Effects of soil nitrogen availability on rhizodeposition in plants: a review

Soil contains the majority of terrestrial carbon (C), forming the foundation for soil fertility and nutrient cycling. One key source of soil C is root-derived C, or rhizodeposits, which signal and sustain microbes that cycle nutrients such as nitrogen (N). Although N availability can affect rhizodeposition both quantitatively and qualitatively, these effects remain poorly understood due to conflicting results among studies. Here, we review studies examining the influence of soil N availability on rhizodeposition. We conduct a quantitative analysis of the response of various rhizodeposition C pools to N availability, and assess methodological aspects potentially underlying the highly variable results among studies. We also review impacts of N availability on the composition and quality of rhizodeposits. Effects of N on rhizodeposition were strongly dependent upon the specific C pools considered and the units for reporting those pools. N additions increased nearly all rhizodeposit C pools when expressed on a per plant basis, and decreased rhizodeposition per unit fixed C for several C pools, while no rhizodeposition C pools were significantly altered when expressed per unit root mass. Nevertheless, N effects were generally mixed due to a combination of variation in experimental methods and species-specific responses. Overall, our review indicates several key challenges for better understanding the mechanistic links between N availability, plant physiology, and microbial function. Identifying such links would substantially improve our ability to predict C- and N-dynamics in changing ecosystems.