Magnitude and temperature sensitivity of tephra-derived soil carbon pools across a mean annual temperature gradient in tropical montane wet forest

Soils represent the largest terrestrial sink of carbon (C), potentially offsetting some effects of anthropogenic increases in atmospheric CO₂ and associated climate change. Tephra-derived soils typically have a large capacity to adsorb and retain C due to large exposed mineral surface area. However, little is known about how these soils will respond to climate change over decades to centuries. To isolate the effects of mean annual temperature (MAT) on soil C storage in tropical montane forests, soils were collected along a 5.2°C MAT gradient located on the northeastern slope of the Mauna Kea Volcano on the Island of Hawaii. Sites along this gradient constrained many variables that impact soil C such as disturbance history, soil type and age, vegetation, and soil water availability, providing the unique opportunity to investigate differences in soil C dynamics associated with temperature. Although our previous work found that total C storage and radiocarbon-based mean residence time of bulk soil did not differ along this MAT gradient, other studies have shown that at higher temperatures there is a smaller contribution of readily degradable C due to rapid and extended decomposition. As a result, these soils should be relatively resistant to increased decomposition under warmer conditions. Therefore, we hypothesized that the amount of soil C within unprotected, easily decayed C pools would decrease with increasing MAT and that sensitivity to increased temperature also would decrease with MAT. To test these hypothesis, soil collected from 0-15 cm section of the organic to mineral transition layer was physically separated into C pools with sequential density fractionation, and then incubated at a range of temperatures (16-26°C) consistent with current conditions and IPCC projections. We found, consistent with previous findings of the bulk soil from the gradient, that there was no relationship between MAT and C pool size, distribution among C pools of varying mean residence times, or temperature sensitivity of these C pools. These findings are consistent with other work demonstrating the importance of mineralogy, specifically organo-Al complexes, in sorption and stabilization of C inputs to tephra-derived soils, which occur in this system. Taken together, this and prior studies provide a better understanding of how tephra-derived soils will respond to warming temperatures in terms of their ability to continue to store and retain large quantities of C.