Modeling the role of root exudation and plant-microbe interactions in the response of soil respiration to P fertilization

Root exudation is a substantial carbon (C) flux from plants to soils and a key pathway by which vegetation influences soil respiration. Especially in phosphorus (P) limited ecosystems, enhanced nutrient availability as a consequence of root exudation links plant C allocation to microbial activity and soil respiration. However, we have limited knowledge of how root exudation and soil microbial activity modulate soil respiration when P limitation is alleviated through fertilization. Previous studies have showed that the response of soil respiration to P fertilization is ambiguous and dependent on the ecosystem but the underlying causes often remain unidentified.

Here we used the microbial-explicit terrestrial biosphere model QUINCY-JSM, including an implementation of dynamic root exudation based on plant carbon surplus and nutrient deficiency, to investigate the role of plant-soil interactions in the response of soil respiration to P fertilization. The root exudation implementation was previously tuned and tested for the Eucalyptus Free Air Carbon Enrichment (EucFACE) experiment, where the role of increased root exudation under CO₂ fertilization on soil organic matter cycling and soil respiration in a P-limited forest was evaluated. This experiment has now been fertilized with P, and here we take advantage of this modification to the experiment by simulating the EucFACE experiment under P fertilization. We investigate how microbial stoichiometry and P availability influenced simulated responses. In agreement with measurements, our model reproduced a decrease in soil respiration on P addition. Our simulations reveal root exudation as a key driver in this response: P fertilization alleviated plant P limitation, leading to a decrease in root exudation by up to 40 %. Consequently, the reduced C supply to the rhizosphere decreased microbial respiration up to 10 % and soil respiration up to 5 %. However, in simulations with high microbial P demand, microbes out-competed plants for the additionally available P and therefore suppressed the feedback to root exudation.

Our results highlight the role of root exudation in modulating soil respiration response to nutrient addition and the influence of soil microbial stoichiometry and baseline soil P availability. We make recommendations for further research by identifying critical variables for future modeling and observational studies.