Landscape dependent changes in soil properties due to long-term cultivation and subsequent conversion to native grass agriculture

Abstract

On farmland in undulating landscapes, soil organic carbon (SOC) stocks depend on landscape position. In the North American Prairie Pothole region, we compared a native prairie reference site with a nearby farm undergoing transition to perennial agriculture (“restoration”) after a century of producing annual crops. We quantified legacy effects of farming at four upland landscape positions (to 0.9-m soil depth) and three wetland positions (to 1.0-m soil depth). We also quantified short-term (4years) changes in SOC stocks (to 0.15-m soil depth) during restoration, and how these changes were impacted by historic erosion.Surface (to 0.05-m soil depth) measurements indicated degradation of the cropland soil relative to the prairie at all landscape positions due to less soil organic matter (SOM) and altered soil properties (e.g., water aggregate stability and microbial activity). All upland and wetland positions of the farm lost SOC stocks (Mgha−1) relative to the prairie in the top 1.5 Ggha−1 of soil (~14-cm depth). However, when considering a larger mass of soil, 4.5Ggha−1 (~39-cm depth), loss of SOC stocks was significant (p<0.001) only at the summit (46Mgha−1) and shoulder (63Mgha−1) landscape positions. Differences in SOC stocks between farm and prairie were smaller and not significant at the backslope, footslope, and in wetlands.Contrary to expectations, sites of soil deposition did not accumulate soil carbon after restoration. Accretion of soil C during restoration differed according to the severity of historic erosion (p<0.001), with severely eroded soils gaining soil C at the fastest rate. Historic loss of clay at the shoulder and backslope and subsoil compaction in the wetlands may prevent these landscape positions from full restoration of soil C stocks, net primary productivity, and historic vegetation.