Annual cycles of physical and biological properties in an uncultivated and an irrigated soil in the San Joaquin Valley of California

Abstract

The temperature, moisture and CO 2 content of an uncultivated and an irrigated soil in the western San Joaquin Valley of California were monitored for more than 1 year in order to determine the degree to which irrigated agriculture affected the annual variations of these important ecological properties. The uncultivated soil underwent strong seasonal changes in moisture and CO 2, the highest values being obtained in the late winter and early spring and the lowest occuring in the late summer and early autumn. In contrast, as adequate moisture for biological activity was available year round in the irrigated soil, the CO 2 levels varied in response to variations in temperature. The organic C content of the irrigated soil was half that of the uncultivated soil (1.8 versus 3.6 kg m −2), presumably owing to an enhanced level of organic matter decomposition in the irrigated soil. Calculated annual rates of soil respiration for the two soils were nearly identical (0.25 g m −2 h −1). Based on estimates of annual biomass production in the uncultivated soil, it was estimated that the mean residence time of soil organic matter was approximately 61 years. Although this estimate indicates a relatively rapid turnover of C, it is consistent with previous work indicating rapid nutrient cycling in annual grasslands.