Aspectos da ciclagem de nutrientes em ecossistemas ripários sob diferentes usos do solo

Abstract

Gallery forests play a strategic role in maintaining the balance of aquatic environments. The volume and water quality of small streams are regulated by the interaction of hydrological, biological and geological flows. In small streams, the main sources of matter and energy come from the riparian vegetation, and the decomposition of allochthonous leaf litter is a key process in the nutrient cycling. The decomposition process in a local scale is mainly regulated by the quality of leaf litter and by the water characteristics, which may influence the colonization by decomposers and detritivorous. Small streams are more sensitive to environmental changes, thus their study is very relevant for the understanding of nutrient cycling. This work was developed at Peti Environmental Station in Minas Gerais state, where two preserved streams and two others, impacted either by eucalyptus plantations or by mining activities, were selected. Nutrient flows were estimated by wet deposition, runoff, water flow and input of organic matter. The dynamic decomposition of allochthonous leaf litter was evaluated and nutrient flows through the decomposition process were estimated. Mining activity changed the water quality in the Brucutu stream, increasing conductivity, pH and nutrient concentrations (ammonium, inorganic carbon and total phosphorus). Eucalyptus plantations resulted in soil compaction, increased runoff volume and conductivity in the Eucaliptal stream. Changes in nitrogen processing were observed in both streams, probably due to inhibition of the nitrification process, affected by allopathic substances released by the eucalyptus or by the high dissolved metals content in the mining areas, which altered the microbial activity performance. The eucalyptus leaf detritus had faster decomposition rate than the detritus of native species, and a slower decomposition process was observed when both detritus were incubated in impacted streams. The decomposition was even slower in the stream impacted by mining activity that showed indications of nitrogen immobilization. From this study, we can conclude that anthropogenic activities in the watershed resulted in changes in the hydrological and biological fluxes of nutrients, in the stream water quality and in the nutrient processing in the water.