Soil properties along cultivation and fallow time sequences on vertisols in northeastern Mexico

Rain-fed agriculture in alternation with natural fallow is widespread in semiarid northeastern Mexico, but little information on changes in soil properties, soil degradation, and natural rehabilitation is available. The effects of rain-fed agriculture and fallow on soil quality indicators on vertisols in a semiarid area near Linares, Nuevo Leon, were studied. One cultivated time sequence representing 3 to 30 yr of use and one fallow time sequence of 2 to 22 yr were selected. Fifty percent of soil organic carbon (SOC) and 56% of total nitrogen (TN) were lost during the first 4 yr of cultivation, but loss reached approximate equilibrium after 6 yr. The SOC showed 34% recovery and TN showed 62% recovery on sites abandoned for 22 yr. Despite this recovery after 22 yr of fallow, SOC and TN levels reached only 50% of those observed under native vegetation. Water-stable macroaggregation declined by 14% under cultivation, but increased swiftly during fallow, showing no significant correlation with SOC. The levels of SOC and TN depletion under conventional rain-fed agriculture observed are very difficult to mitigate by natural fallows in economically-viable time periods. However, the precise impact of these changes on aggregation properties of these vertisols and the long-term sustainability of present cultivation-fallow practices is not clear. Further research to determine the precise influence of individual soil organic constituents on physical properties of these vertisols is in progress.