Ciclagem biogeoquímica de nutrientes em Eucalyptus dunnii Maiden em uma microbacia hidrográfica experimental do bioma Pampa

Abstract

The planting of large areas with exotic species is a consequence of the evolution of an entire industrial structure; it aims to meet the demand for forest products. The use of these species requires knowledge on the techniques of soil preparation, fertilization, weed competition, and residue management and harvest intensity, among others. The objective of this study was to evaluate aspects of biogeochemical cycling of nutrients in Eucalyptus dunnii established in a watershed located in the Pampa biome, municipality of Alegrete/RS in area belonging to Stora Enso S.A. The following parameters were evaluated monthly: the rainfall, within and outside the forest stand, litterfall deposition, the total biomass and the output of nutrients by runoff from the watershed. Precipitation was sampling through twelve collectors of internal precipitation, twelve collectors solution resulting from stemflow and three external collectors. The runoff was sampling with aid of container for water collection of 500 ml, made in the spillway of a flume. Sampling of litterfall was obtained through sixteen collectors of 0.5 m² of floor area, to evaluate the fractions of leaves and miscellany, in addition, sixteen areas of collecting thick branches. The determination of biomass occurred after the cutting of twelve trees, considered representative of four diameter classes, with the excavation of their respective roots. The rainfall within the forest stand, litterfall deposition and biomass were evaluating in four installments. The rainfall, biomass production and litterfall and the output of ecosystem nutrients by runoff were evaluating for quantity, the concentration of elements and the contribution thereof. It was founded that the total rainfall was 1385.28 mm yr-1 corresponding to the average of 2012 and 2013 value and the internal precipitation was 91.4% and the stemflow was 1.3%, where 7.61% were intercepting by the canopy. The annual effective precipitation was 1242.69 mm, corresponding to approximately 90% of the local rainfall. It has been found that the interaction of rain with the results in the leaching of surface of the tissue, increasing in 421% the concentration of K, 24.0% the concentration of Ca and 185% the concentration of Mg. In the internal precipitation, the order of predominance was Cl > K+ > SO4- > Ca²+ > Mg²+ and the stemflow, in the order of predominance was K+ > Cl > Mg²+ > Ca²+ > SO4- . The total biomass of the stand was 67.49 Mg ha-1, with a decreasing sequence of accumulation of biomass: wood stem > root > trunk bark > branches > leaves. The total quantity of nutrients in kg ha-1, was: 211.51 of N; 22.12 of P; 199,88 of K; 39,70 of Ca; 86.42 of Mg and 25.05 of S; and the micronutrients in g ha-1, was 562.57 of B; 401.46 of Cu; 9913.28 of Fe; 31877.82 of Mn and 766.96 of Zn. There was accumulation of Mn in biomass components, in addition that the highest concentrations of nutrients are in the tissues of the canopy. The average litterfall production was 7.0 Mg ha-1 yr-1. The forming material of the litterfall consists of sheets, contributing the majority of the deposition (64.3%) followed by twigs (17.5%), thick branches (10.2%) and miscellaneous (7.9%), being Ca the most representative element into leaves, twigs and boughs, presenting only in the miscellaneous fraction, less than N. The P and S elements reported the lowest concentrations. The total transfer of macronutrients was 163.0 kg ha-1 and micronutrients was 9.7 kg ha-1, the macronutrient levels found in litterfall followed the descending order Ca > N > K > Mg > S > P and micronutrient followed the descending order of Mn > Fe > B > Zn > Cu. Three harvesting systems were simulated, completely tree above the ground; wooden trunk + bark and stem wood only. In all simulations, the balances were positive. The total retention of crop residues on the forest site can provide a higher percentage of return of nutrients to the soil. The weight of the harvest Eucalyptus dunnii Maiden, planted in sandy soil and low fertility affected, albeit positively, differently biogeochemical cycling of nutrients