Quantificação dos efeitos da introdução de Pinus taeda nas comunidades microbianas do solo e no ciclo do nitrogênio em uma floresta tropical montana nativa com Araucaria angustifolia

Abstract

Land-use change has led to the loss of about 90% of the tropical forest cover in the Atlantic Forest domain, Brazil, with significant impacts on the water and biogeochemical cycles and ecosystem services provisioning. Soil microorganisms play an important role in the cycling of nutrients in terrestrial ecosystems, and their community composition and function are influenced by the interaction of a series of biotic and abiotic factors. These interactions are inevitably altered by land-use change but their effects on below-ground communities and processes rarely quantified, such as in the case of the introduction of exotic species that replace native forest ecosystems, with consequences for ecological and biogeochemical processes.This work aimed to quantifying the effects of replacing native A. angustifolia-dominated montane forests with Pinus taeda plantations on (i) soil microbial communities (archaea and bacteria) and (ii) the nitrogen (N) cycle. To do this, we quantified microbial biomass and characterized the spatial and seasonal (wet and warm vs. dry and cool) composition/potential activity of genes associated with the N cycle of soil archaeal and bacterial communities in a native forest stand and a pine plantation in the State Park of Campos do Jordão, where A. angustifolia is best preserved in its northern distribution within the Atlantic Forest biogeographical domain. Mineral soil and the organic layer were sampled at the peaks of the wet and dry seasons. We report the results for the native and exotic conifer species for the frequency/abundance of genes nifH, AmoB, AmoA, associated with the N cycle, and relate these to soil variables and microbial biomass, estimated based on organic N, and carbon (C) concentrations of the soil samples.We found that the native forest with A. angustifolia presented higher values of microbial immobilization of C (MBC) and C (MBN), NO3and NH4 + in the organic layer, and that both the organic and mineral topsoil in A. angustifolia showed higher values of MBC and MBN compared with those in the pine plantation. Regarding the potential activity of enzymes involved in the N cycle, the abundance of nifH gene was negatively correlated with soil NH4 + . Accordingly, the greatest abundance of nifH was found in the topsoil of the pine plantation in correspondence with the lowest concentration of NH4 + . The abundance of amoA genes related to ammonium oxidizing archaea (AOA) and ammonium oxidizing bacteria (AOB) was negatively correlated with the concentration of NH4 + . The pine plantation showed a higher abundance of bacterial and archaeal amoA genes when compared with the araucaria forest. This natural experiment is an open door for investigating the activity and function of the soil microbiota in tropical mountain ecosystems. Our results will contribute with data to biogeochemical models to project the impacts of land use (pine plantations) and the dynamics of the carbon and nitrogen cycle associated with mycorrhizal fungi.