Trends in stream nitrogen concentrations for forested reference catchments across the USA

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Trends in stream nitrogen concentrations for forested reference catchments across the USA
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To examine whether stream nitrogen concentrations in forested reference catchments have changed over time and if patterns were consistent across the USA, we synthesized up to 44 yr of data collected from 22 catchments at seven USDA Forest Service Experimental Forests. Trends in stream nitrogen presented high spatial variability both among catchments at a site and among sites across the USA. We found both increasing and decreasing trends in monthly flow-weighted stream nitrate and ammonium concentrations. At a subset of the catchments, we found that the length and period of analysis influenced whether trends were positive, negative or non-significant. Trends also differed among neighboring catchments within several Experimental Forests, suggesting the importance of catchment-specific factors in determining nutrient exports. Over the longest time periods, trends were more consistent among catchments within sites, although there are fewer long-term records for analysis. These findings highlight the critical value of long-term, uninterrupted stream chemistry monitoring at a network of sites across the USA to elucidate patterns of change in nutrient concentrations at minimally disturbed forested sites.

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over the last century the nitrogen cycle has been dramatically altered nitrogen in synthetic fertilizers intensive agriculture practices and fossil fuel emissions has seeped into
many aquatic environments were alter stream functions and dynamics despite policies to prevent nitrogen pollution nitrogen concentrations in streaming ground have increased across more experience of the United States and are expected to continue to do so much of what we know about how stream water
quality changes over time comes from studies were the surrounding land has been altered by human users in contrast we know little about temporal trends in nitrogen dynamics in
streams with minimal human impacts the streams can serve as references for evaluating catchments more heavily modified face the U.S. forest service maintains a network of experimental forests and ranges for decades researchers from many institutions have been collecting data on the
hydrology and stream chemistry from undisturbed watersheds in this network these watersheds provides unique opportunities to evaluate long-term trends in the relative absence of changes in land use on land cover the external force network covers BIOMS throughout the United States and part of the some sites have 40 years of uninterrupted data and sites stream samples are collected the weekly in this study we analyzed
dreams in organic nitrogen from 20 to force reference catchments in 7 experimental forest sites represented diverse climatic hydrologic and vegetation conditions ranging from the tropical conditions of query go to the unit but cool Pacific Northwest we found that stream nitrate declines in the Pacific Northwest in the northeast and imported ego however these trends were not consistent at national or local scales some sites in the mountain west and south increasing trends in nitrate concentration adding to these complex spatial patterns of trends catchments within an experimental for sometimes have opposing trends for the same nitrogen species for the same period of time this suggests that the controls on stream nitrogen concentrations may vary among and within external force and that extrapolation of trends from 1 catchment to a neighboring 1 should be made with extreme prudence we also found that the inclusion of additional years in the analysis change the direction of the trend and that's 3 nitrate and ammonium did not show consistent trends or coincident timing intron ship this highlights that even when trends for short periods are statistically significant they're not necessarily indicative of longer term patterns caution is needed when extrapolating trends over time and across species of nitrogen long-term
research and reference sites provides a unique opportunity to document changes in nitrogen concentrations understanding how nutrient concentrations are changing over time reference frames is vital for informing best management practices are aimed at protecting water resources