Magnitudes, patterns, controls and mitigation potentials of net ecosystem carbon balances across wetlands in China

Abstract

China contains Asia’s largest wetlands, which include diverse wetland types, e.g., peatlands, marshes and mangroves; these wetlands are found from inland to coastal regions. Over the past four decades, considerable amounts of natural wetlands have been drained, thus severely decreasing their carbon (C) sequestration ability. Recent rewetting has been initiated to restore wetland ecosystem services and mitigate carbon dioxide (CO₂) emissions. However, the magnitudes, patterns, controls and mitigation potentials of annual net ecosystem C balances (NECB) have not been adequately studied, leading to large uncertainties in predicting wetland C cycles under climate change and anthropogenic activities across different wetlands in China. These influencing factors include the combinations of vertical net ecosystem exchange of CO₂ (NEE) and horizontal C removal through biomass harvest, as well as dissolved organic and inorganic C (DOC and DIC) exports. Here, using the annual measurement of NECB from 148 sites across different wetlands in China, we found that the annual NECB varied widely from –9413 to + 8760 kg C ha⁻¹ yr⁻¹; these fluxes were collectively influenced by wetland status, wetland location, and wetland type. Drainage turned natural wetlands from net C sinks to net C sources (p < 0.05), regardless of whether wetlands were inland or coastal, while rewetting significantly (p < 0.05) reversed the net C loss trends from drained wetlands to net C uptake in rewetted wetlands. Coastal wetlands had significantly greater (p < 0.05) annual C uptake than inland wetlands, with coastal mangroves exhibiting the highest uptake rates. Eighty percent of the spatial variations in the annual NECB were explained (p < 0.0001) by the spatial variations in the mean annual temperature (MAT) and water table depth (WTD) across different wetlands in China. Scenario analysis showed that drained wetlands should be rewetted promptly; postponing rewetting causes larger emissions from continued C losses from drained wetlands.