Rooting depth and water use of summer cover crops in a semi-arid cropping environment

Summer cover crops could add biomass (groundcover), fix nitrogen (legumes) and create biopores in Mediterranean-type cropping systems, but soil water use, especially from the subsoil, may reduce yields of the following winter cash crop. Research on rooting patterns/depth of summer cover crops and consequences for soil moisture storage in Mediterranean-type cropping systems is limited. To assess the rooting patterns and water use of a range of legume (sunn hemp, cowpea, mung bean, lablab), non-legume broadleaf (buckwheat, radish) and cereal (millet) summer cover crops in a Mediterranean-type environment, two field trials (2020 and 2021) and one glasshouse experiment were conducted. Root distribution, rooting depth, root diameter, water use and shoot biomass were evaluated. Millet produced the most biomass of any single species in the field (around 4 t ha-1) and had significantly higher water use efficiency (8.4 g L-1) than all legumes (4–6 g L-1). Millet and buckwheat exhibited dense, shallow root systems that exploited water in the profile surface, while legumes, particularly cowpea, had greater root density in the subsoil, resulting in greater subsoil water use. All legumes had significantly greater average root diameter (0.5–0.7 mm) below 40 cm than millet, buckwheat and radish (0.2–0.4 mm). Dual species mixes with millet or cowpea generally led to increased biomass production and increased rooting and water use in deeper soil layers compared to single species cover crops. The results suggest that millet may be preferable in seasons where the key objective of a summer cover crop is to provide ground cover with minimal water use, whereas in wetter seasons, legumes may be beneficial in drying the subsoil and potentially creating biopores. Ultimately, all cover crops significantly reduced stored soil water compared to a chemical fallow, which will likely affect cash crop yield in many seasons.