Soil carbon stocks in stable tropical landforms are dominated by geochemical controls and not by land use

Soil organic carbon (SOC) dynamics depend on soil properties derived from the geoclimatic conditions under which soils develop and are in many cases modified by land conversion. However, SOC stabilization and the responses of SOC to land use change are not well constrained in deeply weathered tropical soils, which are dominated by less reactive minerals than those in temperate regions. Along a gradient of geochemically distinct soil parent materials, we investigated differences in SOC stocks and SOC ( delta C-14) turnover time across soil profile depth between montane tropical forest and cropland situated on flat, non-erosive plateau landforms. We show that SOC stocks and soil delta C-14 patterns do not differ significantly with land use, but that differences in SOC can be explained by the physicochemical properties of soils. More specifically, labile organo-mineral associations in combination with exchangeable base cations were identified as the dominating controls over soil C stocks and turnover. We argue that due to their long weathering history, the investigated tropical soils do not provide enough reactive minerals for the stabilization of C input in either high input (tropical forest) or low-input (cropland) systems. Since these soils exceeded their maximum potential for the mineral related stabilization of SOC, potential positive effects of reforestation on tropical SOC storage are most likely limited to minor differences in topsoil without major impacts on subsoil C stocks. Hence, in deeply weathered soils, increasing C inputs may lead to the accumulation of a larger readily available SOC pool, but does not contribute to long-term SOC stabilization.

Automated summary

Soil organic carbon (SOC) dynamics in deeply weathered tropical soils are not well understood. In this study, we compared SOC stocks and turnover in montane tropical forest and cropland. We found that land use did not significantly alter SOC, but differences in SOC could be explained by soil physicochemical properties. Labile organo-mineral associations and exchangeable base cations were identified as the main controls over SOC stocks and turnover. Our findings suggest that increasing C inputs in deeply weathered soils may not lead to long-term SOC stabilization.