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Land application is increasingly used for the disposal and treatment of effluents. We investigated how irrigation of tertiary‐treated domestic effluent influenced 14 soil biochemical properties in a Monterey pine (Pinus radiata D. Don) forest on volcanic soils. The soils were irrigated with either effluent or water at two loading rates (49 and 74 mm wk−1). Surface soils (0–5 cm) were collected from the effluent‐irrigated and adjacent nonirrigated control sites annually for 3 yr and for 2 yr from the water‐irrigated sites. Effluent irrigation significantly (P < 0.05) increased several soil properties including pH, invertase activity, denitrification, mineralizable N, and extractable nitrate. These increases were not observed in the water‐irrigated soils suggesting that the changes resulted from effluent chemistry rather than additional 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 extractable ammonium in the effluent‐ and water‐irrigated soils. Both rates of effluent application had the same effect on soil properties indicating that the threshold rate that changed soil properties was ≤49 mm of effluent per week.

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