Complementary nitrogen use by legume and grass enhances plant productivity irrespective of earthworm species

Plant mixtures typically lead to higher productivity compared to monocultures. Yet, most studies on plant mixtures focus on aboveground interactions, while little attention is given to interactions below the ground. Earthworms are key components of the belowground subsystem and are closely associated with plant roots; however, how different earthworm species affect the direction and strength of plant mixture - productivity relationships remains largely unclear. We conducted a microcosm study including monoculture legume, monoculture grass and legume-grass mixture as well as five earthworm species. To specifically focus on earthworm - plant root interactions, leaf litter was excluded from the microcosms. Plant mixtures increased the productivity of both legumes and grasses by 3.56 g (65.0 %) and 0.60 g (139.5 %) dry mass per plant individual, respectively, likely due to the complementary nitrogen use by legumes and grasses [10.20 g (58.0 %) and 0.43 g (173.6 %) higher total amount of nitrogen in individual plants in mixture, respectively], but potentially also due to less competition for other resources such as light and water, particularly for legumes, as also supported by a lower microbial water stress indicator (cy/pre, ratio of cyclopropyl phospholipid fatty acids to its monoenoic precursors) in plant mixture. In addition, grasses enhanced the microbial activity as shown by higher microbial specific respiration (10.24 ± 0.34, 11.44 ± 0.44 and 12.30 ± 0.42 μl O 2 mg −1 C mic h −1 for the legume, grass and mixture treatments, respectively), thereby maximizing nitrogen uptake in plant mixtures. While the increase in plant growth was rather uniform, earthworm species differentially affected nitrogen uptake of both legumes and grasses, with Allolobophora chlorotica facilitating nitrogen uptake of plants more than other earthworm species, especially in grass monocultures [0.09 g (35.5 %) higher amount of total nitrogen in individual plants in presence of A. chlorotica compared to other earthworm species]. Overall, our study highlights the general importance of plant mixtures for sustainable agriculture, but also points to earthworm-specific effects on plant nitrogen uptake that can mitigate the poor performance of grass monocultures. • Plant nitrogen stable isotopes suggest complementary nitrogen use between legume and grass. • Plant mixtures reduced the intra-specific competition between plants, maximizing productivity of both legume and grass. • A. chlorotica facilitated nitrogen uptake of grasses more than other earthworm species studied. • Earthworms can potentially mitigate the poor performance of grass monocultures.