Cover crops increase belowground N retention substantially in corn cropping systems: Results from a 15 N residue swapping experiment

Background and aims Cover crops can potentially increase nitrogen (N) retention in agroecosystems. However, only a small fraction of the N released from cover crop residues may be transferred to the grain crop, and N losses to the surrounding environment can occur following termination. We applied 15 N tracer in fall in two years to plots of a cover crop field experiment to assess the cover crop effect on N retention and to quantify N transfer from cover crops to the subsequent corn crop. Methods By swapping 15 N-labelled and non-labeled cover crop shoots reciprocally between paired plots, 15 N from cover crop shoots and the belowground pool (soil and cover crop roots) were tracked separately. Both cover crop monocultures (oats or red clover) and mixtures (3-way or 10-way mixtures) were compared, and for winter-hardy species, we examined both fall and spring termination. Results At termination, cover crops increased 15 N retention by 2–3 times relative to the no cover crop control (20–60% of the added 15 N), with the majority retained belowground. At corn harvest, 25–50% of 15 N was recovered in soil, and less than 10% in corn. 15 N recovery for cover crop treatments was up to 25% higher than that of the no cover crop control, and spring termination of clover resulted in the greatest belowground 15 N retention. There were no significant differences in corn 15 N recovery between cover crop monocultures and mixtures. Conclusion Our results demonstrate the effectiveness of cover crops in increasing N retention in the belowground N pool.