A. Roig

Nitrogen transformation during organic waste composting by the Rutgers system and its effects on pH, EC and maturity of the composting mixtures.

The evolution of the different forms of nitrogen during the composting of several wastes was studied, as well as its relation to the pH, electrical conductivity and parameters of maturity of the composts obtained. Four mixtures were prepared from different organic materials: sewage sludge, municipal solid waste, brewery sludge, sorghum bagasse, cotton waste and pine bark. The evolution of the different forms of nitrogen during composting depended on the material which supplied the nitrogen to the mixtures and the organic matter (OM) degradation rate during composting. The greatest concentration of ammonium was observed during the first weeks of composting, coinciding with the most intense period of OM degradation, and ammonium then decreased gradually to reach final values of below 0.04%. The use of urea as a nitrogen source in the mixtures led to high ammonium levels during the first weeks as a result of its rapid hydrolysis. The nitrification process began only when the temperature of the mixtures had dropped below 40 degrees C and its intensity depended on the quantity of ammonium present when the process began. The highest concentrations of NO3-N were always produced at the end of maturation, reaching values of 0.52%, 0.53%, 0.12% and 0.20% in the four mixtures studied. Nitrogen losses during composting depended on the materials used and on the pH values of the mixtures. Mixtures with the highest lignocellulose content showed the lowest losses (below 25%), while those containing municipal solid waste lost more than 40% of the initial content. Statistically significant correlations at a high probability level were found between the NO3-N concentration and pH and electrical conductivity. confirming that nitrification was responsible for the falling pH values and increasing electrical conductivity. The ratio of NH4-N and NO3-N concentrations was shown to be a clear indicator of the maturity of the mixtures during composting, the final values of 0.08, 0.04, 0,16 and 0.11 for the four mixtures being equal to, or below the maximum value established as a maturity index in other materials.

The effects of soil amendments on heavy metal bioavailability in two contaminated Mediterranean soils

Two heavy metal contaminated calcareous soils from the Mediterranean region of Spain were studied. One soil, from the province of Murcia, was characterised by very high total levels of Pb (1572 mg kg(-1)) and Zn (2602 mg kg(-1)), whilst the second, from Valencia, had elevated concentrations of Cu (72 mg kg(-1)) and Pb (190 mg kg(-1)). The effects of two contrasting organic amendments (fresh manure and mature compost) and the chelate ethylenediaminetetraacetic acid (EDTA) on soil fractionation of Cu, Fe, Mn, Pb and Zn, their uptake by plants and plant growth were determined. For Murcia soil, Brassica juncea (L.) Czern. was grown first, followed by radish (Raphanus sativus L.). For Valencia soil, Beta maritima L. was followed by radish. Bioavailability of metals was expressed in terms of concentrations extractable with 0.1 M CaCl2 or diethylenetriaminepentaacetic acid (DTPA). In the Murcia soil, heavy metal bioavailability was decreased more greatly by manure than by the highly-humified compost. EDTA (2 mmol kg(-1) soil) had only a limited effect on metal uptake by plants. The metal-solubilising effect of EDTA was shorter-lived in the less contaminated, more highly calcareous Valencia soil. When correlation coefficients were calculated for plant tissue and bioavailable metals, the clearest relationships were for Beta maritima and radish.

Biochar and denitrification in soils: when, how much and why does biochar reduce N2O emissions?

Agricultural soils represent the main source of anthropogenic N2O emissions. Recently, interactions of black carbon with the nitrogen cycle have been recognized and the use of biochar is being investigated as a means to reduce N2O emissions. However, the mechanisms of reduction remain unclear. Here we demonstrate the significant impact of biochar on denitrification, with a consistent decrease in N2O emissions by 10–90% in 14 different agricultural soils. Using the 15N gas-flux method we observed a consistent reduction of the N2O/(N2 + N2O) ratio, which demonstrates that biochar facilitates the last step of denitrification. Biochar acid buffer capacity was identified as an important aspect for mitigation that was not primarily caused by a pH shift in soil. We propose the function of biochar as an “electron shuttle” that facilitates the transfer of electrons to soil denitrifying microorganisms, which together with its liming effect would promote the reduction of N2O to N2.

