Stable carbon isotope ratio and composition of microbial fatty acids in tropical soils

The soil microbial community plays a critical part in tropical ecosystem functioning through its role in the soil organic matter (SOM) cycle. This study evaluates the relative effects of soil type and land use on (i) soil microbial community structure and (ii) the contribution of SOM derived from the original forest vegetation to the functioning of pasture and sugarcane (Saccharum spp.) ecosystems. We used principal components analysis (PCA) of soil phospholipid fatty acid (PLFA) profiles to evaluate microbial community structure and PLFA stable carbon isotope ratios (delta13C) as indicators of the delta13C of microbial substrates. Soil type mainly determined the relative proportions of gram positive versus gram negative bacteria whereas land use primarily determined the relative proportion of fungi, protozoa, and actinomycetes versus other types of microorganisms. Comparison of a simple model to our PLFA delta13C data from land use chronosequences indicates that forest-derived SOM is actively cycled for appreciably longer times in sugarcane ecosystems developed on Andisols (mean turnover time = 50 yr) than in sugarcane ecosystems developed on an Oxisol (mean turnover time = 13 yr). Our analyses indicate that soil chronosequence PLFA delta13C measurements can be useful indicators of the contribution that SOM derived from the original vegetation makes to continued ecosystem function under the new land use.     

Carbon deposition in soil rhizosphere following amendments with compost and its soluble fractions, as evaluated by combined soil-plant rhizobox and reporter gene systems

We determined the organic carbon released by roots of maize plants (Zea mays L.) when grown in soils amended with compost and its soluble fractions. In rhizobox systems, soil and roots are separated from the soil of a lower compartment by a nylon membrane. Treatments are applied to the upper compartment, while in the lower compartment luminescent biosensors measure the bioavailable organic carbon released by roots (rhizodeposition). The rhizobox-plants systems were amended with a compost (COM), its water extract (TEA), the hydrophobic (HoDOM) and hydrophilic (HiDOM) fractions of the dissolved organic matter (DOM) extracted from the compost. After root development, the lower untreated compartments were sampled and sliced into thin layers. The bioavailable organic carbon in each layer was assessed with the lux-marked biosensor Pseudomonas fluorescens 10586 pUCD607, and compared with total organic carbon (TOC) analyses. The TOC values ranged between 8.4 and 9.6 g kg(-1) and did not show any significant differences between bulk and rhizosphere soil samples in any treatment. Conversely, the biosensor detected significant differences in available C compounds for rhizosphere soils amended with various organic materials. Concentrations of available organic compounds in the first 2 mm of soil rhizosphere were 1.69 (control), 1.09 (COM), 2.87 (HiDOM), 4.73 (HoDOM) and 2.14 (TEA)micromol Cg(-1) soil g(-1) roots. The applied rhizobox-biosensor integrated method was successful in detecting and quantifying effects of organic amendments on organic carbon released by maize plant roots. This approach may become important in assessing the carbon cycle in agricultural soils and soil-atmosphere compartments.     

Diazinon leaching through a sandy soil amended with different humic materials

The water leaching of diazinon (O,O‐diethyl‐O‐2‐isopropyl‐6‐methylpyrirnidin‐4‐yl phosphoroth‐ioate) through soil columns, was studied after column amendments with two well characterized humic acids (HA), in both liquid and solid state, and with the original raw organic materials, an oxidized coal and a leonardite, from which the HA were extracted. The percolation curves and the pesticide distribution over the soil columns showed that the addition of the raw organic materials and the solid HAs reduced significantly the mobility of the pesticide along the soil column. The oxidized coal was more effective than the leonardite original material; the different origin of the two carbon‐rich materials had an influence on the diazinon movement along the soil columns and such difference was enhanced with increasing addition rates. Moreover, incubation at field capacity for two months of the soil columns treated with raw oxidized coal and leonardite, largely enhanced the described effects on pesticide behaviour. A complete adsorption of diazinon on columns and a practical absence of leaching was observed when the HA from both materials were added in dissolved form. These results were explained with the swelling of the humic micelles in water and the enhanced availability of inner hydrophobic surfaces for the strong adsorption of diazinon. The water diffusion into the solid humic materials after two months incubation, also explains their high pesticide retention capacity. This work indicates the usefulness of either solid o dissolved humic substances, with the proper hydrophobic character, in preventing the vertical leaching of non‐polar organic pesticides in soils.     

Spatial and seasonal dynamics of phosphorous and physicochemical variables in the Negro River Estuary (Argentina): a preliminary approach

Environmental Science and Pollution Research - Nutrient discharge into rivers and estuaries and the factors that control it need to be further understood to decrease the risk of harmful algae...     

