Changes in microbial activity after abandonment of cultivation in a semiarid Mediterranean environment

Several organic matter fractions together with biological and biochemical parameters that might serve as markers of microbial activity were determined in an area previously dedicated to agriculture but where the soils had been abandoned for different lenghts of time (3 to 45 yr). Recently abandoned agricultural soils showed lower organic matter levels than soils not exposed to anthropic activity (natural soils). During the years immediately following abandonment the soils show lower levels of total organic C, humic substances and labile fractions of organic matter than natural soils. Soils abandoned for longer than 15 yr begin to recover their organic fractions after the appearance of spontaneous vegetation. The lower slopes appear to accumulate a greater quantity of total organic C and some labile fractions of C, such as water soluble C. Soil abandonment has a negative influence on soil microbial activity, biomass C and oxidoreductases such as dehydrogenase and catalase, although these values begin to rise again after several years. Soils abandoned for long periods of time and in the process of recovery show higher oxidoreductase values than natural soils. The activity of hydrolases belonging to the N, C, and P cycles (ureases and proteases, beta-glucosidase, and phosphatase) point to the negative influence of the agricultural practices, indicating that human activity does not favor adequate establishment of the cycles of these important nutrients. The cessation of agricultural activity leads to the recovery of soil hydrolase activities. Except in isolated cases, the hydrolase activities values determined did not appear to be affected by the position on the slope of the plot from which the samples were taken.