Self-regenerating black medic cover crop provides agronomic benefits at low nitrogen

Abstract

A black medic (Medicago lupulina L.) cover crop is able to regenerate from seed annually and produce biomass at the end of each growing season, but its long-term effectiveness on crop productivity, water use efficiency (WUE), and soil nutrient status within a no-till cropping system is unclear. A field experiment was established in 2003 in Saskatchewan in a 3-yr crop rotation [flax (Linum usitatissimum L.)–oat (Avena sativa L.)–wheat (Triticum aestivum L.)]. Treatments included cover crop (black medic or no medic), and N fertilizer (20, 60, and 100% of recommended N) arranged in a split-split plot design. Over 10 yr, medic aboveground fall biomass averaged 625 kg ha–1 (range 0–1,868 kg ha–1) and was greatest at the lowest N rate, 879 kg ha–1. Medic increased grain yield at 20% N fertilizer; no effect was observed at higher N rates. Medic increased tiller density and kernel weight at 20% N, indicating that medic positively influenced the crop throughout the entire life cycle. Medic presence did not affect grain N or P status. Medic did not affect level of available soil N in fall but consistently increased level of soil available P. This extended to the 30–60 cm soil depth in the 100% vs. lower N rates, suggesting medic roots may have influenced P cycling. In oat stubble, medic increased spring soil water and WUE. In conclusion, black medic improved crop productivity at the low N rate but improved available soil P at all N rates, warranting further research.