### 2.27.3.2. Carbon and Nitrogen Conservation[¶](#carbon-and-nitrogen-conservation "Permalink to this headline")

Because of the long timescales involved with below-ground carbon and nitrogen dynamics, it is more important that these state variables be adjusted properly when subgrid areas change. Carbon and nitrogen variables are adjusted with the following three-step process:

1.  Patch-level (i.e., vegetation) state variables are adjusted for any changes in patch areas; this may lead to fluxes into column-level (i.e., soil) state variables (2) Column-level (i.e., soil) state variables are updated based on the fluxes generated in (1)
    

3.  Column-level (i.e., soil) state variables are adjusted for any changes in column areas First, patch-level (i.e., vegetation) state variables are adjusted for any changes in patch areas. This includes changes in column or land unit areas, even if the relative proportions of each patch remain constant: the relevant quantities are the patch weights relative to the gridcell.
    

For a patch that decreases in area, the carbon and nitrogen density on the remaining patch area remains the same as before (i.e., expressed as g per m2 patch area). Because the area has decreased, this represents a decrease in total carbon or nitrogen mass (i.e., expressed as g per m2 gridcell area). The lost mass meets a variety of fates: some is immediately lost to the atmosphere, some is sent to product pools (which are lost to the atmosphere over longer time scales), and some is sent to litter pools.

For a patch that increases in area, the carbon and nitrogen density on the new patch area is decreased in order to conserve mass. This decrease is basically proportional to the relative increase in patch area. However, a small amount of seed carbon and nitrogen is added to the leaf and dead stem pools in the new patch area.

Next, column-level (i.e., soil) state variables are updated based on any fluxes to soil pools due to decreases in patch areas. This step is needed so that any lost vegetation carbon and nitrogen is conserved when column areas are changing.

Finally, column-level state variables are adjusted for any changes in column areas. Similarly to patches, for a column that decreases in area, the carbon and nitrogen density on the remaining column area remains the same as before (i.e., expressed as g per m2 column area). This represents a decrease in total carbon or nitrogen mass on the gridcell, and this lost mass is tracked for each gridcell. After these mass losses are summed for all shrinking columns, they are distributed amongst the growing columns in order to conserve mass. Thus, a growing column’s new carbon density will be a weighted sum of its original carbon density and the carbon densities of all shrinking columns in its gridcell.

This operation makes some simplifying assumptions. First, as described in section [2.27.2](#transient-landcover-reconciling-changes-in-area), we only track net area changes, not gross changes. Second, we assume that growing columns all grow proportionally into each of the shrinking columns.

Non-vegetated land units (e.g., glacier) do not typically track soil carbon and nitrogen. When columns from these land units initially shrink, they are assumed to contribute zero carbon and nitrogen. However, when they grow into previously-vegetated areas, they store any pre-existing soil carbon and nitrogen from the shrinking columns. This stored carbon and nitrogen will remain unchanged until the column later shrinks, at which point it will contribute to the carbon and nitrogen in the growing columns (exactly as would happen for a vegetated column).

In contrast to water and energy (section [2.27.3.1](#transient-landcover-water-and-energy-conservation)), no special treatment is needed for carbon and nitrogen states in columns that newly come into existence. The state of a new column is derived from a weighted average of the states of shrinking columns. This behavior falls out from the above general rules.

