Evaluation of nitrogen isotopes as indicators of nitrate contamination sources in an agricultural watershed

Abstract

Determining the relative contributions of nitrate contamination in agricultural watersheds characterized by applications of both mineral and animal waste nitrogen to fields is important in evaluating the environmental consequences of these practices. Here, the use of nitrate-nitrogen (NO 3 − -N) stable isotopes to identify contaminant sources was examined in surface waters of an agricultural watershed to investigate whether NO 3 − -δ 15 N fractionation processes obscure source signatures. This study was conducted in a 500 ha headwater agricultural watershed within the St. Lawrence lowlands where fertilization with mineral N and hog manure is a common practice. Sampling within the watershed was designed to monitor NO 3 − concentrations and δ 15 N temporal trends at a series of locations that allowed evaluation of water flowing in watershed streams and entering these streams via different hydrological pathways. This required analysis of stream water, tile drain water, and streamside water transported through the soil matrix. These data demonstrated that NO 3 − -δ 15 N values often exceeded the maximum δ 15 N source of approximately +12.5‰ that was associated with hog manure in the watershed. The δ 15 N values and NO 3 − concentrations during such periods were consistent with a pattern of denitrification between precipitation events within the watershed, with values repeatedly exceeding +15‰ in tile drain and stream samples during extended periods with little rainfall. Tile drain NO 3 − concentrations were consistently elevated (4-38 mg l −1 N) relative to streamside soil and shallow groundwater exiting the same field (0.013-0.812 mg l −1 N). These data suggest that NO 3 − -N stable isotopes cannot be used as contaminant source indicators in the surface waters of this watershed between precipitation events, and that further evaluation of their use during storm events is required.

A model-based assessment of the potential use of compound-specific stable isotope analysis in river monitoring of diffuse pesticide pollution

“…6d), since they induced discharge of more recently applied and thus less degraded pesticide. This agrees with previous findings about nitrate contamination where rainfall events led to a fast mobilization of soil nitrate with a relatively depleted δ 15 N-value, while increasing denitrification in soil between rainfall events resulted in progressively enriched δ 15 N-values (Kellman and Hillaire-Marcel, 2003). In contrast to the simulation of the extreme rainfall event in scenario 2, the isotope ratios did not drop to the initial values of the pesticide application area in scenario 3, which implies the absence of pesticide transport via surface runoff.…”

δ15N and δ13C reveal differences in carbon flow through estuarine benthic food webs in response to the relative availability of macroalgae and eelgrass

“…Nutrients transported from the watershed to the estuary carry isotopic signatures that are characteristic of land uses on the watershed (McClelland et al 1997. Nitrate from atmospheric sources typically have δ 15 N signatures between 0.5 and 8 ‰ (Kreitler & Jones 1975, Gormly & Spaulding 1979, Kellman & Hillaire-Marcel 2003 15 N signatures of nitrate from synthetic fertilizers range from -3 to 3 ‰ (Freyer & Aly 1974, Macko & Ostrom 1994, Kellman & Hillaire-Marcel 2003, and nitrate from human and animal wastes typically have a δ 15 N signature that is enriched relative to other sources of N ranging from 9 to 25 ‰ (Kreitler et al 1978, Kreitler & Browning 1983, Rolston et al 1996. Several studies have shown an overall shift in producers and consumers towards heavier δ 15 N signatures as a result of increasing urbanization of watersheds leading to increases in the N loads and relative contribution by wastewater (McClelland et al 1997, Yamamuro et al 2003, Tewfik et al 2005, Martinetto et al 2006.…”

A study of tile drain nitrate - ?15N values as a tool for assessing nitrate sources in an agricultural region

“…Nitrate in precipitation typically has low ␦ 15 N values ͑Heaton 1986͒. The ␦ 15 N values of NO -3 are retained during transport unless it undergoes a removal via microbial denitrification, resulting in an upward shift ͑enrichment͒ in ␦ 15 N values ͑Mariotti et al 1982;Mariotti 1986;Kellman and Hillaire-Marcel 1998;Kendall 1998;Kellman and Hillaire-Marcel 2003͒. An investigation of NO -3 N stable isotopes in surface water sources of an agricultural watershed in Eastern Canada ͑Kellman and Hillaire-Marcel 1998; Kellman and Hillaire-Marcel 2003͒ pointed to complications as a result of fractionations that altered original source signatures during N cycling processes. Transport via tile drains, however, provide a mechanism for rapidly moving water and leached NO -3 from field surfaces and therefore may avoid this complication.…”