Mineral control of organic carbon mineralization in a range of temperate conifer forest soils

Coupled climate-ecosystem models predict significant alteration of temperate forest biome distribution in response to climate warming. Temperate forest biomes contain approximately 10% of global soil carbon (C) stocks and therefore any change in their distribution may have significant impacts on terrestrial C budgets. Using the Sierra Nevada as a model system for temperate forest soils, we examined the effects of temperature and soil mineralogy on soil C mineralization. We incubated soils from three conifer biomes dominated by ponderosa pine (PP), white fir (WF), and red fir (RF) tree species, on granite (GR), basalt (BS), and andesite (AN) parent materials, at three temperatures (12.5°C, 7.5°C, 5.0°C). AN soils were dominated by noncrystalline materials (allophane, Al-humus complexes), GR soils by crystalline minerals (kaolinite, vermiculite), and BS soils by a mix of crystalline and noncrystalline materials. Soil C mineralization (ranging from 1.9 to 34.6 [mg C (g soil C)⁻¹] or 0.1 to 2.3 [mg C (g soil)⁻¹]) differed significantly between parent materials in all biomes with a general pattern of AN<BS<GR. We found significant negative relationships between Fe-oxyhydroxides, Al-oxyhydroxides, and Al-humus complex content and soil C mineralization, suggesting mineral control of C mineralization. Modeled decomposition rates and mineralizable pool size increased with increasing temperature for all parent materials and biomes. Further, δ¹³C values of respired CO₂ suggest greater decomposition of recalcitrant soil C compounds with increasing temperature, indicating a shift in primary C source utilization with temperature. Our results demonstrate that soil mineralogy moderates soil C mineralization and that soil C response to temperature includes shifts in decomposition rates, mineralizable pool size, and primary C source utilization.