EFFECT OF SURFACE-CASTING EARTHWORMS ON THE TRANSPORT OF PHOSPHORUS AND NITROGEN IN SURFACE RUNOFF FROM PASTURE

Treatment of a soil under permanent pasture with carbaryl (a broad spectrum carbamate biocide) resulted in a 2-fold increase in the volume of surface runoff. This was attributed to a 3-fold reduction in infiltration rate as a result of litter accumulation at the soil surface in the absence of surface-casting earthworm activity. The amounts of dissolved inorganic P (DIP), NH+4-N, and NO-3-N in surface runoff from pasture treated with carbaryl (1.18, 9.53 and 4.25 kg ha-1 yr-1, respectively) were appreciably greater than those from untreated pasture (0.31, 1.63 and 0.52 kg ha-1 yr-1). This was attributed to the large amounts of DIP, NH+4-N, and NO-13-N released from decomposing litter. Following incubation at 4°C for 18 days the release of DIP, NH+4-N and NO-3-N from litter was 160, 1600 and 950 μg g-1, respectively. Losses of particulate P and sediment in surface runoff were lower in the absence (0.31 and 290 kg ha-1 yr-1, respectively) than in the presence (0.56 and 1120 kg ha- yr-1) of surface casts, pointing to the importance of surface casts as a source of sediment. Surface casts accounted for 45 and 75%, respectively, of the annual loading of particulate P and sediment in surface runoff. Nevertheless, the total loss in surface runoff of P and N forms was increased substantially when the production of earthworm casts was eliminated. © 1979.