EFFECT OF IRRIGATION WITH SEWAGE EFFLUENT ON DECOMPOSITION OF LITTER IN PINUS-RADIATA FORESTS

Two Pinus radiata D. Don forests (aged 18 and 25 years) were spray-irrigated for 32 months with domestic sewage effluent from a secondary oxidation pond. Effluent was applied at rates of 25 and 50 mm week-1. Irrigation decreased litter organic matter by 43%-57% but there was no change in organic matter in the soil (0-50 mm). Annual loss-constants determined from a litter-bag study in one forest approached 1.0 in the irrigated plots, compared to 0.48 in the control. Irrigation increased the pH and the availability of P, Ca and Mg in the soil. Increased concentrations of N, P, Ca and Mg in irrigated litterbags were closely correlated with increases in the rate of decomposition of the organic matter. The effect of irrigation on decomposition rates was due, in part, to the maintenance throughout the year of a more suitable environment in the litter for decomposers, particularly during dry summer periods. © 1990.