Accordingly, nitrates potentially leaching from a paddy field are
modelled according to a dual N and water mass balance approach
suggested by Pathak et al. (2004). N inputs include fertiliser, precipitation,
irrigation water and soil (N stock, immobilisation). N
outputs include losses in surface runoff, deep percolation, harvested
and exported crop components (mostly rice grain), soil
losses (erosion), mineralisation, volatilisation and nitrification
processes. The difference in N stored in pre-cultivation soil and in
post-cultivation soil is considered negligible because these soils
have maintained long-term stable nitrogen contents under the
same cropping systems for years. Similarly, the organic matter
dynamic is deemed balanced over time, with equal mineralisation
and immobilisation (straw). Other components, such as biological
nitrogen fixation, groundwater contribution, and exports by weeds,
are ignored (Pathak et al., 2004).