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 irrigation of dairy farm effluent (DFE) onto the land, to protect surface water quality. The rate at which the resistant organic matter from DFE accumulates in the soil and the effect of any accumulation on other soil organic matter (SOM) related pools, such as microbial biomass, are, however, unknown. This information is necessary to determine the long-term impact and sustainability of land-applied DFE. In this paper we report on changes over 4 years in organic carbon (C-org) and total nitrogen (N-t) from a soil receiving a high application rate of DFE. Soil microbial biomass (C-mic) measurements were also included to test the hypothesis that C-mic or the C-mic/C-org ratio can be used as an early indicator of changes in SOM. The regular irrigation with DFE at the high rates used in this study increased the C-mic, pH, C-org, and N-t of the soil receiving the effluent. The time series of C-mic showed that this measurement is suitable as an early indicator 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 to be reduced concurrently with the gradually increasing N mineralisation from the accumulating organic matter.