Manipulation of sward diversity is a more effective management strategy than addition of microbial inoculants in intensively managed grassland

Abstract

Abstract Background and aims Diversifying grassland sward composition and application of microbial inoculants are potential alternative routes to facilitate enhanced nutrient acquisition by plants, but their relative effects have been rarely tested in grasslands. In a two-year field experiment, we investigated the impact of inoculants and sward types on forage yield, nutrient uptake, and the soil microbiome. Methods We implemented a fully factorial experiment with inoculants (eight levels; arbuscular mycorrhizal fungi (AMF), Paraburkholderia phytofirmans PsJN and Bacillus sp. P5 (P5), applied singly and in combination), and sward types (three levels: grass-only, grass and legume, mixture of grass, legume, and herb) as the two factors. Results In both years, there was a very strong effect of sward type, with the grass + legume and grass + legume + herb swards having much higher yields and nutrient uptake (N, P, K, S, Ca, Mg, Zn, Cu) than grass-only. Generally, there was no effect of the inoculants on yield and nutrient uptake, with limited exceptions involving AMF + P5. The best-performing microbial inoculant increased forage yield by 597 kg/ha/year, while switching from grass-only to one of the other sward types increased forage yield by 3932 kg/ha/year (grass + legume) and 4693 kg/ha/year (grass + legume + herb). The inoculants persisted in plots for > 1 year after application. Inoculants and sward type significantly affected the overall prokaryotic and fungal community structures. Conclusion Overall, under controlled field plot conditions, including legumes and herbs in a grass sward proved to be a far better farm-scale management strategy for increasing grassland forage yield and nutrient uptake than the application of microbial inoculants.