Lithological impact on topsoil organic carbon storage of karst forest soils shaped by aggregate pool complexity and their molecular composition

Preservation of soil organic carbon (SOC) of forestlands has been challenged with land degradation and climate warming. However, the impact of lithology on SOC preservation in karst forestlands has been poorly addressed. Topsoil core samples of forestlands were collected from karst terrains in central Guizhou, Southwest China, over sandstone (SS), dolomite (DS), and limestone (LS) bedrocks. The changes in SOC pool distribution and molecular composition were examined through size and density fractionation of water-stable aggregates coupled with biomarker assays. The soils on DS (sandy loam) and LS (loam) were finer than those on SS (loamy sand) in texture. 1:1 clay minerals dominated in SS (100 %), while 2:1 clay minerals in DS (90.6 %) and LS (77.2 %). For the mass proportion of macroaggregates, the soil on DS (45.3 %) was significantly higher by 31 %–33 % compared to those on SS and LS. Similarly, for the mean weight diameter of aggregates, the soil on DS was greater by 27 %–29 % than those on SS and LS. Furthermore, the topsoil SOC on DS (51.0 g kg⁻¹) was significantly higher by 24 % than that on SS and by 15 % than that on LS. Following the trend of clay mineralogy, the storage of particulate OC on DS (25 g kg⁻¹) was significantly higher by 108 % than that on SS and by 39 % than that on LS. Plant-derived lipids (PL, including cutin, suberin, long-chain fatty acids, and phytosterols) were abundant in macroaggregates and microaggregates compared to the silt-clay fractions, while microbial-derived lipids (ML) remained similar across aggregate size fractions. Moreover, PL preservation followed the order of LS > DS > SS. Notably, the DS soils exhibited higher abundances of lignin phenols and ML, while the SS soils had lower abundance of lignin phenols but higher abundance of p-hydroxy phenols. Thus, lithology impacted SOC accumulation via shaping pool distribution and selective preservation of plant-derived OCs in soil aggregates, via modulation of soil texture and clay mineralogy. Forest soils on dolomite provided higher SOC storage through enhanced aggregation with the rich 2:1 clay minerals.