IMPACT OF LAND-APPLIED TERTIARY-TREATED EFFLUENT ON SOIL BIOCHEMICAL-PROPERTIES

Land application is increasingly used for the disposal and treatment o f effluents. We investigated how irrigation of tertiary-treated domest ic effluent influenced 14 soil biochemical properties in a Monterey pi ne (Pinus radiata D. non) forest on volcanic soils. The soils were irr igated with either effluent or water at two loading rates (49 and 74 m m wk(-1)). Surface soils (0-5 cm) were collected from the effluent-irr igated and adjacent nonirrigated control sites annually for 3 yr and f or 2 yr from the water-irrigated sites. Effluent irrigation significan tly (P < 0.05) increased several soil properties including pH, inverta se activity, denitrification, mineralizable N, and extractable nitrate . These increases were not observed in the water-irrigated soils sugge sting that the changes resulted from effluent chemistry rather than ad ditional water loading. Phosphatase activity decreased with both water - and effluent-irrigation. No changes were observed in total N, total C, basal respiration, microbial biomass, sulfatase activity, or extrac table ammonium in the efflueut- and water-irrigated soils. Both rates of effluent application had the same effect on soil properties indicat ing that the threshold rate that changed soil properties was less than or equal to 49 mm of effluent per week.