Stoichiometry constrains microbial response to root exudation- insights from a model and a field experiment in a temperate forest

[摘要] Plant roots release a wide range of chemicals into soils. This process,termed root exudation, is thought to increase the activity of microbes andthe exoenzymes they synthesize, leading to accelerated rates of carbon (C)mineralization and nutrient cycling in rhizosphere soils relative to bulksoils. The nitrogen (N) content of microbial biomass and exoenzymes mayintroduce a stoichiometric constraint on the ability of microbes toeffectively utilize the root exudates, particularly if the exudates are richin C but low in N. We combined a theoretical model of microbial activitywith an exudation experiment to test the hypothesis that the ability of soilmicrobes to utilize root exudates for the synthesis of additional biomassand exoenzymes is constrained by N availability. The field experimentsimulated exudation by automatically pumping solutions of chemicals oftenfound in root exudates ("exudate mimics") containing C alone or C incombination with N (C : N ratio of 10) through microlysimeter "rootsimulators" into intact forest soils in two 50-day experiments. Thedelivery of C-only exudate mimics increased microbial respiration but had noeffect on microbial biomass or exoenzyme activities. By contrast,experimental delivery of exudate mimics containing both C and Nsignificantly increased microbial respiration, microbial biomass, and theactivity of exoenzymes that decompose low molecular weight components ofsoil organic matter (SOM, e.g., cellulose, amino sugars), while decreasingthe activity of exoenzymes that degrade high molecular weight SOM (e.g.,polyphenols, lignin). The modeling results were consistent with theexperiments; simulated delivery of C-only exudates induced microbialN-limitation, which constrained the synthesis of microbial biomass andexoenzymes. Exuding N as well as C alleviated this stoichiometricconstraint in the model, allowing for increased exoenzyme production, thepriming of decomposition, and a net release of N from SOM (i.e.,mineralization). The quantity of N released from SOM in the modelsimulations was, under most circumstances, in excess of the N in the exudate pulse,suggesting that the exudation of N-containing compounds can be a viablestrategy for plant-N acquisition via a priming effect. The experimental andmodeling results were consistent with our hypothesis that N-containingcompounds in root exudates affect rhizosphere processes by providingsubstrates for the synthesis of N-rich microbial biomass and exoenzymes.This study suggests that exudate stoichiometry is an important andunderappreciated driver of microbial activity in rhizosphere soils.

[学科分类] 地球化学与岩石