Evaluation of the nutrient content in biomass of Eucalyptus species from short rotation plantations in Uruguay

Forest biomass is an interesting option for bioenergy generation, however sustainability aspects should be carefully considered. In this context, the nutrient exportation is a key factor for reach a sustainable biomass production by an adequate harvest decision about what tree's fractions will export less nutrients of farms. In this work biomass production per fraction and nutrient export for three eucalypt species (Eucalyptus benthamii, E. dunnii and E.grandis) planted at four planting densities: 2220, 3330, 4440 and 6660 trees per hectare, in two different sites, were evaluated at the age of 76 months. The assessed nutrients were: P, Ca, Mg, K and N in bark, branches, leaves, and stemwood fractions. Nutrients use efficiency and the balance between available cations in the soil (Ca, Mg and K) vs their export in the different tree components were assessed. The effect of the interaction (species x planting density) for almost all the variables evaluated was not significant in both sites. The nutrients concentration is slightly affected by the planting density while the nutrient export is significantly related to this parameter in both sites. Bark is the fraction with the higher Ca and Mg concentration values, whereas the higher concentrations of N and K were found on the leaves, meanwhile debarked stemwood is the fraction with the lower nutrient concentration. In the short term, K would be the nutrient that will exhaust first, although the use of stemwood allows an important reduction in the use of cations. Even with low planting densities it is possible to obtain high biomass yields and similar to those achieved with the lowest spacing in particular at the Paysandu site.