Jim Rasmussen and Henning Høgh-Jensen Origin and composition of Dissolved Organic C and N from grass-clover mixtures Jim Rasmussen and Henning Høgh-Jensen Department of Agricultural Sciences The Royal Veterinary and Agricultural University Højbakkegårds Allé 30, DK-2630 Tåstrup, Denmark E-mail: jjr[a]kvl.dk Background: The C and N dynamics in perennial grass-clover mixtures are not fully understood although such mixtures dominate temperate grassland. The co-existence of clover and grass involves both competition for and transfer of nutrients between the species. The nutrients may originate from leaky root systems, from a rapid turnover of the fine root systems, or from degradation of more stabile organic material. The aim of the study is - to determine the origin of DOC and DON in grass- clover mixtures - to determine the compo- sition of DOC and DON from the species - to investigate the transfer of C and N between grass and clover Method: In a third year grass-clover ley at the KVL research fields, mezotrons were installed to a depths of 20, 40 and 60 cm in January 2003. In June 2004 suction cups will be installed underneath the mezotrons. In the middle of June leaf labeling of either grass or clover will be done using 15N- and 14C urea. At day 3, 7, 14 and 28 after the labeling samples from the leaves of grass and clover and of the percolating water will be taken. Samples will be analyzed for content of 15N and 14C. Attempts will be made to fractionate DOC and DON in the soil solution in classes according to molecular weight and proportion of aromatic compounds. After the last field sampling mezotrons will be excavated and the distribution of 15N and 14C between plant and soil compartments will be determined. 40 cm 20 cm 60 cm Suction cups Archived at http://orgprints.org/3542