Extraction and cycling of nutrients in eucalyptus sp plantations in uruguay and its effect on the sustainability of the system of production

Doctoral thesis abstract

Resumo

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.