Forbs differentially affect soil microbial community composition and functions in unfertilized ryegrass-red clover leys

Increasing plant diversity in agroecosystems is proposed to enhance multiple ecosystem services. Adding selected forbs such as caraway (Carum carvi L.) and plantain (Plantago lanceolata L.) to productive grass-clover mixtures can further enhance forage yields, root biomass, uptake of mineral nutrients and improve animal performance. Yet, it remains unclear whether and how adding these forbs to grass-clover mixtures can influence soil microbial communities and associated soil carbon (C) and nitrogen (N) cycling. Based on a three-year grassland experiment varying in species diversity and composition with and without fertilizer application, we determined soil microbial community composition and functions related to C and N cycling under laboratory incubations. Results showed that inclusion of caraway modified soil microbial community composition by enhancing fungal-to-bacterial phospholipid fatty acids of the ryegrass-red clover mixture. Adding plantain to the ryegrass-red clover mixture increased the relative decomposition rate of the labile C pool, but not of the recalcitrant C pool. Yet, β-glucosidase activity and net N mineralization were unchanged due to inclusion of either forb. Moreover, fertilization with cattle slurry generally weakened these forb-induced changes in soil microbial properties. These findings demonstrate that adding selected forbs to unfertilized grass-clover leys can modify soil microbial community composition and associated C and N cycling, implying a potential for promoting long-term soil C sequestration through enhanced fungi-to-bacteria ratio, but a limited role in improving soil N fertility.