Soil microbial activities in tree-based cropping systems and natural forests of the Central Amazon,Brazil

Little information is available about the factors controlling soil C and N transformations in natural tropical forests and tree-based cropping systems. The aim of this work was to study the effects of single trees on soil microbiological activities from plantations of timber and non-timber species as well as species of primary and secondary forests in the Central Amazon. Soil samples were taken in the primary forest under Oenocarpus bacaba and Eschweilera spp., in secondary regrowth with Vismia spp., under two non-timber tree species ( Bixa orellana L. and Theobroma grandiflorum Willd.), and two species planted for wood production ( Carapa guianensis Aubl. and Ceiba pentandra). In these soils, net N mineralization, net nitrification, denitrification potential, basal and substrate-induced respiration rates were studied under standardized soil moisture and temperature conditions. Individual tree species more strongly affected N transformations, particularly net nitrification, than C respiration. Our results suggest that soil C respiration can be affected by tree species if inorganic N becomes a limiting factor. We found a strong correlation among almost all microbiological processes suggesting close inter-relationship between C and N transformations in the studied soils. Correlation analysis between soil chemical properties and microbiological activities suggest that such strong inter-relationships are likely due to competition between the denitrifying and C-mineralizing communities for NO ₃ ^-, which might be an important N source for the microbial population in the studied soils.     

N2O and CH4 emission from soil amended with steam‐activated biochar

Steam‐activation increased CH₄ emission of stover biochar but decreased it for wood biochar by 14%^–70%. Biochar generally increased CH₄ emission but reduced N₂O emission by 10%–41%. Emission of N₂O was 17% lower for maize‐stover biochar compared to Eucalyptus‐wood biochar, and 3% lower for 350°C compared to 550°C pyrolysis temperature. Emission of CH₄ was 21% higher for activated stover biochar compared to Eucalyptus‐wood biochar and 10% lower for 350°C compared to 550°C pyrolysis temperature. No difference in net CO₂ equivalent was observed among biochar grades.