Screening suitable legumes for living mulches to support nitrogen dynamics and weed control in a durum wheat-forage sorghum crop sequence

Relay intercropping of service legumes (living mulch) is a valuable agroecological practice to support nutrient availability, yield and non-chemical weed control in a crops sequence, if suitable legumes are chosen. This study tested the suitability of eight legume living mulches for relay intercropping in durum wheat (Triticum durum Desf.) evaluating their effects on the subsequent forage sorghum (Sorghum bicolor L. Moench) in a Mediterranean low-input cereal-based cropping system. Legumes include perennial (Medicago sativa, Medicago lupulina, Trifolium repens, Hedysarum coronarium), annual (Trifolium incarnatum, Trifolium resupinatum) and annual self-seeding (Trifolium subterraneum, Medicago polymorpha) species. A plot experiment repeating two times a wheat-sorghum crop sequence, was carried out in Central Italy to assess the effects of relay intercropping of legumes on agronomical and economic performance of the associated wheat and subsequent sorghum. The legumes were undersown in the already developed durum wheat in late winter. They were maintained after wheat harvest and used as green manure by biomass chopping and plowing in the following spring at sorghum sowing time. None of the intercropped legumes affected N uptake or grain protein content of the companion wheat with respect to the control (wheat sole crop). In the spring before sorghum sowing, N input from legume biomass residues was assessed and it sensibly changed according to legume species, ranging from 1.2 (T. incarnatum) to 182 kg ha−1 (H. coronarium). The legume treatments significantly affected the N uptake in the subsequent sorghum and it ranged from 20.2 to 172.6 kg ha−1. Without the use of additional external nitrogen input, average sorghum dry biomass production preceded by H. coronarium (13.3 t ha−1), T. repens (11.8 t ha−1), T. subterraneum (11.0 t ha−1) and M. sativa (9.3 t ha−1) was in line with the productive level under conventional conditions. Biomass residues of M. sativa significantly reduced the total weed biomass in sorghum by 65% compared with the control. In sorghum, preceding legumes such as T. resupinatum and T. incarnatum promoted dicotyledonous weed growth. Hedysarum coronarium and T. repens were the best legumes for relay intercropping in the low-input wheat-sorghum crop sequence under Mediterranean conditions of this study according to agronomic and economic evaluation. These legumes were able to increase soil nitrogen content allowing to significantly reduce external nitrogen fertilization while optimizing sorghum production. When gross income is calculated at cropping system level, most legumes provide a positive economic balance.