J.J. Schröder

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 300xa0kg 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 (Pu2009<u20090.05) 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 0.10–0.33xa0year−1 during the year of application and 0.10xa0year−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 7–10xa0yearly 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.

Effects of nitrification inhibitors and time and rate of slurry and fertilizer N application on silage maize yield and losses to the environment

Field experiments with silage maize during eight years on a sandy soil in The Netherlands, showed that dicyandiamide (DCD) addition to autumn-applied cattle slurry retarded nitrification, thus reducing nitrate losses during winter. Spring-applied slurry without DCD, however, was on average associated with even lower losses and higher maize dry matter yields.Economically optimum supplies of mineral N in the upper 0.6 m soil layer in spring (EOSMN), amounted to 130–220 kg ha−1. Year to year variation of EOSMN could not be attributed to crop demand only. According to balance sheet calculations on control plots, apparent N mineralization between years varied from 0.36 to 0.94 kg ha−1 d−1. On average, forty percent of the soil mineral N (SMN) supply in spring, was lost during the growing season. Hence, the amounts of residual soil mineral N (RSMN) were lower than expected. Multiple regression with SMN in spring, N crop uptake and cumulative rainfall as explanatory variables, could account for 79 percent of the variation in RSMN.Postponement of slurry applications to spring and limiting N inputs to economically optimum rates, were insufficient measures to keep the nitrate concentration in groundwater below the EC level for drinking water.