Evolution of organic matter and nitrogen during co-composting of olive mill wastewater with solid organic wastes

Abstract Four olive mill wastewater (OMW) composts, prepared with three N-rich organic wastes and two different bulking agents, were studied in a pilot plant using the Rutgers system. Organic matter (OM) losses during composting followed a first-order kinetic equation in all the piles, the slowest being the OM mineralisation rate in the pile using maize straw (MS). The highest N losses through NH3 volatilisation occurred in the mixtures which had a low initial C/N ratio and high pH values during the process. Such losses were reduced considerably when MS was used as the bulking agent instead of cotton waste (CW). N fixation activity increased during the bio-oxidative phase before falling during maturation. This N fixation capacity was higher in piles with a lower NH4+-N concentration. Only the composts prepared with OMW, CW and poultry manure or sewage sludge reached water-soluble organic C (CW) and NH4+-N concentrations and CW/Norg and NH4+/NO3– ratios within the established limits which indicate a good degree of compost maturity. Increases in the cation-exchange capacity, the percentage of humic acid-like C and the polymerisation ratio revealed that the OM had been humified during composting. The germination index indicated the reduction of phytotoxicity during composting.

Characterization of olive mill wastewater (alpechin) and its sludge for agricultural purposes

Abstract Ten samples of olive mill wastewater (OMW) taken from different mills in southern Spain and other ten of OMW sludges from evaporation ponds were analysed. The aim was to study the composition of these wastes and to find relationships which would make it possible to use easily determinable parameters to ascertain their composition. Compared with other organic wastes, these materials had a high potassium concentration, a similar organic matter content and notable levels of nitrogen, phosphorus, calcium, magnesium and iron. The highest potassium concentrations were observed in the OMWs, while the sludges showed higher levels of the other nutrients, especially iron. The dry matter of the OMWs was significantly correlated with most of the parameters studied but, in the sludges, the only correlation was between the ash content and the total organic carbon and total nitrogen concentrations. The regression equations obtained permitted a rapid characterization of the OMWs from their dry matter content.

Bio-degradation of olive mill wastewater sludge by its co-composting with agricultural wastes

The use of maize straw (MS) or cotton waste (CW) as bulking agents in the composting of olive mill wastewater (OMW) sludge was compared by studying the organic matter (OM) mineralisation and humification processes during composting and the characteristics of the end products. Both composts were prepared in a pilot-plant using the Rutgers static-pile system. The use of CW instead of MS to compost OMW sludge extended both the thermophilic and bio-oxidative phases of the process, with higher degradation of polymers (mainly lignin and cellulose), a greater formation of nitrates, higher total nitrogen losses and a lower biological nitrogen fixation. The CW produced a compost with a more stabilised OM and more highly polymerised humic-like substances. In the pile with CW and OMW sludge, OM losses followed a first-order kinetic equation, due to OM degradation being greater at the beginning of the composting and remaining almost constant until the end of the process. However, in the pile with MS and OMW sludge this parameter followed a zero-order kinetic equation, since OM degraded throughout the process. The germination index indicated the reduction of phytotoxicity during composting.

Growth of ornamental plants in two composts prepared from agroindustrial wastes

Two composts prepared from agroindustrial wastes were assayed as substrates: C1 from brewing waste (yeast and malt) plus lemon tree prunings; and C2 from the solid fraction of olive mill wastewater plus olive leaves. Sixteen substrates were prepared by combining each compost with Sphagnum peat or a commercial substrate (CS) in different proportions. The nutrients (N and K) provided by the composts, which acted as slow-release fertilisers, influenced especially the development of calendula, although the physical and physico-chemical properties such as total pore space and electrical conductivity (EC) were also relevant. On the other hand, in the salt-sensitive calceolaria hybrid, EC and chloride concentration were the main factors influencing growth. Adequate substrates for the development of calendula can be prepared by mixing C1 at up to 75% with peat or at up to 50% with CS, and C2 at up to 50% with peat or CS. For calceolaria, the substrate should have a lower proportion of compost, C1 at up to 50% and C2 at up to 25%, both mixed with peat or CS. Therefore, composts of agroindustrial origin such as these can be used as an alternative to peat and CSs for growing ornamental plants. provided the mixture contains at least 25% peat or CS.

