侵蚀泥沙粒级对有机碳迁移转化的影响

Substantial research has dedicated to erosion-induced changes of soil organic carbon (SOC) pools. However, most of the studies just applied the differences of the SOC pools between eroding and depositional sites, without properly accounting for the potential changes of SOC during transport. The transport of SOC is associated with sediment classification. Conventional dry-/wet-sieving merely fractionates soil by particle size, hence cannot reflect the potential influence of shape, porosity, and density to the movement patterns of sediment particles. Along the heavily eroded slopes on the Chinese Loess Plateau, soil layers originated from different sources (e.g., tillage layer, parent loess soil and ancient red soil) have been exposed, forming a patchwork which is simultaneously subject to rainfall and erosion events. In order to investigate the potentially distinctive transport patterns of these soil layers, this project plans to compare the soil classes fractionated by two methods: wet-sieving and settling velocity. The total amount of SOC, labile and stable fractions of SOC, δ13C, CO2 emissions and microbial communities of individual classes will be measured and compared, with the aim to identify the effects of selective transport and preferential deposition of individual sediment particles to the spatial redistribution and mineralization of eroded SOC. By comparing the differences of microbial communities in mixed particles originated from different layers yet with the same settling velocity, this study also intends to capture the potential influences of micro-climate conditions newly formed by preferential deposition processes onto the biogeochemical changes of SOC in depositional areas. The understanding of settling-velocity induced selective transport or preferential deposition of eroded SOC can also help to advance our current understanding of soil erosion-transport-deposition to global carbon cycling.

土壤侵蚀是影响有机碳循环的重要过程之一。然而，评价侵蚀对有机碳源汇的影响大多依赖于侵蚀区和沉积区有机碳储量的差异，忽略了迁移过程中的有机碳转化。有机碳的迁移转化与泥沙颗粒的运动规律密切相关，而泥沙的运动规律由颗粒大小、性状、孔隙度、密度等因素共同决定。传统湿筛法仅依赖于粒径大小，在反映泥沙运动规律方面存在片面性。本项目针对黄土高原地区因常年侵蚀形成的不同土层出露现象 (耕层土壤、黄土母质、第三纪红土)，拟利用经本人改进的沉降管法，更客观的反映坡面不同出露土层颗粒的运动规律，即选择性迁移和优先沉积过程；通过对比不同沉降速度级别中的有机碳总量、组分和转化，揭示泥沙颗粒的优先沉积对有机碳迁移和空间分布的影响；进一步探究坡面不同来源但相同沉降速度的颗粒在同一处混合沉积后，微环境重组所诱发的微生物群落结构改变对有机碳组分转化的影响，为深入理解土壤侵蚀-迁移-沉积全过程对有机碳循环的影响提供理论依据。

土壤有机碳在坡面上的空间分布与泥沙颗粒迁移距离有关。然而，传统的湿筛或干筛法只简单地通过粒径大小进行颗粒分选，并未考虑颗粒的形状、密度、孔隙度对其运动规律的影响，无法准确反应泥沙颗粒在迁移过程所发生的沉降分选现象。本项目利用速度沉降管法，以土壤颗粒沉降速度分选粒级为基本单位，对比传统湿筛以粒径大小为粒级分选单位，测定不同来源、不同粒级有机碳含量、稳定性同位素13C自然丰度、CO2释放量和微生物群落结构分布规律，结果表明：1) 沉降速度分级法与湿筛法在分离土壤颗粒过程中的原理本质不同，对土壤颗粒的重量分布以及有机碳含量和微生物群落特性等方面也产生截然不同的影响。充分突出了沉降速度分级法在研究泥沙侵蚀沉积规律方面的优势，综合考虑了颗粒大小、形状、孔隙度以及密度等因素对其运动规律的影响，有效克服了传统湿筛法仅依靠颗粒尺寸大小进行分离的片面性，更适用于研究泥沙颗粒侵蚀沉积过程中的分选和重分布规律。2) 不同坡位土壤颗粒组成差异明显，主要表现为先粗骨化，后细化。侵蚀热区粗骨化处地形凸度明显，母质层出露，有机碳含量仅是弱侵蚀区的一半。同时，此处CO2释放速率偏低，对温度变化极为敏感，且微生物群落多样性较弱。3) 最小沉降粒级(小于20微米)的细菌和真菌群落结构与其他沉降粒级显著不同，分别以变形菌和子囊菌为主，相对丰度占比分别达到86%和50%，致使其Alpha和Beta多样性均显著降低。但当细小颗粒在坡脚沉积区富集后，泥沙中的细菌或真菌群落并没有表现出相应的趋同性，反而有小幅度的多样化表现。这些结果表明不同侵蚀坡位微生物群落结构和多样性不仅与颗粒分异特征有关，很大程度上还受不同坡位环境条件的影响。本研究结果揭示了土壤颗粒沉降速度在侵蚀迁移分选过程对有机碳空间分布的作用机理，并进一步探究粒级间微生物群落结构异质性对坡面微生物空间分布格局和有机碳矿化影响，为改进侵蚀模型中土壤可迁移性参数和深化侵蚀碳循环理论提供依据。