Fluxo de nitrogênio no sistema solo-planta em áreas de sucessão secundária na Amazônia Central

Abstract

Deforestation implies changes of landscape and land use with direct and indirect effects on climate change and biogeochemical cycles. Particularly, in the Amazon region, advancing agriculture, particularly cattle pasture, it is still primarily responsible for the increase in deforested areas. Whereas the pastures become unsustainable due to low soil fertility and low use of production technology in a short time, the pastures are abandoned giving way to emerging secondary vegetation typical of these impacted areas. To understand the ecophysiological strategies related to the absorption, metabolism and the use of nitrogen (N) in pioneer tree species more common in areas of abandoned pastures in Central Amazonia, studies on N dynamics in soil-plant system of forest succession were performed in order to relate the flow of N in pioneer trees in a successional chronosequence of secondary vegetation in different periods of precipitation. The study was conducted in abandoned pastures aged 0- 19 years located in the experimental area of the Biological Dynamics of Forest FragmentsPDBFF. The levels of total amino acids, amino acids, NO3- and NH4+ were determined in the soil, roots, xylem sap and leaves of Vismia japurensis, Vismia cayennensis, Bellucia dichotoma, Laetia procera and Goupia glabra. Additionally, we analyzed the activity of nitrate reductase (NRA) in leaves of five species. In soils, it was found that capoeiristas forward in the regeneration process, there was an increase in mineral-N and also found that the prevalence of the two forms of inorganic nitrogen (NO3- and NH4+) in the soil are related to different periods of precipitation. This is the reason NO3-/NH4+ in soil solution undergoes changes with increasing age of abandonment, tended to decline more pronounced in periods of low rainfall. The levels of soluble amino acids (AA-total) in the soil did not differ between periods of precipitation, except for a class of older age at a depth of 10-20 cm, where the nutrient content of organic was higher in the dry season. The content of AA-total in soils was higher for the older age class, although not verified any increasing trend with the increase of age of abandonment. The levels of AA-totals were higher when compared to levels of inorganic nitrogen (NO3- and NH4+) in both periods of precipitation. B. dichotoma showed greater capacity for assimilation and transport of nitrogen, since it showed higher inorganic nitrogen (NO3- and NH4+) in the xylem sap compared with other species, and exhibited higher values of AA-totals in their leaves. The roots, NO3- content was higher than for B. dichotoma and also for V. japurensis. This kind of Vismia also showed higher levels of NO3- in leaves, followed by G. glabra which in turn, had a higher content of AA-total xylem. L. procera exhibited greater ability to reduce NO3- in leaves, besides being the only species that showed an increase of the content of NO3- leaf as a function of increasing age of abandonment, but only in the rainy season. L. procera was the only species that transported asparagine (ASN), demonstrating that must have high NRA in the root system. Furthermore, L. procera also showed higher content of pigments chloroplastid. Species possessing greater ability to accumulate nitrogen in the leaves, in the form of NO3- were: G. glabra and V. japurensis whereas B. dichotoma is the species that carries larger amounts of this mineral nutrient to the shoot, however, the accumulation of nitrogen for this species is in organic form. Given these results, we can infer that the soil where the plant is installed pioneer on a time scale larger, the predominant content of organic-N, however, the N-mineral increases as the coops advance in the process of regeneration. The pioneer species exhibit different strategies absorption and nitrogen use, especially B. dichotoma which exhibited higher contents of different forms of nitrogen in the soil-plant interface and L. procera, due to its greater ability to reduce NO3- in the shoots.