A concrete mechanistic understanding about how climatic variables, microbial activity, soil mineralogy, and carbon (C) chemistry interact to determine soil C storage is necessary to accurately predict feedbacks to global change.
This research used soils from three montane forest sites along soil age gradients on basaltic lava flows in Hawaii to assess interacting effects of warming and soil mineralogy on the fate of newly added C in soils. All three sites have similar elevation, climate, vegetation, topography and parent material, but vary substantially in parent material age resulting in a distinct soil mineralogy gradient ranging from dominantly primary minerals at the youngest site (Thurston ~300 yrs.) to increasing amounts of metastable noncrystalline secondary minerals in the older sites (Laupahoehoe ~ 20,000 years and Kohala ~150,000 yrs.).
Soils were collected from 0-10 cm depths and incubated in the lab at 16 °C (ambient temperature) and 26 °C (elevated temperature). 13C-labeled glucose and glycine were added to track the fate of C with warming across sites. Carbon dioxide fluxes were measured periodically during incubation. 13C was tracked in microbial biomass, soil solution, and soil matrix after 4 days and 8 months.
Results/Conclusions
The youngest soil had the lowest microbial biomass C, soil organic carbon (SOC) content, and heterotrophic respiration. Microbial biomass C didn’t show significant change when incubated at the elevated temperature during first 4 days but declined when incubated for 8 months across sites.
Warming showed a significant effect on the retention of 13C substrates in soils and in microbial biomass. First, there was significant uptake and retention of added 13C substrates in microbial biomass and soil matrix during the first 4 days of incubation. Also, glycine 13C retention in soil declined at elevated temperature during the first 4 days relative to ambient temperature, with no difference for glucose. After 8 months, greater losses of 13C were observed at the elevated temperature for both substrates, with a larger effect for glycine 13C. Eight months of warming also reduced 13C retention in microbial biomass pool relative to ambient temperatures for both substrates. These results show that warming decreases the retention of simple C substrates in soil, and also affects which C pool is retained, with somewhat different responses by different C substrates.