Evaluating the decomposition of agricultural legume species to estimate nutrient recycling in Mediterranean climates

Legumes enhance the sustainability of Mediterranean farming systems by increasing soil nutrient levels, improving soil water and carbon retention, and reducing reliance on synthetic fertilisers through biological nitrogen fixation. However, the environmental and biological complexity of residue decomposition necessitates a clear understanding of how different crop residues decompose in synchrony with the nutrient requirements of subsequent cereal or oilseed crops. To address this, litterbag experiments were conducted at two distinct agroecological zones within the Western Australian Wheatbelt (Kendenup: 730 mm rainfall, Brown Gravelly Chromosol; Konnongorring: 350 mm rainfall, Grey Chromosol sandy duplex). Residues from nine legume species (grain legumes: Lupinus angustifolius, Vicia faba ; traditional pastures: Trifolium subterraneum, Vicia sativa, V. villosa ; novel ley legumes: Ornithopus sativus, Trigonella balansae, Trifolium spumosum ) and wheat ( Triticum aestivum ) were monitored from summer to winter (60 weeks) to assess decomposition patterns and associated C and N dynamics. Decomposition rates were significantly higher at the high-rainfall Kendenup site, with all species showing 70–85% biomass loss by 60 weeks compared to 50–75% at Konnongorring, reflecting moisture as the primary driver of decomposition. Across both sites, T. aestivum exhibited the slowest decomposition and highest N immobilisation (final C:N ~ 35), followed by grain legumes L. angustifolius and V. faba (final C:N 30–40). Vetch species ( V. sativa, V. villosa ) and T. spumosum decomposed most rapidly, achieving final C:N ratios of 11–19. Novel ley legumes O. sativus and Trig. balansae displayed intermediate decomposition rates and achieved a C:N ratio of 25–35, potentially optimising N release synchrony with subsequent crop demand. These findings provide the first comparative assessment of second-generation ley legumes ( O. sativus , Trig. balansae ) alongside traditional pasture and grain legumes under field conditions in Mediterranean environments. Results demonstrate that novel pasture legumes provided sustained N release, potentially optimising nutrient synchrony with subsequent cereal crops better than either fast-decomposing traditional pastures or low N fixer grain legume residues. This species-specific decomposition information addresses a critical knowledge gap in managing N cycling in evolving Australian farming systems, where increases in N harvest index and changes to ley systems have altered N dynamics. This study demonstrates that, while edaphoclimatic conditions were strongly associated with residue decomposition in the Mediterranean environments of Western Australia, species selection substantially influenced the timing of N release. Novel ley legumes offer intermediate decomposition patterns that may better balance availability and consistency of N release compared to traditional legume options, with potential implications for managing nutrient synchrony, cereal biomass, and grain quality (protein and grain size) in evolving rotational systems.