Soil C Dynamics in a 26 Year CRP Chronosequence On An Amarillo Fine Sandy Loam.

The Conservation Reserve Program (CRP) sequesters more carbon (C) on private lands than any other federally administered program, but the rate and maximum amount of sequestered SOC is dependent on inherent soil properties (e.g. texture), local climate, and initial restoration efforts. We estimated the change in SOC dynamics in a 26-yr chronosequence of cultivated fields (predominantly cotton) restored to C4 grasses (monoculture and mixed) on an Amarillo fine sandy loam. In 2012, 7 dryland cropland fields (0 years) and 16 CRP fields, that ranged from 6 to 26 years under CRP, were sampled (0-10 and 10-30 cm) and in situ soil respiration (CO₂) was measured. Metabolic quotient (qCO₂) was calculated as the ratio of CO₂-flux to microbial biomass C. Particulate organic matter (POM) was isolated using a combination of size and density fractionation techniques. POM-C and whole-SOC content and δ¹³C were measured to determine relative fraction (Fc) of C from C₄ plants. qCO₂ decreased with years restored (from 3.6 to 1.4 µg CO₂-C mg^-1 C_mic h^-1). Whole-SOC was not significantly affected by years restored, however POM-C and the microbial biomass C-to-SOC ratio increased with years restored (0.63 to 1.38 Mg ha^-1 and 0.02 to 0.05, respectively) indicating an increase in the more labile C pools. Within the POM fraction, SOC at 0-yrs was primarily from C₃ sources (cotton) as opposed to relict C₄ grasses (62 and 38%, respectively). After 13-yrs in CRP, the proportion of POM-C from C₄ grasses increased to 85% with no further increase measured up to 26 years under restoration. To determine potential for ecosystem recovery 3 native short-grass prairie fields have been identified and will be examined for the same soil parameters.