Modeling the carbon cost of plant nitrogen acquisition: Mycorrhizal trade‐offs and multipath resistance uptake improve predictions of retranslocation

Accurate projections of the future land carbon (C) sink by terrestrial biosphere models depend on how nutrient constraints on net primary production are represented. While nutrient limitation is nearly universal, current models do not have a C cost for plant nutrient acquisition. Also missing are symbiotic mycorrhizal fungi, which can consume up to 20% of net primary production and supply up to 50% of a plant's nitrogen (N) uptake. Here we integrate simultaneous uptake and mycorrhizae into a cutting‐edge plant N model—Fixation and Uptake of Nitrogen (FUN)—that can be coupled into terrestrial biosphere models. The C cost of N acquisition varies as a function of mycorrhizal type, with plants that support arbuscular mycorrhizae benefiting when N is relatively abundant and plants that support ectomycorrhizae benefiting when N is strongly limiting. Across six temperate forested sites (representing arbuscular mycorrhizal‐ and ectomycorrhizal‐dominated stands and 176 site years), including multipath resistance improved the partitioning of N uptake between aboveground and belowground sources. Integrating mycorrhizae led to further improvements in predictions of N uptake from soil ( R 2  = 0.69 increased to R 2  = 0.96) and from senescing leaves ( R 2  = 0.29 increased to R 2  = 0.73) relative to the original model. On average, 5% and 9% of net primary production in arbuscular mycorrhizal‐ and ectomycorrhizal‐dominated forests, respectively, was needed to support mycorrhizal‐mediated acquisition of N. To the extent that resource constraints to net primary production are governed by similar trade‐offs across all terrestrial ecosystems, integrating these improvements to FUN into terrestrial biosphere models should enhance predictions of the future land C sink.