M. J. Molina

Influence of organic matter transformations on the bioavailability of heavy metals in a sludge based compost

The agricultural use of anaerobically digested sewage sludge (ADSS) as stable, mature compost implies knowing its total content in heavy metals and their bioavailability. This depends not only on the initial characteristics of the composted substrates but also on the organic matter transformations during composting which may influence the chemical form of the metals and their bioavailability. The objective of this work was to examine the relationships between the changes in the organic matter content and humus fractions, and the bioavailability of heavy metals. A detailed sampling at 0, 14, 84, and 140 days of the composting process was performed to measure C contents in humic acids (HAs), fulvic acids, (FAs) and humin, the total content of Zn, Pb, Cu, Ni, and Cd, and also their distribution into mobile and mobilisable (MB), and low bioavailability (LB) forms. Significant changes of C contents in HA, FA, and Humin, and in the FA/HA, HA/Humin and C(humus)/TOC ratios were observed during composting. The MB and LB fractions of each metal also varied significantly during composting. The MB fraction increased for Zn, Cu, Ni, and Cd, and the LB fraction increased for Pb. Stepwise linear regressions and quadratic curve estimation conducted on the MB and LB fractions of each metal as dependent on the measured organic variables suggested that Zn bioavailability was mainly associated to percentage of C in FAs. Bioavailability of Cu, Ni and Cd during composting was associated to humin and HAs. Pb concentration increased in the LB form, and its variations followed a quadratic function with the C(humus)/TOC ratio. Our results suggest that the composting process renders the metals in more available forms. The main forms of metal binding in the sludge and their availability in the final compost may be better described when metal fractionation obtained in sequential extraction and humus fractionation during composting are considered together.

Stabilisation of sewage sludge and vinasse bio-wastes by vermicomposting with rabbit manure using Eisenia fetida

Changes in the chemical characteristics and biological parameters of Eisenia fetida were assessed by VER using (CO+VE) rabbit manure (Vo) spiked with sewage sludge (SS) or vinasse (V). Seven mixtures were used: Vo, control; Vo+SS at 10%, 30%, and 50% (SS1, SS2, and SS3); Vo+V at 10%, 30%, and 50% (V1, V2, and V3). SS vermicomposts had higher humus, nutrient and total metal contents, but less soluble salts (EC) than V vermicomposts. The number and weight of worms were higher in Vo, followed by SS, at decreasing doses. V3 showed the smallest number and size. The EC of the initial mixtures explained reduced weight, whereas EC and avP2O5 accounted for lower numbers. Vermicomposting is an efficient biowaste recycling technology, but the total amount and composition of soluble salts in food influence the quality of end products and are of primary importance for biological parameters of worms.

Influence of organic matter transformations on the bioavailability of heavy metals

The agricultural use of anaerobically digested sewage sludge (ADSS) as stable, mature compost implies knowing its total content in heavy metals and their bioavailability. This depends not only on the initial characteristics of the composted substrates but also on the organic matter transformations during composting which may influence the chemical form of the metals and their bioavailability. The objective of this work was to examine the relationships between the changes in the organic matter content and humus fractions, and the bioavailability of heavy metals. A detailed sampling at 0, 14, 84, and 140 days of the composting process was performed to measure C contents in humic acids (HAs), fulvic acids, (FAs) and humin, the total content of Zn, Pb, Cu, Ni, and Cd, and also their distribution into mobile and mobilisable (MB), and low bioavailability (LB) forms. Significant changes of C contents in HA, FA, and Humin, and in the FA/HA, HA/Humin and C(humus)/TOC ratios were observed during composting. The MB and LB fractions of each metal also varied significantly during composting. The MB fraction increased for Zn, Cu, Ni, and Cd, and the LB fraction increased for Pb. Stepwise linear regressions and quadratic curve estimation conducted on the MB and LB fractions of each metal as dependent on the measured organic variables suggested that Zn bioavailability was mainly associated to percentage of C in FAs. Bioavailability of Cu, Ni and Cd during composting was associated to humin and HAs. Pb concentration increased in the LB form, and its variations followed a quadratic function with the C(humus)/TOC ratio. Our results suggest that the composting process renders the metals in more available forms. The main forms of metal binding in the sludge and their availability in the final compost may be better described when metal fractionation obtained in sequential extraction and humus fractionation during composting are considered together.

API ZYM assay to evaluate enzyme fingerprinting and microbial functional diversity in relation to soil processes

