Exotic Earthworms Accelerate Plant Litter Decomposition in a Puerto Rican Pasture and a Wet Forest
Z. G. Liu and X. M. Zou
Ecological Applications
Vol. 12, No. 5 (Oct., 2002), pp. 1406-1417
Abstract:
Tropical land-use changes can have profound influence on earthworms that
play important roles in regulating soil processes. Converting tropical forests to pastures
often drastically increases the abundance of exotic earthworm populations such as Pontoscolex
corethrurus. We initiated this study to examine the influence of exotic earthworms
on the decomposition of plant leaves and roots in a tropical pasture and a wet forest of
Puerto Rico. We employed two treatments: control with natural earthworm population, and
earthworm reduction using an electroshocking technique. Decomposition rates of plant
leaves on the ground surface and root materials within the surface mineral soil were estimated
using a litterbag technique. To understand the role that exotic earthworms play in
altering plant litter decomposition, we also compared soil CO2 evolution rates, soil microbial
biomass, and physical and chemical soil properties between the controls and earthwormreduced
plots during a one-year period. Earthworm populations in the electroshocked enclosures
were reduced by 85% and 87% as compared with pasture and forest controls by
the end of the experiment. Earthworm reduction significantly decreased the annual decay
rates of plant leaves but had no effects on those of plant roots in both pasture and forest
sites. Although the control plots had less mass remaining on every litterbag collecting date,
significant treatment effects on leaf decomposition occurred only after 240 d in both sites.
The decay rates were greater when organic materials had low carbon to nitrogen or phosphorus
ratios. Soil respiration was also decreased in the earthworm-reduced plots. In contrast,
soil microbial biomass C was not affected by earthworm reduction. Furthermore, there
were no significant differences between the two treatments in soil bulk density, moisture
content, pH, or temperature at either site. Our results suggest that exotic earthworms may
accelerate leaf litter decomposition by elevating rates of litter consumption/digestion or
microbial activity, rather than by improving soil physical/chemical conditions or altering
microbial biomass.