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TECHNICAL REPORTS

Waste Management

Soil Water Nitrate and Ammonium Dynamics under a Sewage Effluent–Irrigated Eucalypt Plantation

^a School of Forest and Ecosystem Science, The Univ. of Melbourne, 500 Yarra Blvd., Melbourne, VIC 3122, Australia
^b School of Biological, Earth and Environmental Sciences, The Univ. of New South Wales, Biological Sciences Bdg., Randwick, NSW 2031, Australia
^c Ecosystems Research Group, School of Plant Biology, The Univ. of Western Australia, 35 Stirling Hwy., Crawley WA 6009, Australia

^* Corresponding author (sjlive{at}unimelb.edu.au).

Received for publication April 10, 2007. Managed forests and plantations are appropriate ecosystems for land-based treatment of effluent, but concerns remain regarding nutrient contamination of ground- and surface waters. Monthly NO₃–N and NH₄–N concentrations in soil water, accumulated soil N, and gross ammonification and nitrification rates were measured in the second year of a second rotation of an effluent irrigated Eucalyptus globulus plantation in southern Western Australia to investigate the separate and interactive effects of drip and sprinkler irrigation, effluent and water irrigation, irrigation rate, and harvest residues retention. Nitrate concentrations of soil water were greater under effluent irrigation than water irrigation but remained <15 mg L^–1 when irrigated at the normal rate (1.5–2.0 mm d^–1), and there was little evidence of downward movement. In contrast, NH₄–N concentrations of soil water at 30 and 100 cm were generally greater under effluent irrigation than water irrigation when irrigated at the normal rate because of direct effluent NH₄–N input and indirect ammonification of soil organic N. Drip irrigation of effluent approximately doubled peak NO₃–N and NH₄–N concentrations in soil water. Harvest residue retention reduced concentrations of soil water NO₃–N at 30 cm during active sprinkler irrigation, but after 1 yr of irrigation there was no significant difference in the amount of N stored in the soil system, although harvest residue retention did enhance the "nitrate flush" in the following spring. Gross mineralization rates without irrigation increased with harvest residue retention and further increased with water irrigation. Irrigation with effluent further increased gross nitrification to 3.1 mg N kg^–1 d^–1 when harvest residues were retained but had no effect on gross ammonification, which suggested the importance of heterotrophic nitrification. The downward movement of N under effluent irrigation was dominated by NH₄–N rather than NO₃–N. Improving the capacity of forest soils to store and transform N inputs through organic matter management must consider the dynamic equilibrium between N input, uptake, and immobilization according to soil C status, and the effect changing microbial processes and environmental conditions can have on this equilibrium.

Abbreviations: SOM, soil organic matter