MODELING THE RESIDUAL N-EFFECT OF SLURRY APPLIED TO MAIZE LAND ON DAIRY FARMS IN THE NETHERLANDS

Quantification of the residual N effect of manuring is required to max imize the financial returns of farming systems and to avoid contaminat ion of the environment. This is of special concern to maize land in Th e Netherlands, since it has been used for manure disposal during more than 25 years. The decomposition rate of soil organic N was estimated from data of a long-term field experiment and used in a simulation mod el. Subsequently, the model was used to estimate the effects of Dutch manuring practice on maize land. The time course of the nitrogen (N) m ineralization rate was estimated for three scenarios: i) following act ual manure applications which have declined with time (A scenario); ii ) assuming continuous applications in accordance with the present and anticipated legislation (P scenario); iii) assuming applications of 20 0 kg mineral fertilizer N ha(-1) yr(-1) only (M scenario). We estimate d that the actual mineralization rate (following the A scenario) in 19 95 was 23-31 kg N ha(-1) yr(-1) higher than when manure had been appli ed at moderate rates (following the P scenario). Corresponding estimat es for the year 2005 still amounted to 18-19 kg N ha(-1) yr(-1). Our c alculations suggest that it may be difficult to maintain soil organic N pools with mineral fertilizer only. Consequently, the mineralization rate following the M scenario decreased with time as did the yields o f silage maize; The magnitude of the residual effect found in the pres ent study, indicates that there is need and scope for fine tuning of N fertilizer recommendations. The simple model used in this paper seems a suitable tool to explore the magnitude of the residual effect of ma nuring.