Impacts of converting existing cropland to grassland on ...€¦ · Impacts of converting existing...
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Impacts of converting existing cropland to grassland on losses of nitrogen, phosphorus, and sediment across CONUS Eric G. Booth and Christopher J. Kucharik University of Wisconsin-Madison Overview Increasing the proportion of perennial grassland across the continental US (CONUS) is seen as a potential strategy for improving water quality. This is reflected in long-standing and continued support for programs such as the USDA Conservation Reserve Program. We leveraged previous research (link) on the impacts of recent intensification of corn on existing US cropland to determine the impacts of converting varying levels of existing cropland to grassland. We simulated corn, soy, wheat, and grassland at a 2.5 arc-minute spatial resolution on all cropland in the CONUS. We output the median field-level losses of nitrogen (via potential nitrate leaching to groundwater or flux past a soil depth of 1.5 m), phosphorus (via runoff), and sediment (via runoff) over the 2007-2016 period. We then use estimated crop frequency data for corn, soy, and wheat (Figure 1) to determine the current losses from corn, soy, and wheat. Then we replace varying levels (1, 5, 10, 25, and 50%) of the existing cropland base with grassland and determine the changes in impacts to water quality metrics. Results Average crop frequency data (Figure 1) shows widespread cultivation of corn across the Midwest, Atlantic Coastal Plain, Mississippi Delta, and parts of the West including the California Central Valley. Soy cultivation is also quite widespread but most intense in the Mississippi Delta region, while wheat is generally low throughout the Corn Belt but dominant in the Great Plains (Figure 1). Simulated estimates of current losses of nitrogen via nitrate leaching from corn, soy, and wheat show substantially higher amounts within the Corn Belt where intensive production of corn and soy with high nutrient inputs occurs, while phosphorus and sediment losses are highest in parts of western Iowa due to high manure P inputs, relatively steep slopes, and higher proportions of erosion-susceptible corn and soy (Figure 2). Reductions in nitrate leaching, phosphorus loss, and soil erosion associated with grassland replacement of cropland (Table 1) are the largest in the areas where current values are also quite large (i.e., high loss areas have the most room for improvement) (Figures 3-5). Percent reductions in nitrate leaching generally follow the percentage of cropland replaced by grassland with 10 and 50% replacement leading to 9.7 and 48.4% reductions, respectively (Table 2). However, the response in terms of phosphorus and sediment is more muted. For phosphorus loss, 10 and 50% grassland replacement only lead to 7.6 and 37.9% reductions, respectively; while for soil erosion, these levels lead to 8.3 and 41.6%. This differential response is due to very minimal nitrate loss in grassland systems that do not receive nitrogen inputs (e.g. fertilizer and manure). In contrast, a relatively larger amount of phosphorus (mostly dissolved) is lost from grassland systems because soil phosphorus has built up due to previous agricultural land management and can remain over longer periods of time (legacy effect). The reduction is also slightly less for soil erosion (compared to nitrate) because grassland
Transcript of Impacts of converting existing cropland to grassland on ...€¦ · Impacts of converting existing...
Impacts of converting existing cropland to grassland on losses of nitrogen, phosphorus, and sediment across CONUS
Eric G. Booth and Christopher J. Kucharik
University of Wisconsin-Madison
Overview
Increasing the proportion of perennial grassland across the continental US (CONUS) is seen as a potential strategy for improving water quality. This is reflected in long-standing and continued support for programs such as the USDA Conservation Reserve Program. We leveraged previous research (link) on the impacts of recent intensification of corn on existing US cropland to determine the impacts of converting varying levels of existing cropland to grassland. We simulated corn, soy, wheat, and grassland at a 2.5 arc-minute spatial resolution on all cropland in the CONUS. We output the median field-level losses of nitrogen (via potential nitrate leaching to groundwater or flux past a soil depth of 1.5 m), phosphorus (via runoff), and sediment (via runoff) over the 2007-2016 period. We then use estimated crop frequency data for corn, soy, and wheat (Figure 1) to determine the current losses from corn, soy, and wheat. Then we replace varying levels (1, 5, 10, 25, and 50%) of the existing cropland base with grassland and determine the changes in impacts to water quality metrics.
