Representing leaf and root physiological traits in CLM improves global carbon and nitrogen cycling predictions

Abstract

In many ecosystems, nitrogen is the most limiting nutrient for plant growth and productivity.However, current Earth System Models (ESMs) do not mechanistically represent functional nitrogen allocation for photosynthesis or the linkage between nitrogen uptake and root traits. The current version of CLM (4.5) links nitrogen availability and plant productivity via (1) an instantaneous downregulation of potential photosynthesis rates based on soil mineral nitrogen availability, and (2) apportionment of soil nitrogen between plants and competing nitrogen consumers assumed to be proportional to their relative N demands. However, plants do not photosynthesize at potential rates and then downregulate; instead photosynthesis rates are governed by nitrogen that has been allocated to the physiological processes underpinning photosynthesis. Furthermore, the role of plant roots in nutrient acquisition has also been largely ignored in ESMs. We therefore present a new plant nitrogen model for CLM4.5 with (1) improved representations of linkages between leaf nitrogen and plant productivity based on observed relationships in a global plant trait database and (2) plant nitrogen uptake based on root-scale Michaelis-Menten uptake kinetics. Our model improvements led to a global bias reduction in GPP, LAI, and biomass of 70%, 11%, and 49%, respectively. Furthermore, water use efficiency predictions were improved conceptually, qualitatively, and in magnitude. The new model's GPP responses to nitrogen deposition, CO 2 fertilization, and climate also differed from the baseline model. The mechanistic representation of leaf-level nitrogen allocation and a theoretically consistent treatment of competition with belowground consumers led to overall improvements in global carbon cycling predictions.

Earth System Model Needs for Including the Interactive Representation of Nitrogen Deposition and Drought Effects on Forested Ecosystems

“…Nitrogen uptake (when included) is often less complex in ESMs; most N uptake is from a bulk N mineralization pool that depends on supply and demand rather than root biomass or profile, although some recent work has focused on uptake as a function of root biomass [106]. Essentially, plants are given every opportunity to extract all available N necessary for growth.…”

“…More recent developments have led to the development of the Fixation and Uptake of Nitrogen (FUN) model [108], which expands N acquisition to include processes of passive uptake, active uptake, re-translocation, and symbiotic N fixation through a C cost. Other advances in the Community Land Model (CLM) expand N uptake to include methods using Michaelis-Menten equations [106,110] or equilibrium chemistry approximation [111,112]. Additional processes such as abiotic (i.e., mineral surface) competition for soil nutrients (Nutrient COMpetition model, [111]) are anticipated for the next generation of the CLM and the Accelerated Climate Model for Energy (ACME).…”

“…• Optimize nutrient uptake • Increase tissue allocation to respond to limiting resource CLM/ACME [106] Improve form and function of roots…”

“…Losses are from plant uptake, immobilization, leaching, and nitrification/denitrification processes. Models can represent N limitation in different ways, including using N to scale photosynthesis [105,106], downscaling potential gross primary productivity (GPP) to reflect N availability [38,84,85,107], defining a C cost of N uptake [108], optimizing N allocation for leaf processes [109], or adapting a flexible C:N ratio for N allocation [106]. Nitrogen uptake is scaled depending on demand, based on stoichiometry (see Section 5.2) and availability, where photosynthesis and decomposition may be downscaled.…”

“…For others, support needs to come from observation and the empirical community to develop robust methodology for inclusion in models. • Effects N in the leaf with influences on photosynthesis Fixation and Uptake of Nitrogen (FUN) model [108] Leaf Utilization of Nitrogen for Assimilation (LUNA) [109] Community Land Model (CLM)/Accelerated Climate Model for Energy (ACME) [106] Adaptive dynamics approach to C partitioning…”