Microbial contribution to organic carbon accumulation in volcanic ash soils

Purpose: Physico-chemical mechanisms of soil organic carbon (SOC) protection in volcanic ash soils (Andisols) are well understood. However, this role of microbes in the process is unclear. This study was conducted to understand the microbial contribution to SOC accumulation in Andisols, in comparison with non-volcanic ash soils (Inceptisols). Methods: Soils were collected from agricultural land, coniferous forest, and deciduous forest (n = 7 per land-use for Andisol, n = 3 for Inceptisols). Soils were analyzed for SOC and physico-chemical properties related to SOC protection (clay content, aggregate size distribution, pH, and active Al and Fe concentration). Soil microbial biomass C (MBC), composition, respiration, and metabolic quotient (qCO₂) were further analyzed. Results: Forest Andisols had greater SOC compared to agricultural Andisols and Inceptisols from all land-use types. The greater SOC content in forest Andisols was associated with the complexation of SOC with clay and active Al and Fe. The physico-chemical protection of SOC in forest Andisols was further related to lower percentage of total SOC respired as CO₂ (% R_CO2/SOC) and qCO₂, which indicates that microbes assimilate more C than they respire. Microbial composition as well as the inter-correlation among qCO₂, R_CO2/SOC, and SOC content suggested that microbes in forest Andisols are likely to incorporate C derived from the decomposition of plant-derived C rather than indigenous SOC, contributing to SOC accumulation in the soils. Conclusions: Our study suggests that microbial contribution is critical for enhanced SOC in forest Andisols in addition to the inherent soil physico-chemical properties, but not in agricultural Andisols.