Temporal trends in nitrogen isotope values of nitrate leaching from an agricultural soil

The concentration and δ15N of NO3− in leachate from two undisturbed and unfertilized soil lysimeters, one conventionally tilled and one with no tillage, was determined on a bi-weekly basis from March through November, 1993 to assess the origins and transformations of NO3− leaching from an agricultural soil. Concentrations of NO3− in leachate from the tilled lysimeter were approximately twice those from the non-tilled lysimeter throughout the year and are consistent with observations that tilling favors the mineralization process in soils. Although no difference in δ15N between lysimeters was evident, NO3− from both lysimeters exhibited considerable isotopic variability, ranging from −3.9‰ in March to a late summer maximum of 9.6‰. Low δ15N values in the spring and fall were indicative of NO3− derived from soil organic matter and affected by fractionation during mineralization. High values in late summer indicated NO3− originating from soils that had been influenced to a small extent by fractionation during denitrification. Weighted mean δ15N values for NO3− from the conventionally tilled and non-tilled lysimeters were 1.7 and 3.2‰, respectively, and are depleted in 15N by greater than 4‰ relative to soil organic matter. These δ15N values would normally be indicative of NO3− derived from fertilizers, however, in this study they reflect an origin from soil organic matter and are depleted in 15N in response to fractionation during mineralization. The wide range of δ15N values in this study illustrates that sampling of NO3− in soil leachate at one point in time is clearly not sufficient to assess origins or identify the predominant microbial processes occurring in soils. Our results indicate that NO3− in soil leachate is subject to considerable isotopic variation and that δ15N may provide more information on the predominance of microbial processes in soils than on origins.