Microbial Responses to Disturbance Following the Conversion of CRP Land to Cropland Under Water Limiting Conditions.

The conversion of Conservation Reserve Program (CRP) lands to croplands potentially reduces ecological benefits such as high soil health condition that resist to disturbance (i.e. extreme weather). Soil CO₂ flux primarily comes from root and microbes’ respirations and is sensitive to changes in soil biomass and activity caused by environmental and mechanical disturbances. To investigate the impacts of CRP to cropland conversion on soil health, specifically on biomass and activity, we measured in situ soil CO₂ flux monthly from November 2012 to December 2015 in two systems: three CRP lands (>23 yr) and three adjacent converted CRP lands (CCRP, >23 yr CRP converted to dryland crop production within 1-2 yr as of 2012) on the Southern High Plains. The most metabolically active (more soil CO₂ flux) period was from May to October when most of the precipitation were received (265, 466, and 555 mm from 2013 to 2015). Accumulated CO₂ flux in CCRP was greater than CRP in 2013 (2.9 vs. 2.7 Mg CO₂-C ha^-1  as of September), which was likely because the drought limited biomass growth in both systems but the incorporation of organic residues during the conversion relieved the drought stress in CCRP slightly. However, accumulated CO₂ flux was lower in CCRP than CRP in 2014 and 2015 and the difference of accumulated CO₂ flux between two systems increased (CRP-CCRP: -0.2, 0.5, and 2.1 Mg CO₂-C ha^-1 from 2013 to 2015), which was likely due to depletion of the organic residues from previous CRP in CCRP. We also measured soil microbial biomass carbon (MBC) in every November from 2012 to 2015 to calculate metabolic quotient (qCO₂ =CO₂/MBC). The qCO₂ increased in 2013 and 2014 in CCRP but did not change in CRP, which was another indicator that CCRP depleted the organic residues from previous CRP without much assimilation.