相反，同时添加碳氮模拟分泌物的处理组，其微生物呼吸速率、微生物生物量以及分解土壤有机质（soil organic matter, SOM）等低分子量组分（如纤维素、氨基糖）的胞外酶活性均显著提升，同时降低了降解多酚、木质素等高分子量土壤有机质的胞外酶活性。模型模拟结果与实验结果一致：仅添加碳的模拟分泌物处理会引发微生物氮限制，进而制约微生物生物量与胞外酶的合成。在模型中，同时向分泌物中添加氮与碳可缓解该化学计量约束，从而提升胞外酶产量、激发有机质分解过程，并实现土壤有机质中氮的净释放（即矿化作用）。在多数模拟场景中，模型预测从土壤有机质中释放的氮量均高于模拟分泌物脉冲输入的氮量，这表明含氮根系分泌物可通过激发效应（priming effect）成为植物获取氮素的可行策略。本研究的实验与模拟结果均验证了我们的假说：根系分泌物中的含氮化合物可作为合成富氮微生物生物量与胞外酶的底物，进而调控根际过程。