Agricultural reclamation effects on ecosystem CO₂ exchange of a coastal wetland in the Yellow River Delta

Little is known about the impacts of agricultural exploitation of coastal wetlands on ecosystem CO₂ exchange, although coastal wetlands have been widely reclaimed for agricultural use across the world. We measured net ecosystem CO₂ exchange (NEE) and its major components, gross primary production (GPP) and ecosystem respiration (R_eco) using an eddy covariance flux technique in a natural coastal wetland (reed) and an adjacent, newly reclaimed farmland (cotton) in the Yellow River Delta, China. The results showed that agricultural reclamation changed the ecosystem CO₂ exchange of the coastal wetland at three distinct levels. Initially, the conversion from the wetland to farmland changed the light response parameters (α, A_max, and R_{eco, day}) of NEE and temperature sensitivity (Q₁₀) of R_eco mainly by changing the dominant vegetation type. Over the growing season, NEE, R_eco and GPP were significantly correlated with LAI at both sites and aboveground biomass at the farmland site. Next, the reclamation of wetland modified the diurnal and seasonal dynamics of ecosystem CO₂ exchange. Significant differences in diurnal variations of NEE between the wetland and farmland sites were found during the growing season (with the exception of June and July). Seasonal means of daily GPP and R_eco values at the wetland site were higher than those at the farmland. Ultimately, the agricultural reclamation altered the CO₂ sequestration capacity of the coastal wetland. The cumulative NEE in the wetland (-237.4gCm^-2) was higher than that in the farmland (-202.0gCm^-2). When biomass removal was taken into account, the farmland was a strong source for CO₂ of around 131.9gCm^-2 during the growing season. Overall, land use changes by reclamation altered ecosystem CO₂ exchange at several ecological scales by changing the dominant vegetation type and altering the ecosystem's natural development.