Nitrate retention is an important ecosystem service provided by streams which reduces nitrogen loads to downstream ecosystems. However, most inferences about nitrate retention and uptake in streams are based on measurements at sub-annual scales. Our objective was to evaluate variation in nitration retention and solute concentrations at weekly to decadal time scales. We measured nitrate and chloride concentration, nitrate flux and nitrate retention in Emmons Creek, a groundwater-dominated, nitrate-rich stream in central Wisconsin during a 10-year period. We used a two-station mass balance approach to measure nitrate retention while accounting for groundwater inputs. Surface water nitrate concentration, nitrate yield and nitrate retention exhibited strong seasonal variation. Seasonal Kendall tests revealed positive trends for surface water discharge, nitrate and chloride concentrations and groundwater chloride concentration, but not nitrate retention. Generalized additive mixed models (GAMM), accounting for serial autocorrelations, also indicated positive trends in surface water nitrate and chloride concentrations but no trend for nitrate retention. Nitrate retention averaged 370 mg NO 3 -N m −2 d −1 (± 316 SD) and was highest during the spring (512 mg NO 3 -N m −2 d −1 ± 319), followed by summer (460 ± 322), fall (340 ± 281) and winter (165 ± 212). Nitrate retention rates in Emmons Creek were relatively high compared to most published estimates. However, our results suggest that Emmons Cr. may have saturated its capacity to retain nitrate during a period when nitrate yield from its watershed increased, which is an important finding given the increases in N availability in many regions of earth.

中文翻译：

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威斯康星州沙平原河流中硝酸盐保留和溶质趋势的十年记录：多个尺度的时间变化

硝酸盐保留是河流提供的重要生态系统服务，可减少下游生态系统的氮负荷。然而，大多数关于河流中硝酸盐截留和吸收的推论都是基于次年尺度的测量。我们的目标是评估每周到十年时间尺度内硝化保留和溶质浓度的变化。我们在 10 年内测量了 Emmons Creek 的硝酸盐和氯化物浓度、硝酸盐通量和硝酸盐截留量，Emmons Creek 是威斯康星州中部以地下水为主、富含硝酸盐的河流。我们使用两站质量平衡方法来测量硝酸盐保留，同时考虑地下水输入。地表水硝酸盐浓度、硝酸盐产量和硝酸盐截留量表现出强烈的季节性变化。季节性 Kendall 测试显示地表水排放呈积极趋势，硝酸盐和氯化物浓度和地下水氯化物浓度，但不保留硝酸盐。广义加性混合模型 (GAMM) 考虑了连续自相关，也表明地表水硝酸盐和氯化物浓度呈正趋势，但没有硝酸盐保留趋势。硝酸盐保留平均为 370 mg NO 3 -N m -2 d -1 (± 316 SD) 并且在春季最高 (512 mg NO 3 -N m -2 d -1 ± 319)，其次是夏季 (460 ± 322) )、秋季 (340 ± 281) 和冬季 (165 ± 212)。与大多数公布的估计值相比，Emmons Creek 的硝酸盐保留率相对较高。然而，我们的结果表明 Emmons Cr。在其流域的硝酸盐产量增加期间，其保留硝酸盐的能力可能已经饱和，