The increased soil aggregate stability and aggregate-associated carbon by farmland use change in a karst region of Southwest China

Land use change can significantly alter the distribution of different soil aggregates, thereby influencing aggregate stability and aggregate-associated organic carbon (OC). However, there is minimal research on the variations in the distribution of soil aggregates, aggregate stability, and aggregate-associated OC following land use change from farmland to afforested land in the karst regions of Southwest China. Undisturbed soil samples were collected from farmland (FL) and three afforestation patterns (bamboo forest: BA; landscape tree planting: LAT; and orange orchards: ORO) converted from farmland in a karst region of Southwest China. These four land uses have similar geographical characteristics (e.g., elevation and slope aspects) and previous framing practices. The distribution of macroaggregate (>0.25 mm), microaggregate (0.053–0.25 mm) and silt + clay fractions (<0.053 mm) and the aggregate-associated OC contents under the four land uses were measured at the 0–10 cm and 10–20 cm depths. The results showed that afforestation converted from FL significantly increased the macroaggregate and microaggregate contents, geometric mean diameter (GMD), and structural stability index (SSI) but decreased the silt + clay fraction content. Compared with FL, the values at the 0–20 cm depth under BA, LAT and ORO increased by 108.79%, 34.89% and 36.56%, respectively, for GMD and by 239.84%, 94.05%, and 123.41%, respectively, for SSI. Land use change and soil aggregate size had obvious influences on soil organic carbon (SOC) content. The SOC content in the bulk soil, macroaggregate, microaggregate, and silt + clay fraction at the 0–20 cm depth after afforestation increased by 26.83%, 45.57%, 27.14%, and 13.80%, respectively, compared to FL. The contributions of macro- and microaggregates to the increase in SOC content in bulk soil were 146% and 78%, respectively, while the silt + clay fractions had a negative effect with a value of 125% after afforestation. The GMD and SSI were positively correlated with the macroaggregate content and SOC content in the bulk soil and different soil aggregates. Although the increase in SOC content in bulk soil and soil aggregates under BA and ORO was significantly higher than that for LAT, the soil aggregate stability under ORO was lower than that under BA. These findings suggest that afforestation increases aggregate ability, SOC content in bulk soil and aggregate-associated OC and that BA might be the optimum choice among the three afforested lands compared for soil sustainability from the perspective of improving carbon sequestration and soil erosion.