Soil properties, enzyme activity, and microbial functional diversity coevolve with soil processes. Consequently, activity and diversity of soil enzymes could be used for evaluating the natural processes in ecosystems and their changes but, for applied management strategies, useful cost-effective methods and synthetic indicators are required. The aim of this study was to use API ZYM (API) assay to evaluate the fingerprint of soil enzyme activity and microbial functional diversity in relation to soil processes. To strengthen the results, a parallel measurement of enzyme activities was performed with microplate-fluorometric (FA) assay. Seven soil profiles classified as Haploxeralf, located in two Mediterranean areas under xeric moisture and mesic temperature soil regimes, were selected. The Synthetic Enzyme Index (SEI) was calculated as the sum of 16 enzyme activities determined according to the two methods (SEI–API and SEI–FA). Moreover, the enzyme activities were used to assess the microbial functional diversity through the calculation of the Shannon’s diversity index (H’ API and H’ FA). The results showed a relationship between SEI–API and SEI–FA which was described by a linear model. Moreover, a significant linear regression was also found between the Shannon’s diversity indices calculated according to the two methods, H’ API and H’ FA. Significant differences were found from surface A to deep Bt genetic horizons in a decreasing gradient of enzyme activity and microbial functional diversity. For subsurface horizons, the SEI–API was significantly higher in E than AB horizons, suggesting that API method can detect the different biochemical behaviour in eluviation processes occurring in Haploxeralfs. API ZYM assay proved to be suitable for a general biochemical evaluation and for assessing microbial functional diversity in soils of different origin, composition, and land uses. For this reason, it can be proposed as a simple cost-effective method to use during the soil survey.

A combined equation to estimate the soil pore-water electrical conductivity: calibration with the WET and 5TE sensors

Affordable, commercial dielectric sensors of the frequency domain reflectometry (FDR) and capacitance–conductance (CC) types estimate the dielectric permittivity (eb) and electrical conductivity (σb) of bulk soil. In this work, an equation was obtained to estimate the pore-water electrical conductivity (σp), which is closely related to the soil salinity in contact with plant roots, from eb and σb data, by combining the simplified dielectric mixing (SDM) model that relates eb to the soil volumetric water content (θ), with the Rhoades equation that relates θ and σb to σp. This equation was calibrated with measurements of eb and σb obtained with the Delta-T WET (FDR) and the Decagon 5TE (CC) sensors, in 20 pots filled with a clay loam soil and arranged as combinations of four levels of soil moisture with five levels of soil salinity. The calibrations were performed against reference θ and σp values. The σp was calculated with the chemical equilibrium model SALSOLCHEMEC and used as a more reliable reference than the electrical conductivity of the soil wetting water. For both sensors, the SDM model on the one hand, and the Rhoades equation on the other, provided the most accurate estimations using the least number of parameters regarding their respective alternatives, i.e. the third-order polynomial and the Hilhorst equation. The combined equation for estimation of σp subsequently provided root mean square deviations of 3.1 (WET) and 4.1 (5TE) dS m–1, which decreased to 1.5 and 2.6 dS m–1 for θ >0.22 m3 m–3, and σb 0.22 m3 m–3 and σb <3.7 dS m–1.

Advances in validating SALTIRSOIL at plot scale: First results

SALTIRSOIL (SALTs in IRrigated SOILs) is a model for the medium to long term simulation of soil salinity in irrigated, well-drained lands. Once the algorithms were verified, the objective of our study was to validate SALTIRSOIL under one of several water quality and management scenarios in Mediterranean agriculture. Because drip and surface are the most common irrigation systems in irrigated agriculture in Valencia (Spain), the validation was performed with climate, soil, irrigation water (composition and management) and crop (species and management) information from an experimental plot surface irrigated with well water and planted with watermelon that has been monitored since the late spring of 2007. To carry out the validation, first we performed a global sensitivity analysis (GSA). Second, we compared simulated soil saturation extract composition against measured data. According to the GSA, SALTIRSOIL calculations of soil salinity seem to be most affected by climate (rainfall and evapotranspiration) with 60% of explained soil salinity variance, water salinity with 26% of explained variance, and then irrigation with 4%. According to the closeness of the first comparisons between predictions and measurements, SALTIRSOIL does not seem to be affected by any systematic error, and as a consequence, neither inclusion of new parameters nor calibration of the others already included would be needed at least for surface irrigation. The validation of SALTIRSOIL continues under other water quality and irrigation management scenarios.

Aridity Induces Nonlinear Effects of Human Disturbance on Precipitation-Use Efficiency of Iberian Woodlands

The effects of ecosystem degradation are pervasive worldwide and increasingly concerning under the present context of global changes in climate and land use. Theoretical studies and empirical evidence increasingly suggest that drylands are particularly prone to develop nonlinear functional changes in response to climate variations and human disturbance. Precipitation-use efficiency (PUE) represents the ratio of vegetation production to precipitation and provides a tool for evaluating human and climate impacts on landscape functionality. Holm oak (Quercus ilex) woodlands are one of the most conspicuous dry forest ecosystems in the western Mediterranean basin and present a variety of degraded states, due to their long history of human use. We studied the response of Iberian holm oak woodlands to human disturbance along an aridity gradient (that is, semi-arid, dry-transition and sub-humid conditions) using PUE estimations from enhanced vegetation index (EVI) observations of the Moderate-Resolution Imaging Spectroradiometer (MODIS). Our results indicated that PUE decreased linearly with disturbance intensity in sub-humid holm oak woodlands, but showed accelerated, nonlinear reductions with increased disturbance intensity in semi-arid and dry-transition holm oak sites. The impact of disturbance on PUE was larger for dry years than for wet years, and these differences increased with aridity from sub-humid to dry-transition and semi-arid holm oak woodlands. Therefore, aridity may also interact with ecosystem degradation in holm oak woodlands by reducing the landscape ability to buffer large changes in vegetation production caused by climate variability.