THE RELATIONSHIP BETWEEN CARBON FLUXES, PRECIPITATION, AND SOIL PROPERTIES ALONG A CLIMATE GRADIENT, KOHALA VOLCANO, HAWAI'I

The Hawaiian Islands are home to pronounced environmental gradients produced by the interaction of high topography with prevailing NE tradewinds. In order to investigate the effect of changes in temperature and precipitation on soil and ecosystem processes, we established a transect along the leeward (southwestern) flank of Kohala mountain, Hawai’i, on 350,000 year old basalt flows and ash deposits.

Site elevations ranged from 550 m to 1560 m over 6 km, with a corresponding mean annual precipitation (MAP) range from 700 mm to 2400 mm [1]. The highest sites are located in native mesic forest, while the low and intermediate elevation sites were on cleared pasture with non-native kikuyu grasses.

Observations were made in the months of March and April from 2010 to 2013. Measured properties at each site include pH, soil moisture, CO₂ efflux, and soil temperature. Volumetric water content ranged from 2% to 65% (fully saturated). Soil pH ranged from 7.6 to 3.6. At the lower elevation sites, soils contained carbonate while the low pHs at the high elevations indicate soil exchange sites occupied by H⁺ and Al³⁺. Soil pHs are mostly between 6 and 7.5 up to ca. 1800 mm MAP, with no distinct trend with rainfall or elevation. Above 1800 mm MAP soil pH declines approximately linearly with increasing MAP, reaching values < 3.6.

Soil CO₂ efflux was measured using a LiCOR 6400 gas exchange system. Average soil CO₂ efflux rates were < 1 to 21 µmol m^-2 s^-1 in pasture sites and ­­­1.6 to 14.2 µmol m^-2 s^-1 in forest sites. Soil efflux rates were positively correlated with rainfall in the pasture sites, presumably reflecting increased net primary production. Lower CO₂ efflux rates in the forest sites may reflect lower net primary productivity and/or soil saturation that drastically decreases gas diffusivity. We computed soil CO₂ profiles using a reaction-diffusion model. Calculated CO₂ concentrations reach 15´ atmospheric levels for the high productivity sites. The forest sites hold substantially more CO₂both in the standing crop of biomass and in soil carbon than do the pasture sites at similar rainfall levels.

[1] Giambelluca, T.W., Q. Chen, A.G. Frazier, J.P. Price, Y.-L. Chen, P.-S. Chu, J.K. Eischeid, and D.M. Delparte, 2013: Online Rainfall Atlas of Hawai‘i. Bull. Amer. Meteor. Soc. 94, 313-316.