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.
In a pilot study, we investigated how irrigation of secondary sewage effluent onto steeply sloping land affected soil physical, chemical, and biochemical properties, the composition of soil‐ and surface‐waters and the vegetation on the site. The 3.36‐ha site received up to 44 mm effluent/wk (over a 7–11 h period), for 65 wk. Irrigation significantly improved total‐ and Olsen‐P status of the soils and greatly enhanced nitrification potential. Respiration increased with increasing soil water content, but microbial biomass was not greatly affected by irrigation. Soil phosphatase activity decreased with increasing P fertility. Soil physical properties were not affected by effluent and hydraulic conductivities were sufficient to conduct water into and through the soil profiles. Soil‐ and surface‐water NO−3‐N concentrations increased markedly, especially in the second half of the trial when soil nitrification rates were also high. However, the streamwater NO3‐N concentrations remained well below the drinking water limit concentration of 11.3 g m−3. In contrast, streamwater NH+4‐N and PO3−4‐P concentrations remained low and results indicated that concentrations of PO3−4‐P in river water, resulting from a full‐scale irrigation scheme, would not exceed the target limit level of 0.0056 g m−3. Irrigation accelerated natural successional changes in the vegetation, with a decline in undesirable fire‐prone and shrnbby species and an increase in native trees and tree ferns. These results demonstrated that, in the short term at least, a carefully designed and implemented irrigation scheme on steepland could renovate secondary sewage effluent, without adversely affecting soil properties and surface water quality.
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