Fallow replacement cover crops impact soil water and nitrogen dynamics in a semi-arid sub-tropical environment

Conventional dryland cropping systems rely upon frequent and lengthy fallow periods to conserve soil water and mineral nitrogen to stabilize crop production. However, this is associated with depletion of soil organic matter and decreased fallow efficiency. Intensifying cropping systems by planting cover crops has been touted as a means to stem soil organic matter loss and improve fallow efficiency. We investigated whether the manipulation of cover crop functional trait diversity and sowing proportions (utilizing Poaceae, Fabaceae, and Brassicaceae) could provide complementary functions that improve soil water and mineral N management during fallow. Grass-legume mixtures represented the best compromise between biomass production (> 4000 kg DM ha(-1)), N retention (142 kg N ha(-1)), N supply via biological N fixation (35 kg N ha(-1)) and maintained an additional 70 mm of water at the end of fallow period. Regardless of functional trait type, cover crops increased N retention but maintained similar soil mineral N content at the end of fallow period. However, soil water effects were functional trait-specific, and there were significant soil water deficits with brassica-dominated cover crops. Soil water accumulation post cover crop termination was significantly higher in cover crops compared with conventional fallows, but the overall fallow efficiency was higher in the conventional fallow. This study demonstrates that cover crops are not universally beneficial, and careful selection of cover crop functional traits in mixtures could enhance fallow soil water and N management in semi-arid sub-tropical drylands.