Mineral Control On Organic Carbon Decomposition From Soils of Diverse Eco-Regions.

Temperature, moisture content and substrate quality influence the decomposition of soil organic carbon (SOC), but stabilization by sorption onto soil minerals is still inadequately understood. This study examined the influence of the mineral fraction on the decomposition of native SOC and of added glucose C on a global suite of soils.  Five soil orders were selected from four contrasting climate zones (Mollisol from temperate, Ultisol and Oxisol from tropics, Andisol from sub-arctic and Gelisol from arctic region). Soils from the A horizon were fractionated into particulate OC (POC) and mineral associated OC (MOC) by a size-based method. Fractions were incubated at 20ºC and 50% water holding capacity in the dark after the addition of unlabeled D-glucose (0.4 mg C g fraction^-1) and U-¹⁴C -glucose (17760 disintegration per minute g fraction^-1). Respiration of glucose ¹⁴C and native C were measured throughout the experiment, while microbial biomass C (MBC) and dissolved OC were measured at days 4, 30 and 150. Results revealed that 64 to 84% of added glucose ¹⁴C was respired from POC and 62 to 70% from MOC, and more than half of the cumulative respiration occurred within 4 days. Native C respiration varied widely across soil types and fractions: 12 to 46% of native C was respired from POC and 3 to 10% was respired from MOC fractions. The respiration of native MOC was much less than that of native POC, suggesting that the chemical structure of native C and interaction with soil minerals in the MOC fraction effectively decreased respiration. In contrast, glucose ¹⁴C derived respiration in the POC and MOC did not vary as much as the native C respiration, indicating a limited protective effect of sorption on the decomposition of added glucose. This study provided evidence that native OC was more stabilized on the mineral fraction than in the particulate fraction, but that recently added glucose was equally stabilized in both fractions.