Nitrogen is removed from soil by plant uptake and
microbial denitrification, which requires anoxic conditions,
a source of readily degradable carbon and
nitrogen in the form of nitrate. Denitrification
removed only 2% of applied nitrogen (1000 kg N/ha/
year loading) from cropped land (Russell & Cooper,
1987). However, the contribution of denitrification
to nitrogen removal depends on the concentration,
and probably the type, of COD present in the irrigation
water. Total nitrogen (N20+N2) emission rates
from soils irrigated with anaerobically treated wastewater
at very high nitrogen-loadings (about 1000
kg/ha.year) were only one-third those of land irrigated
with primary-treated wastewater (Russell et
al., 1993), therefore increasing the danger of nitrate
leaching. Presumably, denitrification would be even
further reduced in aerobically treated wastewater, in
which COD levels are little higher than 1-200 mg/1.