稻田转为菜地初始阶段温室气体排放特征

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

[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·hm^-2·a^-1),这主要归功于后两年降雨量的增加引起了土壤水分含量的升高。稻田转为菜地显著减少了CH4排放,减少量相当于稻田CH4年累积排放量的83%-100%。菜地第一年的CH4累积排放量(31.22 kg CH4-C·hm^-2)显著高于第二年(0.45 kg CH4-C·hm^-2)和第三年(0.89 kg CH4-C·hm^-2),表明稻田转菜地对CH4排放的影响具有时间滞后效应。稻田是弱的N2O排放源(1.35-3.49 kg N2O-N·hm^-2·a^-1),其转为菜地显著增强了N2O排放。菜地第一年的N2O累积排放量(95.12 kg N2O-N·hm^-2)显著高于第二年(38.28 kg N2O-N·hm^-2)和第三年(40.07 kg N2O-N·hm^-2)。菜地土壤异养呼吸对N2O排放的影响在第一年明显高于第二、三年,表明稻田转为蔬菜种植的第一年,有机质矿化对N2O排放有重要贡献。在100年尺度CO2当量下,稻田转为蔬菜种植第一和第二年的综合增温潜势(GWP)相对于稻田分别显著增加了390%和98%,主要是由于增加的N2O增温潜势超过了减少的CH4增温潜势。但是,稻田转为菜地的第三年,菜地的GWP((16.72±3.25)Mg CO2-eq·hm^-2)与稻田((14.84±1.39)Mg CO2-eq·hm^-2)相比无显著差异,主要是由于减少的CH4增温潜势完全抵消了增加的N2O增温潜势。这些研究结果表明稻田转菜地对GWP的影响主要集中在该土地利用方式转变的第一年。[结论]稻田转为菜地显著减少了CH4排放,增加了N2O排放,增强了菜地第一和第二年的综合增温潜势。有机质矿化过程对新转菜地第一年较高的N2O排放有重要贡献。这些研究结果表明了评价土地利用方式转变初始阶段温室气体排放特征的重要性,便于及时采取有效管理措施缓解温室气体排放,实现环境友好型农业可持续生产。

[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(Rh),and the dependence of N2O fluxes on Rh 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 MgCO2-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.