The SOC and S are the two variables ordinarily used as
indicators of farmland soil quality, and they are also important
indicators of diffuse N loading. The horizontal
fitting lines for SOC
and S in the paddy rice sub-basins showed that they did not have a
strong influence. Because most diffuse N was in Org-N form, a
higher SOC in the soil improved the attachment of Org-N to soil
particles and decreased the transport chance. The spatial
correlations of the four metal indicators followed the same
pattern. The metal contents in paddy rice sub-basins was
negatively correlated with diffuse N loading, which meant that
the paddy rice sub-basins with higher metal contents had lower
loading risk. However, the situation in upland sub-basins was the
opposite. The completely contrary correlations meant that the
discharge principle was different in upland sub-basins with the
paddy rice sub-basins. In the upland sub-basins, the Org-N was
mainly absorbed by the soil and transported during rainfall events.
Under the aquatic condition of paddy rice sub-basins, the irrigation
accelerated the diffuse N discharge.