Earthworms effects on soil respiration in a sub-tropical wet forest in Puerto Rico

Carbon storage, as well as other ecosystem services of soils, is an emergent property on the landscape scale that can be attributed to mechanisms occurring on scales orders of magnitude smaller. These processes can be greatly accredited to the actions of microorganisms. However, soil and litter macro- and mesofauna play major regulatory roles over the rate and magnitude of these processes. This study aimed to tease out the influence of an earthworm community on soil carbon and nitrogen cycling in a sub-tropical wet forest in northeastern Puerto Rico.

In this study, earthworms were reduced using electrical extraction in manipulated plots (40-225 volts, 0.1-1.2 amps, 1hr applications) for the 15-month duration of the experiment; initiated in January 2011. Experimental plots (1m², 0.5m deep) were established in the summer of 2010, isolated with porous plastic mesh to restrict earthworm migration in a randomized block design of four blocks containing one plot of each treatment. Earthworm populations were assessed via hand-sorting upon experiment termination. Monthly soil respiration measurements were collected with an Environmental Gas Monitor closed chamber system (EGM-4, PP-Systems) in each plot during mid-day.

A pattern of greater monthly mean soil respiration in control plots compared to earthworm-reduced plots emerged over the year with an accumulated difference between treatments of 5.9 µmol CO₂ m^-2 s^-1. The peak mean soil respiration rate for reduced plots (4.4 µmol CO₂ m^-2 s^-1, October) lagged behind the peak for control plots (4.9 µmol CO₂ m^-2 s^-1, September) by one month, which was the only mean respiration for the reduction treatment that was greater than the respective mean control respiration. Additionally, these respiration peaks coincide with one of the two litterfall peaks recorded in this forest previously between August and November.

These findings suggest that earthworms may contribute to the rate and quantity as well as the timing of carbon released as CO₂ from tropical forest soils. Furthermore, earthworm community loss due to land use or disturbance, or conversely the introduction of earthworms in ecosystems previously devoid of them, may have implications on carbon budgets formerly not considered.