The contribution of upland denitrification to nitrate removal in soils, and the factors controlling denitrification, were investigated in the Rotorua Land Treatment System (RLTS), New Zealand. The RLTS is forested with radiata pine and located on free-draining soils formed from pumiceous parent materials. In land treatment systems, a large proportion of the nitrogen added in the wastewater is thought to either be utilised by the cover crop, or by soil microbial processes. An important soil microbial process that is often assumed to occur is denitrification. However, the contribution of upland denitrification to nitrogen renovation, and the effects of wastewater application on the soil denitrifying population, is poorly understood in forested land treatment systems.
An initial study was undertaken to establish a suitable method for measuring in situ denitrification rates in the RLTS. Denitrification enzyme activity (DEA) was measured at different soil depths (litter, 0-5, 5-10, 10-20 and 20-40 cm) in three topographic positions of the RLTS (ridge, midslope and toeslope). In addition, in situ denitrification rates were measured, using an acetylene-inhibition technique, at various time intervals before and after irrigation to determine how frequently soil cores needed to be taken to quantify denitrification losses after wastewater irrigation. It was concluded that it was necessary to collect cores from the uppermost 10 cm of the soil profile (including the litter layer), on a daily basis between irrigation events, and repeatedly throughout the year, to accurately estimate annual denitrification rates in the RLTS.
Denitrification rates were measured in the RLTS over a period of 12 months. The spatial variability of in situ denitrification rates was investigated by using a nested field design that divided the RLTS into four stages (irrigation block, topographic position, field site and sample point). In situ denitrification rates were measured between irrigation events, to establish how daily denitrification rates varied after wastewater irrigation, and on 21 different occasions during the year, to establish how daily denitrification rates varied seasonally. Annual denitrification rates of 2.4 and 1.7 kg N ha⁻¹ yr⁻¹ were recorded for the wastewater-irrigated and unirrigated soils, respectively. Daily denitrification rates were spatially and temporally variable, with coefficients of variation greater than 100%. Differences in denitrification rates between irrigation blocks contributed significantly more to spatial variability than differences between or within topographic positions. Denitrification rates varied seasonally, with greatest losses occurring in the late summer and autumn. Daily denitrification rates also varied from day-to-day after irrigation. However, the day-to-day pattern of denitrification after irrigation changed throughout the year. Over 12 months, temporal effects contributed more than spatial effects to the overall variation in denitrification rates.
Soil moisture content, nitrate concentration, respiration, DEA and temperature were measured during the 12 month field trial to determine their effects on in situ denitrification rates. Using multiple regression analysis, soil and environmental properties could only explain up to 29% of the variation in in situ denitrification rates.
Laboratory studies showed that denitrification rates were very small when soil moisture contents were less than 80%. During the field trial, water-filled porosity was low, and in 84% of the samples collected (n = 4527), soil moisture contents were less than the critical threshold value required for denitrification. Therefore, it was proposed that in situ denitrification rates were small in the RLTS because soil moisture contents were low and generally less than the critical moisture content required for denitrification.
The size of the denitrifying population was also found to be small in the RLTS. Under optimum laboratory conditions, potential denitrification rates at 25 °C were 13.4 kg N ha⁻¹ yr⁻¹ in the wastewater irrigated soils. However, potential denitrification rates would be expected to be less at average field temperatures (11 °C). Laboratory studies, using disturbed soil samples, suggested that the size of the denitrifying population in the wastewater irrigated soils was limited by soil aeration. When oxygen availability in irrigated soils was limited, the size of the denitrifying population increased to a greater extent than measured in the field. However, adding carbon and nitrate to anaerobic soils did not further increase the denitrifying population in comparison to controls in the irrigated soils. Wastewater-irrigation changed the factors limiting denitrifiers in the RLTS. In the irrigated soils, denitrification is limited by soil aeration, while in the unirrigated soils denitrifiers were limited by both soil aeration and nitrate. Furthermore, wastewater irrigation altered the short-term response of denitrifiers to anaerobiosis. Under low oxygen conditions, in the laboratory, denitrifiers in the wastewater-irrigated soils produced enzymes earlier, and at a greater rate, than soil that had no history of wastewater irrigation.
It was concluded that wastewater needs to contain sufficient nitrogen to increase soil nitrate concentrations and should be applied to soils which are less free-draining soils than the soils used in this study, if upland denitrification is to contribute significantly to nitrogen removal in a forested land treatment system. A conceptual model is proposed to assist in establishing the likelihood of upland denitrification significantly contributing to nitrogen removal in a forested land treatment system
