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.

中文翻译：

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东北地区湿地还田对生态系统碳收支和温室气体排放的影响


湿地提供了巨大的碳 （C） 汇，是调节气候变化的战略区域。然而，自 1700 年以来，大面积的湿地已经消失，主要用于转化为农田。目前，很少有研究全面评估湿地转为农田后 C 预算和温室气体排放的变化。在这里，我们通过结合涡度相关和腔室方法测量了以芦苇为主的湿地和邻近湿地转化的大豆农田的年二氧化碳 （CO2）、甲烷 （CH4） 和一氧化二氮 （N2O） 排放量。我们将农田的生物量去除包括在完整的 C 和 GHG 核算中。每年，南方松湿地都是大气中 CH4 的重要来源（50 ± 1 g CH4 m\u20122），但强 CO2（\u201217 ± 162 g CO2 m\u20122）和弱 N2O（\u20120.1 kg N2O ha\u20121）汇，共同形成了一个大的 C 汇（94 ± 44 g C m\u20122）和温室气体净源（180 ± 164 g CO2-eq m\u20122）。将 P. australis 湿地转化为大豆农田摧毁了大气中的 CO2 和 N2O 汇，并形成了 CO2（140 ± 149 g CO2 m\u20122 yr\u20121）和 N2O（1.1 ± 0.2 kg N2O ha\u20121 yr\u20121）的净来源。同时，这种转换将 CH4 排放量大大降低到 1.2 ± 0.5 g CH4 m\u20122 年\u20121。综上所述，大豆农田的直接大气碳净来源为 39 ± 41 g C m\u20122 yr\u20121，而温室气体预算为 203 ± 150 g CO2-eq m\u20122 yr\u20121。此外，农田中的谷物和秸秆在收获过程中被移除，造成 142 ± 18 g C m\u20122 yr\u20121 的 C 损失，最终将温室气体预算增加到 722 ± 165 g CO2-eq m\u20122 yr\u20121。因此，湿地-农田转化的全部温室气体债务增加了数十倍，达到 542 ± 233 g CO2-eq m\u20122 yr\u20121，其中 95.8% 归因于生物质去除。 总体而言，我们的研究有助于人们越来越认识到湿地转化为农田的碳损失风险，并强调了秸秆返回在减轻农业活动期间气候影响方面的重要性。