Effect of wheat-maize straw return on the fate of nitrate in groundwater in the Huaihe River Basin, China.

Straw return is becoming a routine practice in disposing of crop residues worldwide. However, the potential effect of such operation on the chemistry of local groundwater is not well documented. Here, shallow groundwater in an area where wheat-maize straw return is practiced was analyzed, and the seasonal changes in the nitrate concentration and the isotope compositions of NO3- and H2O were determined along two flow paths. Measured δD and δ18O in waters indicated that the groundwater was mainly recharged by atmospheric precipitation, while measured δ15N and δ18O in nitrate suggested that the sources for groundwater NO3- included urea fertilizer, soil nitrogen, and sewage/manure. Reduced NO3- concentrations coincided with an enrichment of organic matter in the groundwater of the straw return area, revealing an environmental condition that facilitates nitrate reduction, whereas increased δ15N-NO3- and δ18O-NO3- along the flow path suggested the occurrence of denitrification. Further analyses showed that, compared to the cases in the absence of straw return, as much as 80% and 90% of groundwater nitrate was removed in low and high water seasons in the straw return area, pointing to a potential positive effect of straw return to groundwater quality.