Results
Average crop frequency data (Figure 1) shows widespread cultivation of corn across the Midwest, Atlantic Coastal Plain, Mississippi Delta, and parts of the West including the California Central Valley. Soy cultivation is also quite widespread but most intense in the Mississippi Delta region, while wheat is generally low throughout the Corn Belt but dominant in the Great Plains (Figure 1). Simulated estimates of current losses of nitrogen via nitrate leaching from corn, soy, and wheat show substantially higher amounts within the Corn Belt where intensive production of corn and soy with high nutrient inputs occurs, while phosphorus and sediment losses are highest in parts of western Iowa due to high manure P inputs, relatively steep slopes, and higher proportions of erosion-susceptible corn and soy (Figure 2). Reductions in nitrate leaching, phosphorus loss, and soil erosion associated with grassland replacement of cropland (Table 1) are the largest in the areas where current values are also quite large (i.e., high loss areas have the most room for improvement) (Figures 3-5).
Percent reductions in nitrate leaching generally follow the percentage of cropland replaced by grassland with 10 and 50% replacement leading to 9.7 and 48.4% reductions, respectively (Table 2). However, the response in terms of phosphorus and sediment is more muted. For phosphorus loss, 10 and 50% grassland replacement only lead to 7.6 and 37.9% reductions, respectively; while for soil erosion, these levels lead to 8.3 and 41.6%. This differential response is due to very minimal nitrate loss in grassland systems that do not receive nitrogen inputs (e.g. fertilizer and manure). In contrast, a relatively larger amount of phosphorus (mostly dissolved) is lost from grassland systems because soil phosphorus has built up due to previous agricultural land management and can remain over longer periods of time (legacy effect). The reduction is also slightly less for soil erosion (compared to nitrate) because grassland
can still lose a small amount of sediment compared with cropland. These results highlight the concept that responses for different water quality constituents following land-use or management practice change will not be uniform.
Figure 1. Average frequency of corn, soy, wheat, and sum of all three based on the USDA National Cropland Frequency Layer
Figure 2. Annual average losses of nitrate-nitrogen, phosphorus, and sediment from corn, soy, and wheat (median values over 2007-2016 simulation period) based on current crop frequencies in each grid cell.
Table 1. Average annual water quality impacts (+/- one standard deviation) across CONUS cropland grid cells over 2007-2016 time period for current corn, soy, wheat cropping systems and 5 scenarios with varying levels (1, 5, 10, 25, and 50%) of grassland replacement of cropland. Average values reflect proportion of a grid cell devoted to each land cover type.
Scenario NO3 Leaching [kg/ha] P Yield [kg/ha] Sediment Yield [tons/km2] Current (2008-2018) 14.7 ± 15.1 0.101 ± 0.174 8.74 ± 19.3 1% converted to grassland 14.5 ± 15.0 0.101 ± 0.173 8.67 ± 19.1 5% converted to grassland 13.9 ± 14.4 0.098 ± 0.167 8.38 ± 18.5 10% converted to grassland 13.2 ± 13.7 0.094 ± 0.160 8.02 ± 17.7 25% converted to grassland 11.1 ± 11.5 0.082 ± 0.139 6.93 ± 15.3 50% converted to grassland 7.6 ± 7.8 0.063 ± 0.103 5.11 ± 11.3
Table 2. Grassland expansion impact. Annual average changes in water quality impacts due to scenarios of varying levels of grassland replacement compared to current proportions of corn, soy, and wheat on cropland in CONUS.
