The saturation of N cycling in Central Plains streams: 15N experiments across a broad gradient of nitrate concentrations

Abstract

We conducted 15 NO 3 -stable isotope tracer releases in nine streams with varied intensities and types of human impacts in the upstream watershed to measure nitrate (NO 3 -) cycling dynamics. Mean ambient NO 3 -concentrations of the streams ranged from 0.9 to 21,000 lg l -1 NO 3 --N. Major N-transforming processes, including uptake, nitrification, and denitrification, all increased approximately two to three orders of magnitude along the same gradient. Despite increases in transformation rates, the efficiency with which stream biota utilized available NO 3 --decreased along the gradient of increasing NO 3 -. Observed functional relationships of biological N transformations (uptake and nitrification) with NO 3 -concentration did not support a 1st order model and did not show signs of MichaelisMenten type saturation. The empirical relationship was best described by a Efficiency Loss model, in which log-transformed rates (uptake and nitrification) increase with log-transformed nitrate concentration with a slope less than one. Denitrification increased linearly across the gradient of NO 3 -concentrations, but only accounted for~1% of total NO 3 -uptake. On average, 20% of stream water NO 3 -was lost to denitrification per km, but the percentage removed in most streams was <5% km -1 . Although the rate of cycling was greater in streams with larger NO 3 -concentrations, the relative proportion of NO 3 -retained per unit length of stream decreased as NO 3 -concentration increased. Due to the rapid rate of NO 3 -turnover, these streams have a great potential for short-term retention of N from the landscape, but the ability to remove N through denitrification is highly variable.

Saturation of NO3−uptake in prairie streams as a function of acute and chronic N exposure

“…The effects of chronic vs acute NO 3 2 enrichment were tested by comparing the relationship between log(x)-transformed U t and NO 3 2 concentrations found by O'Brien et al (2007) with data from the compiled short-term NO 3 2 additions (Fig. 3).…”

“…Uptake was measured based on 15 NO 3 2 tracer loss from stream water over the length of the stream reach. A more detailed description of the methods used in these measurements is provided by O'Brien et al (2007).…”

“…We compiled data from isotopic tracer and shortterm nutrient additions (for both NH 4 + and NO 3 2 ) from previous studies (Dodds et al 2000, 2002, O'Brien et al 2007 to determine overall trends in stream N uptake with respect to concentration within the Kings Creek basin and to determine the extent of N uptake saturation.…”

“…An alternate representation of the relationship between V f and C is described by efficiency-loss model described by O'Brien et al (2007), which involves a continuous decrease in V f with increasing C (following a power relationship with an exponent between 0 and 21). The efficiency-loss model appears as a linear decline on a plot of log(x)-transformed V f and log(x)-transformed C. We did a linear regression on log(x)-transformed V f and log(x)-transformed C to test for efficiency-loss across both the NO 3 2 and NH 4…”

“…Bernot et al (2006) found that NO 3 2 uptake was saturated in high-N agricultural streams, a result that supported this prediction. However, O'Brien et al (2007) and Mulholland et al (2008) found that NO 3 2 uptake, nitrification, and denitrification did not saturate in streams across a wide gradient of chronic N loading. Instead, they found a pattern of efficiency loss as NO 3 2 concentrations increased.…”