Pool distribution and molecular composition of organic matter among water-stable aggregate size fractions of karst topsoil across a lithologic sequence from Guizhou, Southwest China

Soil organic matter (SOM) is a key player in soil functioning and services in forest lands, which had been subject to accelerated land degradation particularly in karst terrain in Southwest China. So far, there had been poor knowledge of pool and molecular composition of SOM associated with soil aggregates across lithologic origins of karst soil. In this study, undisturbed topsoil (0-10 cm) samples were collected in forest lands on sandstone (SS), dolomite (DS) and limestone (LS) sedimentary rocks in a karst terrain from Guizhou, Southwest China. Changes in SOM pool distribution and molecular composition of water-stable aggregates were explored using size and density fractionation and GC/MS detection of extracted biomarkers. The OC content ranged from 41.05 g kg^-1 on SS to 50.94 g kg^-1 while the mean weight diameter of sand-free soil water-stable aggregates ranged from 420.9 μm on SS to 544.4 μm on DS, across the lithologic sequence. With biomarker molecular assay, the higher SOC storage was relevant to the higher abundance of plant-derived organics (lignin, cutin, suberin, wax and phytosterols) in macro- and micro- aggregates. Whereas, the higher OC in silt & clay fraction of topsoil on DS and LS could be explained by the higher abundance of microbial lipids plus cutin and suberin. Also, the higher ratio of (Ad/Al)v to (Ad/Al)s of silt-clay fraction pointed to a stronger degradation of lignin thereby. Thus, the forest soil of dolomite and limestone origin preserved a relatively high level of SOC storage in topsoil, mainly with accumulation of POC physically protected in macro- and micro- aggregates. Moreover, the high SOC of topsoil on dolomite could also be attributed to enrichment of SOC in the clay silt fraction, mainly with mineral association of microbially degraded OCs.