Small farm ponds are ubiquitous throughout the Midwestern United States, and have potential to play a role in nutrient processing and storage. Here we describe efforts to understand N and P fate and transport in a small (550 acre) mixed-use (corn and soybeans plus restored tallgrass prairie and savanna oak forest) watershed draining into a ~10-acre artificial pond in central Iowa. Nitrate was monitored continuously at high temporal resolution by optical nitrate sensors at the pond inflow and outflow in addition to discrete sampling for ammonia, total nitrogen, phosphate, total phosphorous, and other geochemical parameters. During the 2017 field season, nitrate concentrations in the pond-inflow peaked in late May/early June at &gt;10 mg N/L, and declined to less than 1.0 mg/L by October. Nitrate concentrations in the pond outfall were ~3.0 mg N/L in mid-late Spring to &lt;0.2 mg N/L during most summer months, with in-pond nitrate removal constituting 60-95% of the nitrate load. Geochemical depth profiles of the pond during summer months indicated large-scale hypoxia (&lt;2 mg O2/L) and anoxia (&lt;0.1 mg O2/L) and detectable Fe(II), coincident with high nitrate-N removal and visible algal blooms. While denitrification is likely an important N removal pathway, we observed ammonification and increasing dissolved organic nitrogen at depth, which suggest potential for internal N cycling and downstream N transport. Concerning phosphorous, dissolved phosphate concentrations were generally &lt;0.1 mg P/L, and N/P ratios suggested that the pond is P-limited. High temporal resolution nitrate monitoring allowed us to measure the impact of hydrologic events and in-stream nitrate processing on N delivery to the pond. During stable hydrologic conditions, we observed clear diurnal nitrate signals, with amplitudes up to 20% of average daily nitrate concentration. Prior to canopy closure over the stream channel, the diurnal nitrate signal appeared linked to in-stream primary production, with nitrate concentration declining steadily during the hours of peak photosynthesis. In later summer, as tree canopy began to limit light availability, nitrate concentrations peaked during the daylight hours, suggesting evapotranspiration concentration of stream nitrate. Our results demonstrate the importance of high temporal resolution measurements for nutrient mass balance studies, particularly in lower-order streams where significant in-stream nutrient processing is likely.

Nutrient processing and storage in agricultural streams and ponds as reavealed by high frequency observation

255th National Meeting (2018)

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