Modeling the carbon costs of plant nutrient uptake: Opportunities and challenges

Kara E. Allen, Manaaki Whenua--Landcare Research, Lincoln, New Zealand, Renato K. Braghiere and Joshua B. Fisher, NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, Renato K. Braghiere and Joshua B. Fisher, Joint Institute for Regional Earth System Science and Engineering, University of California, Los Angeles, CA, Richard P. Phillips, Department of Biology, Indiana University, Bloomington, IN, Jennifer S. Powers, Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, MN, Jennifer S. Powers, Department of Plant and Microbial Biology, University Of Minnesota, St. Paul, MN, Christopher A. Walter and Edward R. Brzostek, Department of Biology, West Virginia University, Morgantown, WV

Background/Question/Methods
Plants allocate a substantial portion of their available carbon (C) to acquiring key nutrients, such as nitrogen (N) and phosphorus (P). In many ecosystems, N and P either limit or co-limit net primary production (NPP), making the mechanisms plants use to acquire these nutrients costly. However, representations of C-N-P dynamics in terrestrial biosphere models that account for energy invested in multiple nutrient acquisition pathways are scarce, limiting our ability to accurately predict terrestrial C uptake under future climate scenarios. To fill this knowledge gap, we integrated P dynamics into an existing plant nutrient uptake model that estimates the C cost of N acquisition from soil based on the cost of allocating C to leaf resorption and root/root-microbial uptake. Within the new model framework (Fixation and Uptake of Nutrients; FUN 3.0), we incorporated the direct C cost of P uptake, as well as N costs of synthesizing phosphatase enzymes to extract P from soil. We confronted and validated FUN 3.0 against empirical estimates of canopy, root, and soil P pools from temperate and tropical forest sites. We then ran model experiments to examine the extent to which the costs of P acquisition varied as a function of nutrient availability and mycorrhizal association.
Results/Conclusions
Across both tropical and temperate forest sites, FUN 3.0 accurately predicted empirical estimates of N and P translocation to leaves. Overall, C costs for acquiring P were highest in the tropical forest sites, where a large proportion of C was invested in N to support P uptake, enhancing N fixation in these systems. Model simulations with reduced nutrient availability illustrated that P limitation is not only an important process in the tropics but can also limit NPP through co-limitation with N in temperate forests. The addition of FUN 3.0 into an earth system model (E3SM), resulted in a better representation of the global C cycle in comparison to multiple observation driven datasets within ILAMB, ultimately downregulating NPP. Collectively, FUN 3.0 provides a novel framework for predicting how coupled N and P limitation may impact terrestrial C uptake. However, detailed N and P budgets are needed from a variety of ecosystems and environmental gradients to improve model projections. While further model development is needed to better represent flexible stoichiometry and future changes in biological nitrogen fixation under climate change.