The sustainable management of forest plantations by keeping the harvest residues on site improves the soil’s chemical, physical and biological properties while constituting an important nutrient reserve. Our objectives were: (a) to identify and quantify the characteristics of Eucalyptus dunnii, Eucalyptus grandis and Eucalyptus globulus that affect the decomposition rates of harvest residues, as well as indicators that can explain the process and (b) to quantify the potential recycling of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) to the soil from residue decomposition and the quantitative and qualitative differences between the species. We analyzed the information of five commercial plantations of Uruguay. At the harvest, the biomass of leaves, thin and thick branches, bark and their respective N, P, K, Ca and Mg contents were quantified. At each site, bags with samples of the different residues were left to decompose and were periodically collected throughout 24 months. Eucalyptus dunnii presented the largest amounts of residues of all parts. The decomposition rates of the different residues depended on their chemical constitution, part size and the species. Eucalyptus dunnii leaves showed the shortest half-life (0.94 years), while the bark of the same species presented the longest (5.62 years). Total nitrogen and carbon (total and soluble) contents, which can be easily determined, emerged as good predictors for half-life estimation. The release patterns of nutrients depended more on their dynamics in the plant and their parts than on the species itself. The results highlight the importance of nutrient recycling to ensure the sustainability of the productive system in the medium and long term.

The sustainability of forest production is based on an adequate balance between soil nutrient supply and its demand by forest plantations, which may vary depending on the species and the site. In Uruguay, the low to medium fertility of the soils under forest production makes this aspect relevant. The objectives of the study were: (a) to quantify the concentration and distribution of nutrients in the harvested biomass of Eucalyptus dunnii, Eucalyptus grandis and Eucalyptus globulus in the predominant soils of the zones of greater forestry aptitude of Uruguay; and (b) to compare the nutrient use efficiencies of the three mentioned species in different soil types, as a criterion to evaluate the sustainability of the productive system in the long term. In 29 sites located in the three main forest areas of the country, nine trees with average diameter at breast height (DBH) and height were harvested at eight to eleven years of age. The aerial biomass and nutrient uptake (nitrogen, phosphorus, potassium, calcium and magnesium) in commercial wood, bark, leaves and branches were quantified, and two nutrient use efficiency indexes (nutrient use efficiency and biological utilisation coefficient) were calculated. Although the zones offered different conditions for growth and nutrient uptake, E. dunnii presented the highest extraction of nutrients in commercial wood, considering all sites, these indexes being lower for E. dunnii. Therefore, when planting E. dunnii, its greater use of the soil’s resources must be considered, particularly in soils that are naturally less fertile. Likewise, E. grandis presented a greater extraction of cations than E. globulus because of more extensive growth.

To ensure the sustainable management of the production of Eucalyptus sp. in low fertility soils in Uruguay, it is necessary to quantify nutrient exports with the harvest, as well as characterize the decomposition of residues and the recycling of nutrients to the soil. The objectives were: a) to quantify the concentration and distribution of nutrients in the aerial biomass harvested from E. dunnii, E. grandis and E. globulus in soils suitable for forestry; b) to compare the nutrient use efficiencies of the species in the different soil types, as a criterion to evaluate the sustainability of the production system; c) to quantify the characteristics that affect the decomposition rates of residues and identify indicators that can explain the process, and d) to quantify the potential recycling of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) to the soil from the remains and in relation to quantitative and qualitative differences of the species. In each plantation (29), the soil was characterized and trees of average size were harvested, quantifying the aboveground biomass and the extraction of N, P, K, Ca and Mg in commercial wood and harvest residues. In 5 plantations, covering the three species, the decomposition of residues was studied for two years. Nutrient utilization efficiency was lower in E. dunnii in relation to the other two species, for all nutrients and regardless of the planting site. The biological utilization coefficient showed lower values for cations (K, Ca and Mg) in E. dunnii compared to the other two species, and, also, for N and P, compared to E. grandis. E. dunnii exerts greater pressure on the soil resource, which must be considered for sustainable management. Decomposition rates of the residues depended on their chemical constitution, particle size and the species, with the contents of total N and C (total and soluble) being good predictive tools to estimate their half-life. After two years, most of the K was released (91 %), a little more than half of the Mg and P, and a third of the N and Ca. The release patterns depended more on its structural complexity in the plant and the fraction where they were present than of the species itself, this recycling being essential to ensure sustainability in the medium and long term.