Changes in Soil Organic Carbon Stocks and Their Quality Due to Land Use Changes in a Semiarid Region of Mexico.

Land use changes modify biomass production, i.e. the quantity and quality of organic debris entering the soil as well as their turn over rates, affecting soil organic matter (SOM) stocks. We compared SOM stocks and their quality under a mesquite forest, xerophytic shrub vegetation, rain-fed maize, and wastewater irrigated alfalfa-maize rotation systems with different lengths of time under irrigation in Central Mexico. We aimed to document how wastewater irrigation affects C stocks and their quality in Vertisols and Leptosols. We collected composite surface soil samples (0 to 30 cm) from Leptosols and Vertisols, and samples in Vertisol profiles from each horizon. Samples were analyzed for total organic carbon, and mid infrared spectra of all samples were recorded. The spectra of pretreated sample aliquots to destroy organic compounds were substracted from those of untreated samples, to eliminate the signals of the mineral phase. Multivariate statistical analyses allowed to classify the spectra into groups according to the structural characteristics of the organic fraction.

Smallest SOM stocks were found under rain-fed maize (3 Mg ha^-1 in 30 cm), and largest under mezquite (12 Mg ha^-1 in 30 cm). In the alfalfa-maize rotation under wastewater irrigation SOM stocks doubled during the first 40 years until an new apparent steady state was reached at 6 Mg ha^-1.

Multivariate analyses showed that the structural characteristics of the SOM changed in relation to soil depth and length under irrigation. SOM in rain-fed soils was more hydrophobic and more metabolized by microorganisms, while the one from irrigated soils had larger amounts of labile compounds as cellulose, sugars (simple carbohydrate structures) and proteins, which have a more hydrophilic character. Lignine in surface horizons of long term irrigated soil showed more mono-substituted aromatic structures, while lignine in deeper soil horizons or irrigated for less time have more ortho- and para-substitutions, showing a smaller condensation degree.