Carbon Stocks and Soil C Dynamics: an Investigation of C Sequestration Potential in a Eucalyptus grandis Plantation in Hawaii

Tropical forests are important for many reasons, one of which is their ability to transfer large quantities of CO2 from the atmosphere to living biomass thereby potentially offsetting climate change. If the biomass is then harvested for commercial use, the stored carbon (C) is released back to the atmosphere. As a result, commercial rotational forestry is generally considered C neutral. However, the growth and harvest of forests also affects the soil C cycle through inputs of below ground biomass in proportion to above ground biomass. With sustainable management practices, soil can be a long-term sink for C, shifting the C balance of the system and providing a climate offset. This study examines the C stocks and dynamics of an E. grandis plantation located in Hawaii. The study has two parts: 1) A snapshot of C resources in the plantation, and 2) An investigation of change in soil C stock and pool size with afforestation. Above ground biomass C was calculated from measurements of the E. grandis trees and ranged from 40-67 Mg C/ha. Below ground biomass C was estimated from published allometric equations and was 16-27 Mg C/ha. 55 preliminary soil cores from 0-30 cm were collected in a 400 m2 plot in the plantation. Strong spatial dependence was observed in a sample variogram constructed from this data, and cumulative organic C in the top 0.4 t ranged from 120-580 Mg C/ha. To identify the effect of E. grandis afforestation on changes in soil C stock and pools, we compared adjacent pastureland and forested plots in a paired design with six sites. The paired plots constrained elevation, climate, and soil series, so that the effects of conversion from pasture to E. grandis plantation could be evaluated. Soil is physically separated into fractions that have different C turnover times: the labile pool which decomposes rapidly, the intermediate (or intra-aggregate) pool which turns over on a decadal scale, and the mineral-associated pool, which can reside in the soil for hundreds to thousands of years. Previous data showed that in the surface 0-15 cm mineral soil, land use change from pasture to E. grandis resulted in a ≈ 30% decrease in total soil C stock specifically due to losses of the most labile soil C pools. This was expected, as grasses tend to input larger amounts of root biomass C in the surface soil. Therefore, it is hypothesized that soil under E. grandis is effective at storing C within fractions associated with longer term C sequestration. It is also hypothesized that soil under E. grandis will contain more C than the pasture soil in the deeper soil (30 cm to 1m) due to large differences in rooting depth. The rooting depth of E. grandis may account for a higher degree of organo-mineral association as there is more mineral surface area available for root exudate C to bind to in the rhizosphere. Furthermore, the lack of disturbance in the E. grandis plantation compared to grazed pastureland may lead to greater physical protection within aggregates; again leading to increased C sequestration.