Disentangling the effects of temperature and reactive minerals on soil carbon stocks across a thermal gradient in a temperate native forest ecosystem

Effects of global warming on soil organic carbon (C) can be investigated by comparing sites experiencing different temperatures. However, observations can be affected by covariance of temperature with other environmental properties. Here, we studied a thermal gradient in forest soils derived from volcanic materials on Mount Taranaki (New Zealand) to disentangle the effects of temperature and reactive minerals on soil organic C quantity and composition. We collected soils at four depths and four elevations with mean annual temperatures ranging from 7.3 to 10.5 °C. Soil C stocks were not significantly different across sites (average 162 MgC ha⁻¹ to 85 cm depth, P > .05). Neither aluminium (Al)-complexed C, nor mineral-associated C changed significantly (P > .05) with temperature. The molecular characterisation of soil organic matter showed that plant-derived C declined with increasing temperature, while microbial-processed C increased. Accompanying these changes, soil short-range order (SRO) constituents (including allophane) generally increased with temperature. Results from structural equation modelling revealed that, although a warmer temperature tended to accelerate soil organic C decomposition as inferred from molecular fingerprints, it also exerted a positive effect on soil total C presumably by enhancing plant C input. Despite a close linkage between mineral-associated C and soil organic C, the increased abundance of reactive minerals at 30–85 cm depth with temperature did not increase soil organic C concentration at that depth. We therefore propose that fresh C inputs, rather than reactive minerals, mediate soil C responses to temperature across the thermal gradient of volcanic soils under humid-temperate climatic conditions.