关键词: 稻田, 菜地, 土地利用方式, CH₄, N₂O, 综合增温潜势

Abstract:

【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.

Key words: rice paddy, vegetable field, land-use conversion, CH₄, N₂O, global warming potential (GWP)