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

Summary

Carbon (C) and nitrogen (N) dynamics in ryegrass-clover mixtures were investigated in field mezotrons 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 14C- and 15N-enriched urea and the fate of the two tracers was studied for three months during summer. Transfer of 14C occurred mainly from ryegrass to clover, whereas the largest transfer of 15N was in the opposite direction. The average transfer of N from clover was 40% (±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 14C transferred from ryegrass to clover was 1.7% (±0.1, n=9) of the 14C-activity in the total aboveground biomass found in the unlabelled clover, and with a transfer from clover to ryegrass being 0.4% (± 0.1, n=9). 15N-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. 14C was found solely as 14CO2 in the pore water indicating that dissolved organic carbon (DOC) did not originate from fresh root deposits. Transfer of 14C between the two species in the mixed crop alongside with high transfer of 15N despite a large percolation of pore water indicates that part of the N transfer occurred either in the form of organic compounds or by direct transfer via mycorrhizal mycelium. 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 14C-allocation within the receiving species and the root turnover rate in the soil.