Interactive Feedbacks of Climate, Mineralogy and Microbiological Communities on Soil Carbon: A Deep Soil Warming Experiment.

Soils that have a high potential to drawdown carbon (C) from the atmosphere, such as soils derived from volcanic ash, could aid in mitigating climate change while long term solutions are enacted. In Hawaiian Andisols organo-mineral associations are strong soil C-protection mechanisms leading to large soil C stocks. Existing evidence from non-Andic soils suggests that increased soil temperature escalates microbial activity in the soil and consequently soil respiration rates throughout the soil profile, implying declining C stocks if metabolic losses outweigh input gains. However, how the intensive warming predicted with climate change by 2100 will impact C cycling and storage in Andisols is unknown. On a 200 m² hillslope of a wet montane Andisol, soil was heated using a randomized design that allowed for a temperature gradient of ambient to +4°C across sampling depths (20, 40, 60, 80, and 100 cm). After a year of deep soil warming during which soil respiration and flux throughout the soil profile was measured, the warming response ranged from 3 to 95 μmol CO 2 m ^-3 hr ^-3 across the gradient of temperature and depth. A Kruskal Wallis test indicated that overall, the amount of C released from the soil did not significantly increase in response to warming at depth. Generalized additive modeling, confirmed the hypothesis that poorly and non-crystalline minerals (hydroxylamine hydrochloride extractable Al + 0.5Fe), derived from the volcanic ash parent material, was the primary driver of the lack of CO ₂ response. Other significant drivers of the CO ₂ response were volumetric water content (38-45%) with a parabolic relationship and depth with a negative relationship. Bacterial and fungal diversity measured by high-throughput sequencing at 20 and 40cm were significantly more diverse than those at 60, 80 and 100cm. The significant relationship of mean CO ₂ produced in response to warming and poorly and non-crystalline minerals suggests the strong organo-mineral protection mechanisms found in Hawaiian Andisols could be considered as a C management tool.