Soil carbon storage does not vary with temperature along a 5°C mean annual temperature gradient in Hawaiian tropical montane wet forests

Background/Question/Methods

In forest ecosystems, annual CO₂ release from soil to the atmosphere (soil respiration; F_S) accounts for ≥50% of gross primary productivity. Tropical broadleaf forests, in particular, account for ~30% of the global annual F_S budget. Numerous studies have documented increased F_S with increasing temperature. This is often interpreted as evidence that future warming will accelerate heterotrophic losses of belowground carbon and result in a positive feedback between F_S and warming. Alternatively, because total belowground carbon flux (TBCF) increases with mean annual temperature (MAT), increased F_S with warming may result from increased belowground substrate supply with no change in soil carbon storage and, therefore, no positive feedback on warming. In this study we quantified carbon flux into (TBCF) and out of (F_S) soil, mineral soil carbon pools to 91.5 cm depth, and soil carbon mean residence time (MRT) in nine tropical montane wet forest plots across a 5°C MAT gradient (13-18°C). Importantly, plots did not vary in overstory composition, disturbance history, substrate type/age, or soil water balance, allowing us to isolate the impact of future warming on soil carbon pools and fluxes. This study addresses a topic of considerable importance to understanding feedbacks between terrestrial carbon cycling and global climate change.

Results/Conclusions

Across our gradient, F_S increased linearly and positively with MAT (~95 g C m^-2 yr^-1 per 1°C; P=0.04; r²=0.42), but TBCF also increased linearly and positively with MAT (~85 g C m^-2 yr^-1 per 1°C; P=0.04; r²=0.43), supporting the hypothesis that increased belowground carbon supply drives increased F_S with temperature. Across all plots, mineral soil carbon averaged 24,792 g C m^-2 (±1,273 S.E.) and, critically, did not vary with MAT at any depth: 0-10cm (r²=0.12; m=-161.9); 10-30cm (r²=0.02; m=81.4); 30-50cm (r²=0.03; m=107.9); 50-100cm (r²=0.13; m=536.4); 0-100cm (r²=0.05; m=563.8). While soil carbon storage did not vary with MAT, preliminary data indicate that soil carbon MRT declines sharply at 30-50cm depth with increasing MAT (~5,000 yrs at 15°C to ~2,000 yrs at 17°C). To date, evidence for a strong impact of warming on soil carbon storage has been inconclusive. The data presented here strongly suggest that warming, by itself, will not decrease soil carbon storage and, therefore, will not result in a positive feedback with warming. However, our data do suggest that warming will decrease soil carbon MRT, with important implications for ecosystem carbon storage in a warming world that will likely include increases in the frequency and intensity of natural disturbances.