CRP to cropland conversion: Short-term impacts on soil quality in the Southern High Plains

Background/Question/Methods

The Conservation Reserve Program (CRP) began in 1985 to stabilize marginal lands and reduce soil erosion. Secondary benefits were later realized including potential increases in carbon (C) sequestration and biodiversity. Each year, however, thousands of acres expire from CRP contracts. These CRP lands may be converted back to croplands and reverse the benefits established during restoration. Texas leads the nation in acreage enrolled in CRP and in expiring contracts; the majority of this land is restored grasslands located in the Southern High Plains (SHP). The objective of this study was to investigate how conversion of CRP to croplands affected soil quality, metabolic functioning, and microbial community composition. Methods used included the soil management assessment framework to calculate a soil quality index (SQI), seven soil enzyme activities (EAs), microbial biomass carbon (MBC), and fatty acid methyl ester profiling. Soil samples from three SHP locations with two management treatments were sampled in November 2012. Management treatments included CRP grasslands and CRP fields that were converted to non-irrigated croplands within 1-2 years at the time of sampling. All CRP fields had been under restoration for 23-25 years. Three replicates of each treatment with three sub-samples were collected at three depths (0-10cm, 10-30cm, 30-50cm).   

Results/Conclusions

Management did not significantly affect SQI, EAs, MBC, and the ratios of each EA:MBC. However, there was a consistent pattern of higher SQI, EA, MBC and lower ratios for converted cropland than CRP. The mean SQI in cropland and CRP was 52.3 and 48.5, respectively. Mean MBC and SOM was also higher in cropland (125.3mg kg^-1 and 20.5 mg ha^-1, respectively) than CRP (88.9mg MBC kg^-1 and 17.9 mg SOM ha^-1). Cropland also had lower EA:MBC, which is an indication of enhanced microbial metabolic efficiency. Management did significantly affect microbial community composition with relatively higher arbuscular mycorrhizal fungi abundance and lower relative abundance of Gram positive bacteria in CRP than converted cropland. The trends of enhanced soil quality variables and metabolic functioning in converted croplands could be an artifact of increased degradation of organic residues via termination of grasses and tillage. Increased organic materials and fertilization practices in converted fields stimulates microbial activities and reduces nutrient acquisition stress. However, samples were collected following two years of extreme drought, which may have preserved SOM. A return to normal precipitation levels may facilitate rapid decomposition, loss of SOM, and reduced MBC and EAs. Continued sampling efforts will test this hypothesis.