Connecting root activity, nitrogen mineralization, and plant allocation in temperate forest: A new modeling approach

Background/Question/Methods

Plants allocate carbon (C) to free-living and symbiotic microbes to increase their access to nitrogen (N) resulting in important downstream effects on soil C storage. The strength of coupling of C and N belowground appears to differ between temperate forest trees that associate with ecto- (ECM) and arbuscular mycorrhizae (AM).  N cycling in ECM stands is tightly coupled with C allocation to rhizosphere microbes, whereas N cycling in AM stands is driven by microbes inhabiting the bulk soil. Models that simulate carbon and nitrogen cycling at ecosystem and global scales generally lack mechanistic representations of plant-microbial interactions and these tradeoffs between AM and ECM fungi. As a result, many models are unable to capture key empirical responses to global change including the sustained CO₂-fertilization effect on NPP and associated increases in N uptake. We present a new model that couples predictions of C allocation to AM and ECM rhizospheres (Fixation & Uptake of N model) with resulting impacts on soil C and N through a microbial-focused soil decomposition model (CORPSE-N). We validated the coupled model across a 45-plot gradient in mycorrhizal association in Indiana where we have extensive measurements of C allocation and N cycling both above- and belowground.

Results/Conclusions

The coupled models accurately simulated the carbon and nitrogen cycles relative to observations, including soil carbon and nitrogen pools, nitrogen mineralization, soil CO₂ emissions, and plant nitrogen uptake.  Increased root exudation stimulated soil nitrogen mineralization and plant nitrogen uptake, and the magnitude of this effect was highly sensitive to the size of the rhizosphere. AM-plots had lower rates of root exudation but more rapidly-decomposing soil due to differences in litter properties compared to ECM-plots. ECM plots had more decomposition-resistant litter and soils, but also higher rates of root exudation and stronger responses of soil carbon and nitrogen mineralization to root exudation.  Model simulations indicated that the increased root exudation resulting from rising plant nitrogen demands (for example, due to CO₂ fertilization) had the potential to decrease soil carbon stocks through enhanced priming effects, but that ECM soils were more vulnerable than AM soils.   These results illustrate the importance of root-soil interactions in supporting plant growth as well as controlling soil carbon storage. This is one of the first models to successfully couple plant carbon allocation to below-ground priming and nitrogen mineralization in a framework that can be easily integrated into large-scale land surface models.