Mapping Remnant Grasslands across the Great Plains of North America to Guide Soil Health Restoration

Background/Question/Methods

Conversion of the grasslands of the Great Plains of North America into cropland and rangeland has resulted in substantial degradation and loss of soil health, a fundamental feature of these ecosystems. In particular, soil organic matter content is a major contributor to the establishment and maintenance of healthy grasslands, but has severely declined through erosion and oxidation resulting from clearing, cultivation, and overgrazing. Consequently, to address the simultaneous and interconnected problems of food security, freshwater scarcity, and loss of both biodiversity and carbon-sequestering abilities, it is essential to revitalize grassland soil health, through either improved agricultural practices, or more direct reclamation or restoration action. After continuous and extensive degradation over 150 years, there are now only a few intact healthy grasslands remaining to act as reference systems or benchmarks to help guide land managers, restorationists, and researchers. Currently, the extent of organic matter and soil carbon loss is variously being estimated through long-term erosion monitoring, evaluations of restoration chronosequences, modeling efforts, or localized paired comparisons of cultivated and remnant sites. A clearer benchmark for the historic condition of grassland soil health would provide context for how to plan for the future of these ecosystems and give scope for their recovery.

Results/Conclusions

We have developed an extensive map of soil organic matter levels representative of the Great Plains biome prior to grassland conversion. A literature review produced 140 soil data sets collected from uncultivated grassland remnants, ranging from surveys conducted in the early 1930s to comprehensive soil health assessments including microbial community composition in remnants as recently as 2020. Overall, soil organic matter concentrations from remnant soils were higher in surface layers and extended deeper into the soil profile than current soils exhibit. A comparison of measured modern day soil carbon levels with samples from across a transect of prairie remnants shows a mean loss of 3.4 percentage points in soil carbon concentration in the top 5 cm, roughly equivalent to 21.6 MgC ha-1 within this layer alone. This work will provide a basis for comparison with the recent coordinated assessment of current nationwide soil carbon levels conducted by USDA-NRCS scientists in 2016 (Rapid Carbon Assessment – RaCA) to develop a spatially explicit map of soil degradation across the Great Plains. The information will help prioritize soil health restoration, highlight untapped carbon sequestration potential, and form a basis for outreach in grassland restoration and protection of remnant grasslands.