Tracing crop residue N into subsequent crops: Insight from long-term crop rotations that vary in diversity

Diversified grain crop rotations have shown yield advantages, but it is unclear if this is related to improved N supply. Crop residue-N supply could be an important driver of differences observed in contrasting crop rotations. To address this, we focused on a long-term field trial near Guelph Ontario that compares different grain crop rotations and tillage practices. We hypothesized that diversified crop rotations increase the transfer of residue-derived N to subsequent crops. From 2017 to 2018, we used a ¹⁵N technique to measure N turnover in a simple corn and soybean rotation compared to a diversified rotation where winter wheat and red clover were included. Although crop yields were greater in the diverse vs. simple rotation, the long-term rotation had no impact on crop residue N turnover to the subsequent crop—a finding that did not support our hypothesis. The recovery of above- plus below-ground residual-N by the subsequent crops varied by crop type and sequence: 9.7–13.5% of corn residue-N was recovered in the soybean seed, 26 % of soybean residue-N was recovered in the corn grain, and 30 % of winter wheat/red clover residue-N was recovered in the corn grain. Below-ground residue-N recoveries in subsequent grain or seed were 4- to 10-times greater than above-ground residues; in terms of the total contribution, below-ground residue-N supplied 8- to 18-times more N than above-ground residues. Indigenous soil N or fixed N were principal contributors to crop N uptake; more so than the crop residues or fertilizer combined. At this site, an accumulated legacy of N fertilizer applications is likely responsible for building a soil N reservoir for crop production, explaining yield benefits observed by diversifying the rotation—but also masking any effect that diversification may have on residue-N turnover and subsequent crop N uptake. To improve N use efficiency, diversifying crop rotations should be accompanied with reduced N fertilizer applications, and better account for below-ground residual N pools as well as legume N inputs.