Denitrification Rates in a Wastewater‐Irrigated Forest Soil in New Zealand

Abstract

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 denitrification to N removal in a land treatment system by measuring in situ denitrification rates for 12 mo in a Monterey pine (Pinus radiata D. Don) forest irrigated with tertiary‐treated wastewater. The variability of denitrification rates was investigated using a nested field design that divided the land treatment 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 soil cores collected daily, for six consecutive days on 21 occasions throughout the year. Soil moisture content, NO 3 concentration, available C, denitrifying enzyme activity, and temperature also were measured. The annual denitrification rate in the irrigated soil was 2.4 kg N ha−1 yr−1, and only slightly higher than the unirrigated soil (1.7 kg N ha−1 yr−1). Temporal effects (i.e., seasonal and day‐to‐day variation) contributed more than spatial effects to the overall variation in denitrification rates. Multiple regression analysis showed that soil factors could only explain 29% of the variation in denitrification rates. Soil water‐filled porosity was low in the land treatment system, and less than the critical threshold value (74% water‐filled porosity) determined in a laboratory study. We concluded that denitrification in this land treatment system studied was limited by excessive aeration in the free‐draining soils.