Plant-derived lipids play a crucial role in forest soil carbon accumulation

Plant and microbial residues are two main sources of soil organic carbon (SOC). While recent studies have extensively examined the distribution of microbial necromass in different ecosystems, how plant residues (in particular, non-lignin components) contribute to SOC accumulation is less clear, especially in forests which make up 50% of the global soil carbon storage. Filling this knowledge gap will help us better understand SOC accumulation patterns and their response to land-use changes. Here, we analyze plant- and microbial-derived biomarkers (including lignin phenols, amino sugars, free and hydrolysable lipids) in the topsoil of major forest types in China and compare their distribution patterns together with the existing data (for lignin phenols and amino sugars) in forests and grasslands distributed globally. At the global scale, forests contain significantly less microbial necromass in SOC compared with grasslands, suggesting higher contribution of plant-derived components to forest SOC. However, plant-derived lignin phenols do not seem to play a major role in SOC accumulation, given their negative relationship with SOC contents. Instead, leaf- and root-derived hydrolysable lipids constitute a much higher proportion of SOC than lignin phenols in the investigated forests of China, even compared to grassland soils. Moreover, in contrast to lignin phenols, both SOC contents and the relative abundance of hydrolysable plant lipids in SOC increase with decreasing soil pH, increasing reactive iron and aluminum contents and with increasing lignin oxidation (indicated by acid-to-aldehyde ratios) in these forest soils. These results suggest that with increasing lignin decomposition, plant lipids and SOC accumulated via (oxyhydr)oxide protection. Collectively, our results demonstrate differential importance of plant-derived components in SOC accumulation in forests versus grasslands and highlight that plant lipids play a more important part than lignin in forest SOC accumulation. Quantitative investigations on the distribution of plant-derived lipids in addition to lignin in forest soils may help to elucidate pathways and hotspots of plant component-dominated SOC accrual.