Nitrogen isotope indicators of seasonal source variability to groundwater

A nitrogen isotope study of soil water and groundwater in southern Indiana, USA, in 1991-1992 demonstrated considerable variations in nitrate degradation processes compared to an earlier investigation in 1986-1987. Although N-fertilizers were applied in May 1991, the delta(15)N values in soil water decreased in February 1992, indicating its delayed release into the system after substantial rainfall. The delta(15)N values of groundwater decreased from + 12.3 parts per thousand in November 1991 to + 11.3 parts per thousand in February 1992, and to + 7.5 parts per thousand in March 1992. The increased residence time of nitrate in the soil resulted in increased denitrification, ammonia volatilization and plant uptake, and reduced threat to the groundwater quality. The 1986-1987 study in the area reported that excessive rainfall during the summer rapidly transported the nitrate to deeper horizons and drastically reduced volatilization and microbial reduction of nitrate, thus increasing the immediate threat to the groundwater quality in the area. The present study demonstrated that nitrogen isotopic signatures can be used to determine the effects of local soil type, rainfall, and land-use practices on the fate of nitrate in the subsurface.