Land application has become a widely applied method for treating wastewater. However, it is not always clear which soil-plant systems should be used, or why. The objectives of our study were to determine if four contrasting soils, from which the pasture is regularly cut and removed, varied in their ability to assimilate nutrients from secondary-treated domestic effluent under high hydraulic loadings, in comparison with unirrigated, fertilized pasture. Grassed intact soil cores (500 mm in diameter by 700 mm in depth) were irrigated (50 mm wk(-1)) with secondary-treated domestic effluent for two years. Soils included a well-drained Allophanic Soil (Typic Hapludand), a poorly drained Gley Soil (Typic Endoaquept), a well-drained Pumice Soil formed from rhyolitic tephra (Typic Udivitrand), and a well-drained Recent Soil formed in a sand dune (Typic Udipsamment). Effluent-irrigated soils received between 746 and 815 kg N ha(-1) and 283 and 331 kg P ha(-1) over two years of irrigation, and unirrigated treatments received 200 kg N ha(-1) and 100 kg P ha(-1) of dissolved inorganic fertilizer over the same period. Applying effluent significantly increased plant uptake of N and P from all soil types. For the effluent-irrigated soils plant N uptake ranged from 186 to 437 kg N ha(-1) yr(-1), while plant P uptake ranged from 40 to 88 kg P ha(-1) yr(-1) for the effluent-irrigated soils. Applying effluent significantly increased N leaching losses from Gley and Recent Soils, and after two years ranged from 17 to 184 kg N ha(-1) depending on soil type. Effluent irrigation only increased P leaching from the Gley Soil. All P leaching losses were less than 49 kg P ha(-1) after two years. The N and P leached from effluent treatments were mainly in organic form (69-87% organic N and 35-65% unreactive P). Greater N and P leaching losses from the irrigated Gley Soil were attributed to preferential flow that reduced contact between the effluent and the soil matrix. Increased N leaching from the Recent Soil was the result of increased leaching of native soil organic N due to the higher hydraulic loading from the effluent irrigation.

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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