The pool size ratio of POM to MAOM (mineral-associated organic matter) and the molecular abundance ratio of PL (plant-derived lipids) to ML (microbe-derived lipids) appeared to be indicative of SOM accumulation following forest restoration. Calculated as per the Shannon diversity index (H’), changes in SOM pool complexity and molecular diversity were parallel to the SOM accumulation trend. With biomarker analyses, the relative abundances of plant-derived organics (lignin, cutin, suberin, wax and phytosterols), mostly protected in aggregates, increased, while those of microbe-derived OC, predominantly mineral bound, decreased in response to prolonged forest restoration. Compared to that of CL, SOM content was increased by 24%, 79% and 181%, mass proportion of macroaggregates increased by 136%, 179% and 250%, and particulate organic matter (POM) increased by 13%, 108% and 382%, respectively at RL10, RL20 and RL40. SOM changes were explored using the size and density fractionation of water-stable aggregates, 13C isotopic signalling and biomarker analyses as well as 13C solid-state NMR assays. In this study, undisturbed topsoil (0–10 cm) samples were collected across a decadal chronosequence of forest stands (RL10, RL20 and RL40) restored for 10, 20 and 40 years following maize cropland (CL) abandonment in a karst terrain of Guizhou, Southwest China. Fast accumulation of soil organic matter (SOM) following forest restoration shifted from cropland has been widely reported, but how the pools and molecular composition change across soil aggregate fractions remains unclear.
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