Smithsonian Environmental Research Center, P.O. Box 28, 647 Contees Wharf Road, Edgewater, Maryland 21037-0028, USA.
Ecol Appl. 2011 Jul;21(5):1679-95. doi: 10.1890/10-0789.1.
Watershed analyses of nutrient removal in riparian buffers have been limited by the geographic methods used to map buffers and by the statistical models used to test and quantify buffer effects on stream nutrient levels. We combined geographic methods that account for buffer prevalence along flow paths connecting croplands to streams with improved statistical models to test for buffer effects on stream nitrate concentrations from 321 tributary watersheds to the Chesapeake Bay, USA. We developed statistical models that predict stream nitrate concentration from watershed land cover and physiographic province. We used information theoretic methods (AIC(c)) to compare models with and without buffer terms, and we demonstrate that models accounting for riparian buffers better explain stream nitrate concentrations than models using only land cover proportions. We analyzed the buffer model parameters to quantify differences within and among physiographic provinces in the potentials for nitrate loss from croplands and nitrate removal in buffers. On average, buffers in Coastal Plain study watersheds had a higher relative nitrate removal potential (95% of the inputs from cropland) than Piedmont buffers (35% of inputs). Buffers in Appalachian Mountain study watersheds were intermediate (retaining 39% of cropland inputs), but that percentage was uncertain. The absolute potential to reduce nitrate concentration was highest in the Piedmont study watersheds because of higher nitrate inputs from cropland. Model predictions for the study watersheds provided estimates of nitrate removals achieved with the existing cropland and buffer distributions. Compared to expected nitrate concentrations if buffers were removed, current buffers reduced average nitrate concentrations by 0.73 mg N/L (50% of their inputs from cropland) in the Coastal Plain study watersheds, 0.40 mg N/L (11%) in the Piedmont, and 0.08 mg N/L (5%) in the Appalachian Mountains. Restoration to close all buffer gaps downhill from croplands would further reduce nitrate concentrations by 0.66 mg N/L, 0.83 mg N/L, and 0.51 mg N/L, respectively, in the Coastal Plain, Piedmont, and Appalachian Mountain study watersheds. Aggregate nitrate removal by riparian buffers was less than suggested by many studies of field-to-stream transects, but buffer nitrate removal is significant, and restoration could achieve substantial additional removal.
河岸带缓冲带的养分去除分析受到用于绘制缓冲带的地理方法和用于测试和量化缓冲带对溪流养分水平影响的统计模型的限制。我们结合了考虑连接农田和溪流的水流路径上缓冲带流行度的地理方法以及改进的统计模型，以测试美国切萨皮克湾 321 条支流流域对溪流硝酸盐浓度的缓冲效应。我们开发了从流域土地覆盖和地形省份预测溪流硝酸盐浓度的统计模型。我们使用信息理论方法（AIC(c)）比较了带有和不带有缓冲项的模型，并证明了考虑河岸带缓冲带的模型比仅使用土地覆盖比例的模型更好地解释了溪流硝酸盐浓度。我们分析了缓冲模型参数，以量化不同地形省份内和之间农田硝酸盐损失和缓冲带硝酸盐去除的潜力差异。平均而言，沿海平原研究流域的缓冲带具有更高的相对硝酸盐去除潜力（从农田输入的 95%），而皮埃蒙特的缓冲带（输入的 35%）则较低。阿巴拉契亚山脉研究流域的缓冲带处于中间水平（保留农田输入的 39%），但这一比例不确定。由于农田输入的硝酸盐较高，皮埃蒙特研究流域的硝酸盐绝对去除潜力最高。研究流域的模型预测提供了根据现有农田和缓冲带分布实现的硝酸盐去除量的估计。与去除缓冲带后预期的硝酸盐浓度相比，如果去除缓冲带，目前的缓冲带将降低沿海平原研究流域的平均硝酸盐浓度 0.73mgN/L（来自农田的 50%），降低皮埃蒙特的硝酸盐浓度 0.40mgN/L（11%），降低阿巴拉契亚山脉的硝酸盐浓度 0.08mgN/L（5%）。恢复到农田下游的所有缓冲带缺口，将分别使沿海平原、皮埃蒙特和阿巴拉契亚山脉研究流域的硝酸盐浓度进一步降低 0.66mgN/L、0.83mgN/L 和 0.51mgN/L。河岸带缓冲带的硝酸盐总去除量小于许多田间到溪流横断面研究的建议，但缓冲带硝酸盐去除量是显著的，恢复可以实现更多的去除量。
