从湿地到农田:围垦对生态系统碳排放的影响

湿地围垦转化为农田直接影响碳循环过程,但之前的众多研究忽略了相关人为活动如农资生产、农用器械使用等所产生的碳排放.为了更全面认识湿地围垦为农田所导致的这种变化,以崇明岛为研究地区,基于通量观测和生命周期评价,本文分别探讨当考虑和不考虑人为活动伴随的碳排放时,生态系统总碳排放的变化.结果表明: 如果只考虑生态系统与大气间的碳通量,农田仍表现为碳汇,但与自然湿地相比,其碳排放增加了10.47 t (CO₂-eq)·hm^-2;当将农业生产中人为活动碳排放纳入计算后,崇明岛自然湿地和围垦农田的碳排放总量分别为-15.38和6.54 t (CO₂-eq)·hm^-2,碳排放增加了21.92 t (CO₂-eq)·hm^-2,其中,人为活动碳排放为11.45 t (CO₂-eq)·hm^-2;田间种植和农资生产的碳排放共占农田生命周期碳排放总量的84.6%,化肥的生产施用是农田生命周期碳排放的主要来源之一.围垦使生态系统乃至区域尺度的碳源汇属性发生变化,需重新评估其影响;同时,为了达到低碳农业的目的,需减少化肥施用、提高化肥使用效率.

Effects of reclamation on ecosystem carbon emission: Wetland conversion to cropland

The reclamation of wetlands for agricultural uses affects carbon cycling directly, how-ever, previous studies ignored carbon emissions caused by human activities such as fertilizer production and use of agricultural machinery. To more thoroughly understand the changes in ecosystem carbon emission caused by land reclamations, we used flux observation and life cycle assessment to estimate total ecosystem carbon emission with and without accompanying carbon emissions due to human activities on Chongming Island. The results showed that croplands acted as carbon sink if only carbon fluxes between the ecosystems and atmosphere were considered. However, carbon emission of croplands increased by 10.47 t (CO₂-eq)·hm^-2 compared to wetlands. When carbon emissions caused by human activities were considered, total carbon emission of wetlands and reclaimed croplands respectively were -15.38 and 6.54 t (CO₂-eq)·hm^-2, and the carbon emission due to wetland reclamations increased by 21.92 t (CO₂-eq)·hm^-2, the carbon emissions caused by human activities were 11.45 t (CO₂-eq)·hm^-2. The carbon emissions of agricultural material production and rice cultivation accounted for 84.6% of the total emission in their life cycles, and thus the production and application of fertilizer were largely responsible for the carbon emission. In sum, we need to reassess reclamation effects because it changed carbon sink/source properties at ecosystem and even regional scales. And we should reduce fertilizer use and improve fertilizer use efficiency to achieve low-carbon agriculture.