Abstract
Abstract. The extent to which terrestrial ecosystems slow climate change by sequestering carbon hinges in part on nutrient limitation. We used a coupled carbon–climate model that accounts for the carbon cost to plants of supporting nitrogen-acquiring microbial symbionts to explore how nitrogen limitation affects global climate. To do this, we first calculated the reduction in net primary production due to the carbon cost of nitrogen acquisition. We then used a climate model to estimate the impacts of the resulting increase in atmospheric CO2 on temperature and precipitation regimes. The carbon costs of supporting symbiotic nitrogen uptake reduced net primary production by 8.1 Pg C yr−1, with the largest absolute effects occurring in tropical forest biomes and the largest relative changes occurring in boreal and alpine biomes. Globally, our model predicted relatively small changes in climate due to the carbon cost of nitrogen acquisition with temperature increasing by 0.1 ∘C and precipitation decreasing by 6 mm yr−1. However, there were strong regional impacts, with the largest impact occurring in boreal and alpine ecosystems, where such costs were estimated to increase temperature by 1.0 ∘C and precipitation by 9 mm yr−1. As such, our results suggest that carbon expenditures to support nitrogen-acquiring microbial symbionts have critical consequences for Earth's climate, and that carbon–climate models that omit these processes will overpredict the land carbon sink and underpredict climate change.
Highlights
The magnitude of carbon (C) uptake by the terrestrial biosphere strongly depends on the availability of nutrients to support net primary production (NPP) (Wang et al, 2010; Zaehle et al, 2015; Wieder et al, 2015)
Compared to the Community Atmosphere Model (CAM) runs where N was obtained at no cost, when we included the C cost of symbiont-mediated N acquisition (i.e., CAM-Fixation and Uptake of Nitrogen (FUN)), C uptake by the terrestrial biosphere was more strongly constrained by N availability
NPP and leaf area index (LAI) were affected with the strongest relative effects occurring at the poles and the strongest absolute effects occurring near the Equator
Summary
The magnitude of carbon (C) uptake by the terrestrial biosphere strongly depends on the availability of nutrients to support net primary production (NPP) (Wang et al, 2010; Zaehle et al, 2015; Wieder et al, 2015). Given the magnitude of these C expenditures, Earth system models (ESMs) that do not account for the costs of supporting symbiotic microbes may overestimate NPP and the ability of terrestrial ecosystems to slow climate change. Plant associations with mycorrhizal fungi such as arbuscular mycorrhizae (AM) and ectomycorrhizae (ECM), or with N-fixers, are critical for the uptake of soil nutrients and, as such, impact C and nutrient cycling (Phillips et al, 2013; Wurzburger et al, 2017). Recent data syntheses have shown that ECM and AM ecosys-
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