Forage legume impact on soil fertility and N balance

Summary

Dairy production systems in Europe are to a large extent based on ley-arable rotations. In the ley phase of such rotations nitrogen accumulation occurs as a result of (1) organic carbon accumulation in soil not disturbed by tillage operations and (2) a considerable nitrogen surplus in grasslands, particularly under grazing regimes where a large part of the N in ingested grass is recycled to soil via urine and faeces. The accumulation of N and C in grasslands starts soon after establishment, the rate asymptotically declining with age and depends on practices such as fertiliser level, animal feed composition, stocking density, length of grazing and the botanical composition of the sward. In these pasture systems, key perennial legumes are white clover (Trifolium repens L.) red clover (Trifolium pratense L.) and lucerne (Medicago sativa). They are used because of their valuable contribution to production, feed quality and N inputs via biological fixation of atmospheric N2 (Ledgard et al., 2010)
Grassland cultivation almost always results in a substantial residual effect and the mineralization of N often exceeds the requirement of the succeeding crop. Thus, there is a high risk of nitrate leaching following sward cultivation. Management practices to control nitrate losses include delayed ploughing until late winter or spring, the use of efficient catch crops after ploughing and a reduction in fertilizer N application to cereals after ploughing.
The objective of this paper is to illustrate by examples the importance of management for N fertility building and efficient utilization in crop rotations containing forage legumes.