Adding worms during composting of organic waste with red mud and fly ash reduces CO2 emissions and increases plant available nutrient contents

Alkaline industrial wastes such as red mud and fly ash are produced in large quantities. They may be recycled as bulking agent during composting and vermicomposting, converting organic waste into soil amendments or plant growth media. The aim of this study was to assess the microbial parameters, greenhouse gas emissions and nutrient availability during composting and vermicomposting of household waste with red mud and fly ash 15% (dry weight). CO₂, CH₄ and N₂O emissions were monitored during 6 months in controlled laboratory conditions and microbial biomass and phospholipid acids, N and P availability were analysed in the end-products. Higher CO₂ emissions were observed during vermicomposting compared to composting. These emissions were decreased by red mud addition, while fly ash had no effect. Nitrate (NO₃-N) content of the end-products were more affected by worms than by alkaline materials, while higher ammonium (NH₄-N) contents were recorded for composts than vermicomposts. Red mud vermicompost showed higher soluble P proportion than red mud compost, suggesting that worm presence can counterbalance P adsorption to the inorganic matrix. Final composts produced with red mud showed no harmful heavy metal concentrations. Adding worms during composting thus improved the product nutrient availability and did not increase metal toxicity. From a practical point of view, this study suggests that for carbon stabilisation and end-product quality, the addition of red mud during composting should be accompanied by worm addition to counterbalance negative effects on nutrient availability.

Does grassland introduction into cropping cycles affect carbon dynamics through changes of allocation of soil organic matter within aggregate fractions?

Implementation of ley grassland into crop rotation could have positive influence in soil ecosystem services such as C storage. The periodical changes of land-use plus the in situ labelling given by the introduction of maize crops under ley grassland induce differences in soil organic matter (SOM) that could be traced either by stable isotopes or by the characterization of plant biomarkers such as lignin derived phenols. Evaluation of SOM dynamics is often limited by the complexity of soil matrix. To override these limitations, a hierarchical approach to decompose the soil mosaic into aggregates has been proposed in this study. Soil and plant samples were collected from a long-term experimental area in Lusignan (western France). Soils from four different treatments (bare fallow, permanent maize, permanent grassland, and ley grassland based on 6years of grassland followed by 3years of maize) were sampled, fractionated into water stable aggregates, and finally analysed for carbon, nitrogen, and lignin contents, as well as for ¹³C isotopic signature. Soils under ley and permanent grassland stored higher amount of SOM in larger aggregates and preserved more efficiently the lignin stocks than the corresponding samples under permanent maize. Contemporary, finer fraction of ley grassland showed higher mean residence time of organic carbon, probably due to a legacy effect of the previous years under grassland. Even if maize derived SOM was identified, the grassland footprint was still dominating the ley grassland soils, as described by the principal component analysis. Strong correlation between these results and the quality and stoichiometry of the vegetal litter returned to soil were found, evidencing the needs for a comprehensive evaluation at a molecular level of all the parameters modified by land-use changes, including tillage, to understand the potential for carbon storage of different agroecosystems.

Change of the chemical composition and biodegradability of the Van Soest soluble fraction during composting: a study using a novel extraction method

Van Soest fractionation is widely employed to characterize exogenous organic matter. The soluble fraction of Van Soest fractionation (SOL, extracted using hot water and then neutral detergent) often increases in line with compost maturity, although it is generally considered as labile. We have developed an alternative extraction method that comprises four successive steps (extraction using hot water, sodium tetraborate, dichloromethane/methanol and chelating resin) in order to clarify the chemical nature of the SOL fraction and explain its biodegradability. This method was tested on municipal solid waste compost sampled during the thermophilic phase (MSWi) and after 8 months of composting (MSWm). Both methods extracted similar proportions of organic matter. The composition of the residues was similar in MSWm although differences were noted for the extraction of polysaccharides and lipids in the case of MSWi. The hot water extractable fraction decreased during composting. Its high biodegradability in MSWi was linked to the high polysaccharide content revealed by pyrolysis-GC/MS and FTIR spectroscopy. The increase in the sodium tetraborate extractable fraction mainly explained the increase in the SOL fraction during composting. This was made up of N-containing compounds, polysaccharides and lipids in the immature compost, and a majority of N-containing compounds in the mature compost. During composting, the stabilization of organic matter in the SOL fraction extractable by sodium tetraborate and EDTA might principally involve N-containing structures through the formation of complexes of organic matter with metal ions, especially Ca(2+), which may be broken down during extraction of the Van Soest soluble fraction. These mechanisms still need to be investigated.

