稻田甲烷氧化和传输过程的稳定性碳同位素分馏效应

Methane oxidation is a natural regulation for methane emission, and is important in reduction and regulation of greenhouse gases. Among the technologies for methane oxidation determination, the stable C-isotope technology, which is widely used in paddy soils, has advantages since it need not to destroy the natural construction of the study objects and can bring about more accurate and representive results. However, the previous studies generally used the C-isotopic composition of the methane in soil solution, aerenchyma to represent the C-isotopic composition of the oxidized methane, and artificially ignored the isotope fractionation effects of the whole or part of the methane transport processes. It would be the explanation that the methane oxidation rate derived using C-isotope were much lower than that derived using other methods. Indeed, the change in C-isotopic composition of the emitted methane is influenced by oxidation and transport. Therefore, clarifying and quantifying the fractionation effects of oxidation and different transport pathways (diffusion, bubble and plant) are essential for determining the methane oxidation in paddy soils. However, rare studies have focused on this issue. Therefore, based on field and laboratory experiments and specially-designed installations, this study aims to determine the methane fluxes of different transport pathways ((diffusion + bubble, and plant), quantify the C-isotope fractionation effects of these pathways and the seasonality, and then clarify the methane oxidation and the influencing factors. The results would be helpful for understanding the characteristics and mechanisms of methane oxidation in paddy soils, and estimation of methane production and emission.

甲烷氧化是稻田甲烷排放的自然调节过程，其机理的研究对稻田甲烷排放调控具有重要意义。在土壤甲烷氧化研究中，稳定性碳同位素法因具有不破坏测定对象的优点而被广泛应用。然而，现有研究主要采用土壤或植株某些部位的甲烷同位素组成来推断甲烷氧化，忽略了甲烷传输过程或部分过程的同位素分馏效应，往往低估了甲烷氧化作用。稻田排放甲烷碳同位素受土壤氧化和传输过程的综合影响，区分和定量甲烷氧化及不同传输路径（植株传输、扩散和冒泡）的分馏效应，是确定稻田甲烷氧化的基础，然而现有研究少有涉及。本项目结合田间试验和室内模拟试验，采用特定装置，分离稻田甲烷排放中植株和冒泡+扩散两种传输过程并确定其排放通量，明确不同传输路径同位素分馏效应以及季节变化特征，结合土壤氧化过程同位素分馏特征，定量稻田甲烷氧化率。研究成果不仅有助于深入认识稻田甲烷氧化机理，也可为我国稻田甲烷产生和排放估算提供科学依据。

结合田间试验、模拟试验以及室内培养试验，获得了稻田甲烷不同传输路径（植物传输、冒泡+扩散）排放通量和季节变化，测定了土壤甲烷氧化以及甲烷传输过程的碳同位素分馏特征，分析了不同甲烷传输路径碳同位素分馏过程的季节规律及影响因子。得出的主要结果包括：1）稻田甲烷排放具有明显的季节特征，土壤传输过程和植株传输过程具有相同的季节变化规律，其中植株传输占有稻田甲烷排放的绝大大部分（80%以上）；2）无论是土壤传输还是植株传输，其主要影响因素是土壤Eh电位和土壤温度；3）土壤传输和植株传输与气象因子的关系差异主要体现在气温上，土壤传输与气温相关性较低，而植株传输则与其有较高的相关性，这主要是因为植株的生长以及光合蒸腾作用受到气温的影响；4）在田间，土壤传输和植株传输在甲烷排放同位素分馏上具有一定补偿效应，两者相关系数为-0.6（负相关）；5）甲烷排放同位素分馏补偿效应主要发生在甲烷排放较低时期，而在排放旺季，同位素分馏现象并不明显且两种传输途径无明显补偿关系；6）甲烷传输速率是同位素分馏的首要影响因素；7）土壤甲烷氧化能力具有一定的季节波动特征，但在数量上差别并不明显，进一步研究表明，土壤氧化能力与NO3--N（硝态氮，r=0.93，P=0.006）、DOC（可溶性有机碳，r = 0.96，P = 0.002）、TN（全氮，r = 0.74，P = 0.093）、TC（全碳，r = 0.46，P = 0.356）、NH4+-N（铵态氮，r = 0.47，P = 0.344）正相关，而与pH（r = -0.51，P = 0.298）负相关；8）土壤甲烷氧化同位素分馏特征主要与甲烷氧化速率有关，且与土壤活性铁含量呈显著负相关（-0.92，P = 0.026）。