Denitrification rates in a wastewater-irrigated forest soil in New Zealand

Denitrification is considered to be an important N removal process in land- based wastewater treatment systems, although in situ denitrification rates have rarely been reported. We investigated the contribution of denitrificat ion to N removal in a land treatment system by measuring in situ denitrific ation rates far 12 mo in a Monterey pine (Pinus radiata D. Don) forest irri gated with tertiary-treated wastewater. The variability of denitrification rates was investigated using a nested field design that divided the land tr eatment system into four spatial components (irrigation block, topographic position, field site, and sample plot) and two temporal components (sample period, sample day). Denitrification was measured using undisturbed sail co res collected daily, far six consecutive days on 21 occasions throughout th e year. Soil moisture content, NO3 concentration, available C, denitrifying enzyme activity, and temperature also were measured. The annual denitrific ation rate in the irrigated soil was 2.4 kg N ha(-1) yr(-1), and only sligh tly higher than the unirrigated soil (1.7 kg N ha(-1) yr(-1)) Temporal effe cts (i.e., seasonal and day-to-day variation) contributed more than spatial effects to the overall variation in denitrification rates. Multiple regres sion analysis showed that sap factors could only explain 29% of the variati on in denitrification rates. Soil water-filled porosity,vas law in the land treatment system, and less than the critical threshold value (74% water-fi lled porosity) determined in a laboratory study, We concluded that denitrif ication in this land treatment system studied was limited by excessive aera tion in the free-draining soils.