Legume Cover Crops Enhance Nutrient Cycling and Biomass Production in a Lettuce–Lettuce–Oat Cropping Sequence in a Pot Experiment

This study evaluates the performance of seven cover crop species, five legumes, one grass, and one brassica, and their residual effects on a lettuce–lettuce–oat rotation. A completely randomised design with eight treatments and three replicates was established in 12 L pots to assess biomass production, nutrient accumulation, and the effects of cover crops on subsequent crops and soil properties. Legumes produced significantly higher biomass (97.0–152.9 g pot−1) than non-legumes (7.5–11.1 g pot−1). Nitrogen (N) accumulation was also significantly higher in legumes (2.2–3.1 g pot−1) than in non-legumes (0.1 g pot−1). More than 96% of the N accumulated by legumes was derived from the atmosphere, as estimated using the difference method, highlighting the importance of atmospheric N fixation to legume N accumulation under the conditions of this experiment. Differences among legume species were also observed. Although legumes showed lower concentrations of phosphorus (P) and other nutrients in plant tissues, this was attributed to a dilution effect associated with greater biomass production rather than restricted nutrient uptake. Cover crops had marked effects on subsequent cash crops. Crops grown after legumes showed significantly higher biomass, N status, and photosynthetic performance than those following non-legumes or the control. These effects were strongest in the first crop but persisted throughout the rotation. In contrast, non-legumes did not differ from the control. N recovery proved to be a sensitive indicator of these differences, confirming the key role of legumes in supplying N through residue mineralisation. Legumes also influenced soil properties by increasing nitrate availability and promoting slight soil acidification, potentially associated with enhanced nitrification. Overall, the results highlight the importance of species selection in cover cropping, with legumes playing a central role in improving subsequent crop productivity.