Changes from previous version

The previous versions of SWAT+ have separate pools for soil organic N (two pools, active and stable) and organic P (one pool) and do not include explicitly $S_C$, $R_C$, or $M_C$ pools. Nitrogen and P decompose separately, excepting the common effect of soil moisture and temperature on the actual decomposition rate and the indirect control of decomposition through $R_{CN}$ and $R_{CP}$. The higher the ratios the lower the decomposition rate, but no immobilization of mineral N or P occurs. Of the N and P decomposed from residues, 20% is allocated to the active soil organic N and the soil organic P pools, respectively, and 80% is allocated to the nitrate ($NO_3$) and P in the soil solution. The 20% allocated to the N and P organic pools is roughly similar to the humification of C described in the new sub-model.

In the new sub-model, mineralization of N from organic pools feeds the $NH_4$ pool instead of the $NO_3$ pool. This will possibly increase volatilization of ammonium because in SWAT the latter is tightly linked to nitrification. Since mineralization and immobilization are explicitly calculated, the fluctuations in mineral $NO_3$, should be more realistically simulated, which has obvious implications for $NO_3$ transport and other processes across the watershed. Tillage accelerates SOM turnover and mixes layers according to the tillage depth. In the previous version tillage had some effects on the surface properties (residue cover) and mixing only involved two layers. No effect of tillage on the organic N or P decomposition rate was previously simulated. The new sub-model also enhances the capacity of SWAT+ to simulate the impact of tillage on nutrient cycling.