EFFECT OF SEWAGE EFFLUENT ON SOIL FAUNA IN A PINUS-RADIATA PLANTATION

Water and nutrients in effluent applied to land may influence soil processes. This study uses late summer and autumn samples to assess changes in litter and soil fauna under a 17-year-old Pinus radiata plantation on dune sands of the Waitarere-Hokio association after 7 years of spray irrigation of sewage effluent. Populations estimated included 12 groups of litter arthoropods, earthworms, enchytraeids, tardigrades, rotifers and nematodes. Three groups of litter arthropods (adult diptera, spiders and aphids) were significantly less abundant in the effluent-treated area than in the unirrigated control. Increased moisture content was apparently responsible for greatly increased populations of earthworms and nematodes in the effluent-treated area. Bacterial-feeding and predatory nematodes showed marked increases, while fungal-feeding nematodes declined significantly. The nematode maturity index (MI) was lower in all treated plots. After effluent had been withheld for 52 days, there were significant increases in isopods and Collembola. Rotifers were most abundant in the control area. It appears that effluent enhanced physical breakdown of the P. radiata litter. Movement of the decayed fragments into the upper layers of sand may have produced conditions unfavourable for enchytraeids and rotifers.