By Chris Jones, Philip W. Gassman, and Keith E. Schilling
The Iowa Departments of Agriculture and Natural Resources and Iowa State University initially developed the Iowa Nutrient Reduction Strategy (INRS; ISU 2019a) in 2012 to provide a framework for mitigating point and nonpoint-source nutrient pollution across the state. A primary goal of the INRS is reducing total nitrogen (TN) and total phosphorus (TP) loads to Iowa streams by 45%, as established in the 2008 Gulf Hypoxia Action Plan (USEPA 2008). The INRS states that nonpoint sources contribute 92% of the TN loads that enter Iowa’s stream system each year, based on a previous statewide nutrient balance study (Libra, Wolter, and Langel 2004). A core aspect of the INRS approach to addressing nonpoint-source TN pollution is the implementation of multiple management practices that are categorized as: nitrogen management (e.g., timing, nitrogen application rate, cover crops), land use (perennial crops, extended rotations, grazed pastures), and edge-of-field (e.g., wetlands, bioreactors, buffers). The INRS reports various statewide scenario analyses, including an assessment of 15 nitrate-N reduction practices that ranks cover crops (28%), wetlands (22%), bioreactors (18%) and perennial crops (18%) as providing the strongest reductions. Adoption of these practices remains low, largely because their economic benefits in terms of crop yield and farm revenue is neutral at best. The INRS scenario finds that various in-field nitrogen management practices, which can enhance farm profitability, offer little potential to reduce statewide stream nitrogen loading (estimated reductions were 0.1–9%).
USDA Census data shows an increase in cover crops in Iowa from 379,614 acres in 2012 to 936,118 acres in 2017 (Dreibus 2019), which likely was due in part to the influence of the INRS. ISU Geographic Information Services also documents extensive use of terraces, grassed waterways, contour buffer strips, and other erosion control practices on cropland landscapes in over 1,700 Iowa watersheds (ISU-GIS 2019). In contrast to practices that trap nitrogen, adoption of erosion control practices is robust because they are necessary to maintain the long-term productive capacity of the farm and can enhance land value. Thus, while Iowa has made progress in reducing soil erosion, nutrient export from nonpoint sources remains severe and pervasive, as evidenced by: (a) measured average nitrate contributions from 1999 to 2016 of 45%, 55%, and 29% from Iowa stream sources to respective overall loadings in the Upper Mississippi River basin, Missouri River basin, and Mississippi-Atchafalaya River basin (Jones et al. 2018c); and, (b) a 73% increase in the five-year running annual average of nitrate-N loading to Iowa’s streams between 2003 and 2018 (Jones and Schilling 2019). Thus, substantial challenges remain regarding the goal of reducing nutrient losses from Iowa cropland.
One possible intervention that warrants more investigation is the practice of fertilizing beyond the nutrient needs of Iowa crops. Although this is a contributor to elevated stream nitrate statewide, certain areas with concentrated livestock, especially hogs, are most likely to receive nitrogen inputs well beyond crop needs (Jones et al. 2018b; Jackson et al. 2000). Mitigation of nutrient over-application “hotspots,” which can occur due to excessive combinations of manure and fertilizer nutrient applications on specific land parcels (Teshager et al. 2017; Secchi and Mcdonald 2019), could have disproportionately large benefits for statewide stream nitrate loading.
Recent research reveals that hotspots may be occurring in regions of intensive livestock production in Iowa, such as the Floyd and North Raccoon River watersheds (figure 1), which drain portions of northwest and north-central Iowa (Jones et al. 2018a; 2018b). We further explore the implications of achieving overall statewide water quality goals based on an evaluation of the nutrient balance and corresponding in-stream nitrate water quality indicators for the Floyd and North Raccoon River watersheds, which represent different ecoregions in Iowa but with similar intensive livestock production.
We derive corn and soybean areas and yields for each watershed from USDA-NASS (USDA 2019) county-level data based on the area portion of each county within the watershed, and base the nitrogen content of harvested grain on Blesh and Drinkwater (2013) and USDA (2009). We obtained commercial nitrogen-fertilizer sales data from Gronberg and Spahr (2012) and the Iowa Department of Land Stewardship (IDALS). We derive watershed-level data from the county data by adjusting the total amount based on the area portion within the respective watershed. For years with missing county-level data, we estimate commercial fertilizer amounts by using the calculated rate per corn area for years where data exists, then we adjust the watershed total based on the number of corn acres for the respective year. We derive watershed livestock populations from USDA-NASS (USDA 2019) and the Iowa DNR AFO database (IDNR 2019b), and use Iowa Geological Survey values (IGS 2006) to calculate manure N content; however, we do not consider poultry manure due to the absence of reliable county-level data for most years. We calculate soybean nitrogen fixation based on the approach in Barry et al. (1993). We obtained water quality data (stream nitrate concentrations and loads) from the Iowa DNR ambient water monitoring program (IDNR 2019a), and calculate stream N loads (mass) by multiplying concentrations by daily USGS discharge readings, and use linear interpolation to estimate concentrations on non-analysis days.
