Summary:

## Vegetation Carbon and Nitrogen Cycling in CLM5

### Introduction

The Community Land Model (CLM) includes a prognostic treatment of the terrestrial carbon and nitrogen cycles, covering natural vegetation, crops, and soil biogeochemistry. The model tracks carbon and nitrogen state variables for various vegetation pools, including leaves, stems, roots, and grains. It also accounts for short-term and long-term storage of non-structural carbohydrates and labile nitrogen, as well as growth respiration reserves and maintenance respiration demands.

### Vegetation Pools and Fluxes

The vegetation component of CLM includes 23 carbon state variables and 22 nitrogen state variables, representing the different tissue types and storage pools. The seasonal timing of new growth and litterfall is also modeled, responding to environmental factors such as temperature, soil moisture, daylength, and crop management practices.

The prognostic vegetation properties, including leaf area index (LAI), stem area index (SAI), tissue stoichiometry, and vegetation heights, are utilized by the biophysical model to couple the carbon, water, and energy cycles.

### Decomposition and Soil Organic Matter

In addition to the vegetation pools, CLM includes a series of decomposing carbon and nitrogen pools as vegetation breaks down into coarse woody debris (CWD), litter, and soil organic matter. The decomposition model and methods for rapidly equilibrating the decomposition pools are discussed in a separate chapter.