Immobilisation, remineralisation and residual effects in subsequent crops of dairy cattle slurry nitrogen compared to mineral fertiliser nitrogen

About 50-60% of dairy cattle slurry nitrogen is ammonium N. Part of the ammonium N in cattle slurry is immobilised due to microbial decomposition of organic matter in the slurry after application to soil. The immobilisation and the remineralisation influence the fertiliser value of slurry N and the amount of organic N that is retained in soil. The immobilisation and the remineralisation of N-15-labelled dairy cattle slurry NH4-N were studied through three growing seasons after spring application under temperate conditions. Effects of slurry distribution (mixing, layer incorporation, injection, surface-banding) and extra litter straw in the slurry on the plant utilisation of labelled NH4-N from slurry were studied and compared to the utilisation of N-15-labelled mineral ferriliser. The initial immobilisation of slurry N was influenced by the slurry distribution in soil. More N was immunobilised when the slurry was mixed with soil. Surface-banding of slurry resulted it) significant volatilisation losses and less residual N-15 in soil. Much more N was immobilised after slurry incorporation than after mineral fertiliser application. After 2.5 years the recovery of labelled N in soil (0-25 cm) was 46% for slurry mixed with soil. 42%, for injected slum; 22% for surface-banded slurry and 24% for mineral fertiliser N. The total N uptake in a ryegrass cover crop was 5-10 kg N/ha higher in the autumn after spring-application of cattle slurry (100-120 kg NH4-N/ha ) compared to the mineral fertiliser N reference, but the immobilised slurry N (labelled NI) only contributed little to the extra N uptake in the autumn. Even in the second autumn after slurry application there was an extra N uptake in the cover crop (0-10 kg N/ha). The residual effect of the cattle slurry on spring barley N uptake was insignificant in the year after slurry application (equivalent to 3% of total slurry IN). Eighteen months after application. 13% of the residual N-15 in soil was found in microbial biomass whether it derived from slurry or mineral fertiliser. but the remineralisation rate (% crop removal of residual N-15) was higher for fertiliser- than for slurry-derived N, except after surface-banding. Extra litter straw in the slurry had a negligible influence on the residual N effects in the year after application. It is concluded that a significant part of the organic N retained in soil after cattle slurry application is derived from immobilised ammonium N, but already a few months after application immobilised N is stabilised and only slowly released. The immobilised N has negligible influence on the residual N effect of cattle slum, in the first years after slurry application, and mainly contributes to the long-term accumulation of organic N in Soil together with part of the organic slurry N. Under humid temperate conditions the residual N effects of the manure can only be optimally utilised when soil is also covered by plants in the autumn, because a significant part of the residual N is released in the autumn, and there is a higher risk of N leaching losses on soils that receive cattle slurry regularly compared to soils receiving only mineral N fertilisers.