Influence of growing Eucalyptus trees for biomass on soil quality

Recommendations • Avoid harvesting at too young stand age: Eucalyptus plantations in Brazil are intensively managed systems with a high biomass production compared to many other countries. This production level is reached through fertilization and - in some regions - irrigation. There is an ongoing tendency of harvesting the plantations at younger age. That will result in plantations with a short cropping duration, which are more comparable to agricultural systems than to classic forestry, both from the perspective of management intensity and the influence on environmental factors. Such shorter-lived plantations may be more negative to the environment than longer-lived plantations. • The area in Brazil covered with native forest or savannah is declining and becoming fragmented, resulting in enhanced numbers of endangered species. It has been suggested that Eucalyptus plantations may serve as a refuge for native species and connect habitat fragments. However, biodiversity is higher in plantations with long rotation cycles that are extensively managed than in arable crops and poor rangelands. Moreover, as Eucalyptus is exotic to South America, fewer organisms are adapted to this tree species. All in all, the biodiversity in Eucalyptus plantations is lower than in natural forest, mainly enabling survival of generalist species without specific habitat requirements or species with high dispersal capacity. In any case, longer-term stands will have less negative effects on biodiversity than very short-lived stands. Spatial and temporal stand design might enhance the function of Eucalyptus stands as refugia or corridor for species and migrants among original forest and savannah systems. This needs further study. • The effect of Eucalyptus plantations on soil organic matter (SOM) depends on the previous land use: if the plantation is established on natural forest soil, SOM content will first decline and after some decades increase. However, if the plantation is established on poor soils with low SOM content, the SOM will increase. Soil microbial characteristics differ between Eucalyptus plantations and native vegetation, but differences become smaller when the age of the plantation increases. Litter accumulation is higher in Eucalyptus plantations than in native vegetation, independent of plantation age, probably due to low nutritional quality and low decomposition rate of Eucalyptus litter. Therefore, effects of new Eucalyptus plantations can only be assessed properly when the local conditions are known. • The concentration of nutrients in leaves, branches and bark is higher than in the stem wood. However, over time trees allocate relatively more biomass to wood, so that, although the nutrient concentration in the wood is low, the total amount of nutrients in the wood per unit of planted area steadily increases. Large amounts of especially macronutrients N, P and K, but also of micronutrients B, Cu, Fe and Zn are removed with the harvested wood, with a need of fertilization to replace nutrient stocks. Leaving harvest residues in the field limits the removal of especially base cations Ca, Mg and Mn that are present in larger amounts particularly in the bark. From a nutrient removal perspective, it may be wiser to harvest branches than bark for biomass. Removing harvest residues without supplying nutrients via fertilizers will cause a decrease in productivity of successive rotations due to nutrient removal. In addition, leaving harvest residues in the field will increase water availability, prevent soil erosion, protect the soil from compaction by harvesting machines and reduce the germination of weeds. However, there is a lack of information on the amount of residues that needs to be left in the field to sustain biomass production. Therefore, in the planning phase of tree plantations for biofuel and other biomass-related production, site-specific recommendations are required accounting for soil type, land surface steepness, climate, length of the rotation and how these factors influence residue retention and its effect on soil quality and soil functioning. • Biochar as a rest product from biofuel production may be used to return nutrients to the field and to increase the carbon content of the soil. Addition of biochar also may increase cation exchange capacity (CEC), water holding capacity, drainage capacity, the pH of acid soils and abundance of soil microbes. However, downsides of using biochar is that it may decrease the effectiveness of pesticides and herbicides through adsorption of these chemicals, and the possibility of introducing toxic compounds to the soil. As biochar is very stable, some of these effects may be long-lasting. Wood ash, another rest product of biofuel production, is noted for its acid neutralizing capacity and supply of base cations Ca, Mg and K. Bringing back wood ashes to soil may increase growth conditions for the trees and does not seem to harm soil organisms in the long term, provided that the wood ash is not contaminated with ashes (and toxic compounds) from other sources