As found in prior publications, this study confirm that steam C content was appreciably higher in conifers (P. caribaea) than the selected broadleaf species (50.8% vs. 48.2%, respectively). Our results indicate that the widely used 50% wood C content figure is a fair rule of thumb when there is lack of information. We recognized that our estimation method of C content did not include chemical forms of volatile C. As shown by [40] the volatile C fraction is nonnegligible, averaging 2.2% with a high variation among species in China.
4.3. Balance of soil nutrient reserves and aboveground nutrient content
biomass and bioenergy 35 (2011) 1779e1788 1787
Nutrients with very unstable indices (>100%) are P and K, except for P in V. ferruginea (87%) and K in P. caribaea (59%). Ca and Mg resulted in unstable stability indices that range from 16% to 68% depending on species, and N is the nutrient with the less unstable values ranging from 1.5 to 4. Hence, given that these sites are growing on Ultisols and Oxisols, with minimal to no weatherable minerals that can resupply bioavailable forms of soil nutrients; the soil nutrients reserves on these sites may not support many rotations of trees without nutrient amendments. In general terms P. caribaea resulted with the better nutrient stability indices in comparisons with broadleaf species.This species has the lowest concentrations of nutrients in the bark and steam, with the exception of P (V. ferruginea has the lowest value). At present, forest plantations in Costa Rica concentrates in two species T. grandis and G. arborea and are often planted on infertile and degraded soils that are notable for their low nutrients reserves to sustain a good tree growth under intensive management. Taking this into account, if managers are seeking productivity models that guarantee sustainability, plans should be considered that includes residues management and nutrient amendments to takingplace on medium to high fertility soils, nutrient losses by harvesting could be relatively small and probably be replaced by soil weathering and natural inputs such as rainfall.