Wet–dry cycles affect dissolved organic carbon in two California agricultural soils

During Californiaʹs hot, dry summers, irrigated soils are subjected to frequent wet–dry cycles and surface layers dry to near air–dry conditions between irrigations. We investigate whether wet–dry cycles enhance soil dissolved organic carbon (DOC) concentrations. This research follows up on previous observations of higher DOC concentrations in the surface (0–2 cm) than deeper (2–15 cm) soil layer late in the growing season, even when soils were moist throughout the profile. We also investigate whether DOC contents correspond to other measures of C available to microorganisms. All measurements were made on soils stabilized at −0.03 MPa water potential for 48 h at 25°C to avoid the initial pulse of microbial activity which follows re-wetting of dry soils. After 3 months during the summer field season, DOC concentrations increased 2.5-fold in the surface 0–2 cm layer and 1.20 to 1.35-fold in the 2–15 cm layer in soils under both organic (N inputs of cover crop and manure) and conventional (inorganic N inputs) management for irrigated tomatoes. In microcosms exposed to wet–dry cycles for 3 months, DOC concentrations increased by 70%, while in microcosms maintained at −0.03 MPa for 3 months DOC remained constant. The increase in DOC in both field and microcosm soils exposed to wet–dry cycles indicates that wet–dry cycles contribute to higher background DOC contents. The greater DOC increase in the field than microcosms may be due to evaporation causing upward movement of water and concentrating DOC at the soil surface, or to greater C availability in the field due to the presence of plant roots. Respiration and microbial biomass C (MBC) remained constant or declined slightly in both soil layers and microcosm treatments over the growing season, counter to the trends in DOC concentration. Therefore DOC contents measured under moist soil conditions do not appear to consistently indicate C availability to microorganisms. The percentage of labile DOC, as measured by a bioassay, declined in the surface layer of the organic field soil and in organic and conventional soils in both microcosm treatments over the 3 month experiment, possibly indicating that roots were a continuing source of labile DOC in the lower field layers. Reflecting the higher organic inputs to the organic than conventional soil, DOC, MBC and respiration rates were 2–2.5 times higher in the organic than conventional soil throughout the experiments, however the percentage of labile DOC was approximately twice as high in the conventional soil as in organic soil.