Meta-Analysis of Cover Cropping Systems: The Effects of Cover Crops on Subsequent Cash Crop Yields and Nitrogen Contribution.

The integration of cover crops (CC) in cropping systems is recommended to retain post-harvest nutrients and minimize soil erosion, although the potential benefits of CC to crop productivity have been reported to vary across regions, soils, or under different farm practices. To address the variability in CC performance, we performed a meta-analysis of data from 139 field experiments published in peer-reviewed (67) or grey (20) literature to quantify (1) the effect of CC on cash crop yield (corn, soybean, cereals) under northern humid temperate climate, (2) the potential nitrogen (N) contribution of CC with the amount of N accumulated in the CC aboveground biomass (N_CC), and (3) the effective N contribution of CC to cash crop yield (compared to bare fallow) measured by the fertilizer equivalency (FE), the inorganic N credit and the apparent N credit. The overall effect of CC on cash crop yields was significant in corn and cereal production with 16% (n=1685) and 22% (n=493) yield increases, respectively. The overall effect of CC on corn yield was highly modulated by CC types with best performance attributed to legumes (21%) and mixes with legumes (16%). Grass CCs decreased (-4%) in corn yield, whereas non-legume broadleaf CCs had no effect. Yield increases lessened as corn N fertilization increased, although significant CC benefits to subsequent corn (9%) was still noticed at 120 and 60 kg applied N/ha in soils with low (<2%) and medium (2-5%) organic matter content (SOM), respectively. Above 5% SOM, corn yield increases were not significant. Mixes with legumes (MIX) or legumes alone presented the highest N_CC with 115 and 95 kg N/ha, respectively. The legumes FE averaged 86 kg N/ha [CI (95%)=69 to 100], significantly higher than the MIX with 57 kg N/ha [CI (95%)=39 to 79]. Termination timing and CC species significantly impacted FE.