Pore structure characteristics and organic carbon distribution of soil aggregates in alpine ecosystems in the Qinghai Lake basin on the Qinghai-Tibet Plateau

Soils in alpine ecosystems serve as important carbon sinks. Soil aggregates provide physical protection to soil organic carbon (SOC) and pore networks of them can greatly influence SOC sequestration. However, previous studies mainly focused on SOC dynamics at ecosystem or regional scale in cold alpine regions. The lack of quantification of alpine soil aggregate pore structure limits our understanding of SOC sequestration mechanisms of alpine soil. In this study, three typical alpine ecosystems in the Qinghai Lake basin of Qinghai-Tibet Plateau (QTP) were selected: Kobresia pygmaea meadow (KPM), Potentilla fruticosa shrubland (PFS), and Achnatherum splendens steppe (ASS). Soil pore structure, SOC and their relationships in aggregates were examined through sieving procedure and X-ray computed tomography (CT) scanning. The results showed that the mass proportion of small macroaggregates (SMA, 0.25–2 mm) was highest and they accounted for 36–51% of all aggregate fractions in the above three alpine ecosystems. The pores of soil aggregate were found to be dominated by micropores (<30 μm), which accounted for over 90% of the pore numbers. Aggregates of the ASS ecosystem had highest mean pore volume (7.761 × 10³ μm3 in average) while soil aggregates of the KPM ecosystem had the highest pore number density (4.137 × 10-5 no. μm−3 in average). The highest total organic carbon (TOC) content was mostly observed in the mA (microaggregates, 0.053–0.25 mm) fractions in the three ecosystems. Particulate organic carbon (POC) content was higher than mineral-associated organic carbon (MAOC) content of aggregates. Pores of < 15 μm served as indirect indicators for SOC protection in soil aggregates. Pores of 15–30 μm in aggregates might provide voids for SOC decomposition. TOC and MAOC content were positively correlated with pore surface area density while POC content was positively correlated with pore equivalent diameter and mean volume of aggregates. SOC was mainly affected by soil water content (SWC) in KPM and PFS ecosystems while it was mainly affected by soil particle compositions in the ASS ecosystems. Our results revealed the structural characteristics of aggregates of alpine soil and their relationships with the carbon storage function, which are helpful to understand the micro-scale mechanism of carbon sink of alpine soil.