In short term, cover crops can counteract N leakage after the main crop, reduce erosion, increase evaporation, etc. In the long term, the conservation and build up of hummus and organic C-level leads to an increased soil life and a better soil structure and mineral management. This all helps to increase the yield of the crops following. In addition, cover crops have the property of fixating N, leading to a lower amount of manure needed and less N will be leached and pollute nearby water courses.
The analysis of Adalla et al. (2019) demonstrated that cover crops significantly (p < 0.001) decreased N leaching and significantly (p < 0.001) increased SOC (soil organic carbon) sequestration without having significant (p > 0.05) effects on direct N2O emissions. Cover crops could mitigate the NGHGB (net GHG balance) by 2.06 ± 2.10 Mg CO2‐eq ha−1 year−1. One of the potential disadvantages of cover crops identified was the reduction in grain yield of the primary crop by ≈4%, compared to the control treatment. This drawback could be avoided by selecting mixed cover crops with a range of legumes and non‐legumes, which increased the yield by ≈13%. These advantages of cover crops justify their widespread adoption.
However, to increase the effectiveness of cover crops, field management techniques should be optimized to the local climatic conditions, water resources, soil and cropping systems. The genetics of cover crop species could be improved to provide deeper rooted crops, which have higher N use efficiencies, better nitrate scavenging abilities and lower N leaching potential. Deep rooted species could help with cover crop resilience, for example deeper delivery of C in the soil profile. It is also important to adjust timings and dates of the planting and kill of the cover crops, to avoid competition with the primary crop, to improve their effectiveness and avoid trying to establish cover crops when soil conditions are sub-optimal
Cover cropping can comprise a single species or a mixture of species and can use annual, biennial or perennial vegetation. Cover crops can be killed (or ploughed‐in) in winter or spring, or grazed, and incorporated in soils by tillage to prevent competition with the primary crop, and to promote mineralization of organic N. They can also be left on the soil surface over the fall and winter periods, until a primary crop in no‐till is planted, to provide weed control and N inputs.
Different types of cover crops:
- Legumes (e.g. alfaalfa, vetches and clover)
- Non-legumes (e.g. spinach, canola and flax)
- Grasses (e.g. ryegrass and barley)
- Brassicas (e.g. radishes and turnips)
- Direct: evaporation, protection against water/wind erosion, inhibition of weed development, organic nematode control
- Indirect: stable organic matter (hummus) for structure and fertility of the soil