Optimizing multifunctionality in agroecosystems is a major goal of sustainable farming. Organic amendments play a crucial role in achieving sustainable management, improving crop yields while enhancing soil functions and protecting soil biodiversity. The objective of this study was to assess the effectiveness of various amendments with contrasting C:N ratios in optimizing soil functions and crop yield in a low-carbon, semi-arid cereal system. We hypothesized that organic amendments promote multifunctionality and that organic matter quality, in terms of its C:N ratio, is the main driver of soil nutrient availability and microfaunal diversity in response to amendments in a 2-year wheat-barley rotation in central Spain. Our findings reveal that amendments with high C:N ratios did not impact soil C accrual or soil respiration. However, with increasing C:N ratios, beta-glucosidase activity declined by 41.8 %, soil food web structure was depleted, plant parasitic nematodes increased, and food webs became bacterial-dominated. In our experiment, neither organic amendments nor NPK fertilizers consistently improved crop yield, but fresh manure promoted grain N uptake and quality in the first year, while amendments with a high C:N ratio increased crop yield by 52.48 % the second year. Organic amendments generally enhanced agroecosystem multifunctionality, and a negative relation between multifunctionality scores and pest pressure was observed.