Objectives Agricultural use of compost sludge is the main way of sludge recycling. The rich organic matter in composted sludge has different effects on soil carbon components and pH. However, it is unclear whether different amounts of composted sludge affect the organic-inorganic carbon of acidic paddy soil under dry farming conditions.

Methods A 2-year wheat-maize successive pot experiment was conducted with five sludge application rates 0 (CK), 3.75, 7.50, 37.50, 75.00 t/hm² with three replicates according to GB4284–2018 agricultural sludge application standard. Before planting winter wheat in October each year, each pot was filled with the corresponding weight of sludge and soil. In June and October of each year, the topsoil samples were collected after wheat and maize harvest, and the effects of different sludge application rates on organic, inorganic carbon components and pH of acid paddy soil were analyzed.

Results The pH of the paddy soil under 37.50 t/hm² sludge dosage was 0.21, 0.51, 0.40 and 0.70 units higher than that of CK in the four successive growing seasons. Application of composted sludge increased the soil organic carbon (SOC), readily oxidizable carbon (ROC), dissolved organic carbon (DOC), humus carbon (HSC), calcium carbonate (CC) and activated calcium carbonate (ACC) content with increase in sludge application rate. The SOC, DOC, ROC, HSC, CC, and ACC under sludge dosage of 75.00 t/hm² was significantly higher than those under CK. The ROC and DOC contents under sludge application of ≥ 7.50 t/hm² were significantly higher than those of CK. The soil CC content under the sludge application of ≥ 3.75 t/hm² was significantly higher than that under CK. The sludge application rate of ≥ 7.50 t/hm² significantly increased carbon pool management index of different crop growth seasons. CC content was positively correlated with soil pH in acid paddy soil. Organic carbon and inorganic carbon also showed a positive correlation.

Conclusions Application of 37.50–75.00 t/hm² of composted sludge could increase the active soil organic and inorganic carbon contents, increase the pH of acidic paddy soil significantly under dry farming.