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

Background and aimsCover 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 N-15 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.MethodsBy swapping N-15-labelled and non-labeled cover crop shoots reciprocally between paired plots, N-15 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.ResultsAt termination, cover crops increased N-15 retention by 2-3 times relative to the no cover crop control (20-60% of the added N-15), with the majority retained belowground. At corn harvest, 25-50% of N-15 was recovered in soil, and less than 10% in corn. N-15 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 N-15 retention. There were no significant differences in corn N-15 recovery between cover crop monocultures and mixtures.ConclusionOur results demonstrate the effectiveness of cover crops in increasing N retention in the belowground N pool.

Automated summary

The study found that cover crops can effectively increase nitrogen retention, especially in the belowground N pool. In the cover crop treatments, 25% more nitrogen was recovered compared to the control group, and spring termination of red clover resulted in the highest belowground nitrogen retention.