Irrigation of an allophanic soil with dairy factory effluent for 22 years:responses of nutrient storage and soil biota

Long-term application of wastewater adds large amounts of carbon (C), nitro gen (N), and phosphorus (P) to soils, and their effects on soil quality are not fully known. We compared the distribution of C, N, P, and Olsen P in t he top 0.75 m of an allophanic soil after 22 years irrigation with dairy fa ctory effluent with that in a non-irrigated soil. Earthworm species, biomas s and distribution, microbial biomass, microbial activity, and relative use of substrates were measured to evaluate the contribution of biological pro cesses to cycling and redistribution of total C. Total C did not differ bet ween irrigated and nonirrigated soil, although there was less total C in th e 0-0.1 m layer and more total C at 0.1-0.5 m in the effluent-irrigated soi l. Microbial biomass C and basal respiration activity were increased by 4- and 1.6-fold, respectively, in the 0-0.1 m layer of the irrigated soil. Mea surements of relative use of substrates indicated that the greater microbia l biomass in the effluent-irrigated soil was supported by the inputs of ava ilable C (particularly lactose) in the effluent rather than by greater deco mposition of the organic C in the soil. Irrigation increased total N storag e by 2.1 t/ha and total P was increased by 11.5 t/ha. Most of the increase in total N occurred in the 0.1-0.5 m layers, whereas total P was greater at all depths. Olsen P also increased at all depths by 1.3- to 23-fold. Appro ximately 8% of the N and 91% of the P applied during the past 22 years was stored in the 0-0.75 m layer of the profile, with the potential for further P storage. Effluent irrigation increased the total soil nutrient stores, w ithout detrimental effects on total C storage. Changes in nutrient distribu tion at the irrigated site can be partially attributed to leaching and the dominance (155 g/m(2)) of the earthworm Aporrectodea longa, which forms per manent burrows to lower depths.