Impact of wetland conversion to cropland on ecosystem carbon budget and greenhouse gas emissions in Northeast China

Abstract

Wetlands provide a huge carbon (C) sink and represent strategic areas for regulating climate change. However, extensive wetlands have been lost since 1700, primarily for conversion to cropland. Currently, few studies have comprehensively evaluated changes in C budgets and greenhouse gas (GHG) emissions following wetland conversion to cropland. Here, we measured annual carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) emissions from a Phragmites australis-dominated wetland and adjacent wetland-converted soybean cropland by combining eddy covariance and chamber methods. We included biomass removal from cropland in the full C and GHG accounting. Annually, the P. australis wetland was a substantial atmospheric CH₄ source (50 ± 1 g CH₄ m^‒2) but strong CO₂ (‒1217 ± 162 g CO₂ m^‒2) and weak N₂O (‒0.1 kg N₂O ha^‒1) sinks, which collectively shaped a big C sink (‒294 ± 44 g C m^‒2) and net GHG source (180 ± 164 g CO₂-eq m^‒2). Converting P. australis wetland to soybean cropland demolished atmospheric CO₂ and N₂O sinks, and formed net sources of CO₂ (140 ± 149 g CO₂ m^‒2 yr^‒1) and N₂O (1.1 ± 0.2 kg N₂O ha^‒1 yr^‒1). Meanwhile, this conversion greatly reduced CH₄ emissions to 1.2 ± 0.5 g CH₄ m^‒2 yr^‒1. Taken together, soybean cropland was a net direct atmospheric C source of 39 ± 41 g C m^‒2 yr^‒1, while holding a GHG budget of 203 ± 150 g CO₂-eq m^‒2 yr^‒1. Further, grain and straw in cropland were removed during harvest, creating a C loss of 142 ± 18 g C m^‒2 yr^‒1, and eventually increased GHG budget to 722 ± 165 g CO₂-eq m^‒2 yr^‒1. Consequently, the full GHG debt of wetland-cropland conversion increased by dozens of times to 542 ± 233 g CO₂-eq m^‒2 yr^‒1, 95.8% of which attributed to biomass removal. Overall, our study contributes to growing recognition of C loss risks of wetland conversion to cropland and highlights the importance of straw return in mitigating climate impacts during agricultural activities.