The enduring effects of sowing legume-rich mixtures on the soil microbial community and soil carbon in semi-arid wood pastures.

[Aims] Agroforestry systems and legume sowing have been reported as efficient in terms of soil function, gaining momentum as environmentally sustainable agricultural practices. This paper examines how the soil microbial community of Mediterranean pastures responds to legume-enrichment.
[Methods] We used phospholipid fatty acids profiling (PLFA) to measure microbial community composition, community-level physiological profiling (CLPP) to measure their activity and functional diversity, and analysed soil labile and bulk organic carbon in four wood pastures. Each site included a triplet of treatments composed of a native pasture (Control) and two native pastures enriched with a legume-rich mixture sowed ≤ 7 (young) and ≥ 11 (old) years ago, and two habitats (under tree canopy vs open pasture). The sites were fertilized with varying amounts of phosphorous.
[Results] We found large variations among legume-plot ages and habitats, with microbial biomass, diversity and CLPP-activity higher in the legume-plots than in the control plots, and under tree than in open pastures. The community shift after sowing was driven by increased abundance of gram-negative bacteria and arbuscular mycorrhizal fungi, and decreased saprophytic fungi. Soil bulk and labile C stocks steadily increased after the sowing, and were related to legume abundance, P-fertilization and forage yield, both directly and indirectly, through the positive effect on microbial biomass and CLPP-activity.
[Conclusions] The shift in microbial community structure together with the increased soil C stocks, soil P and likely N fixed by legumes form a positive legacy that can help to enhance the productivity and climate-resilience of pastures, although this needs further research.