Figures 2 and 3 illustrate simple nitrogen budgets for both watersheds from 2000 to 2019. We consider commercial N, manure N (dairy, beef, and hog), and soybean fixation from the previous year as inputs, and consider the “surplus” the sum of the inputs minus the harvested grain N. In both the Floyd River and North Raccoon watersheds, N inputs far exceed the N harvested in the grain (Floyd=217%; North Raccoon=140%) over the 19-year period. In fact, the N surplus in the Floyd River watershed exceeds the harvested grain N in every year since 2005 and was nearly double the grain N in the drought year of 2012. Illustrating the importance of N contribution by animals, manure N was 79% of the total input amount for the Floyd River watershed, but only 20% for the North Raccoon watershed. To emphasize, these values do not include poultry manure.
Interestingly, a much higher percentage of the N surplus reaches the stream in the North Raccoon watershed than it does in the Floyd River watershed (63% versus 18%, respectively), likely reflecting landscape and climate differences. Water yield (runoff volume adjusted to watershed area) from the North Raccoon watershed is about 1.8 times that of the Floyd River watershed.
The North Raccoon watershed, situated on the recently-glaciated Des Moines Lobe, is intensely drained with field tiles known to hasten the delivery of nitrate to the stream network. High-nitrate shallow groundwater entering the stream network through alluvial pathways likely drives stream nitrate in the drier Floyd River watershed, where tile is less common.
Because of the abundance of soil N, denitrification, whose contribution as a loss mechanism increases with increasing soil N concentrations, is probably a bigger loss pathway in the Floyd River watershed. Even so, nitrate concentrations in the Floyd River watershed are very high (long-term average of 11.7 mg/L) and are often the highest in Iowa for a stream of that size (i.e., HUC8 level watershed).
Over the 19-year period, annual average concentrations range from 6.2 (2000) to 17.9 mg/L (2016) in the Floyd River watershed and 3.9 (2002) to 18.2 mg/L (2013) in the North Raccoon watershed. Concentrations in both rivers exceed the standard for safe drinking water (10 mg/L) much of the time, with the annual average in the Floyd River and North Raccoon watersheds below 10 mg/L in only four and seven of the 19 years, respectively.
We also derive stream nitrate loads from Jones and Schilling (2019) to evaluate similar statewide data (figure 4). Compared to the analysis above, the statewide data include poultry manure and Minnesota areas draining to Iowa. When Iowa is considered as a whole (including MN areas draining to Iowa), total inputs are 160% of the harvested grain N. Manure N makes up 26% of the input total, a figure that has not substantially changed over the past 20 years. About 32% of the “surplus” eventually finds its way to the outlets of watersheds draining to the Missouri and Mississippi Rivers. Crop yields (calculated as harvested grain N) have clearly increased over the past 20 years, but not nearly as fast as N inputs and stream nitrate loads (table 1).
|N Category||Difference (Mg)||% Change|
Edge-of-field and other N trapping treatments supported by taxpayer-funded cost share, such as cover crops, woodchip bioreactors, saturated buffers, and denitrifying wetlands, are currently highlighted in the INRS as primary practices for reducing nitrate losses from Iowa cropland landscapes. These treatments can be very effective in trapping edge-of-field nitrates and/or specifically removing excess nitrate from subsurface tile drains at a local scale. For example, Castellano et al. (2019) report that bioreactors, saturated buffers, and wetlands respectively intercepted 12–100%, 27–96%, and 25–78% of the nitrate transported in tile drains. However, the overall cost of implementing these practices across the state of Iowa to effectively control nonpoint-source nitrate losses would likely require billions of dollars, which could prove prohibitive.
The nutrient balance analyses reported here for the Floyd and Raccoon River watersheds point to a potential partial alternative and inexpensive solution (i.e., better aligning N inputs with crop needs, particularly in regions with intensive livestock production). There is abundant evidence in the literature that Net Anthropogenic Nitrogen Inputs (NANI) correlate well with stream nitrate in the US Corn Belt (McIsaac et al. 2001; Hong, Swaney, and Howarth 2011; Hong et al. 2012), while Khanal et al. (2014) and Jones et al. (2018a) demonstrate manure-fertilized rotations have higher net N (i.e., difference between inflows and outflows) statewide in Iowa. The long-term excessive in-stream nitrate concentrations documented for the Floyd River, North Raccoon, and other Iowa stream systems impacted by intensive livestock production further underscore the urgent need to improve management of land-applied nutrient inputs in these regions. Thus, we suggest a renewed emphasis on appropriate nitrogen inputs, which would not solve all of Iowa’s water quality problems but could serve as an important step in mitigating excess nitrate export to Iowa’s stream system. One place to begin is with Iowa’s Manure Management Plans, which still allow farmers to apply nutrients based on the archaic and discredited “yield goal” strategy (Rodriguez, Bullock, and Boerngen 2019). Aligning manure nitrogen inputs with economically optimal nitrogen rates (ISU 2019b) would bring an immediate reduction in the N surplus statewide, especially in watersheds where livestock populations are dense.
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