Biochar is known for the improvement of soil health, fertility, crop productivity, and quality in many agro-ecosystems globally, but information regarding fodder yield, quality, and soil microbial activity responses to biochar application remains very limited. The objective of this study was to prepare biochar from invasive weeds, i.e., Parthenium hysterophorus L. and Lantana camara L., and use it as a soil amendment along with inorganic fertilizers for oats (Avena sativa L.) growth, fodder yield, quality, and soil microbial activities in a two-year pot experiment. Treatments were comprised of control, 100% RDF (Recommended dose of fertilizers), 75% RDF along with three doses (2.5, 5.0, and 10 t/ha) of Parthenium hysterophorus L. biochar (PB) and Lantana camara L. biochar (LB), PB (10 t/ha), and LB (10 t/ha). Results showed that application of 75% RDF along with 10 t/ha LB gave significantly higher green (∼8%) and dry (∼7.8%) fodder yield and crude protein (∼6%) and decreased acid detergent fibre (ADF) and neutral detergent fibre (NDF) by 5.70 and 6.04% as compared to the 100% RDF treatment. The same treatment had a significantly higher population of bacteria (7.33 × 108 colony forming unit (CFU)/g soil), alkaline phosphatase activity (19.56 μg pNP/g soil/h), microbial biomass carbon (156.67 μg/g soil) and dehydrogenase activity (12.59 μg TPF/g/24 h), whereas the maximum fungal population (13.33 × 104 CFU/g soil) and acid phosphatase activity (14.45 μg pNP/g soil/h) were found in 75% RDF along with 10 t/ha PB and control treatment, respectively. This study concluded that application of invasive weed biochar along with inorganic fertilizers can benefit fodder yield and quality of oats by increasing plant height and number of tillers directly and by improving nutrient availability and water holding capacity (WHC) of soil indirectly, besides improving soil health. The findings from this study will provide a potential strategy for invasive weed management. •LB was moderately superior to PB in terms of fodder yield and quality.•Application of biochar at 2.5 t/ha saved 25% fertilizers.•Biochars applied plots had higher soil enzymatic activities than fertilizer alone.•Making biochar would be a promising way to manage invasive weeds for environmental sustainability.