Carbon Cycling and Soil Carbon Storage in Mesic to Wet Hawaiian Montane Forests

Global surveys have shown that increased precipitation in mesic to wet ecosystems increases soil carbon storage, but it is not clear how changes in water availability act to affect soil carbon since water is generally in excess of plant demand within these humid ecosystems. Differences in precipitation can affect soil carbon storage either by altering the biotic processes involved in carbon cycling or by altering abiotic processes such as carbon adsorption on soil minerals. We compared the relative effect of water on net primary productivity and decomposition, and on soil weathering and mineralogy in controlling soil carbon storage in forests. Patterns of ecosystem carbon cycling and soil carbon storage were measured in Hawaiian montane forest sites similar in temperature regime, parent material, ecosystem age, vegetation, and topographical relief, while mean annual precipitation alone varied from 2200 to >5000 mm/yr. Soil carbon storage ranged from 30.9 to 62.5 kg/m2 and increased by a factor of 1.7 with increased precipitation across the gradient. Neither changes in total secondary minerals nor noncrystalline mineral content could explain increased soil carbon as both decreased with increasing precipitation. Aboveground net primary productivity, soil carbon dioxide flux, and plant litter decomposition rates also decreased with increased precipitation. These data suggest that increased soil carbon storage was not stimulated by an increase in net primary productivity, but instead by a decrease in decomposition rates. Decreased decomposition rates corresponded with low soil reduction–oxidation potentials, suggesting that soil oxygen availability was limiting to microbes. While soil oxygen did not appear to affect plant growth directly, the decline in net primary productivity corresponded with decreased nitrogen availability, a consequence of slower decomposition and nutrient release. Thus, in humid environments the most important effect of variation in water on carbon cycling appears to be its control on the diffusion of oxygen into the soil.