Controls on soil nitrogen cycling and microbial community composition across land use and incubation temperature

We conducted a laboratory incubation of forest (Scots pine (Pinus sylvestris) or beech (Fagus sylvatica)), grassland (Trifolium repens/Lolium perenne) and arable (organic and conventional) soils at 5 and 25 °C. We aimed to clarify the mechanisms of short-term (2-weeks) nitrogen (N) cycling processes and microbial community composition in relation to dissolved organic carbon (DOC) and N (DON) availability and selected soil properties. N cycling was measured by 15N pool dilution and microbial community composition by denaturing gradient gel electrophoresis (DGGE), phospholipid fatty acid (PLFA) and community level physiological profiles (CLPP). Soil DOC increased in the order of arable-grassland-forest soil while DON and gross N fluxes increased in the order of forest-arable-grassland soil; land use had no affect on respiration rate. Soil DOC was lower, while respiration, DON and gross N fluxes were higher at 25 than 5 °C. Gross N fluxes, respiration and bacterial biomass were all positively correlated with each other. Gross N fluxes were positively correlated with pH and DON, and negatively correlated with organic matter, fungal biomass, DOC and DOC/DON ratio. Respiration rate was positively correlated with bacterial biomass, DON and DOC/DON ratio. Multiple linear modelling indicated that soil pH, organic matter, bacterial biomass, DON and DOC/DON ratio were important in predicting gross N mineralization. Incubation temperature, pH and total-C were important in predicting gross nitrification, while gross N mineralization, gross nitrification and pH were important in predicting gross N immobilization. Permutation multivariate analysis of variance indicated that DGGE, CLPP and PLFA profiles were all significantly (Plt;0.05) affected by land use and incubation temperature. Multivariate regressions indicated that incubation temperature, pH and organic matter content were important in predicting DGGE, CLPP and PLFA profiles. PLFA and CLPP were also related to DON, DOC, ammonium and nitrate contents. Canonical correlation analysis showed that PLFA and CLPP were related to differences in the rates of gross N mineralization, gross nitrification and soil respiration. Our study indicates that vegetation type and/or management practices which control soil pH and mediate dissolved organic matter availability were important predictors of gross N fluxes and microbial composition in this short-term experiment

Impactos da aplicação de biossólidos na microbiota de solos tropicais Impacts of biosolids amendments on the microbiota of tropical soils

A aplicação de biossólidos de Estações de Tratamento de Esgotos (ETEs) em solos agrícolas e florestais tem sido uma das práticas alternativas preconizadas para a reciclagem desses resíduos orgânicos. No entanto, alguns biossólidos de ETEs podem conter metais e, ou, xenobiontes que poderiam afetar a microbiota. Neste trabalho, os impactos da aplicação de biossólidos das ETEs de Barueri e Franca (SP), com alta e baixa concentração de metais, respectivamente, na microbiota de um solo argiloso (Nitossolo Vermelho eutroférrico típico) e um arenoso (Neossolo Quartzarênico órtico típico) foram determinados em condições de microcosmos. Imediatamente após a adição de diferentes doses de biossólidos ao solo, e depois de 4, 8, 16, 32 e 64 dias de incubação, a respiração basal (RB), C na biomassa microbiana (CB), quociente metabólico (qCO2) e relação CB/C-orgânico do solo (CB/Corg) foram avaliados. No geral, a RB foi maior nos solos com maiores quantidades de biossólidos, sendo os maiores acréscimos verificados logo após a aplicação dos biossólidos. No solo arenoso, decréscimos significativos do CB foram observados nos tratamentos com as doses mais elevadas de biossólidos. O qCO2 foi maior nos solos com doses mais elevadas de biossólidos, mas diminuiu com o aumento do período de incubação. Independentemente do tipo de solo, CB/Corg foi maior nos solos que não receberam biossólidos, em relação aos solos que receberam biossólidos ricos em metais. A relação CB/Corg nos solos tratados com biossólidos ricos em metais diminuiu significativamente entre 4 e 16 dias de incubação, não sofrendo alterações posteriormente. Esses dados indicam que a aplicação de biossólidos nos solos analisados, independentemente do teor de metais, pode causar um estresse transiente na comunidade microbiana, dependendo da dose aplicada, e que alterações na estrutura das comunidades microbianas podem estar ocorrendo.<br>The amendment of agricultural and forest soils with biosolids from sewage treatment plants (STPs) is an alternative to recycle these organic residues. However, biosolids from STPs may contain metals and/or xenobiotics that can affect soil microorganisms. In this study, the impacts of biosolids from the STPs of Barueri and Franca (São Paulo, Brazil), containing high and low metal concentration, respectively, on the microbiota of a Nitisol (clayey) and a Dystric Arenosol (sandy) were determined in microcosm. Immediately after biosolids application and 4, 8, 16, 32 and 64 days after incubation, the basal respiration (BR), C in microbial biomass (CB), metabolic quotient (qCO2), and CB/soil organic C ratio (CB/Corg) were evaluated. In general, RB was higher in soils amended with the highest amounts of biosolids, and the greatest increments were observed immediately after biosolids application. In the sandy soil, statistically significant decreases in CB were observed in the treatments with the highest amounts of biosolids. The qCO2 was higher in soils with the highest amounts of biosolids, but decreased during incubation. Regardless of the soil type, CB/Corg was higher in soils without biosolids when compared to soils with metal-rich biosolids. The CB/Corg ratio in soils amended with metal-rich biosolids decreased significantly between 4 and 16 days after incubation, and leveled off thereafter. These data indicate that the amendment of the tested soils with biosolids, independently of the metal content, may cause a transient stress in the microbial community, depending on the applied dose, and that changes in the structure of the microbial communities may have occurred