Differentiating between the supply of N to wheat from above and belowground residues of preceding crops of pea and canola

Belowground N comprises an important component of total residue N, but the N from this pool is often not included in studies that use N-15 approaches to trace residue-derived N in succeeding crops. The objective of this greenhouse study was to differentiate between N supplied from aboveground and belowground crop residues, including rhizodeposits, of pea and canola to wheat using N-15 labeling. A cross N-15-labeling approach was used in which wheat was grown on N-15-labeled belowground residues and non-labeled aboveground residues and vice versa. On average, the amount of N in wheat derived from belowground residues was almost twice the amount from aboveground residues. The higher input of both aboveground and belowground residue biomass and hence residue N from canola compared to pea tended to increase the residue-derived N in wheat from canola (6.4 %) relative to pea (4.7 %). However, differences in the percent recovery of N-15 based on the amount of residue-N-15 initially applied revealed that a higher proportion of belowground residue N from pea (13.4 %) was recovered in wheat compared to the corresponding aboveground residue N from pea (8.8 %), and both belowground and aboveground residue N from canola (6.5 and 7.3 %, respectively). The total supply of N to wheat from preceding pea and canola crops was relatively low, likely due to wide C/N ratios. This study demonstrates the importance of belowground residue N to the supply of N to succeeding crops as well as differences between species.