Physical and biological drivers of forest soil carbon cycling

Soil respiration, a significant process of the global carbon cycling, is very sensitive to soil temperature, moisture and other biotic and abiotic factors. However, the relative contribution of soil biota to soil respiration, and how earthworms, the important engineering species of many terrestrial ecosystems influence soil carbon cycling are not fully understood. We carried out a laboratory experiment and a field experiment to investigate: (1) how important are the effects of earthworms compared with other environmental factors on soil respiration; (2) How do earthworms influence soil C cycling through their impacts on leaf litter, soil organic matter and soil aggregates and (3) How these physical and biological factors interact with each other on soil C cycling. In the laboratory experiment, we set up single and combined earthworm species treatments with various densities of earthworm species ( Lumbricus rubellus and Octolasion lacteum) to study the effects of different species of earthworms on soil respiration. We also conducted a field experiment in two forest stands at the Smithsonian Environmental Research Center, Edgewater, Maryland, in which we manipulated earthworm density and substrate supply in the experimental enclosures to study the contribution of environmental factors to soil respiration and the role of earthworms in regulating soil C cycling.;We found biological factors could contribute 5% of the variation in soil respiration. Earthworm activities stimulated soil respiration in both laboratory and field manipulation experiments. Their effects were dependent on substrate supply and earthworms species composition, as different species have different strengths in regulating carbon cycling. Earthworms could change the carbon cycling not only directly by stimulating soil respiration, but also indirectly by influencing the temperature sensitivity of soil respiration; not only in a short term, but also in a long term by increasing the physical protection of soil carbon within soil aggregates.;In our laboratory and field experiments, we used the chamber method to measure soil respiration, which is labor intensive and could not monitor soil respiration continuously. Gas well method is an alternative method to measure soil respiration and could compensate for the disadvantages of chamber method. We set up a laboratory experiment to test the application of gas well method at different temperature and moisture regimes with and without leaf litter. Millington-Quirk model and Fcp model had more accurate predictions than Penman model, while they also had the tendency to underestimate CO2 efflux at high temperature and overestimate CO2 efflux at low temperature. The presence of leaf litter layer caused underestimation of CO2 efflux by gas well models, depending on how much CO2 is produced by lead litter decomposition. The gas well models need to be improved for practical use in the future.