侵蚀黑土逐层沉积结构异质性及其对温室气体排放的影响

    Heterogeneous settling and bedding of black soil particles and their impacts on greenhouse gas emissions

    • 摘要: 为探究沉积区不同沉积层次结构及理化性质差异对温室气体排放的影响,该研究取典型侵蚀黑土进行粒级沉降分选,将颗粒由粗到细分成6个粒级(L1~L6),并按不同泥沙迁移情景,将粗细颗粒依次逐层沉积,形成6种不同深度和层次结构的沉积土柱(S1~S6):S1仅由最粗粒级L1组成,S6则由L1~L6从粗到细、自下而上依次逐层沉积而成。将不同土层结构土柱进行恒温恒湿培养,并监测CO2、N2O和CH4的排放速率。结果表明:1)不同沉积土层孔隙度由L2层的23.79%减少至L6的1.00%,而含水率则由L2层的11.16%增加到L6层的41.02%。2)碳氮含量集中分布在L2~L4层。3)7 d培养期间,温室气体平均释放速率总体随沉积层数增加而显著降低(P<0.05),也表现为气体通量并未随着沉积土层增厚而增加。逐层沉积过程使得粗细颗粒分离,粗疏颗粒在下层优先沉积,而细密颗粒在上层堆叠,致使孔隙度和气体相对扩散系数随颗粒自下而上随逐层沉积而逐渐降低,仅1.5 cm厚的细密沉积层便可有效抑制温室气体向土表排放。该研究通过颗粒沉降分选和逐层沉积,从细观过程刻画沉积分层和掩埋效应对温室气体排放的影响,对深化沉积环境温室气体排放机理具有重要意义。

       

      Abstract: Particle heterogeneity is one of the key indicators to estimate the variability in the soil properties and compositions. This study aims to explore the effects of the soil particles settling and bedding on the greenhouse gas emissions from the depositional area. The layer-specific physicochemical properties were also considered during this time. Specifically, one type of Mollisol (local people named as the black soil) was sampled from a typical eroding slope in the northeastern China. Then six classes were fractionated by their respective settling velocities. Six structures were obtained to bed the differently-sized soil particles in sequence: S1 was the thinnest structure with only > 500 μm particles L1, whereas S6 was the thickest structure with the six particle classes being layered in sequence from the coarsest particles L1 at the bottom to the finest particles L6 on top. The emissions of greenhouse gases CO2, N2O and CH4 were finally collected from the incubated structures on a daily basebasis. The results showed that: 1) The coarse particles settled first (L1) on the bottom, whereas, the fine particles settled the last and then covered on the top (L6). The porosity gradually decreased from 23.79% of coarse layer L1 to 1.0% of fine layer L6. By contrast, the soil water content increased generally at the highest moisture content of 68.78%, as the soil particles were settled with the L5. 2) Carbon and nitrogen were rich in the L2-L4 layer, where the soil organic carbon and total nitrogen were peaked in the L2 layer (with 27.34 and 2.35 g/kg, respectively), and the maximum dissolved organic carbon and nitrogen appeared in the L3 and L4 layer (with 519.31, and 19.69 mg/kg, respectively). 3) The average greenhouse gas emission rates decreased significantly during the seven incubation days, as the sedimentary layers were thicker (P<0.05). Correspondingly, there was no increase in the gas flux with the thickening of the sedimentary soil layer. The layer-by-layer deposition process was separated from the coarse and fine particles, where the coarse particles were preferentially deposited as the lower layers, while the fine ones were stacked over forming upper layers. Consequently, there was a gradual decrease trend of the porosity and relative diffusion coefficient of gas from the bottom to the top. As such, it is expected to effectively inhibit the transfer of greenhouse gases via the depositional profile. Overall, the layer-by-layer settling and bedding can be used to reconstruct the deposition process in the field, and then to effectively capture the potential impacts of depositional layer structure on greenhouse gas emissions. The finding can overcome the limitations of traditional sampling, indicating the outstanding variations of small-scale layers and bedding. The settling and bedding patterns can also be characterized to quantify the greenhouse gas emissions from different depositional settings.

       

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