Soil acidification enhanced soil carbon sequestration through increased mineral protection

As a significant land carbon sink, highly acid subtropical forests in southern China continued to accumulate a significant amount of soil carbon under elevated acid deposition, yet the mechanism of how soil organic carbon (SOC) and its two components: particulate (POC) and mineral-associated (MAOC) organic carbon increased remain unclear. We aim to assess which mechanism and drivers dominated the accumulation of SOC and its two fractions under elevated acid deposition. We conducted a 11-year field acid addition experiment to study how acid deposition affected the accumulation of SOC and its fractions. Lignin phenols and amino sugars were used as two tracers for plant- and microbe-derived carbon. We found that both POC (0–20 cm) and MAOC (10–20 cm) were significantly increased by acid addition. Acid addition significantly reduced the contributions of fungal-, bacterial- or total microbial residue carbon to SOC but significantly increased the plant-derived soil carbon in both soil depths. The increase of lignin phenol and suppressed soil organic matter decomposition from soil microbes suggested that soil POC increased with acid addition. Soil acidification strongly enhanced MAOC accumulation through increased lignin and mineral protection by iron-aluminum oxides and cations. Our results showed that increased mineral protection of plant-derived carbon was the dominant driver of the increased SOC sequestration under acid addition. This finding identified the dominant pathway for SOC accumulation in a highly acidic subtropical forest and provides new insights into understanding how plant-soil-mineral interact under increasing acid deposition in the region.