Carbon storage and weathering in soils across a high elevation climate gradient on Mauna Kea, Hawai’i

Background/Question/Methods Volcanic soils retain the largest and most persistent soil carbon pools of any ecosystem. However, the mechanisms governing soil carbon accumulation during initial phases of weathering are not well understood.We examined secondary mineral production and organic carbon storage across a high altitude (3563 - 3013 m) 20 ky climate gradient on Mauna Kea in Hawaii. Four elevation sites were selected (~250-500 mm mean annual precipitation,~0-3 C mean annual temperature) which range from arid-periglacial high-altitude sites that support virtually no plants to slightly lower altitude sites that support a sparse mix of shrubs and grasses. Mass loss and pedogenic transformations were used to characterize the extent of weathering, leaching, changes in soil mineralogy and carbon accumulation with the short-range-ordered (SRO) minerals.Results/Conclusions These reactive-phase minerals increase in abundance with decreasing elevation.  However carbon accumulation patterns across the climate gradient are largely decoupled from these trends.  The results suggest that after 20ky, pedogenic processes have altered the nature and composition of the volcanic parent material such that it is capable of retaining soil C even where organic acid influences from plant material and leaching from precipitation is severely limited. Comparisons with lower elevation soils on Mauna Kea and other moist mesic (2500mm precipitation) sites on Hawaii suggest that given their weathering and secondary mineral status these soils have reached only between 1-15 % of their capacity to retain carbon.Changes in soil carbon composition and amount across a broader precipitation (250-2500mm) and temperature range on Mauna Kea indicate that the rate of carbon supply to the subsoil, driven by coupling of precipitation and plant (organic) matter inputs, is a governing factor of forms and amount of soil organic matter accumulation, while soil mineralogy remained relatively uniform.