Conversion of Conservation Reserve Program land back to cropland: Changes in soil carbon and nitrogen dynamics during the first five years

In the United States,Texas has one of the largest areas of land enrolled as well as expiring from the Conservation Reserve Program (CRP), and most of these restored grasslands are in the Southern High Plains region. Crop market forces and agency reductions in the number of hectares that can reenroll in CRP have resulted in many farmers considering recultivating this land. Converting grasslands to conventionally tilled annual croplands could reverse the accumulated benefits afforded from perennial grasslands.Three soil profile depths (0 to 10, 0 to 30, and 0 to 50 cm) from three long-term (>23 years) CRP and three converted CRP (C-CRP) sites were assessed annually from 2012 to 2015 (representing one to five years following conversion). Soil health indicators related to soil carbon (C) and nitrogen (N) dynamics were evaluated including soil organic C (SOC), total N (TN), particulate organic matter C and N (POM-C and POM-N), permanganate oxidizable C (POXC), microbial biomass C and N (MBC and MBN), in situ soil carbon dioxide (CO₂) efflux, and metabolic quotient (qCO₂). Redistribution of C following CRP conversion was detected in the MBC stock in 2012 with values higher in C-CRP than CRP at 0 to 30 cm and 0 to 50 cm profiles but not at 0 to 10 cm. The increase was short-lived with lower MBC in C-CRP than CRP in subsequent years. Most C and N stocks were lower in C-CRP compared to CRP across all depth profiles. The greatest losses of soil C and N stocks occurred at 0 to 10 cm, with the magnitude of loss varying by indicator and soil depth profile. Five years following conversion, SOC was reduced 2.7 Mg ha^-1 at 0 to 10 cm, and averaged across all sample years, SOC was reduced by 4.3 Mg ha^-1 at 0 to 50 cm. Drought stress was present from 2011 to 2014 and exerted an additional pressure to both systems, but the CRP system responded more positively to increased precipitation in 2014 and 2015. In CRP, the qCO₂, an indicator of microbial stress, remained below 1.0 throughout the study, and MBC increased by 81% from 2012 to 2014, when drought stress was lessened. In contrast, MBC decreased and qCO₂ doubled under C-CRP from 2012 to 2014. Overall, this study showed negative impacts on soil C and N stocks within five years after CRP to C-CRP conversion.