Effects of Spartina invasion on the soil organic carbon content in salt marsh and mangrove ecosystems in China

Coastal wetlands are large reservoirs of soil carbon (C) and have been invaded globally by the exotic species Spartina alterniflora. However, the effects of these invasions on soil C content remain unclear. We performed a meta-analysis of 2479 soil organic C (SOC) content observations collected from 91 field studies conducted in coastal China, the world's largest introduced range for S. alterniflora. Spartina alterniflora invasions had no significant effect on the SOC content in vegetated native wetlands in coastal China. S. alterniflora increased the SOC content in the top 30 cm of salt marshes after invading only one of the nine marsh types, which was dominated by a dwarf succulent species, Suaeda salsa in the northern subtropics and decreased the SOC content in the top 60 cm of mangroves dominated by Kandelia obovata and mixed communities after invasion in the southern subtropics. S. alterniflora invasions did not significantly affect the SOC content in the other salt marsh or mangrove ecosystems. Moreover, the SOC content in S. alterniflora-invaded ecosystems increased only on a decadal scale and then decreased, rather than increasing monotonically. Synthesis and applications. Our findings reveal that the SOC content of S. alterniflora-invaded ecosystems is similar to or lower than that of most native ecosystems. As soil bulk density is often reduced by S. alterniflora invasions, the C storage capabilities of vegetated native ecosystems might be substantially reduced after S. alterniflora invasions. Moreover, the SOC content in these invaded ecosystems gradually decreased after long-term S. alterniflora invasions (>20 years). Therefore, the displacement of native salt marsh or mangrove species by S. alterniflora may not be considered a natural climate solution to increase the C sink in coastal China. Future efforts are required to control S. alterniflora preferentially in coastal wetlands where S. alterniflora invasions do not increase the SOC content, and to conserve and restore native coastal wetlands, which are the core areas of the World Natural Heritage site serving as China's Migratory Bird Sanctuaries that have been severely threatened by S. alterniflora invasions, to increase ecological integrity and C sinks and to mitigate climate change.