Asymbiotic nitrogen fixation in the phyllosphere of the Amazon forest: Changing nitrogen cycle paradigms.

Biological nitrogen fixation is a key process for the maintenance of natural ecosystems productivity. In tropical forests, the contribution of asymbiotic nitrogen fixation (ANF) to the nitrogen (N) input has been underestimated, even though few studies have shown that ANF may be as important as symbiotic nitrogen fixation in such environments. The inputs and abiotic modulators of ANF in the Amazon forest are not completely understood. Here, we determined ANF rates and estimated the N inputs from ANF in the phyllosphere, litter and rhizospheric soil of nine tree species in the Amazon forest over time, including an extreme drought period induced by the El Niño-Southern Oscillation. Our data showed that ANF rates in the phyllosphere were 2.8- and 17.6-fold higher than in the litter and rhizospheric soil, respectively, and was highly dependent on tree taxon. Sampling time was the major factor modulating ANF in all forest compartments. At the driest period, ANF rates were approximately 1.8-fold and 13.1-fold higher than at periods with higher rainfall, before and after the extreme drought period, respectively. Tree species was a key modulator of ANF in the phyllosphere, as well as N and Vanadium concentrations. Carbon, molybdenum and vanadium concentrations were significant modulators of ANF in the litter. Based on ANF rates at the three sampling times, we estimated that the N input in the Amazon forest through ANF in the phyllosphere, litter and rhizospheric soil, was between 0.459 and 0.714 kg N ha-1 yr-1. Our results highlight the importance of ANF in the phyllosphere for the N input in the Amazon forest, and suggest that changes in the patterns of ANF driven by large scale climatic events may impact total N inputs and likely alter forest productivity.

Fine root biomass and root length density in a lowland and a montane tropical rain forest, SP, Brazil / Biomassa de raízes finas e densidade do comprimento radicular em uma floresta tropical chuvosa de terras baixas e montana, SP, Brasil

Fine roots, <2 mm in diameter, are responsible for water and nutrient uptake and therefore have a central role in carbon, nutrient and water cycling at the plant and ecosystem level. The root length density (RLD), fine root biomass (FRB) and vertical fine root distribution (VRD) in the soil profile have been used as good descriptors of resource-use efficiency and carbon storage in the soil. Along altitudinal gradients, decreases in temperature and radiation inputs (depending on the frequency of fog events) may reduce decomposition rates and nutrient availability what might stimulate plants to invest in fine roots, increasing acquisition of resources. We evaluated the seasonal variation of fine root parameters in a Lowland and Montane forest at the Atlantic Rain Forest. We hypothesized that, due to lower decomposition rates at the Montane site, the FRB and RLD at soil surface will be higher in this altitude, which can maximize the efficiency of resource absorption. FRB and RLD were higher in the Montane forest in both seasons, especially at the 0-5 layer. At the 0-5 soil layer in both sites, RLD increased from dry to wet season independently of variations in FRB. Total FRB in the top 30 cm of the soil at the Lowland site was significantly lower (334 g.m-2 in the dry season and 219 g.m-2 in the wet season) than at the Montane forest (875 and 451 g.m-2 in the dry and wet season, respectively). In conclusion, despite the relevance of FRB to describe processes related to carbon dynamics, the variation of RLD between seasons, independently of variations in FRB, indicates that RLD is a better descriptor for studies characterizing the potential of water and nutrient uptake at the Atlantic Rain Forest. The differences in RLD between altitudes within the context of resource use should be considered in studies about plant establishment, seedling growth and population dynamics at the Atlantic Rain Forest. At the ecosystem level, RLD and it seasonal variations may improve our understanding of the Atlantic rain forest functioning in terms of the biogeochemical fluxes in a possible scenario of climate change and environmental changes.

Raízes finas, <2 mm de diâmetro, são as principais responsáveis pela absorção de água e nutrientes e, portanto, têm um papel central nos ciclos carbono, água e nutrientes, desde o nível da planta até o ecossistêmico. A densidade do comprimento radicular (DCR), a biomassa de raízes finas (BRF) e a distribuição vertical de raízes finas (DVR) no perfil do solo têm sido utilizados como bons descritores da eficiência no uso de recursos e de estocagem de carbono no solo. Ao longo de gradientes altitudinais, a diminuição da temperatura e da radiação solar (dependendo da frequência de eventos de neblina) podem reduzir as taxas de decomposição e disponibilidade de nutrientes, o que poderia estimular o aumento do investimento das raízes finas para maximizar a absorção de água e nutrientes. O presente estudo avaliou a variação sazonal de parâmetros radiculares nas florestas ombrófilas densas de Terras Baixas (FODTB) e Montana (FODM) na Mata Atlântica. A hipótese foi a de que o investimento em BRF e DCR seria maior na FODM, o que poderia maximizar a eficiência na absorção de recursos. A BRF e a DCR foram maiores na FODM em ambas as estações, especialmente na profundidade de 0-5 cm. A BRF total nos primeiros 30 cm de solo na FODTB foi significativamente menor (334 g.m-2 na estação seca e 219 g.m-2 na chuvosa) do que na FODM (875 e 451 g.m-2 nas estações seca e chuvosa, respectivamente). Na profundidade de 0-5 cm em ambas as altitudes, a DCR aumentou da estação seca para chuvosa independentemente de variações na BRF. Apesar da relevância da BRF para descrever processos relacionados à dinâmica de carbono, a variação da DCR entre estações, independente de variações na BRF, indica que a DCR é um melhor descritor para estudos caracterizando o potencial de absorção de água e nutrientes na Floresta Atlântica. As diferenças da DCR entre altitudes dentro do contexto de uso de recursos devem ser consideradas em estudos sobre estabelecimento, crescimento de plântulas e dinâmica de populações na Floresta Atlântica. No nível ecossistêmico, as variações sazonais da DCR podem aumentar nosso entendimento sobre o funcionamento da Floresta Atlântica em termos de fluxos biogeoquímicos em um possível cenário de mudanças climáticas e ambientais.