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Abstract:The natural abundance of nitrogen and oxygen isotopes in nitrate can be a powerful tool for identifying the source of nitrate in streamwater in forested watersheds, because the two main sources of nitrate, atmospheric deposition and microbial nitrification, have distinct υ18 O values. Using a simple mixing model, we estimated the relative fractions in streamwater derived from these sources for two forested watersheds with markedly different streamwater nitrate outputs. In this study, we monitored υ 15 N and υ 18

[1] Snowpack, snowmelt, precipitation, surface water, and groundwater samples from the Loch Vale watershed in Colorado were analyzed for d 15 N and d 18O of nitrate to determine the processes controlling the release of atmospherically deposited nitrogen from alpine and subalpine ecosystems. Although overlap was found between the d 15 N (NO3) values for all water types (À4 to +6%), the d 18 O (NO3) values for surface water and groundwater (+10 to +30%) were usually distinct from snowpack, snowmelt, and rainfall values (+40 to +70%). During snowmelt, d18 O (NO3) indicated that about half of the nitrate in stream water was the product of microbial nitrification; at other times that amount was greater than half. Springs emerging from talus deposits had high nitrate concentrations and a seasonal pattern in d18 O (NO3) that was similar to the pattern in the streams, indicating that shallow groundwater in talus deposits is a likely source of stream water nitrate. Only a few samples of surface water and groundwater collected during early snowmelt and large summer rain events had isotopic compositions that indicated most of the nitrate came directly from atmospheric deposition with no biological assimilation and release. This study demonstrates the value of the nitrate double-isotope technique for determining nitrogencycling processes and sources of nitrate in small, undisturbed watersheds that are enriched with inorganic nitrogen.

A study was conducted to determine whether NO3– stable isotopes (δ15N and δ18O), at natural abundance levels, could discriminate among NO3– sources from sites with different land uses at the basin scale. Water samples were collected from 24 sites in the Mississippi River Basin from five land-use categories: (1) large river basins (>34 590 km2) draining multiple land uses and smaller basins in which the predominant land use was (2) urban, (3) undeveloped, (4) crops, or (5) crops and livestock. Our data suggest that riverine nitrates from different land uses have overlapping but moderately distinct isotopic signatures. δ18O data were critical in showing abrupt changes in NO3– source with discharge. The isotopic values of large rivers resembled crop sites, sites with livestock tended to have δ15N values characteristic of manure, and urban sites tended to have high δ18O values characteristic of atmospheric nitrate.

Abstract:Nitrate NO 3 and other nutrients discharged by the Mississippi River are suspected of causing a zone of depleted dissolved oxygen (hypoxic zone) in the Gulf of Mexico each summer. The hypoxic zone may have an adverse affect on aquatic life and commercial fisheries. The amount of NO 3 delivered by the Mississippi River to the Gulf of Mexico is well documented, but the relative contributions of different sources of NO 3 , and the magnitude of subsequent in-stream transformations of NO 3 , are not well understood. Forty-two water samples collected in 1997 and 1998 at eight stations located either on the Mississippi River or its major tributaries were analysed for NO 3 , total nitrogen (N), atrazine, chloride concentrations and NO 3 stable isotopes (υ 15 N and υ 18 O). These data are used to assess the magnitude and nature of in-stream N transformation and to determine if the υ 15 N and υ 18 O of NO 3 provide information about NO 3 sources and transformation processes in a large river system (drainage area ¾2 900 000 km 2 ) that would otherwise be unavailable using concentration and discharge data alone.Results from 42 samples indicate that the υ 15 N and υ 18 O ratios between sites on the Mississippi River and its tributaries are somewhat distinctive, and vary with season and discharge rate. Of particular interest are two nearly Lagrangian sample sets, in which samples from the Mississippi River at St Francisville, LA, are compared with samples collected from the Ohio River at Grand Chain, II, and the Mississippi River at Thebes, IL. In both Lagrangian sets, mass-balance calculations indicate only a small amount of in-stream N loss. The stable isotope data from the samples suggest that in-stream N assimilation and not denitrification accounts for most of the N loss in the lower Mississippi River during the spring and early summer months.