Soil Enzyme Activities As Indicators of Soil Health in Long-Term CRP and CRP Converted to Cropland.

The Southern High Plains (SHP) of Texas has 0.91 million hectares enrolled in the Conservation Reserve Program (CRP) as of 2016. Each year, however, thousands of hectares expire from CRP contracts with many hectares returned to crop production. The impacts of this shift in land management on soil biogeochemical potential are unknown. Nine enzyme activities (EAs) were evaluated for C, N, P & S cycling from soil samples (0-10 and 10-30cm) from 26 fields across the SHP of varying years under CRP (0-28-y as of 2014). We then tracked changes in the same EAs from 2012 to 2015 in three 25-y CRP sites that were converted to cropland (C-CRP) in 2010 (1 site) or 2012 (2 sites) and compared these results to adjacent 25-y CRP sites, with an additional depth (30-50cm). The influence of CRP years on EAs was most pronounced in 2012 following extreme drought conditions. All nine EAs at 0-10cm and five EAs at 10-30cm increased linearly with increasing CRP years with up to three-fold increases by 28-y. In 2014, following relief from the drought, only five EAs at 0-10cm and two EAs at 10-30cm increased with CRP years. Regardless of linear response over CRP years, there was a consistent trend of greater EAs in CRP than cropland at both depths and sampling times. Conversion of CRP to cropland resulted in immediate decreases in EAs. Averaged over all sampling years, C-CRP had lower EAs at 0-10cm compared to CRP except for urease (all years) and b-glucosidase and arylsulfatase in 2012. Compared to 2012, the geometric mean of all EAs increased in 2014 in CRP and C-CRP when drought stress was relieved, but CRP had a greater overall increase of 49%, compared to 19% in C-CRP.