Impacts of Prairie Pothole Hydrology on Field-Scale Losses of Nitrogen and Dissolved Phosphorus
Pothole depressions are a common feature of Iowa agricultural landscapes that have potentially large but uncertain implications for managing nitrogen (N) and phosphorus (P) use efficiency and hydrologic losses. It is possible farmed pothole depressions are hotspots of N and P loss to water and/or the atmosphere that generate a disproportionate impact at the field scale. If so, altering cropping strategies in the most poorly drained potholes could yield important gains for water quality, as well as potential economic benefits from more efficient fertilizer use.
Two years of field observations are planned to assess the impact of pothole depressions on hydrologic and gaseous losses of N and P relative to upland soils, on a site planted the first year with conventional corn and the second year with bean.
To characterize hydrologic N and P losses across pothole-to-uplandtopographic gradients, shallow subsurface lysimeters and moisture sensors will be installed at six locations in established plots at the ISU-owned Been farm. These will be along four replicate transects spanning pothole depressions to well-drained ridgetops. The lysimeters will be sampled bi-weekly from April to November for analysis of nitrate, total dissolved N and P, and iron. Leaching fluxes will be calculated as the product of measured concentrations and water infiltration, using water depth measurements and a hydrologic model. To characterize gaseous N losses, researchers will continue ongoing weekly measurements of soil-atmosphere fluxes of N2O using dynamic chambers at these same sites. These measurements will be complemented with monthly measurements of N2 and N2O production from fresh, intact soil cores.
Research findings were presented at the Geological Society of America regional meeting in Ames, and the American Geophysical Union meeting in New Orleans. Results support the primary hypotheses being tested in this study, that farmed potholes disproportionately contribute to field-scale emissions of N2O and leaching of nitrate and phosphate, although these trends vary seasonally. Analysis of dissolved N and P from samples collected in 2017 is complete. These data confirm pothole depressions have disproportionately high loads of both nutrients, due primarily to increased infiltration, not greater nutrient concentrations.
Measurements continued of N2O and CO2 fluxes and nitrate and phosphate leaching from soils spanning a pothole-to-upland gradient under intensive corn cultivation. Data continue to support the primary hypotheses researchers sought to test in this study, that farmed potholes disproportionately contribute to field-scale emissions of N2O and nitrate and phosphate leaching, although these trends vary seasonally. Dissolved P analyses from lysimeter samples collected during 2017 have been completed. These data confirm pothole depressions have disproportionately high concentrations of soluble inorganic P. Upcoming measurements will focus on freeze-thaw contributions to N2O and N and P leaching.
Measurements continue to be taken of N2O and CO2 fluxes and nitrate and phosphate leaching from soils spanning a pothole-to-upland gradient under intensive corn cultivation. Data continue to support the primary hypotheses being tested in this study, that farmed potholes disproportionately contribute to field-scale emissions of N2O and nitrate leaching, although these trends vary seasonally. So far, results are more equivocal for P, and analysis of accumulated samples continue.
Lysimeter data from November 2016 through June 2017 show potholes disproportionately experience soil water infiltration relative to adjacent upland soils. About one-third of the catchment received about two-thirds of the total infiltration volume. Iron reduction was substantial in potholes, indicative of anaerobic conditions in soil microsites. However, nitrate concentrations remained high in pothole soil water, indicating any denitrification was not complete, and not sufficient to attenuate nitrate leaching in these positions. In addition, researchers found concentrations of reduced iron were significant, consistent with the hypothesis that iron reduction in potholes could liberate organic phosphorus. For the first time, the team saw a strong “pothole effect” on N2O emissions immediately following rainfall events, whereby N2O emissions were several times greater in the pothole than in upslope positions. This finding is consistent with the hypothesis that potholes contribute disproportionately to N2O emissions at the landscape scale.
Lysimeter data from November through March show potholes disproportionately experience soil water infiltration relative to adjacent upland soils, with about one-third of the catchment receiving about two-thirds of the total infiltration volume. Nitrous oxide emissions were greatest immediately following snowmelt and declined thereafter. Responses to wet-up were observed following spring rainstorms, as expected. Denitrification rates measured in the laboratory remained low, suggesting this N2O came from nitrification, and was therefore less likely to be affected by pothole hydrology. Prior to N fertilization of corn, there was not a strong pothole effect on N2O emissions. This relationship may change following planting and fertilization.
Researchers began to sample the zero-tension vadose zone lysimeters across a pothole-to-upland transect (catena) at the Been field site, and continued measurements of soil-atmosphere N2O fluxes. Data from the laboratory soil incubations were analyzed and, as predicted, denitrification rates tended to be greater in potholes. However, a measurement taken during a winter thaw showed rates of N2O production were two orders of magnitude greater, and showed no consistent trend with topographic position. This finding highlights the importance of wintertime N losses in Iowa cropping systems. Laboratory measurements of soils across the catena continued, and researchers found that while substantial iron reduction occurred under elevated moisture, this process was not significantly important for phosphorus release. However, five-fold increases in dissolved organic nitrogen associated with elevated moisture and iron reduction were observed.
Eight zero-tension lysimeters were constructed and installed across a pothole-to-upland transect at the Been field site. Lysimeters were sampled twice and discarded to allow for matrix equilibration, and collection of chemical data will begin with the next precipitation event that exceeds field capacity. Laboratory incubations allowed the team to better assess short-term N dynamics following simulated rainfall events. Soils from pothole depressions exhibited greater N2O production within 48 hours after flooding relative to ridgetop soils, supporting the hypothesis that potholes are hotspots of N2O production immediately following significant rain events. Laboratory incubations also indicated plow layer soils from pothole depressions exhibit substantial capacity for iron reduction during saturated conditions.