Rototillage, Disking, and Subsequent Irrigation

Spring and summer tillage are usually followed by irrigation before planting crops in California's summer‐dry Mediterranean‐type climate. Tillage treatments such as rototillage or disking are known to disturb the soil structure to different extents, but little is known about how the intensity of a tillage event and subsequent irrigation affect the microbial biomass, respiration, CO 2 efflux, and mineral N of agricultural soils. We carried out an experiment with a Yolo silt loam (fine‐silty, mixed, superactive, nonacid, thermic Mollic Xerofluvent) with two tilled treatments (rototillage and disked and rolled) and a nontilled control. The soil was subsequently sampled throughout a 17‐d period. Nine days after tillage, all treatments were lightly sprinkler‐irrigated to bring the soil water potential above −10 kPa. After tillage, the soil ammonium and nitrate content increased rapidly relative to the control with highest increases in the disked soil. Mineral N remained higher in the tilled treatments after irrigation. Rototillage and disking increased the CO 2 efflux of the soil within 24 h of the disturbance. The increase was higher in the disked soil, which was more than three times the CO 2 efflux of the control soil at 0.25 h after tillage. This effect may be due to degassing of dissolved CO 2 since microbial respiration did not increase in tilled soils. Irrigation increased the CO 2 efflux of all treatments but this was most pronounced in the control soil, which had an order of magnitude increase in CO 2 efflux after irrigation. An ancillary experiment carried out under similar conditions but with more frequent sampling showed that increases in CO 2 efflux after irrigation were accompanied by increases in soil respiration. This study shows that different tillage implements affect CO 2 efflux, nitrate accumulation, and microbial activity, and thus have different effects on soil and atmospheric environmental quality.