Changes in quality and quantity of soil organic matter stocks resulting from wastewater irrigation in formerly forested land

Land use changes often diminish soil carbon stocks and alter soil organic matter (SOM) quality affecting soil productivity. Therefore, intensively managed agricultural systems are intended to maintain and even increase SOM stocks. Here we compared soil organic carbon (SOC) stocks and their quality in fields under natural vegetation (mesquite forest and xerophytic shrubs), converted first to rain fed maize cultivation and then to irrigated alfalfa-maize rotation systems in a semiarid region of Central Mexico. Fields under the alfalfa-maize rotation have been irrigated for different lengths of time (15, 35, 51, 85 and 100years) with untreated wastewater from Mexico City. We tested if the SOC stocks of the irrigated land use system were similar or even larger than those found under natural vegetation, and investigated if the quality of the SOM changed under the different covers. We collected composite soil surface samples and soil profile samples from fields under the different land uses and irrigation lengths and determined their organic carbon concentrations; we also quantified root residues and analyzed their bromatological composition. Mid infrared spectra of the soil samples were recorded and analyzed by multivariate procedures to determine changes in SOM quality.