Procedures for characterising denitrification rates in a wastewater-irrigated forest soil

In land-based wastewater treatment systems, soil denitrification can remove sufficient added nitrogen to decrease leaching and the threat of water pollution. To establish a suitable sampling strategy for measuring in situ denitrification rates, we investigated the importance of spatial and temporal variability of denitrification in a forest soil regularly irrigated with wastewater. To determine appropriate sampling depths and locations in a catchment, denitrifying enzyme activity (DEA) was measured at 5 sampling depths (litter, 0-5, 5-10, 10-20, 20-40 cm), and 3 topographic positions (ridge, midslope, toeslope) in irrigated and nonirrigated sites. To determine appropriate times for sampling after irrigation, in situ denitrification rates were measured at time intervals before and after irrigation for 1 week, using soil cores and acetylene inhibition. DEA was greatest in the litter layer and decreased with depth. In irrigated soils, DEA was greater than zero in the upper 20 cm of toeslopes, and the upper 10 cm of midslopes and ridge positions. In situ denitrification rates increased immediately after wastewater irrigation, peaking at 24 h, and then decreased to pre-irrigation rates after 3 days, unless it rained. We concluded that soil cores need to be collected from at least the upper 10 cm soil (including the litter layer), and on a daily basis between irrigation events, to quantify denitrification losses from soil regularly irrigated with wastewater.