Influence of cover crops and their traits on the yield of main annual crops grown in tropical and subtropical environments - A meta-analysis

Context or problem: The sustainability of agriculture in tropical regions is threatened by climate change, soil degradation, and food insecurity. Crop diversification through growing cover crops is a promising strategy to mitigate these problems. However, their effects in rotation and as intercrops has not been comprehensibly assessed in the tropics, considering cultural practices, edapho-climatic conditions, and the functional traits of these cover crops. This study aims to conduct a comprehensive meta-analysis to simultaneously analyze, in rotation and intercropping systems, the effect of cover crops and their traits on the yield of main annual crops grown in tropical and subtropical regions. Methods: A meta-analysis was conducted based on 1788 field observations from 191 articles covering 29 tropical and subtropical countries and regions. The analysis accounted for both rotation and intercropping systems, while considering the variability in cultural practices, edapho-climatic conditions, and traits of cover crops. Results: The results showed that the cover crops increased yields of cereals, legumes, and fiber crops by 25 % in rotation and by 7 % in intercrops compared to monocrops. These effects varied greatly depending on the context. In rotation, yield gains were enhanced by crop residue incorporation, nitrogen application, bimodal tropical climates compared to unimodal ones, and altitude. In intercrops, positive effects were associated with the use of Fabaceae as cover crops, staggered sowing, altitude, temperature, and subtropical climates and volcanic soils (Andosols). Yield responses varied depending on the annual crop: in rotation, maize (290 %), soybeans (300 %), and cotton (240 %), relative yield gains were observed in low-yielding environments, while in intercrops only maize (360 %) showed gains under these conditions, and rice (- 30 %) suffered a decline. Analysis of cover crop traits revealed that in rotation systems, aboveground cover crops biomass, root diameter and root density explained part of the relative yield variability and were positively associated with relative yield. In intercrops, cover crops aboveground nitrogen mass was explanatory of annual crop yield. Conclusions: Fabaceae cover crops tend to perform better in intercropping, particularly in subtropical climates and in Andosols. In tropical crop rotations, conserving cover crop residues improves yields, further enhanced by nitrogen fertilization. Cover crops are particularly beneficial in low-yield contexts. Furthermore, the traits of cover crops are insufficiently documented, and their explanatory power remains limited, making the effects of cover crops on annual crop yields unclear. This highlights the importance of conducting further research in this area to gather additional evidence.