Agroforestry reclamations decreased the CO2 budget of a coastal wetland in the Yangtze estuary

Abstract

How does the CO2 budget in coastal wetlands respond to dynamic and composite agroforestry reclamations is unclear. For six consecutive years, CO2 flux data were measured using the eddy covariance (EC) technique to determine the CO2 budget changes and the key factors controlling the variations of net CO2 ecosystem exchange (NEE) over a reclaimed coastal wetland in the Yangtze estuary, China. After agroforestry conversions in spring 2013, the annual NEE decreased substantially from -655.7 (day 109~365 in 2011) and -558.4 (2012) to 258.1 (2013) and -198.1(2014) and persisted on decreasing to -160.8 (2015) and -141.2 (2016) g C m−2 yr−1. |ER/GPP| ratio persistently increased from 0.40 in 2011 to 0.89 in 2016, suggesting a decoupling between ER and GPP in the coastal reclaimed site. The decrease of GPP was possibly attributed more to the decreased wetland vegetation productivity. The increased ER was due to the emergence of young forest (YF) and aquaculture pond (AP) with high net CO2 emissions in the flux footprint. A homogeneity-of-slopes (HOS) analysis showed that the interannual variability (IAV) of NEE was attributed more to the functional changes (19.0 %), rather than the interannual meteorological variations (9.2 %). The functional changes were also verified by the significant decreased Amax and the slightly increased ∂, both of which might be caused by the changed dominant wetland plant species and their growth pattern. The fluctuated Q10 and R10 during the agroforestry reclamations reflected the seasonal changes in the compositions and respiration characteristics of different land-use types within the CO2 flux footprint. Findings imply that agroforestry reclamations in temperate coastal wetlands substantially decrease the CO2 fixation service by changing the ecosystem's structure and function. Our results call for more wise land-use policy and management measures in reclaimed coastal areas around the world to pursue global warming mitigation.