Soil drainage facilitates earthworm invasion and subsequent carbon loss from peatland soil

1. Human activities have been a significant driver of environmental changes with tremendous consequences for carbon (C) dynamics. Peatlands are critical ecosystems because they store similar to 30% of the global soil organic C pool and are particularly vulnerable to anthropogenic changes. The Zoige peatland on the eastern Tibet Plateau, as the largest alpine peatland in the world, accounts for 1 parts per thousand of global peat soil organic C storage. However, this peatland has experienced dramatic climate change including increased temperature and reduced precipitation in the past decades, which likely is responsible for a decline of the water-table and facilitated earthworm invasion, two major factors reducing soil organic carbon (SOC) storage of peatlands. 2. Because earthworms often are more active in low-than in high-moisture peatlands, we hypothesized that the simultaneous occurrence of water-table decline and earthworm invasion would synergistically accelerate the release of SOC from peatland soil. We conducted a field experiment with a paired split-plot design, i.e. presence vs. absence of the invasive earthworms (Pheretima aspergillum) nested in drained vs. undrained plots, respectively, for 3 years within the homogenous Zoige peatland. 3. Water-table decline significantly decreased soil water content and bulk density, resulting in a marked reduction of SOC storage. Moreover, consistent with our hypothesis, earthworm presence dramatically reduced SOC in the drained but not in the undrained peatland through the formation of deep burrows and decreasing bulk density of the lower soil layer over 3 years. The variation in SOC likely was due to changes in above-ground plant biomass, root growth and earthworm behaviour induced by the experimental treatments. 4. Synthesis and applications. We suggest that incentive measures should be taken to prevent further water-table decline and earthworm invasion for maintaining the soil carbon pool in Zoige peatland. Artificial filling of drainage canals should be implemented to increase the water-table level, facilitating the recovery of drained peatlands. Moreover, the dispersal of earthworms and their cocoons attached to the roots of crop plants and tree saplings from low-lying areas to the Zoige region should be prevented.