Soil C Quality Dynamics After Conservation Reserve Program Land Is Converted to Cropland.

The objective of this project is to evaluate changes in soil organic matter quality in Conservation Reserve Program (CRP) sites in the Texas high plains. Fourier transform mid-infrared spectroscopy (MidIR) was used to characterize the chemical composition of the soils and soil fractions. Sampling sites included CRP of different ages, as well as cotton-based cropping systems in 7 counties in west Texas. Three depth intervals from 0-50 cm were sampled.  The particulate organic matter (POM), which is thought to contain the labile soil C and N pools, was obtained from the 0-10 cm layer and analyzed separately. The POM fractionation simplifies MidIR spectral interpretation because interference by clay bands is minimized. Samples from the top layer were also decalcified in order to test for the influence of carbonates in MidIR spectra. Our results show that the POM composition is strongly influenced by the geographic location, but also by the type of management (cotton vs crop). POM under cotton, with higher % LOI, also had higher absorbance at the 3400 cm^-1 OH/NH band, the 2930-2850 cm^-1 aliphatic CH (or carbonate) band. POM samples of different CRP ages had high spectral diversity, and CRP ages between 20-26 y tended to have different spectral properties from 0 y. The POM spectral differences in the CRP chronosequence reflect the higher % LOI in the older CRP, and suggest that labile C is accruing in the POM as CRP ages. Decalcification with HCl vapor caused a reduction in the carbonate bands at 2930-2850 cm^-1 and 2520 cm^-1 as expected, but also affected absorbance at 3400 and 1450 cm^-1, suggesting that the HCl treatment affected organic C bands as well.