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Soil organic carbon retention more affected by altitude than texture in a forested mountain range in Brazil
Despite the importance of tropical forests to the global C cycle, we know little about how their soil organic carbon (SOC) is affected by abiotic factors in highlands and mountain ranges. We investigated the effects of soil texture and altitude on SOC retention under native forests in southeastern Brazil, by sampling to a 1-m depth two pairs of soils of coarser and finer texture, developed respectively from quartzite and mica-schist. One soil pair is sited at 1,060 m, and the other at 1,230 m, along a mountain range. For the 1,060 m altitude, the fine-textured soil (~ 36% clay) contained SOC stocks of 227 Mg ha-1 (0-1 m depth), 100% more than the coarse-textured soil with ~16% clay. Such effect of texture was negligible at 1,230 m, where SOC stocks varied from 205 Mg ha-1 for the quartzite-derived soil with ~12% clay, to 217 Mg ha-1 for the schist-derived soil with ~21 % clay. Furthermore, positive correlations between SOC concentrations with clay+silt contents and specific surface area occurred for the 1,060 m but not the 1,230 m altitude. Such trends suggest that SOC retention in the study area is affected by texture only for altitudes near 1,000 m a.s.l. or lower, whereas at 1,230 m climatic effects of lower temperatures are the predominant factor in the stabilization of SOC through slower decomposition. In addition, 65-80% of total SOC is associated with clays of all soils, indicating a generally high degree of organic matter alteration. Interestingly, at 1,235 m SOC concentration in clay fractions are much higher (up to 17 %) than at 1,060 m, which strongly suggests a possible saturation of the SOC-stabilizing capacity of these soils. In other words, it is likely that at 1,235 m the interaction of SOC with soil mineral components is weaker than at 1,060 m or lower altitudes, and thus SOC stocks are probably more susceptible to decomposition with cultivation or warming.
SR17205 Accepted 31 October 2017
© CSIRO 2017