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

Summary

Forecasting crop nitrogen (N) demand is important for maximising productivity and minimising 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, 1st and 2nd year ley, spring barley/pea undersown with ryegrass, oats undersown with ryegrass and fodder beets) with different management (grazed/cut) and manure type (slurry/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 was included. Type of animal manure did not affect the residual N effect. Crop rotations with grazed grassland had a residual effect that on average was 13% higher than the same rotation without grazing. Ploughing of grasslands clearly increased residual N effects for several years, but grassland age at ploughing was of little importance. Thus, the residual effect of 10-year-old grass-clover ley only marginally exceeded that of undersown grass-clover despite considerable difference in estimated accumulated N-surplus. The results indicate that organic N is easier to mineralise 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 but chemical measures may be difficult to implement in practical farming due to difficulty in representative sampling in systems characterised by huge spatial variability.