Functional root trait based classification of cover crops to improve soil physical properties

Cover crop use is a well-established soil conservation technique and has been proven effective for erosion control and soil remediation in many arable systems. Whereas the obvious protection mechanism of cover crops occurs through the canopy, plant roots perform multiple functions. It is important to consider the soil functions delivered by different root systems in order to increase the uptake of cover crops for sustainable soil and water management. A classification of cover crop root systems up to 0.6 m deep based on functional traits will allow us to better study their potential role in soil bio-engineering; soil structural improvements for hydrological services and soil resource protection. A greenhouse experiment, using large 1 m3 containers filled with loam soil, loose top and compacted subsoil, in which 7 cover crop species (Oat, Rye, Buckwheat, Vetch, Radish, Mustard, Phacelia) were grown for 90 days. Root cores were taken at the end of the experiment, washed and imaged to determine root traits (total root length density, average root diameter, root specific length and root surface area) for both the top and subsoil layers. Root identity was determined from a distinctive combination of single root traits and related to 3 soil functional variables, representing soil structural improvement, runoff mitigation and erosion control. The results showed that total root length and root surface area correlate well with aggregate stability and soil macroporosity. Buckwheat, Mustard and Rye had significantly greater aggregate stability as well as 10, 8 and 7 % greater macroporosity respectively, at the interface with the compacted layer when compared to the control bare soil. Furthermore, average root diameter negatively correlated with soil macroporosity, indicating that cover crop with a fine root system are more beneficial for creating pore-space than those with thicker taproots. Selecting cover crop species with the right root traits is therefore crucial to improve soil health.