Scenario NO3 Leaching P Yield Sediment Yield 1% converted to grassland -0.97% -0.76% -0.83% 5% converted to grassland -4.84% -3.79% -4.15% 10% converted to grassland -9.67% -7.58% -8.31% 25% converted to grassland -24.2% -18.9% -20.8% 50% converted to grassland -48.4% -37.9% -41.6%
Figure 3. Annual average losses of nitrogen via nitrate leaching for varying levels of grassland replacement of existing cropland (left column) and the difference between the grassland replacement scenario and the current losses associated with corn, soy, and wheat (grassland scenario – current)
Figure 4. Annual average losses of phosphorus via runoff for varying levels of grassland replacement of existing cropland (left column) and the difference between the grassland replacement scenario and the current losses associated with corn, soy, and wheat (grassland scenario – current)
Figure 5. Annual average losses of sediment via runoff for varying levels of grassland replacement of existing cropland (left column) and the difference between the grassland replacement scenario and the current losses associated with corn, soy, and wheat (grassland scenario – current)
Methods
Agro-IBIS simulates plant growth, phenology, and the soil-plant-atmosphere cycling of water, energy, momentum, carbon (C), nitrogen (N), and P in natural and managed ecosystems. The structure of Agro-IBIS has been previously described in detail [Kucharik and Brye, 2003; Kucharik et al., 2000] and
validated extensively [Kucharik and Brye, 2003; Kucharik and Twine, 2007; Kucharik et al., 2013; Kucharik et al., 2006; Motew and Kucharik, 2013]. Recently we made two significant modifications to Agro-IBIS. (1) We integrated the variably saturated soil water flow model Hydrus-1D to simulate groundwater-vegetation interactions and effects of shallow groundwater on unsaturated zone processes [Soylu et al., 2014]. (2) We added biogeochemical cycling of P and loss of P to runoff. The terrestrial P module (Figure 4) features P application (manure and fertilizer), transformation, and loss of dissolved P to runoff; in-soil cycling of organic and inorganic forms of P; and loss of particulate-bound P with erosion [Motew et al., 2017]. This approach is based on the SurPhos model, which simulates the release and transport of dissolved P from surface-applied, unincorporated manures [Vadas et al., 2004; Vadas et al., 2005; Vadas et al., 2007].
Historical land cover inputs to Agro-IBIS were developed based on several gridded land cover datasets [Fry et al., 2011; Homer et al., 2007; Homer et al., 2015; Sohl et al., 2016; Sohl et al., 2014] as well as historical county-level USDA Census of Agriculture data [Haines et al., 2018]. Nutrient application rate maps were also created (fertilizer nitrogen (N) and phosphorus (P), manure N and P, manure mass, and manure dry matter fraction) based on historical estimates from the US Geological Survey [Alexander and Smith, 1990; Brakebill and Gronberg, 2017; Gronberg and Spahr, 2012; Gronberg and Arnold, 2017; Mueller and Gronberg, 2013; Ruddy et al., 2006] and the new land cover maps. We assumed no nutrients were applied to grassland. Finally, irrigation maps were created based on the Moderate Resolution Imaging Spectroradiometer (MODIS) Irrigated Agriculture Dataset for the United States (MIrAD-US) [Brown and Pervez, 2014; USGS, 2015] and historical data from the USDA Census of Agriculture.
Climate data from GridMET [Abatzoglou, 2013] for the years 1979-2016 were used as inputs to Agro-IBIS. Years prior to 1979 (spin-up period) were drawn randomly from the 1979-2016 period.
To specify the proportions of corn, soy, and wheat on cropland in CONUS, we used cropland frequency data for over the period of 2008 to 2018 from the USDA National Frequency Layer that is based on the Cropland Data Layer (https://www.nass.usda.gov/Research_and_Science/Cropland/Release/index.php). Varying levels of grassland replacement of cropland occurred proportionally to the current make-up of each grid cell. Calculations of loss for nitrate, phosphorus, and sediment accounted for the proportion of each grid cell devoted to corn, soy, wheat, and grassland. Proportions very rarely added up to a value of 1 reflecting the existence of land cover types other than corn, soy, wheat, and grassland across parts of the CONUS dominated by cropland. Therefore, results shown for water quality impacts are only for the parts of each grid cell that are devoted to corn, soy, wheat, or grassland; and low values can occur both from low nutrient and sediment losses per unit area and from relatively small areas devoted to those four cover types. The following example illustrates the calculation for current conditions and a scenario of 50% grassland replacement:
Current conditions:
Crop Type Proportion of Grid Cell Area
N leaching per crop [kg/ha]
Area-weighted N leaching per crop [kg/ha]
N leaching per grid cell from corn, soy, and wheat [kg/ha]
Corn 50% 63.0 31.5 Soy 25% 24.0 6.0 Wheat 10% 21.0 2.1 39.6
50% grassland replacement scenario:
Crop Type Proportion of Grid Cell Area
N leaching per crop [kg/ha]
Area-weighted N leaching per crop [kg/ha]
N leaching per grid cell from corn, soy, wheat, and grass [kg/ha]
Corn 25% 63.0 15.75 Soy 12.5% 24.0 3.0 Wheat 5% 21.0 1.05 Grass 42.5% 1.3 0.5525 20.35
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