Effect of regular irrigation with dairy farm effluent on soil organic matter and soil microbial biomass

Over recent years regulatory authorities in New Zealand have promoted irrig ation of dairy farm effluent (DFE) onto the land, to protect surface water quality. The rate at which the resistant organic matter from DFE accumulate s in the soil and the effect of any accumulation on other soil organic matt er (SOM) related pools, such as microbial biomass, are, however, unknown. T his information is necessary to determine the long-term impact and sustaina bility of land-applied DFE. In this paper we report on changes over 4 years in organic carbon (C-org) a nd total nitrogen (N-t) from a soil receiving a high application rate of DF E. Soil microbial biomass (C-mic) measurements were also included to test t he hypothesis that C-mic or the C-mic/C-org ratio can be used as an early i ndicator of changes in SOM. The regular irrigation with DFE at the high rates used in this study increa sed the C-mic, pH, C-org, and N-t of the soil receiving the effluent. The t ime series of C-mic showed that this measurement is suitable as an early in dicator of changes in C-org and N-t, whereas a single determination of the C-mic/C-org ratio was not. The sustainability of DFE application onto land in terms of N leaching can be maintained only when the supply of inorganic N is continually matched by the demand of the pasture. This means that inorganic N fertilisation has t o be reduced concurrently with the gradually increasing N mineralisation fr om the accumulating organic matter.