Residual nitrogen effect of a dairy crop rotation as influenced by grass-clover ley management, manure type and age

Forecasting crop nitrogen (N) demand is important for maximizing productivity and minimizing losses to the environment, and includes taking into account residual effects. The residual N effect was estimated in a dairy crop rotation (spring barley undersown with grass-clover, first and second year ley, spring barley/peas undersown with ryegrass, oats undersown with ryegrass and fodder beet) with different management (grazed or cut) and manure type (slurry or deep litter) by anaerobic incubation and plant N-uptake in a pot experiment and in the field. For comparison a 10-year-old grass-clover ley was included. Type of animal manure did not affect the residual N effect. Crop rotations with grazed grassland had a residual N effect that on average was 13% higher than the same rotation without grazing. Ploughing of grassland clearly increased residual N effects for several years, but age of grassland at ploughing was of little importance. Thus, the residual N effect of 10-year-old grass-clover ley only marginally exceeded that of undersown grass-clover, despite considerable difference in estimated N-surplus. The results indicate that organic N is easier to mineralize the more recently it has been formed. Good correlations existed between soil inorganic N in the spring, N released during anaerobic incubation, and plant-available N. However, chemical analyses may be difficult to implement in practical farming due to difficulty of achieving representative samples in systems characterized by huge spatial variability.