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

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.