Increasing cover crop diversity has the potential to improve the resilience of agricultural systems to the extreme weather events associated with climate change. While the advantages of diverse cover crops for crop productivity are well-documented, their role in soil stabilisation remains less explored. Research on how different species contribute to soil binding through rhizosheath development is limited, and it is unclear whether multi-species cover crops provide added benefits for soil and water management.
Firstly, a mesocosm experiment assessed the rhizosheath formation capacity of four common grass and brassica cover crop species in the UK under droughted vs well-watered conditions, with a second mesocosm experiment assessing the diversity effect by mixing Secale cereale and Brassica juncea. Consistent across both experiments, water availability had no effect on specific rhizosheath mass and grasses had a greater soil binding capacity than brassicas. S. cereale maintained its soil binding capacity when grown in a mixture, suppressing the soil binding capacity of B. juncea.
A winter cover crop field trial then assessed the impact of cover crop diversity and associated root diversity on soil erodibility by conducting overland flow simulations on plots containing S. cereale, B. juncea and Vicia faba as monocultures and in all possible species combinations. Individual species identity, rather than species diversity, determined rhizosheath mass, with treatments containing S. cereale having 50% more rhizosheath mass per unit of root dry mass than others. Presence of above-ground cover crop biomass delayed runoff, but neither rhizosheath mass nor species diversity affected soil erodibility.
Finally, rhizosheath persistence in cover crops of varying diversity over time was assessed in the field. While overall rhizosheath mass decreased with plant age, S. cereale maintained the largest rhizosheath out of the three species regardless of time, diversity level or species combination and outcompeted B. juncea and V. faba. Root morphological traits in S. cereale, such as its fibrous rooting system and long, dense root hairs, make this cover crop exceptionally effective at binding soil. Root hair length and density explained only moderate to weak variation in specific rhizosheath mass, therefore other factors like mucilage production are likely to be important. The ability of S. cereale to extend and fill the space equally regardless of sowing rate coupled with its great soil binding capacity, makes this species an excellent choice to include in a cover cropping system for soil retention.