Soil organic carbon retention more affected by altitude than texture in a forested mountain range in Brazil

Abstract

Little is known about how soil organic carbon (SOC) under tropical forests is affected by altitude. We investigated the effects of soil texture and altitude on SOC retention under native forests in Brazil, by sampling two pairs of soils of coarser and finer texture, developed respectively from quartzite and mica-schist. One soil pair was sited at altitude 1060 m, and the other at 1230 m, along a mountain range. For 1060 m, the soil with ~36% clay contained 227 Mg SOC ha–1 (0–1 m depth), the double of the soil with ~16% clay. Such effect of texture was negligible at 1230 m, where SOC stocks were 205 Mg ha–1 for the soil with ~12% clay, and 217 Mg ha–1 for the soil with ~21% clay. Furthermore, there were positive correlations between SOC concentrations with clay + silt contents and specific surface area for 1060 m but not 1230 m altitude, suggesting that SOC retention was affected by texture only for altitudes near 1000 m or lower. At 1230 m, lower temperatures were predominant in SOC stabilisation through slower decomposition. In addition, 65–80% of total SOC was associated with clays, indicating a high degree of organic alteration. Interestingly, at 1235 m, SOC concentration in clay fractions reached 17%, much higher than at 1060 m, strongly suggesting SOC saturation. Thus, at 1235 m the interaction of SOC with soil minerals was probably weaker than at 1060 m or lower, and so SOC stocks are more susceptible to decay.