Objective The aims were to investigate the influence of different carbonate rocks on the stability of soil aggregates and the distribution of organic carbon (C) in karst farmlands, and then to enhance the understanding of structure and C sequestration mechanism of farmland soil in karst area.

Methods The paddy field and dry land of dolomite and limestone parent material in karst area of northwest Guangxi were selected, and undisturbed soil of different depth were collected as differential particule sizes and separated by dry sieving and wet sieving to study the influence of different parent materials on soil aggregate content, aggregate stability, contribution rate and correlation of aggregate organic C in farmland were.

Result ① The mechanical stable aggregates of the two parent materials were mainly distributed in the particle size > 2 mm, and their content increased with the increase of soil depth. The aggregate content of each grain grade decreased with the decrease of particle size. With the increase of soil depth, the large water-stable aggregate decreases gradually and the small water-stable aggregate increases gradually. The aggregate content of each particle size decreased first and then increased or increased gradually with the decrease of particle size. ② Failure rate PAD 0.25, PAD 2, mechanical aggregate MWD and GMD all showed an increasing trend with the increase of soil depth, while the water-stable aggregates MWD and GMD showed a decreasing trend. The MWD and GMD of water-stable aggregates in dolomite parent material were higher than those in limestone parent material. The mechanical stability and water stability MWD and GMD of paddy field were higher than those of dry field under the same carbonate rock development. ③ The soil organic C content decreased with the increase of soil depth. And the soil organic carbon content of the dolomite parent material is higher than that of the limestone parent material. The organic C content of paddy field was higher than that of dry land under the same carbonate rock development. In both mechanical and water-stable aggregates, the contribution rate of organic C were mainly in large aggregates > 0.25 mm. The correlation analysis showed that the organic C contribution rate of each particle size aggregate was significantly positively correlated with the aggregate content, indicating that the higher the aggregate content, the higher the organic C contribution rate.

Conclusion The soil developed from dolomite parent material has higher corrosion resistance and structural stability than limestone parent material. The organic C of farmland soil under the two parent materials is mainly fixed in the large aggregates