Efeitos da limitação nutricional sobre a ciclagem de nutrientes em uma floresta madura na Amazônia Central

Abstract

Tropical forests are widely distributed across a wide variety of soils. However, the forests of the Central and Eastern Amazon are located under soils with low availability of nutrients, originating from the soil parental material, such as P and basic cations. These forests are sustained by efficient nutrient cycling, however the low supply of essential nutrients may be constraining the productivity of these ecosystems, the return and recycling of C and nutrients, as well as the activities of organisms responsible for organic matter decomposition. With the aim of understanding how such limitations regulate the upland forests of the Central Amazon, we conduct a factorial experiment involving the addition of nutrients (N, P and cations) and their interactions and monitored the nutrient contents of the soil and forest for 3 consecutive Years, litter and important nutrient cycling processes, nutrient use efficiency and returned amounts of C and nutrients. After the third fertilization cycle, we conduct a litter decomposition experiment with enriched litter, to study organic matter decomposition processes in these forests, under alleviated nutrient limitation. This involving measuring mass loss, decomposition rates, soil microbial biomass C, N, P, litter biomass P and production of extracellular enzymes, used for C acquisition (Exoglucanase, β-glucosidase, Exochitinase), and P (Phosphatase) and stoichiometric relationships. Nutrient alleviation increased soil P content in all its fractions, and available cations, with a consequent increase in these nutrients in the litter. P addition enhanced the return of C, N and P, confirming that the cycling of organic matter and nutrients is limited by P. Forests in unfertilized areas showed greater efficiency in the use of nutrients while nutrient addition areas experienced a rapid loss of efficiency in the use of P and cations. Nutrient did not accelerate mass loss, and did not promote the production of enzymes for C acquisition. There was a lower production of phosphatase enzymes in areas with P addition, indicating that P limitation act as an inducer of this enzyme production. The litter microbial biomass in fertilized areas accumulated significantly more P than unfertilized areas, and the same occurred for P in soil microbial biomass. The stoichiometric relationships (C:N:P) of the litter, soil and microbial biomass were significantly altered with the addition of P. The changes that occurred in the stoichiometry of the soil, litter and biomass indicate that these organisms are P limited. Nutritional limitation determines the chemical quality and stoichiometry of the litter, and controls the strategies of decomposer microorganisms, acquisition and accumulation of nutrients, especially for P. These limitations when controlling the activity of decomposer microorganisms can determine the quality and quantity of stable organic matter in the soil, in the forest of the Central Amazon.