Quantifying Hotspots of Nitrate and Dissolved Phosphorus Losses from Cropped Depressions and Impacts at the Catchment Scale
Improving understanding of the spatial distribution of “hotspots” for nitrate and phosphorus loading to surface water provides a key opportunity for informing strategic management interventions and their potential improvements to water quality. Where relatively small portions of the landscape contribute disproportionately to nutrient losses, targeting these hotspots could provide disproportionate nutrient reductions with lower overall impact to farm operations. Drained and cropped depressions (former prairie pothole wetlands) that experience intermittent flooding represent likely hotspots for nutrient losses that contribute disproportionately to nitrate and phosphorus loads.
This project will quantify the overall importance of depressions by combining new in-field measurements with ongoing measurements of nutrient loads strategically conducted at the outlets of tile mains. This will allow researchers to calculate the relative and absolute contribution of cropped depressions to the total loads exported from a given catchment, providing a template to assess gains from implementing best management practices on these landscape features.
Sampling will be done within two catchments in the Des Moines lobe that are already instrumented for monitoring close-interval nitrogen and phosphorus loads at catchment outlets. Lysimeters will be deployed over three time periods to assess seasonal relationships between topographic position and nutrient loads. Using measured values of N and P infiltration as a function of topographic position in these basins, researchers will extrapolate the cumulative impact of depressions on total loads measured at the catchment outlet.
Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.
During Jan. – June 2021, researchers continued field sampling and chemical analyses of previously sampled resin lysimeters from study sites. A total of 481 lysimeters were sampled during the 2019 and 2020 crop years and analyzed for nitrate, ammonium and soluble reactive phosphorus.
Over the two years of our study, results showed that depression soils typically had greater leaching of nitrate, ammonium and phosphorus at a depth of 35 cm than uplands did, although results varied among the individual depression and upland transects examined. Median nitrate leaching measured 136 kg N ha-1 y-1 in depressions vs 86 kg N ha-1 y-1 in uplands (P = 0.02). Median ammonium leaching measured 3.5 kg N ha-1 y-1 in depressions vs 2.7 kg N ha-1 y-1 in uplands (P = 0.003). Median phosphorus leaching measured 8.0 kg P ha-1 y-1 in depressions vs. 2.8 kg P ha-1 y-1 in uplands (P = 0.02).
Differences in nitrate and phosphorus leaching between depressions and uplands did not vary between fields managed with and without a cover crop in 2019, nor were they affected by the presence of an establishing perennial biomass crop in 2020. Our data indicate that given the high nutrient availability and propensity for nutrient leaching in depression soils, decreasing fertilizer inputs to these landscape positions may help improve water quality while decreasing economic costs to producers.
During the second half of 2020, field sampling at the study sites and chemical analyses of previously sampled resin lysimeters continued. Analyses of nitrate and soluble reactive phosphorus from the 2018-2019 lysimeters were finished. Findings included:
- On average, nitrate leaching fluxes were greater in topographic depressions (109 kg N ha-1 y-1) than in uplands (80 kg N ha-1 y-1), although the topographic trends differed across the multiple transects that we sampled.
- Topographic differences were even stronger for soluble reactive phosphorus fluxes, which averaged 2.4 kg P ha-1 in depressions and 1.0 kg P ha-1 in uplands.
- Fluxes of sodium, a semi-conservative hydrologic tracer, were similar between depressions and uplands, indicating that differences in total infiltration did not likely explain topographic variation in nitrate and phosphorus fluxes.
Chemical analyses of the 2019-2020 samples continues.
Other activities and accomplishments:
Steven Hall gave a webinar presentation at the Midwest drainage symposium focused on INRC-funded work. He also presented findings in an invited talk at a Foundation for Food and Agriculture Research (FFAR) symposium in January 2021.
The team collected the lysimeters that were installed in 2019 along 13 transects, each spanning an upland to a depression under corn or soybean cultivation. Extractions and chemical analyses began, although lab work is proceeding very slowly due to COVID19 restrictions.
Resin lysimeters had been installed in cultivated corn/soybean fields to assess how soluble nutrient losses varied along topographic gradients from uplands to poorly drained depressional soils. The first batch of lysimeters was sampled in May 2019, reflecting nutrient losses over the previous growing season (2018). Preliminary analyses show that nitrate losses were consistently high across upland-depression gradients at all sites, and losses from depressions exceeded uplands at one site. Considering that the studied depressions had complete crop mortality due to sustained ponding, the high rates of nitrate loss emphasize the poor nitrogen use efficiency of these landscape positions. Analyses of phosphorus are ongoing. In spring 2020, we will harvest the next batch of lysimeters, which spanned a broader array of sites with upland/depression topographic gradients, and where some sites were planted with cover crops.
To quantify the impacts of topographic depressions on losses of nitrate and dissolved phosphorus, researchers installed resin lysimeters across transects spanning depressions to adjacent uplands in two catchments on the Des Moines lobe. A total of 390 lysimeters were installed (in triplicate at each of 10 plots along each of 13 transects). The lysimeters were constructed and prepared throughout the winter of 2018-19 and installed (buried at 35 cm depth) in the field, May-June 2019. The lysimeters will be harvested, extracted and analyzed a year later to measure the integrated nutrient loads.
Lead researcher Steven Hall presented an Iowa Learning Farms webinar on material related to this project in Fall 2018: https://vimeo.com/user42671352/review/295649422/b8d3cb8e11
This is a new project where funds became available in August. Two undergraduate technicians were hired for the project, Participation of willing farmers in our study watersheds was secured. Undergraduate technicians now have constructed the resin lysimeters that will be deployed at field sites to measure nitrate and phosphate leaching across topographic gradients (depressions to uplands) under multiple cropping system types. Lysimeters will be deployed early in 2019.