What Happens to Soil Ecological Properties When Conservation Reserve Program Land Is Disturbed?.

Each year, expiring Conservation Reserve Program (CRP) contracts results in the conversion of restored CRP land back to croplands, potentially reversing multiple ecological benefits including C sequestration potential and microbial biodiversity. We evaluated microbial community composition (fatty acid methyl ester profiles) and a suite of dynamic soil C properties (DCPs) from soil samples (0-10cm, 10-30cm, and 30-50cm) collected from three long-term (>20 y) CRP fields and three CRP fields converted within 1-2 y in 2012 to dryland Crop production in 2012, 2013, and 2014. The DCPs include: microbial biomass C (MBC), particulate organic matter (POM-C), CO₂ flux, and three C cycling soil enzyme activities. Arbuscular mycorrhizal fungi (AMF) and Gram positive bacteria (GM+) were largely responsible for differentiating between the two systems with greater AMF and lower GM+ bacteria in CRP than Crop. At 0-10cm, DCPs were similar between Crop and long-term CRP in 2012, but were different in 2013 and 2014. In general, CRP had greater MBC, POM-C, C-cycling activity than Crop. In contrast, DCPs at 10-30cm were greater in Crop than CRP across the three year sampling period. These increased soil C variables were attributed to increased microbial access to vegetative residues as grasses were terminated and incorporated into the soil during conversion. At 30-50cm, the DCPs were distinguished between Crop and CRP in 2012 only (Crop had greater MBC) but the treatments converged in subsequent years. Initially, CO₂ fluxes were similar between the two systems with CRP producing more CO₂ than Crop in 2013 and 2014. Soil qCO₂ values (mg CO₂-C released per mg MBC), however, indicated enhanced microbial metabolic efficiency in CRP than Crop and supports the need to couple these variables for ecological interpretation. CRP conversion to Crop altered the microbial community composition and decreased DCPs over time in two out of the three depths.