Afforestation influences soil organic carbon and its fractions associated with aggregates in a karst region of Southwest China

Abstract

Variations in soil organic carbon (SOC) and its fractions within soil aggregates in response to land-use change are important to understand the carbon cycles in terrestrial ecosystem. However, responses of total SOC, SOC fractions, and SOC stability in different soil aggregates to land-use change are less addressed, especially in karst regions with serious land degradation. Therefore, bulk soil samples were collected under four land uses with similar geographical characteristics and previous framing practices including farmland (FL), Bamboo forest (BA), landscape tree planting (LAT), and orange orchards (ORO) in a karst region of Southwest China. Contents of total SOC and three carbon fractions based on their degree of oxidizability (F1, very labile; F2, inert; F3, oxidizable resistant) in bulk soil and different soil aggregates (macro-aggregate, micro-aggregate, and silt+clay fraction) were measured. Afforestation significantly increased contents of total SOC and three carbon fractions in bulk soil and soil aggregates, and the influence was more obvious in macro-aggregate than the other aggregates. Contents of total SOC, F1, F2, and F3 under afforestation land increased by 41.73%, 58.19%, 33.91%, and 40.55%, respectively, in bulk soil, by 55.60%, 79.24%, 121.77%, and 43.30%, respectively, in macro-aggregate, by 52.80%, 33.57%, 20.14%, and 75.02%, respectively, in micro-aggregate, and by 26.21%, 35.60%, 29.26%, and 23.75%, respectively, in silt+clay fraction than those under FL. In bulk soil and soil aggregates, proportions of F1, F2, and F3 in total SOC ranged from 0.11 to 0.18, from 0.13 to 0.22, and from 0.60 to 0.73, respectively, suggesting that the stable carbon was the predominant carbon fraction in the study area. Afforestation decreased the values of stability of SOC in macro-aggregate and silt+clay fraction, while it increased the value in micro-aggregate. Although both BA and ORO had higher SOC content in bulk soil than the LAT, but the SOC stability in bulk soil under BA was significantly lower than that under ORO. In conclude, afforestation form FL improved SOC content and altered SOC stability in bulk soil and soil aggregates, and conversion of FL to ORO might be the best choice to increase SOC sequestration in the four land-use types compared in karst regions of Southwest China.

Keywords: Labile carbon fraction; Land-use change; SOC stability; Stable carbon fraction.