|Chemical Routines-in situ|
A daily mass balance of nitrogen (N), phosphorus (P) and organic carbon (C) is computed for each cell (pesticides will be added later). Major components considered are uptake of N and P by plants, application of fertilizers, residue decomposition, and downward movement of nitrogen & phosphorus. The output from each cell includes sediment bound N, soluble N in runoff, sediment bound P, soluble P in runoff, and sediment bound organic carbon. Nitrogen and phosphorus are partitioned into organic and mineral parts and a separate mass balance computed for each. The nitrogen & organic carbon cycles represented in AnnAGNPS are simplifications that track only major nitrogen transformations of mineralization from humified soil organic matter and plant residues, crop residue decay, and fertilizer & plant uptake. Plant uptake of N & P are modeled through a simple crop growth stage index.