稻田转为菜地初始阶段温室气体排放特征

[目的]近年来,随着我国社会经济的快速发展和人们生活水平的提高及膳食结构的改善,越来越多的稻田被转为蔬菜种植,影响了土壤碳氮转化过程及其引起的温室气体排放。因此有必要探究稻田转为蔬菜种植,特别是该土地利用方式转变初始阶段的温室气体(CH₄和N₂O)排放特征及其关键影响因素。[方法]试验选取了长期种植水稻的双季稻田,将其中一部分转为蔬菜种植,另一部分继续种植水稻,每个处理设置了3个重复,按照当地常规模式进行管理。采用静态暗箱一气相色谱法连续3年进行田间原位观测,比较分析稻田和由稻田转变的菜地CH₄和N₂O排放特征及其年际变化差异,明确稻田转为菜地初始阶段CH₄和N₂O排放的关键影响因素。[结果]稻田是重要的CH₄排放源,其第一年的排放强度(183.91 kg CH₄-C·hm^-2·a^-1)明显低于后续两年(241.56-371.50 kg CH₄-C...

Greenhouse Gas Emission During the Initial Years After Rice Paddy Conversion to Vegetable Cultivation

【Objective】In recent years, with the rapid development of social economy, the improvement of people’s living standards and shifting diets and the increasing demands of vegetables result in a considerable share of rice paddy fields conversion to vegetable production in China, thus influencing soil carbon and nitrogen cycling and associated greenhouse gas (GHG) emissions. Therefore, it is necessary to investigate the impacts of land-use conversion from rice into vegetable cultivation on methane (CH₄) and nitrous oxide (N₂O) emissions and their key regulating factors, particularly during initial period upon conversion. 【Method】In this study, six rice paddies subjected to long-term double-rice planting were chosen, and the half of them were converted into vegetable cultivation (Veg) and the remaining still for rice production (Rice), with three replicates of each treatment. The Veg and Rice were managed according to local practices. The fluxes of CH₄ and N₂O from the rice paddy and converted vegetable fields were measured with static chambers from December 2012 to December 2015, so as to investigate the characteristics and inter-annual variation of CH₄ and N₂O emissions and to identify the key factors regulating the two GHGs during the initial period upon conversion. 【Result】Rice paddy acted as an important source of CH₄, and CH₄ emission was significantly lower in the first year (183.91 kg CH₄-C·hm^-2?a^-1) relative to the later two years (241.56-371.50 kg CH₄-C·hm^-2?a^-1), mainly attributed to enhanced precipitation increasing soil water content during the latter two years. Conversion from rice to vegetable cultivation substantially reduced CH₄ emission from Veg by 83%-100% as compared to Rice over the study period. Annual CH₄ emissions from Veg were significantly higher in the first year (31.22 kg CH₄-C·hm^-2) relative to any later years (0.45-0.89 kg CH₄-C·hm^-2), suggesting that this land-use conversion had strong legacy effect on CH₄ emission. Paddy soil acted as a minor source of N₂O (1.35-3.49 kg N₂O-N·hm^-2?a^-1). Rice conversion to vegetable cultivation led to substantial N₂O emission, particularly in the first year during which the cumulative emissions were significantly larger (95.12 kg N₂O-N·hm^-2) than that in the second (38.28 kg N·hm^-2) and third year (40.07 kg N₂O-N·hm^-2). N₂O fluxes from Veg were significantly and positively related to soil heterotrophic respiration rates (R_h), and the dependence of N₂O fluxes on R_h was greater in the first year relative to the subsequent two years. These results suggested that soil organic matter mineralization contributed to N₂O emissions during the first year upon land-use conversion from rice to vegetable production. Land-use conversion from rice to vegetable cultivation significantly increased the global warming potential (GWP) of Veg by 390% and 98% in the first and second year, respectively, relative to Rice, primarily due to the increased GWP of N₂O emission far outweighing the decreased GWP of CH₄ emission. In contrast, the GWP of rice (14.84±1.39 Mg CO₂-eq·hm^-2) was similar to that of Veg (16.72±3.25 Mg CO₂-eq·hm^-2) in the third year after conversion, due to the decreased GWP of CH₄ emission fully offsetting the increased GWP of N₂O emission. These results suggested that land-use conversion from rice to vegetable cultivation had significant impacts on the GWP only at the initial stage upon conversion. 【Conclusion】Land-use conversion from rice to vegetable cultivation significantly decreased CH₄ while increasing N₂O emissions over the whole study period, and increased the GWP only in the first and second year upon conversion. Soil organic matter mineralization significantly contributed to increased N₂O emission from the converted vegetable field. This study suggested that soil GHG emissions in the first years upon conversion were the most important, therefore, which should be considered when evaluating the environmental consequences of land-use conversion. This study also helped us develop effective options to alleviate the effects of land-use conversion on GHG emissions, and for sustainable agricultural production and GHG mitigation.