Abstract
Nutrient limitation is a key source of uncertainty in predicting terrestrial carbon (C) uptake. Models have begun to include nitrogen (N) dynamics; however, phosphorus (P), which can also limit or co-limit net primary production (NPP) in many ecosystems, is currently absent in most models. To meet this challenge, we integrated P dynamics into a cutting-edge plant nutrient uptake model (Fixation and Uptake of Nitrogen: FUN 2.0) that mechanistically tracks the C cost of N uptake from soil based on the cost of allocating C to leaf resorption and root/root-microbial uptake, and the availability of N in soil. We incorporated the direct C cost of P uptake, as well as a N cost of synthesizing phosphatase enzymes to extract P from soil, into a new model formulation (Fixation and Uptake of Nutrients: FUN 3.0). We confronted and validated FUN 3.0 against empirical estimates of canopy, root, and soil P pools from 45 temperate forest plots in Indiana, USA and 18 tropical dry forest plots located in Guanacaste, Costa Rica that vary in P availability and distribution of arbuscular mycorrhizal- (AM) and ectomycorrhizal- (ECM) associated trees. FUN 3.0 was able to accurately predict N and P retranslocation across the temperate and tropical forest sites (slopes of 0.95 and 0.92 for P and N retranslocation, respectively). Carbon costs for acquiring P were three times higher in tropical forest sites compared to temperate forest sites, driving overall higher C costs in tropical sites. In addition, the N costs for acquiring P in tropical forest sites lead to a substantial increase in N fixation to support phosphatase enzyme production. Sensitivity analyses showed that tropical sites appeared to be severely P limited, while the temperate sites showed evidence for co-limitation by N and P. Collectively, FUN 3.0 provides a novel framework for predicting coupled N and P limitation that earth system models can leverage to enhance predictions of ecosystem response to global change.
Highlights
The response of the land carbon (C) sink to environmental changes is controlled by limitation of key nutrients, such as nitrogen (N) and phosphorus (P)
Recent model developments have primarily focused on coupling C-N biogeochemical cycles (Zaehle et al, 2014) and have predicted significant reductions in global net primary productivity (NPP) under increasing atmospheric CO2 compared to C-climate–only models (Hungate et al, 2003; Thornton et al, 2007; Zaehle et al, 2010)
Many of the CN earth system models (ESMs) are parameterized to capture nutrient limitation in northern hemisphere forests, which tend to be most limited by N availability
Summary
The response of the land carbon (C) sink to environmental changes (e.g., increasing atmospheric CO2, nitrogen deposition, warmer temperatures) is controlled by limitation of key nutrients, such as nitrogen (N) and phosphorus (P). Our ability to predict the extent to which environmental change will impact the future land C sink depends on how ecosystem models represent nutrient interactions within their structures (Wieder et al, 2015). Recent model developments have primarily focused on coupling C-N biogeochemical cycles (Zaehle et al, 2014) and have predicted significant reductions in global net primary productivity (NPP) under increasing atmospheric CO2 compared to C-climate–only models (Hungate et al, 2003; Thornton et al, 2007; Zaehle et al, 2010). ESMs that consider C-N-P couplings should facilitate improved predictions of feedbacks to climate change (Fleischer et al, 2019)
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