Soil organic carbon stock and its changes in a typical karst area from 1983 to 2015

Purpose Changes in soil organic carbon (SOC) stock have major impacts on global terrestrial carbon cycling. However, their responses to land use conversions are poorly characterized for karst areas with extremely fragile geology and intensive human disturbance. Methods To investigate the effects of soil type and land use on SOC stock in a typical karst region of southwestern China, 0-15 cm topsoil samples were randomly collected in 2015. Furthermore, in the same locations as the sites in 1983, 0-100 cm stratified profile soil samples (0-10, 10-20, 20-30, 30-50, 50-70, and 70-100 cm) were collected to evaluate the changes in SOC stock as affected by land use conversions from 1983 to 2015. Results The current SOC stock in 0-15 cm differed significantly between Calcisols and Ferralsols, and was highest in the secondary forest, followed by shrubland, grassland, plantation forest, and cropland. Changes in the stratified SOC stock (both recalculated to 0-20, 20-40, 40-60, 60-80, and 80-100 cm intervals in 1983 and 2015) varied among different land use conversions. Average stratified SOC stock decreased after forest degradation and reclamation, except in 80-100 cm. After reforestation, it had decreases in 0-20 and 20-40 cm, whereas it increased in subsoil (40-60, 60-80, and 80-100 cm). However, compared with the cropland in which soil was exposed to continuous conventional tillage, average stratified SOC stock increased in all soil layers after reforestation. Conclusion The increases in SOC stock after reforestation and re-cultivation (short-term reuse in abandoned croplands) indicated the positive role of agricultural abandonment in increasing terrestrial SOC stock.