Carbon cost of plant nitrogen acquisition: global carbon cycle impact from an improved plant nitrogen cycle in the Community Land Model

Abstract

Plants typically expend a significant portion of their available carbon (C) on nutrient acquisition - C that could otherwise support growth. However, given that most global terrestrial biosphere models (TBMs) do not include the C cost of nutrient acquisition, these models fail to represent current and future constraints to the land C sink. Here, we integrated a plant productivity-optimized nutrient acquisition model - the Fixation and Uptake of Nitrogen Model - into one of the most widely used TBMs, the Community Land Model. Global plant nitrogen (N) uptake is dynamically simulated in the coupled model based on the C costs of N acquisition from mycorrhizal roots, nonmycorrhizal roots, N-fixing microbes, and retranslocation (from senescing leaves). We find that at the global scale, plants spend 2.4 Pg C yr(-1) to acquire 1.0 Pg N yr(-1) , and that the C cost of N acquisition leads to a downregulation of global net primary production (NPP) by 13%. Mycorrhizal uptake represented the dominant pathway by which N is acquired, accounting for ~66% of the N uptake by plants. Notably, roots associating with arbuscular mycorrhizal (AM) fungi - generally considered for their role in phosphorus (P) acquisition - are estimated to be the primary source of global plant N uptake owing to the dominance of AM-associated plants in mid- and low-latitude biomes. Overall, our coupled model improves the representations of NPP downregulation globally and generates spatially explicit patterns of belowground C allocation, soil N uptake, and N retranslocation at the global scale. Such model improvements are critical for predicting how plant responses to altered N availability (owing to N deposition, rising atmospheric CO2 , and warming temperatures) may impact the land C sink.

Mycorrhizal association as a primary control of the CO 2 fertilization effect

“…We propose that the CO 2 fertilization effect be quantified based on mycorrhizal type and soil nitrogen status, and that large-scale ecosystem models incorporate mycorrhizal types to account for the differences in biomass enhancement by eCO 2 . Mycorrhizae are ubiquitous, and sort predictably with plant functional type (24,34), making feasible their inclusion in models to capture this microbial influence on global biogeochemistry. Accounting for the influence of mycorrhizae will improve representation of the CO 2 fertilization effect in vegetation models, critical for projecting ecosystem responses and feedbacks to climate change.…”

Evaluating the effect of alternative carbon allocation schemes in a land surface model (CLM4.5) on carbon fluxes, pools, and turnover in temperate forests

“…S1 in the Supplement). As coupled C-N and functional root subroutines are developed for LSMs (Shi et al, 2016), and with better representation of vegetation dynamics (Fisher et al, 2015), we could imagine a dynamic allocation scheme for CLM4.5 based on whether aboveground (light) or belowground (water and nutrients) factors are limiting.…”

“…In CLM4.5 employed here, the roots control water uptake but are not related to nutrient uptake, which limits the potential for dynamic responses to nutrients and CO 2 concentrations (Atkin, 2016;De Kauwe et al, 2014;Hickler et al, 2015;Sevanto and Dickman 2015). Root functionality in LSMs could be enhanced by improving parameterization within models and introducing new components such as dynamic root distribution and root functional traits linked to resource extraction (Warren et al, 2015;Brzostek et al, 2014Brzostek et al, , 2017Shi et al, 2016;Phillips et al, 2016;Iversen et al, 2017).…”

Integration of nitrogen dynamics into the Noah-MP land surface model v1.1 for climate and environmental predictions

“…Because Noah-MP has an interactive vegetation canopy option -which predicts the leaf area index (LAI) as a function of light, temperature, and soil moisture -it is logical to augment this scheme with N limitation and realistic plant N uptake and fixation. The state-of-the-art vegetation N model is the Fixation and Uptake of Nitrogen (FUN) model of Fisher et al (2010), which is embedded into the Joint UK Land Environment Simulator (JULES) (D. B. and the Community Land Model (CLM) (Shi et al, 2016). Modeling the impacts of agricultural management (e.g., fertilizer use) on N leaching is the strength of the Soil and Water Assessment Tool (SWAT) (Neitsch et al, 2011).…”