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

Abstract

Long-term application of wastewater adds large amounts of carbon (C), nitrogen (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 the top 0.75 m of an allophanic soil after 22 years irrigation with dairy factory effluent with that in a non-irrigated soil. Earthworm species, biomass and distribution, microbial biomass, microbial activity, and relative use of substrates were measured to evaluate the contribution of biological processes to cycling and redistribution of total C. Total C did not differ between irrigated and non- irrigated soil, although there was less total C in the 0–0.1 m layer and more total C at 0.1–0.5 m in the effluent-irrigated soil. 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. Measurements of relative use of substrates indicated that the greater microbial biomass in the effluent-irrigated soil was supported by the inputs of available C (particularly lactose) in the effluent rather than by greater decomposition of the organic C in the soil. Irrigation increased total N storage 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. Approximately 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, without detrimental effects on total C storage. Changes in nutrient distribution at the irrigated site can be partially attributed to leaching and the dominance (155 g/m² ) of the earthworm Aporrectodea longa, which forms permanent burrows to lower depths.