Limitations of Using δ¹⁸O for the Source Identification of Nitrate in Agricultural Soils

Abstract

The stable isotopic composition (delta 15N and delta 18O) of nitrate was analyzed in two lysimeter field experiments in order to identify the conditions under which the dual isotope approach can be applied to identify the main source of nitrate in agricultural soils. The first field experiment involved six lysimeters beneath fields that had been fertilized for 10 yr with the same type of fertilizer (NH4NO3; delta 15N = +1.2@1000, delta 18O = +18.6@1000). The isotope ratios of NO3- in the leachate (delta 15N approximately 0@1000; delta 18O approximately +2@1000) could not be interpreted in a conventional way with either fertilizer or soil organic nitrogen as main sources. These results provided clear evidence for the microbial immobilization and subsequent mineralization and nitrification to NO3- (mineralization-immobilization turnover concept). This process masked the original oxygen isotope ratio of the fertilizer source during the summer when microbial activity was high. A second experiment involving the application of Ca(NO3)2 to three lysimeters during the winter confirmed that the dual isotope approach remains valid for the source identification of nitrate under conditions of low microbial activity. The study reveals the limitation of the dual isotope approach to characterize nitrate sources under biologically active conditions and the ability to quantify microbial processes when the main sources can be controlled.

Sources and fate of nitrate in groundwater at agricultural operations overlying glacial sediments

“…Sci. Discuss., https://doi.org /10.5194/hess-2018- Nitrate isotope values in groundwater at the two CFOs studied are generally consistent with previous studies reporting denitrification of manure-derived NO3 -at dairy farms (Wassenaar, 1995;Wassenaar et al, 2006;Singleton et al, 2007;McCallum et al, 2008;Baily et al, 2011 (Mengis et al, 2001;Wassenaar et al, 2006;Xue et al, 2009). To the best of our knowledge, however, no inorganic fertilizers have been applied at these study sites.…”

“…In agricultural areas, multiple sources of NO3 -are common and could include precipitation, soil NO3 -, inorganic fertilizer, manure, and septic waste (Komor and Anderson, 1993;Liu et al, 2006;Pastén-Zapata et al, 2014;Clague et al, 2015;Xu et al, 2015). While source identification is theoretically possible using δ 15 NNO3 and δ 18 ONO3 (particularly with a dual-isotope approach), in practice this can be difficult due to geologic heterogeneity, overlapping source values, and the complexity of biologically mediated reactions 30 (Aravena et al, 1993;Wassenaar, 1995;Mengis et al, 2001;Choi et al, 2003;Granger et al, 2008;Vavilin and Rytov, 2015;Xu et al, 2015). NO3…”

Simplifying and improving the extraction of nitrate from freshwater for stable isotope analyses

“…3 Secondly, biochemical reactions affecting NO 3 À can cause distinctive isotopic fractionation (e.g. denitrification, mineralisation-immobilisation turnover 3,4 ), which gives some insights on nitrate dynamics.…”

Nitrate Biogeochemistry and Reactive Transport in California Groundwater: LDRD Final Report

“…and by the variety of potential processes that affect nitrate Mengis et al, 2001). Successful studies of nitrate behavior and distribution must take into account the many environmental and historical factors that affect nitrate fate and transport (Böhlke and Denver, 1995).…”

“…Many past studies have used 15 N and 18 O in nitrate as tracers of the source and fate of contamination . This dual isotope technique is sometimes limited by the overlap of source isotope values and by the variety of potential processes that affect nitrate Mengis et al, 2001). Successful studies of nitrate behavior and distribution must take into account the many environmental and historical factors that affect nitrate fate and transport (Bohlke and Denver, 1995).…”