We used a recently published, open‐access data set of U.S. streamwater nitrogen (N) and phosphorus (P) concentrations to test whether watershed land use differentially influences N and P concentrations, including the relative availability of dissolved and particulate nutrient fractions. We tested the hypothesis that N and P concentrations and molar ratios in streams and rivers of the United States reflect differing nutrient inputs from three dominant land‐use types (agricultural, urban and forested). We also tested for differences between dissolved inorganic nutrients and suspended particulate nutrient fractions to infer sources and potential processing mechanisms across spatial and temporal scales. Observed total N and P concentrations often exceeded reported thresholds for structural changes to benthic algae (58, 57% of reported values, respectively), macroinvertebrates (39% for TN and TP), and fish (41, 37%, respectively). The majority of dissolved N and P concentrations exceeded threshold concentrations known to stimulate benthic algal growth (85, 87%, respectively), and organic matter breakdown rates (94, 58%, respectively). Concentrations of both N and P, and total and dissolved N:P ratios, were higher in streams and rivers with more agricultural and urban than forested land cover. The pattern of elevated nutrient concentrations with agricultural and urban land use was weaker for particulate fractions. The % N contained in particles decreased slightly with higher agriculture and urbanization, whereas % P in particles was unrelated to land use. Particulate N:P was relatively constant (interquartile range = 2–7) and independent of variation in DIN:DIP (interquartile range = 22–152). Dissolved, but not particulate, N:P ratios were temporally variable. Constant particulate N:P across steep DIN:DIP gradients in both space and time suggests that the stoichiometry of particulates across U.S. watersheds is most likely controlled either by external or by physicochemical instream factors, rather than by biological processing within streams. Our findings suggest that most U.S. streams and rivers have concentrations of N and P exceeding those considered protective of ecological integrity, retain dissolved N less efficiently than P, which is retained proportionally more in particles, and thus transport and export high N:P streamwater to downstream ecosystems on a continental scale.

中文翻译：

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美国溪流和河流中氮和磷的输送因土地利用以及溶解形式和颗粒形式而异。


我们使用最近发布的美国溪流氮 (N) 和磷 (P) 浓度的开放获取数据集来测试流域土地利用是否对氮和磷浓度产生差异影响，包括溶解和颗粒养分部分的相对可用性。我们检验了这样的假设：美国溪流和河流中的氮磷浓度和摩尔比反映了三种主要土地利用类型（农业、城市和森林）的不同养分输入。我们还测试了溶解的无机营养物和悬浮颗粒营养物部分之间的差异，以推断跨空间和时间尺度的来源和潜在的处理机制。观察到的总 N 和 P 浓度经常超过报告的底栖藻类（分别为报告值的 58%、57%）、大型无脊椎动物（TN 和 TP 为 39%）和鱼类（分别为 41%、37%）结构变化的阈值。大多数溶解的氮和磷浓度超过了已知刺激底栖藻类生长（分别为 85%、87%）和有机物分解率（分别为 94%、58%）的阈值浓度。农业和城市土地覆盖较多的溪流和河流中的氮和磷浓度以及总氮：磷比和溶解氮磷比均较高。颗粒物部分随农业和城市土地利用而升高的养分浓度模式较弱。随着农业和城市化程度的提高，颗粒中的氮百分比略有下降，而颗粒中的磷百分比与土地利用无关。颗粒物 N:P 相对恒定（四分位距 = 2–7），并且与 DIN:DIP 的变化无关（四分位距 = 22–152）。溶解的氮磷比随时间变化，但颗粒物除外。 在空间和时间上陡峭的 DIN:DIP 梯度上恒定的颗粒物 N:P 表明，美国流域颗粒物的化学计量很可能是由外部或河流内的物理化学因素控制，而不是由河流内的生物处理控制。我们的研究结果表明，美国大多数溪流和河流的 N 和 P 浓度超过了被认为保护生态完整性的浓度，保留溶解 N 的效率低于 P，后者在颗粒中保留的比例更高，因此将高 N:P 河水输送和输出到大陆范围内的下游生态系统。