Ostrom, N. E., K. E. Knoke, L. O. Hedin, G. P. Robertson, and A. J. Smucker. 1998. Temporal trends in nitrogen isotope values of nitrate leaching from an agricultural soil. Chemical Geology 146:219-227.
The concentration and δ 15N 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 δ 15N between lysimeters was evident, NO3- from both lysimeters exhibited considerable isotopic variability, ranging from minus 3.9 parts per thousand in March to a late summer maximum of 9.6 parts per thousand. Low δ 15N 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 δ 15N values for NO3- from the conventionally tilled and non-tilled lysimeters were 1.7 and 3.2 parts per thousand, respectively, and are depleted in N15 by greater than 4 parts per thousand relative to soil organic matter. These δ 15N 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 N15 in response to fractionation during mineralization. The wide range of δ 15N 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 δ 15N may provide more information on the predominance of microbial processes in soils than on origins.
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