In situ carbon and nitrogen dynamics in ryegrass-clover mixtures: Transfers, deposition and leaching

Carbon (C) and nitrogen (N) dynamics in a third production year ryegrass-clover mixture were investigated in the field. Cylinders (diameter 29.7 cm) were installed to depths of 20, 40 and 60 cm and equipped with suction cups to collect percolating pore water. Ryegrass and clover leaves were cross-labelled with C-14- and N-15-enriched urea and the fate of the two tracers was studied for 3 months during summer. Transfer of C-14 occurred mainly from ryegrass to clover, whereas the largest transfer of N-15 was in the opposite direction. The average transfer of N from clover was 40% (SE +/- 3.1, n = 9) of N in ryegrass, whereas the fraction of N in clover donated by ryegrass was 5% (+/- 1.2, n = 9). The amount of C-14 transferred from ryegrass to clover was 1.7% (+/- 0.1, n = 9) of the C-14-activity in the total above-ground plant biomass found in the unlabelled clover and with a transfer from clover to ryegrass being 0.4% ( 0.1, n = 9). N-15-enriched compounds were not detected in percolating pore water, which may be caused by either dilution from irrigation or low availability of leachable N compounds. C-14 was found solely as (CO2)-C-14 in the pore water indicating that dissolved organic carbon (DOC) did not originate from fresh root deposits. Transfer of C-14 between the two species in the mixed crop alongside with high transfer of N-15 despite a large percolation of pore water indicates that part of the N transfer occurred in non-leachable N-forms. The amount of N transferred between the two species was found to depend on the ratio between dry matter accumulated in the donating and receiving species, the C-14-allocation within the receiving species and the root turnover rate in the soil. (c) 2006 Elsevier Ltd. All rights reserved.