Use of biochar as bulking agent for the composting of poultry manure: Effect on organic matter degradation and humification

The aim of this study was to evaluate the use of biochar (produced by slow pyrolysis of Eucalyptus grandis biomass) as bulking agent for the composting of poultry manure. Three composting mixtures were prepared by the turned-pile system by mixing poultry manure with different organic wastes used as bulking agent (biochar, coffee husk and sawdust) in a proportion of 1:1 (fresh weight). Despite the inert nature of biochar, the composting mixture prepared with biochar underwent an organic matter degradation of 70% of the initial content. The organic matter of the poultry manure-biochar mixture was characterised by a high polymerisation degree of the humic-like substances, with a relative high proportion of humic acids in relation to fulvic acids. At the end of the composting process, the humic acid fraction represented more than 90% of the alkali extractable fraction, reflecting the intense humification of this material. Enrichment of poultry manure with biochar reduced the losses of nitrogen in the mature composts, although the use of sawdust would be more efficient in preserving the organic matter and nitrogen in the mature compost.

Relationships between organic matter and carbon contents of organic wastes

Abstract The values of organic matter of 38 organic wastes obtained by the loss-on-ignition method at 430°C have been correlated with both total organic carbon measured by elemental microanalysis and oxidable organic carbon determined by the traditional method of Walkley and Black, duly adapted to organic materials. Calcination at 430°C for 24 h is the most suitable method for determining the organic-matter content of organic wastes. Linear and quadratic equations for the correlations between organic matter and carbon were tested, and high values of r, significant at the 99·9% probability level, were found. The use of the linear equations is recommended to transform either carbon content into organic matter or vice-versa easily.

Relationships between water-soluble carbohydrate and phenol fractions and the humification indices of different organic wastes during composting

Abstract The present work dealt with the relationships between the degradation and humification processes which the organic matter underwent during the composting of six different organic-waste mixtures. Four of them were prepared by the Rutgers forced-ventilation composting system and the other two by the mobile (turn over) pile system. The main components were: sewage sludge, sorghum bagasse and municipal solid waste. Different degradation rates were observed for the three main components (cellulose, hemicellulose and lignin) of the organic matter during composting. In the case of the first two components, the degree of degradation ranged from 70 to 85% during the whole process, depending on the starting mixture, whereas only 30–50% of the initial concentration of lignin was degraded in the mixtures prepared with municipal solid wastes (MSW) and lignocellulosic materials. Water-soluble carbohydrate and phenol degradation were studied because they have been proposed as precursors of the humification processes. In the experiment described, they had different degradation rates during composting depending on the starting mixture and the composting system used. The water-soluble carbohydrate was the most intensely degraded fraction in the piles prepared with urban refuse although no appreciable degradation was measured in the other three mixtures, whereas there was an appreciable reduction in the water-soluble phenol fraction of all six mixtures during composting, values of less than 0.1% being reached in the mature composts. Humification processes were studied by quantification of the extractable humic-like substances and the generally accepted humification indices: extractable carbon to total organic carbon (EXC/TOC), humic acid carbon to total organic carbon (HAC/TOC), humic acid carbon to extractable carbon (HAC/EXC) and the humic acid carbon to fulvic acid carbon (HAC/FAC) and by determining the cation exchange capacity of the mixture during composting. All the indices increased during composting and followed a similar trend. The humic-like acid fraction was mainly responsible for these changes, showing that the composting involves a process of humification. The cation exchange capacity to total organic carbon ratio showed itself to be a useful humification index during composting since this index clearly increased more than the others. Correlations between some of the above humification indices and the concentrations of water-soluble and less polymerised carbohydrates and phenols indicates the possible influence of these fractions on the humification processes. Significant correlations were found between the phenols and the HAC/TOC and the HAC/EXC ratios, whereas no significant correlations were recorded with the carbohydrate fraction.