2.16. CN Pools

2.16.1. Introduction

CLM includes a prognostic treatment of the terrestrial carbon and nitrogen cycles including natural vegetation, crops, and soil biogeochemistry. The model is fully prognostic with respect to all carbon and nitrogen state variables in the vegetation, litter, and soil organic matter. The seasonal timing of new vegetation growth and litterfall is also prognostic, responding to soil and air temperature, soil water availability, daylength, and crop management practices in varying degrees depending on a specified phenology type or management for each PFT (Chapter 2.20). The prognostic LAI, SAI, tissue stoichiometry, and vegetation heights are utilized by the biophysical model that couples carbon, water, and energy cycles.

Separate state variables for C and N are tracked for leaf, live stem, dead stem, live coarse root, dead coarse root, fine root, and grain pools (Figure 2.16.1). Each of these pools has two corresponding storage pools representing, respectively, short-term and long-term storage of non-structural carbohydrates and labile nitrogen. There are two additional carbon pools, one for the storage of growth respiration reserves, and another used to meet excess demand for maintenance respiration during periods with low photosynthesis. One additional nitrogen pool tracks retranslocated nitrogen, mobilized from leaf tissue prior to abscission and litterfall. Altogether there are 23 state variables for vegetation carbon, and 22 for vegetation nitrogen.

Figure 2.16.1 Vegetation fluxes and pools for carbon cycle in CLM5.

In addition to the vegetation pools, CLM includes a series of decomposing carbon and nitrogen pools as vegetation successively breaks down to CWD, and/or litter, and subsequently to soil organic matter. Discussion of the decomposition model, alternate specifications of decomposition rates, and methods to rapidly equilibrate the decomposition model, is in Chapter 2.21.

2.16.2. Tissue Stoichiometry

As of CLM5, vegetation tissues have a flexible stoichiometry, as described in Ghimire et al. (2016). Each tissue has a target C:N ratio, with the target leaf C:N varying by plant functional type (see Table 2.16.1), and nitrogen is allocated at each timestep in order to allow the plant to best match the target stoichiometry. Nitrogen downregulation of productivity acts by increasing the C:N ratio of leaves when insufficient nitrogen is available to meet stoichiometric demands of leaf growth, thereby reducing the N available for photosynthesis and reducing the \(V_{\text{c,max25}}\) and \(J_{\text{max25}}\) terms, as described in Chapter 2.10. Details of the flexible tissue stoichiometry are described in Chapter 2.19.

Table 2.16.1 Plant functional type (PFT) target C:N parameters.

PFT	target leaf C:N
NET Temperate	58.00
NET Boreal	58.00
NDT Boreal	25.81
BET Tropical	29.60
BET temperate	29.60
BDT tropical	23.45
BDT temperate	23.45
BDT boreal	23.45
BES temperate	36.42
BDS temperate	23.26
BDS boreal	23.26
C3 arctic grass	28.03
C3 grass	28.03
C4 grass	35.36
Temperate Corn	25.00
Spring Wheat	20.00
Temperate Soybean	20.00
Cotton	20.00
Rice	20.00
Sugarcane	25.00
Tropical Corn	25.00
Tropical Soybean	20.00
Miscanthus	25.00
Switchgrass	25.00