The molecular composition of humic substances extracted from green composts and their potential for soil remediation

Humic substances play empirically several essential functions in biogeochemical cycles such as storage of carbon, pollutants, nutrients and water, yet the underlying mechanisms remain poorly known because their precise molecular structure is largely unknown so far. Here, we extracted humic substances from biomass waste of bell pepper, fennel, artichoke, coffee ground, coffee husks, and nursery residues. We analyzed humic extracts by ultra-high resolution Orbitrap Fusion Lumos Tribrid 1 M mass spectrometry, using both positive photoionization and negative electrospray ionization modes, and by ¹³C cross polarization/magic angle spinning nuclear magnetic resonance spectroscopy. We identified 5000–7000 unique organic compounds in humic substances by integrating photoionization with electrospray ionization. The chemical distribution of all components was depicted by nuclear magnetic resonance. Humic substances from green composts are composed by a wide variety of hydrophilic and hydrophobic moieties, thus providing the required biosurfactant properties for effective soil washing capacities, with carboxyl-rich alicyclic molecules, fatty acids, and phenolic acids as major constituents. Overall, our findings provide a major insight in the molecular structure of humic substances, thus opening research on mechanisms ruling the origin, fate and behavior of humic substances.

     

FT-IR spectra of humic substances extracted with dipolar aprotic solvents

Dipolar aprotic solvents (DMSO, DMF and Acetone) in mixtures with dilute HCl extract humic substances by an intermolecular H-bonds disruption mechanism. FT-IR spectra of extracts and difference spectra of unpurified and purified material and thermogravimetric analysis showed that dipolar aprotic solvents produce humic substances with low contents of inorganic impurities and structurally different from common extractants. DMSO and Acetone extract humus particurally rich in aliphatic and proteinaceous components as compared to common extractant.     

Increased Sequestration of Organic Carbon in Soil by Hydrophobic Protection

 The impact of soil organic carbon dynamics on the global carbon cycle is still largely uncertain despite studies of agricultural activities and control emissions of greenhouse gases to the earth's atmosphere. Improved knowledge of organic matter dynamics should lead to reduction in CO₂ emissions. We used stable carbon isotope analysis to detect small changes in organic carbon storage and turnover upon soil treatments with a ¹³C-labeled aliphatic alcohol previously partitioned into soluble humic substances of varying hydrophobicity. We found that labeled organic carbon is increasingly protected from mineralization with increased hydrophobic character of humic matter. The stabilization of organic carbon by hydrophobic protection significantly reduced decomposition during incubation time in soil. Hydrophobic protection can become an useful tool to limit decomposition of fresh organic matter in soil and thus reduce CO₂ emission from agricultural soils on a global scale. Received: 1 March 1999 / Accepted in revised form: 16 June 1999     

Increased retention of polycyclic aromatic hydrocarbons in soils induced by soil treatment with humic substances

The analytical recovery of a mixture of polycyclic aromatic hydrocarbons (PAHs) was determined from a soil before and after oxidation with hydrogen peroxide, and subsequently treated with increasing amounts of an exogenous humic acid and subjected to different incubation periods. The release of PAHs from soil depended on the specific structure and physico-chemical properties of each PAH, and increased with additions of exogenous humic materials for both the oxidized and non-oxidized soil as well as with time of PAH permanence in soil. PAH recoveries were lower in the non-oxidized soil, thereby revealing the importance of native organic matter in increasing PAH retention in soils. This study shows that mobility of PAHs in soils can be controlled by soil conditioning with humic substances.     

Molecular properties of a fermented manure preparation used as field spray in biodynamic agriculture

Manure products fermented underground in cow horns and commonly used as field spray (preparation 500) in the biodynamic farming system, were characterized for molecular composition by solid-state nuclear magnetic resonance [¹³?C cross-polarization magic-angle-spinning NMR (¹³?C-CPMAS-NMR)] spectroscopy and offline tetramethylammonium hydroxide thermochemolysis gas chromatography-mass spectrometry. Both thermochemolysis and NMR spectroscopy revealed a complex molecular structure, with lignin aromatic derivatives, polysaccharides, and alkyl compounds as the predominant components. CPMAS-NMR spectra of biodynamic preparations showed a carbon distribution with an overall low hydrophobic character and significant contribution of lignocellulosic derivatives. The results of thermochemolysis confirmed the characteristic highlighted by NMR spectroscopy, revealing a molecular composition based on alkyl components of plant and microbial origin and the stable incorporation of lignin derivatives. The presence of biolabile components and of undecomposed lignin compounds in the preparation 500 should be accounted to its particularly slow maturation process, as compared to common composting procedures. Our results provide, for the first time, a scientific characterization of an essential product in biodynamic agriculture, and show that biodynamic products appear to be enriched of biolabile components and, therefore, potentially conducive to plant growth stimulation.