Effect of white clover cultivar on apparent transfer of nitrogen from clover to grass and estimation of relative turnover rates of nitrogen in roots

The apparent transfer of N from clover to associated grass was evaluated over a four year period both on the basis of harvested herbage and by taking account of changes in N in stubble and root (to 10 cm depth) in swards with perennial ryegrass and three different white clover cultivars differing in leaf size. The large leaved Aran transferred 15% of its nitrogen while Huia transferred 24% and the small leaved Kent Wild White transferred 34%. When changes in stubble and root N were taken into account the percentage of N transferred was calculated to be 5% less than in harvested herbage only, as the small leaved types had proportionately more N in the roots and stolons, but the large leaved type was probably more competitive towards the grass. Loss of N from clover roots from July to October was compared to that from grass roots in a grass/white clover sward continuously stocked with steers using a method which incorporated tissue turnover and N-15 dilution techniques. Less than 1 mg N m(-2) d(-1) was lost from the grass roots. In contrast 8 mg m(-2) d(-1) were estimated to be lost from clover roots while 12 mg N m(-2) d(-1) were assimilated. It is concluded that clover cultivar and competitive ability on grass have to be taken into account together with the relationship between N turnover in roots and N available for grass growth when modelling N transfer in grass/clover associations.