Modelling the residual N effect of slurry applied to maize land on dairy farms in the Netherlands

The decomposition rate of soil organic N was estimated from data of a long-term field experiment and used in a simulation model. Subsequently, the model was used to estimate the effects of Dutch manuring practice on maize land. The time course of the N mineralization rate was estimated for three scenarios: (i) following actual manure applications which have declined with time (scenario A); (ii) assuming continuous applications in accordance with the present and anticipated legislation (scenario P); (iii) assuming annual applications of 200 kg mineral fertilizer N/ha only (scenario M). The actual mineralization rate (following scenario A) in 1995 was estimated at 23-31 kg N/ha higher than when manure had been applied at moderate rates (following scenario P). Corresponding estimates for the year 2005 were 18-19 kg N/ha per year. The calculations suggest that it may be difficult to maintain soil organic N pools with mineral fertilizer only. Consequently, the mineralization rate following scenario M decreased with time as did the yields of silage maize. The magnitude of the residual effect indicates that there is need and scope for fine tuning of N fertilizer recommendations. The simple model used seems suitable for exploring the magnitude of the residual effect of manures.