Soil health changes from grassland to row crop conversion in the Northern Great Plains

The Northern Great Plains (NGP) landscape is a mosaic of land uses. The NGP comprises 24% of the farmland and nearly 30% of range and pastureland in the United States (USDA, 2018). As a land use priority, grasslands and row crop acreage are often at odds. Indeed, temperate grasslands are commonly thought of as one of the most threatened biomes globally—risking the loss of an extremely biodiverse ecosystem and habitat to numerous threatened and endangered species (Hoekstra et al., 2004).

Grassland displacement for crop production has occurred rapidly over the last half century where an estimated 60% of all native mixed-grass prairie in South Dakota, North Dakota, and Montana have been converted to cropland (Higgins et al., 2002). Between 2008 and 2012, nearly 7.4 million acres of previously uncultivated land was transitioned to cropland nationwide (Lark et al., 2015). Of these 7.4 million acres, 77% was converted from grasslands, located largely in the NGP. Recent research found that as much as 5% of the entire NGP grassland was being converted to cropland each year during this same time period (Wright & Wimberly, 2013). While this trend has slowed somewhat since the period of this study, it still remains a critical issue (Gage et al., 2016).

Land use change is a dynamic process across the NGP—driven by economic forces often underwritten by governmental policies. Programs such as the Sodbuster Provision and the Conservation Reserve Program (CRP), authorized within farm bills, significantly influence the extent of land that is brought into or out of production in any given time period. However, funding for CRP has steadily decreased as Cotton and Acosta-Martínez (2018) noted that CRP enrollment has dropped from a high of 35 million acres in 1995 to 23.5 million acres in 2017 with further decreases expected through 2022. Moreover, a significant portion of this land is returning into production agriculture. It is likely that this land use change has significant environmental impacts, and it is unclear what the impacts are to either soil health or ecosystem functioning as a whole.

The vast majority of grassland conversion across the NGP is for the purposes of growing wheat, corn, and soybeans (Gage et al., 2016; Lark et al., 2015). The method in which this land is converted is likely to have significant impacts on various soil functions. We know that the land manager is the ultimate determinant of soil quality and health; however, very little data exists highlighting the short-term effects of land conversion or tools that can be used by land managers to document this change.

For land managers to understand the impacts resulting from land conversion, simple, comprehensive tools are necessary to provide context to management decisions. One potential tool is the Comprehensive Soil Assessment Tool (CASH), which offers a suite of standard nutrient analyses along with physical and biological tests that represent critical soil functions (Moebius-Clune et al., 2016). The framework can be used to identify physical, chemical, or biological constraints due to management decisions. However, for it to be useful to land managers, it must be responsive on short time scales.

The objectives of this study were to use the CASH to assess the short-term effects of converting land that is considered long-term grassland—similar to that in a CRP contract—to small-grain production. Specifically, this experiment was designed to investigate if, and how, soil health declines are measurable immediately following grassland conversion to row crops using either tillage or no-till practices and if the CASH is a viable tool to document changes in critical soil function.

Experiment Design

A field experiment was conducted at the Cottonwood Field Station in western South Dakota. The research was conducted on a silty clay loam soil with a sand, silt, and clay content of 18, 49 and 33%, respectively (Kettler et al., 2001) and a slope of 0–2%. Grassland conversion took place using either conventional tillage (CT) or no-till (NT) practices. Plots were established in a sequential manner across three growing seasons between 2016 and 2018 (Figures 1 and 2). In each growing season, a new set of conversion plots was established and maintained along with the previous years’ conversions. Therefore, the resulting dataset includes three years of “one year after conversion data,” two years of “two years after conversion data,” and one year of “three years after conversion” data. Precipitation and temperature for the study period were measured on-site (Figure 3). The established three-year crop rotation was hard red spring wheat (HRS, cultivar Surpass) in Year 1, followed by sorghum (cultivar KS-585) in Year 2, and then HRS in Year 3. Nitrogen was applied for both HRS and sorghum at planting as a mid-row band to both CT and NT as 28% urea ammonium nitrate at a rate of 120 lb N/acre, and phosphorus, sulfur, and zinc were applied with the seed as a starter application of 10-25-0-5-0.5 at a rate of 2.7 gal/ac.

