Confronting models with measurements: An optimal allocation model accurately predicts empirical measurements of the rhizosphere marketplace for nitrogen and phosphorus

Background/Question/Methods

Accurate projections of the future land carbon (C) sink by terrestrial biosphere models depend on how they represent the impacts of nutrients on the partitioning of photosynthate-C between above and belowground pools. While empirical research has highlighted the rhizosphere as a hotspot for the trading of photosynthate for soil nitrogen (N) and phosphorus (P) between roots, mycorrhizal fungi, and free-living microbes, this C allocation pathway is missing in the models. P dynamics are also largely absent or poorly parameterized in models despite the ability of P to limit or co-limit net primary production (NPP) in many ecosystems.  Here, we integrated P dynamics into a cutting-edge plant N model (Fixation & Uptake of Nitrogen: FUN; Fisher et al., 2010, Brzostek et al., 2014) that includes representations of how ecto- and arbuscular mycorrhizal trees differ in the rhizosphere marketplace for nutrients. We confronted and validated FUN against empirical estimates of the C allocated belowground by temperate forests trees to mobilize N and P.

Results/Conclusions

Across 45 temperate forest plots that vary in the distribution of arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) associated trees centered on the Morgan-Monroe State Forest AmeriFlux site, we found that FUN predicted much of the variability in the empirical estimates of belowground C allocation.  The N-only version captured the empirical patterns of greater root exudation (R²=0.53) and higher carbon costs of N acquisition (R²=0.77, i.e., C expended per unit N gained) that occur as the dominance of ectomycorrhizal trees increases.  However, there was a bias in the N-only model predictions where costs were under- and overestimated in ECM and AM dominated plots, respectively.  This was solved by integrating the interactions between the C costs of N and P uptake, which eliminated this bias and resulted in ~6% of NPP on average being allocated belowground to gain N and P.  Given that terrestrial biosphere and soil C models are now examining how plant C inputs — often assuming that inputs are a constant fraction of NPP — influence soil decomposition, FUN provides an important, robust bridge between terrestrial biosphere models and microbial-soil C models by dynamically predicting the C allocated by plants to the rhizosphere marketplace.