Long-term nitrogen fertilizer replacement value of cattle manures applied to cut grassland

Manures supply nitrogen (N) to crops beyond the year of application. This N must be taken into account for agronomic and environmental reasons. From 2002 to 2006 we conducted a field experiment on a sandy soil in The Netherlands (52°03″N, 6°18″E) to better quantify this residual N effect. Treatments comprised different time series of mineral fertilizer N or cattle manures of different compositions, all applied at a rate of 300 kg total N ha⁻¹ year⁻¹, whilst compensating for differences in available potassium and phosphorus. Dry matter and N yields of cut grassland responded positively (P < 0.05) to both current manure applications and applications in previous years, whereas mineral fertilizer N affected yields in the year of application only. N yields could be reasonably well predicted with a simple N model, adopting an annual relative decomposition rate of the organic N in manure of 0.10–0.33 year⁻¹ during the year of application and 0.10 year⁻¹ in the following years. Subsequent model calculations indicated that the N fertilizer value (NFRV) of injected undigested cattle slurry rises from an observed 51–53% when slurry is applied for the first time, to approximately 70% after 7–10 yearly applications, whereas it took two to four decades of yearly applications to raise the NFRV of surface applied farm yard manure to a similar level from an initial value of 31%. Manures with a relatively high first year NFRV (e.g. anaerobically digested slurry) had a relatively small residual N effect, whereas manures with a low first year NFRV (e.g. farm yard manure) partly compensated for this by showing larger residual effects. Given the long manuring history of most agricultural systems, rethinking the fertilizer value of manure seems justified. The results also imply that the long term consequences of reduced N application rates may be underestimated if manuring histories are insufficiently taken into account.