Agriculture alters the biogeochemical cycling of nutrients such as nitrogen (N), phosphorus (P), and silicon (Si) which contributes to the stoichiometric imbalance among these nutrients in aquatic systems. Limitation of Si relative to N and P can facilitate the growth of non-siliceous, potentially harmful, algal taxa which has severe environmental and economic impacts. Planting winter cover crops can retain N and P on the landscape, yet their effect on Si concentrations and stoichiometry is unknown. We analyzed three years of biweekly concentrations and loads of dissolved N, P, and Si from subsurface tile drains and stream water in two agricultural watersheds in northern Indiana. Intra-annual patterns in Si concentrations and stoichiometry showed that cover crop vegetation growth did not reduce in-stream Si concentrations as expected, although, compared to fallow conditions, winter cover crops increased Si:N ratios to conditions more favorable for diatom growth. To assess the risk of non-siliceous algal growth, we calculated a stoichiometric index to quantify biomass growth facilitated by excess N and P relative to Si. Index values showed a divergence between predicted algal growth and what we observed in the streams, indicating other factors influence algal community composition. The stoichiometric imbalance was more pronounced at high flows, suggesting increased risk of harmful blooms as climate change increases the frequency and intensity of precipitation in the midwestern U.S. Our data include some of the first published measurements of Si within small agricultural watersheds and provide the groundwork for understanding the role of agriculture on Si export and stoichiometry.

农业改变了氮 (N)、磷 (P) 和硅 (Si) 等营养物质的生物地球化学循环，从而导致水生系统中这些营养物质的化学计量失衡。Si 相对于 N 和 P 的限制可以促进非硅质、潜在有害的藻类群的生长，这对环境和经济有严重的影响。种植冬季覆盖作物可以保留景观中的 N 和 P，但它们对 Si 浓度和化学计量的影响尚不清楚。我们分析了印第安纳州北部两个农业流域中地下瓷砖排水沟和溪流中溶解的 N、P 和 Si 的三年浓度和负荷。Si 浓度和化学计量的年内模式表明，覆盖作物的植被生长并没有像预期的那样降低河流中的 Si 浓度，尽管，与休耕条件相比，冬季覆盖作物增加了 Si:N 的比例，使条件更有利于硅藻的生长。为了评估非硅藻类生长的风险，我们计算了一个化学计量指数来量化相对于 Si 过量 N 和 P 促进的生物量生长。指数值显示预测的藻类生长与我们在溪流中观察到的情况之间存在差异，表明其他因素影响藻类群落组成。化学计量不平衡在高流量时更为明显，这表明随着气候变化增加美国中西部降水的频率和强度，有害水华的风险增加了解农业对硅出口和化学计量的作用。