ABSTRACT.- Supplementing plants with silicon (Si) often improves plant productivity and resilience to biotic and abiotic stresses, but this is mostly reported in highly controlled experimental environments. The ecological consequences of Si supplementation, including environmental benefits and potential risks, are therefore poorly understood and require field-scale evaluation of how Si supplementation affects the wider ecosystem, such as invertebrate communities and soil physicochemical properties. We conducted the first field assessment of how a legume (lucerne; Medicago sativa) agroecosystem and its associated invertebrate communities responded to two levels of Si supplementation (calcium silicate slag), over two years. We quantified seasonal changes in the abundance and diversity of aboveground arthropod communities, crop yield, elemental and nutritional chemistry, and soil pH as well as soil chemistry. The highest rate of Si supplementation increased bioavailable Si in the soil by 181% and soil pH from 5.2 to 6.3, relative to untreated plots, with a significant positive correlation between increased soil bioavailable Si and pH. Si supplementation led to an increase in crop yield by up to 52%; however, the magnitude varied with season. Foliar concentrations of Si tended to increase with Si supplementation, but this increase was marginally significant, potentially due to a dilution effect of higher shoot biomass. Si supplementation did not affect concentrations of most soil elements we quantified or forage quality of lucerne. We recorded over 13,600 arthropods; Si supplementation led to a shift in community structure and overall increased diversity of arthropod functional groups. Notably, the saprophytic fly family, Lauxaniidae was more abundant on Si-supplemented plots compared to untreated plots, potentially due to increased plant turnover. These results indicate that silicon supplementation of a legume agroecosystem, using a by-product of steel production, provides productivity benefits that outweigh some possible detrimental impacts on the ecosystem (i.e. decreased arthropod abundances, toxic metal contamination or reduced forage quality), which we did not detect in our current field study. This management intervention enhances crop yield, so could reduce the need for conventional fertilisers as well as changing soil pH to be more beneficial to crops and some arthropod groups. © 2024 The Authors