The CASH approach was selected to assess soil health during grassland conversion. The focus of the current study is based on seven soil tests that can be broken down into physical, biological, and chemical indicators. Soil samples were collected in the fall following harvest of all crops. All soil replicates were based on a composite of 8 to 12 samples taken from a depth of 0 to 6 inches according to Moebius-Clune et al. (2016). Among soil parameters measured to assess soil health changes were the soil physical properties wet aggregate stability (WAS) and available water-holding capacity (AWC); the soil biological properties soil organic matter (SOM), permanganate oxidizable carbon (POX-C), autoclaved citrate extractable protein (ACE-Protein), and soil respiration (Figure 4); and the soil chemical property soil pH. Data were analyzed statistically using R (R Core Development Team, 2014). Significance was determined at P ≤ .05 (unless otherwise stated) with means separation determined using the Tukey method. Linear correlations were determined using Pearson’s correlation coefficient.

Weather Trends

Temperatures during the study period did not vary significantly from long-term trends. However, all three years of the study were below the long-term average for precipitation—particularly in 2017. The yearly, cumulative growing-season precipitation was 77, 61, and 83% of the long-term average for 2016, 2017, and 2018, respectively (Figure 3).

Comparatively, total ACE-Protein trended slightly lower in this study when compared with larger regional studies. Fine et al. (2017) found an average ACE-Protein of 5.5 mg protein-N/g soil across a range of soils from the Midwestern United States while our study had an average of 4.78, 4.85, and 4.31 mg protein-N/g soil for the grassland, NT, and CT treatments, respectively (Table 1). With a Pearson correlation of 0.89 and 0.84, ACE-Protein correlated strongly with SOM and POX-C, respectively (Table 4). Statistically, NT was greater than the CT while neither conversion treatment differed from the grassland (Table 1). These results suggest that tillage served to decrease the overall organic N pool.

Finally, the general decreasing trend observed in many variables with CT was similar for respiration. Respiration was significantly lower in the CT plots versus the grassland while NT was not statistically different (Table 1). In general, there were still fairly strong correlations between soil respiration and other biological indicators (Table 4). Franzluebbers et al. (2018) found a strong correlation between the flush of CO₂ and net N mineralization. In combination with the decrease of ACE-Protein and POX-C through CT, these results indicate that N availability may be limited to a greater extent in grasslands converted to small grains through intensive tillage.

Chemical Indicators

In contrast to other indicators, pH showed less variability across the field (Table 3). No-till plots had lower pH overall (average of 6.36 vs. 6.55 and 6.44 for the grassland and CT plots, respectively), which was exacerbated during the driest year of the study (Table 3). This pattern is well documented and typically ascribed to the ammonium-based fertilizers being left on the soil surface with NT; hence, a strong pH stratification (Reeves & Liebig, 2016).

Utility and Sensitivity of Soil Health Indicators

The different CASH indicators varied in their response to the treatment effects. However, when taken as a suite of indicators, these protocols can be used to measure soil health in relation to management effects. Strong correlation among indicators, particularly POX-C, ACE-Protein, and SOM, suggests that these measures are sensitive to short-term fluctuations in both carbon- and nitrogen-cycling processes to a varying extent; however, sufficient variability still exists within treatments both among and between years (Cooper et al., 2020). Traditional chemical approaches are effective in increasing agricultural production but fail to identify soil degradation. When combined, the CASH indicators serve as proxies for defining critical physical, biological and chemical properties important to agricultural production and correlated to ecosystem processes. These indicators can serve as benchmarks for targeted studies to determine mechanistic effects of ecological functioning following invasive interventions such as grassland conversion. Overall, our results suggest that the potential for soil health degradation during grassland conversion to row crops is likely to be more severe in dry years. However, these results only address the immediate consequences following conversion and do not address the potential ramifications over a longer time period.

Conclusion

Overall, it is evident that soil health potentially declines rapidly upon conversion from grassland to small grains. In general, the decline was greater under conventional tillage than no-tillage, and these effects occurred within the first year of conversion. This is an intuitive result given the well-documented effects of tillage on a number of different soil health indicators; however, this study provides new data documenting how quickly these changes begin to occur. Moreover, the CASH provides an intuitive framework for monitoring the effects of land use change and can be used by land managers to identify potential soil constraints and formulate potential interventions. These results suggest that several important soil health indicators—notably, wet aggregate stability, ACE-Protein, and soil respiration—are sensitive in the short term to conversion to row crops from grassland based on tillage method. The current study demonstrates how these measures serve as a set of interconnected and reinforcing indicators providing a basis for documenting soil degradation through tillage. Further study is required to establish more robust correlations among the tested soil properties and environmental outcomes.