Carbon Mineralization Controls in Top- and Subsoil Horizons of Two Andisols Under Temperate Old-Growth Rain Forest

The effect of mineral properties on soil organic carbon (SOC) destabilization processes is poorly known. Andisols developed under temperate old-growth rain forest store important amounts of SOC due to strong mineral adsorption capacity. We evaluated the effect of mineral soil properties on microbial activity and native SOC mineralization in an incubation experiment with and without the addition of labile organic carbon (C) and nutrients. Top- and subsoil horizons of two Andisols with contrasting age from temperate old-growth rain forest were sampled. We used the selective dissolution methods to determine the metals complexed with SOC and allophane content. Radiocarbon analysis activity was performed to determine the 14C activity and 14C ages of all soils as a proxy for SOC persistence. Additionally, we carried out a laboratory incubation under controlled conditions with and without 13C labeled cellulose and nutrient solution (NH4NO3, KH2PO4) addition. A lower microbial activity in the subsoils of both Andisols compared to topsoils in accordance with the lower 14C activity at depth was recorded. After cellulose and nutrient additions, both subsoils horizons showed native SOC mineralization after 80 days of incubation. In contrast, these amendments did not induce native SOC mineralization in topsoils showing differences on C turnover rates between top- and subsoil horizons. In both soils, the native SOC mineralization after labile C and nutrient additions was negatively related to Na-pyrophosphate extractable C and may therefore be controlled by the formation of organo-mineral complexes with increasing soil C age. We conclude that the physicochemical soil characteristics evolving during pedogenesis control the SOC stabilization and also destabilization processes in Andisols developed under temperate old-growth rain forests.