Fate of ¹⁵N and ¹⁴C from labelled plant material: Recovery in perennial ryegrass-clover mixtures and in pore water of the sward

The belowground C and N dynamics leading to organic and inorganic N leaching from perennial ryegrass–clover mixtures are not well understood. Based on the hypothesis that four different plant materials would degrade differently, a 16 months field experiment was conducted to determine (i) the source strength of labelled plant residues in dissolved inorganic N (DIN) and dissolved organic N (DON) in pore water from the plough layer, and (ii) the plant uptake of organically bound N. Litterbags containing ¹⁴C- and ¹⁵N-labelled ryegrass or clover roots or leaves were inserted into the sward of a ryegrass–clover mixture in early spring. The fate of the released ¹⁴C and ¹⁵N was monitored in harvested biomass, roots, soil, and pore water percolating from the plough layer. No evidence of plant uptake of dual-labelled organic compounds from the dual-labelled residues could be observed. N in pore water from the plough layer during autumn and winter had a constant content of dissolved organic N (DON) and an increasing content of dissolved inorganic N (DIN). A positive correlation between aboveground clover biomass harvested in the growth season and total-N in pore water indicated that decaying roots from the living clover could be a major source of the 10 kg N ha⁻¹ being lost with pore water during autumn and winter. 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 the living ryegrass. However, ¹⁵N in pore water originating from plant residues only constituted 1.5% of the total dissolved N from the plough layer.