Below ground C and N transformation processes in perennial grass-clover mixtures

Summary

Leaching of N from grass-clover mixtures have been found in both organic and inorganic form. The environmental impact of this loss is however not well understood as leaching of organic C acting as energy source for denitrifying microorganisms could reduce the negative impact of N losses. In the present project the sources of N and C loss from perennial grass-clover swards were investigated in two field experiments using 14C and 15N tracers. The main findings of the project show that fresh plant material (< 1 year) does not contribute significantly to DOC leaching, whereas both fresh ryegrass and clover material was shown to contribute to DON and DIN losses.
Paper I: 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 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% (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 14C transferred from ryegrass to clover was 1.7% (±0.1, n=9) of the 14C-activity in the total aboveground plant 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 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 14C-allocation within the receiving species and the root turnover rate in the soil.
Paper II: The below ground C and N dynamics leading to carbon (C) and nitrogen (N) leaching from perennial ryegrass-clover mixtures are not well understood. A 16 months field experiment was conducted to investigate the fate of dual-labelled ryegrass and clover residues. Litterbags containing 14C- and 15N-labelled ryegrass or clover roots or leaves were inserted into the sward of a first production year ryegrass-clover mixture in early spring. The presence of 14C and 15N was monitored in harvested biomass, roots, soil, and pore water percolating from the plough layer. The large root biomass was found to increase during the study period, whereas small root biomass increased during the growth seasons and decreased in autumn and winter. The increase in large roots corresponded to a net build up of 33 kg N ha-1 during the 16 month experiment. For small roots a net build up of 91 kg N ha-1 was found; this including an off-season decrease of 19 kg N ha-1. Uptake of 15N resembled release patterns from residues in the litterbags, with 14C data showing that only a minor proportion of N was taken up in organic form. Total N leaching from the plough layer increased during autumn and winter, having a fairly constant content of dissolved organic nitrogen (DON) and an increasing content of dissolved inorganic nitrogen (DIN). The presence of 15N in pore water shifted from the DON fraction in autumn to the DIN fraction in late winter, with strong indications that 15N originated from ryegrass. This proportion, however, only constituted one percent of the total N being leached. Positive correlation between harvested clover biomass and total N being leached indicated that decaying clover could be a major source of the 10 kg N ha-1 being lost with pore water during autumn and winter.