Enhanced Rock Weathering Increases Soil Carbon but Reduces Soil Organic Carbon Stability in Subtropical Croplands

Enhanced rock weathering is regarded as a promising carbon dioxide removal method because of its potential to sequester soil inorganic carbon (SIC). However, the influence of enhanced rock weathering on changes in soil organic carbon (SOC) content, fractions and stability remains poorly understood. A randomized block experiment design employing five basalt addition rates (0 (CK), 2.5, 5, 10 and 20 kg·m−2) and four replicates was designed to investigate the influences of basalt addition on SOC and SIC content and stocks, SOC fractions and SOC stability in subtropical cropland, where Zea mays L. and Brassica juncea (L.) Czern were annually rotated. Soil samples were collected from depths of 0–15 cm and 15–30 cm one year after the addition of basalt. The results showed that enhanced rock weathering increased the total carbon content and stock by increasing both the SOC and SIC in a one-year field experiment. Compared with CK, basalt addition rates of 2.5, 5, 10 and 20 kg·m−2 increased the SOC stock by 16%, 23%, 21% and 19%, respectively, and the SIC stock by 37%, 30%, 35% and 32%, respectively. The labile carbon fraction was the primary organic carbon fraction, which accounted for more than 40% of the total SOC content. Enhanced rock weathering altered the content of the very labile carbon fraction due to its high sensitivity to basalt addition, but had little effect on the stable carbon fraction content in a one-year field experiment. Compared with CK, basalt addition increased the very labile carbon fraction content by 12% and 46%, respectively, according to samples from depths of 0–15 cm and 15–30 cm. Under basalt addition rates of 2.5, 5, 10 and 20 kg·m−2, the SOC stability index was 26%, 21%, 17% and 20%, respectively, lower than that under the 0-addition rate in a one-year field experiment, which was 1.63, indicating that enhanced rock weathering reduced the SOC stability. Our findings indicated that enhanced rock weathering increased soil carbon (both of SOC and SIC) sequestration, but reduced the SOC stability in a one-year field experiment in subtropical croplands. These observed trends in changes in soil carbon will be further tested and evaluated as the experiment continues in the future.