Summary To determine to which extent root‐derived carbon contributes to the effects of plants on nitrate reducers and denitrifiers, four solutions containing different proportions of sugar, organic acids and amino acids mimicking maize root exudates were added daily to soil microcosms at a concentration of 150 μg C g −1 of soil. Water‐amended soils were used as controls. After 1 month, the size and structure of the nitrate red...