Geology topics

H. M. Valett

Publications and source records attributed to H. M. Valett.

3 recordsLinked to original sources

USGS2009

Nitrate removal in stream ecosystems measured by 15 N addition experiments: Total uptake

We measured uptake length of 15 NO- 3 in 72 streams in eight regions across the United States and Puerto Rico to develop quantitative predictive models on controls of NO- 3 uptake length. As part of the Lotic Intersite Nitrogen eXperiment II project, we chose nine streams in each region corresponding to natural (reference), suburban-urban, and agricultural land uses. Study streams spanned a range of human land use to maximize variation in NO- 3 concentration, geomorphology, and metabolism. We tested a causal model predicting controls on NO- 3 uptake length using structural equation modeling. The model included concomitant measurements of ecosystem metabolism, hydraulic parameters, and nitrogen concentration. We compared this structural equation model to multiple regression models which included additional biotic, catchment, and riparian variables. The structural equation model explained 79% of the variation in log uptake length (S Wtot ). Uptake length increased with specific discharge (Q/w) and increasing NO- 3 concentrations, showing a loss in removal efficiency in streams with high NO- 3 concentration. Uptake lengths shortened with increasing gross primary production, suggesting autotrophic assimilation dominated NO- 3 removal. The fraction of catchment area as agriculture and suburban-urban land use weakly predicted NO- 3 uptake in bivariate regression, and did improve prediction in a set of multiple regression models. Adding land use to the structural equation model showed that land use indirectly affected NO- 3 uptake lengths via directly increasing both gross primary production and NO- 3 concentration. Gross primary production shortened S Wtot , while increasing NO- 3 lengthened S Wtot resulting in no net effect of land use on NO- 3 removal. ?? 2009.

Limnology and Oceanography↗

USGS2009

Nitrate removal in stream ecosystems measured by 15 N addition experiments: Denitrification

We measured denitrification rates using a field 15 N-NO- 3 tracer-addition approach in a large, cross-site study of nitrate uptake in reference, agricultural, and suburban-urban streams. We measured denitrification rates in 49 of 72 streams studied. Uptake length due to denitrification (S Wden ) ranged from 89 m to 184 km (median of 9050 m) and there were no significant differences among regions or land-use categories, likely because of the wide range of conditions within each region and land use. N 2 production rates far exceeded N 2 O production rates in all streams. The fraction of total NO- 3 removal from water due to denitrification ranged from 0.5% to 100% among streams (median of 16%), and was related to NHz 4 concentration and ecosystem respiration rate (ER). Multivariate approaches showed that the most important factors controlling S Wden were specific discharge (discharge / width) and NO- 3 concentration (positive effects), and ER and transient storage zones (negative effects). The relationship between areal denitrification rate (U den ) and NO- 3 concentration indicated a partial saturation effect. A power function with an exponent of 0.5 described this relationship better than a Michaelis-Menten equation. Although Uden increased with increasing NO- 3 concentration, the efficiency of NO- 3 removal from water via denitrification declined, resulting in a smaller proportion of streamwater NO- 3 load removed over a given length of stream. Regional differences in stream denitrification rates were small relative to the proximate factors of NO- 3 concentration and ecosystem respiration rate, and land use was an important but indirect control on denitrification in streams, primarily via its effect on NO- 3 concentration. ?? 2009.

Limnology and Oceanography↗

USGS2008

Stream denitrification across biomes and its response to anthropogenic nitrate loading

Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing and terrestrial ecosystems are becoming increasingly nitrogen-saturated, causing more bioavailable nitrogen to enter groundwater and surface waters. Large-scale nitrogen budgets show that an average of about 20-25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins, indicating that substantial sinks for nitrogen must exist in the landscape. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks. ??2008 Nature Publishing Group.

Nature↗