Soil carbon accretion and earthworm recovery following revegetation in abandoned sugarcane fields

Assessing the influence of tropical land-use changes on soil organic carbon dynamics and biotic activity can aid the understanding of global C cycling. Economic factors have led to the rapid abandonment of sugarcane fields in Hawaii, and land-use has shifted to exotic tree plantations or sec secondary plant communities. We examined changes in surface soil C and N concentrations (0-0.25 m) and in earthworm and microbial biomass along chronosequences of eucalypt plantations and secondary grass-legume communities developed in abandoned cane fields. While soil bulk density remained unchanged, surface soil C and N concentrations increased from 6.45% and 0.32% in sugarcane field to 9.98% and 0.53% in the 10y old eucalypt plantations, respectively. Earthworms were absent from sugarcane fields, and increased steadily with age of eucalypt plantations and secondary communities. Worm density reached 400 individuals m(-2) in the 10y old plantation. Increases in worm density were associated with increases in soil C and N concentrations and decreases in soil pH. Worm species rich-ness was higher in the grass-legume communities than in the plantations. Whereas total fungal biomass was low, active bacteria and active fungal biomass were high in the sugarcane soils compared with eucalypt plantations and secondary communities. There were no differences between eucalypt plantations and the secondary communities in either total fungal and bacteria biomass or in active fungal and bacteria biomass. Rapid increases of surface soil C concentration and earthworm density following revegetation in the abandoned sugarcane fields demonstrate that alteration in tropical land-use can potentially affect C cycling at the global scale. (C) 1998 Elsevier Science Ltd. All rights reserved.