Nitrogen Cycling Assessment from 15N Labelled Above and below Ground Biomass of Different Forages and Its Impact on Subsequent Potato Yield and Quality.

Nitrogen contribution of the above and belowground biomass of different forage residues to the subsequent crop still needs elucidation. This study used a ¹⁵N isotopic crop residue exchange technique to trace the fate of residues N from red clover (RC), timothy (T), and a mixture of red clover and timothy (M) in a three year potato rotation. This experiment was established at Harrington Research Station, Prince Edward Island (PE), Canada in hollow metal cylinders installed in the field. Forages were seeded in spring 2013, and ¹⁵N labelled fertilizer (¹⁵NH₄¹⁵NO₃, 98 atom % ¹⁵N) was applied in spring 2014. In fall 2014, forage residues were exchanged to achieve a total of four treatments; i). Labeled ¹⁵N vines with unlabeled belowground residues (AG); ii). Labeled ¹⁵N belowground residues (roots and soil) with unlabeled aboveground residues (BG); iii) Labeled ¹⁵N aboveground residues only (AG_only) and; iv) Labeled ¹⁵N roots only (BG_root). One potato plant was seeded in each cylinder in spring 2015. The recovery of ¹⁵N fertilizer in forage crops ranged from 31.58% to 50.06% (RC < M <T) in above ground biomass, whereas it averaged 9.6% in roots, representing around 19 % of total plant ¹⁵N recovery. Total potato dry matter (tuber, vine, root) was proportional to the total biomass incorporated from RC and M (BG_root≈AG_only<BG≈AG), and the opposite was observed for T. Mean whole potato plant ¹⁵N recovery ranged from 0.64% to 4.12%, and there were no significant treatment effects, but a significant crop effect was observed with¹⁵N recovery from both RC and M being approximately 246% higher than from T. The percentage of ¹⁵N derived from forage residues (% Ndfr) was lowest from T from BG_root with 1.57% and up to 66.82% from RC below ground. The percentage of ¹⁵N derived from forage roots in potato phase represented between 29-40% of the %Ndfr from AG residues. Initial results suggest that though root residues contributed somewhat to subsequent potato N nutrition, much of the labeled residue N remained in the soil. Further studies are needed to elucidate the timing of N released from BG forage residues to subsequent crops.