Distribution of C and N mineralization of a sludge compost within particle-size fractions

The contribution of particle-size fractions to carbon (C) and nitrogen (N) mineralization of sludge compost was investigated. Particle-size fractionation was performed using "dry" (sieving of total dry compost) and "wet" (dispersion of compost in water, followed by sieving) fractionation methods, then C and N mineralization of the separated fractions were measured during incubation in soil. The "dry" fractionation did not allow the actual particle-size distribution of compost to be estimated accurately. Out of all the "wet" fractions, the [0-50 microm] fraction was the most significant fraction in compost mass and contributed the most to the N mineralization of sludge compost in soil. Its low degradability, positive N mineralization and similarities with sludge OM suggest that the most humified sludge organic matter was located in this fraction, which would probably contribute to C storage and N availability after compost application in soil. Other fractions (>200 microm) were more readily biodegradable and induced N immobilization.

Ageing processes and soil microbial community effects on the biodegradation of soil (13)C-2,4-D nonextractable residues

The biodegradation of nonextractable residues (NER) of pesticides in soil is still poorly understood. The aim of this study was to evaluate the influence of NER ageing and fresh soil addition on the microbial communities responsible for their mineralisation. Soil containing either 15 or 90-day-old NER of (13)C-2,4-D (NER15 and NER90, respectively) was incubated for 90 days with or without fresh soil. The addition of fresh soil had no effect on the mineralisation of NER90 or of SOM, but increased the extent and rate of NER15 mineralisation. The analyses of (13)C-enriched FAME (fatty acids methyl esters) profiles showed that the fresh soil amendment only influenced the amount and structure of microbial populations responsible for the biodegradation of NER15. By coupling biological and chemical analyses, we gained some insight into the nature and the biodegradability of pesticide NER.

Tracing 2,4-D metabolism in Cupriavidus necator JMP134 with 13C-labelling technique and fatty acid profiling

The use of stable isotope probing of fatty acid methyl esters (FAME-SIP) is a powerful tool to study the microorganisms involved in xenobiotic biodegradation in soil. Nevertheless, it is important to determine how representative these molecules are of microorganisms both qualitatively and quantitatively. Using Cupriavidus necator JMP134 as a simple experimental model, we showed that the (13)C-labelling technique can be used both at a global (here defined as cellular, medium and CO(2)) and molecular level to study the metabolism of 2,4-Dichlorophenoxyacetic acid (2,4-D). Although isotopic fractionation among substrate, biomass and FAME were observed, this technique could be used when using a highly (13)C-labelled substrate. Global (13)C analyses gave similar results to those obtained with traditional (14)C-labelling methods. After 10 days of incubation 59% of ring-C was mineralized and about 30% remained in the liquid medium. A maximum of 11% was incorporated into the biomass after 3 days. The assimilation yield of chain-C into the biomass was about half that of ring-C, suggesting a preferential use of chain-C for energy acquisition. Molecular analysis of the lipid fraction evidenced that the incorporation of the labelled 2,4-D did not correspond to a bioaccumulation of pesticide residues but to the metabolism of the 2,4-D carbons for FAME synthesis. Provided the labelling is located on the benzenic ring, the assessment of (13)C-FAME is a robust method to quantify the incorporation of (13)C into the whole microbial biomass. However, the variability of the (13)C incorporation among FAME due to physiological processes has to be considered in complex biological systems. The coupling of bulk and molecular studies with a simple model as C. necator JMP134 is a good approach for testing FAME-SIP.

Changes in the organic composition of wastewater during biological treatment as studied by NMR and IR spectroscopies

Despite its importance, relatively little is known about the composition and fate of wastewater organic matter (OM) in treatment plants. Monitoring the chemical changes in OM during activated sludge treatment can improve our knowledge of the processes involved in the biological elimination of OM. Direct chemical analyses of treated water OM typically account for about 20% of the OM, and structural information was obtained in this study using Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared (FTIR) spectroscopic techniques. Distinct changes in the OM during wastewater biological treatment were underlined. 13C and 1H NMR showed that aromatic carbons were minor constituents of the samples. Alkyl chains exhibited a more highly branched character in treated water, as compared to long chain aliphatic carbons present in wastewater. Carboxyl signals in the 13C NMR spectrum of wastewater could be due to peptide bonds in proteins, whereas in the treated water spectrum, this signal could be related to the presence of non-proteinaceous nitrogen. Besides the non-degraded compounds, treated water OM could contain recondensation products of simple molecules. Their refractory character probably derives from their complex structures rather than from particular chemical functions, as suggested by the lack of fundamental differences in the chemical structures of wastewater and treated water OM.