Use of a [sup.15][N.sub.2] labelling technique to estimate exudation by white clover and transfer to companion ryegrass of symbiotically fixed N

Backgrounds and aims N rhizodeposition by legumes leads to enrichment of N in soils and in companion plants. N rhizodeposition can be divided into two major components, root exudation and root senescence. Our aim was to quantify N root exudation in white clover (Trifolium repens L.) through an estimation of short-term N rhizodeposition and to assess its impact on N transfer to companion perennial ryegrass (Lolium perenne L.) grown in mixture with clover. Method [sup.15][N.sub.2] provided in the root atmosphere for 3 days was used to estimate transfer of symbiotically fixed nitrogen (SFN) to the growing medium by clover grown in pure stand and to ryegrass by clover grown in mixture for 2 months. Results The proportion of N rhizodeposited over the 3 days increased from 3.5% of SFN in pure stand to 5.3% in mixture. The [sup.15]N-enrichment of ammonium from the adhering substrate shows that a part of the rhizodeposited N was released in the form of ammonium. 4% of the rhizodeposited N was taken up by ryegrass during the labelling period. Conclusions This study showed a significant contribution of root N exudation to the total N rhizodeposition of legumes and in the transfer of N to grasses. Keywords Rhizodeposition * Nitrogen * Clover * Ryegrass * Transfer * Ammonium