However, during the study period, a general increasing trend
was found in groundwater NO3-N concentrations for the same
irrigation conditions in wheat–maize rotations (Figs. 4 and 5). The
increase in groundwater NO3-N concentrations for persistent
irrigation with sewage was over four times as high as that for
persistent well irrigation and nearly double of that for irrigation
that did not alternate between well and sewage water. The
variability of NO3-N concentrations in groundwater may be
explained by other factors, such as soil texture, kind of fertilizer,
nitrate content of sewage, farming practices, and the lateral flow of
groundwater. Thus, we expected a difference in groundwater
nitrate concentration even if land use type and irrigation condition
were unchanged.