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• Effects of pyrolysis conditions on the biochar properties of three different organic waste. • Biochars from rice husk, olive pit and olive prunings were produced and characterized. • C and N content mainly depended on biochar feedstock. • Aryl-C, alkyl–C and hydrophobicity were predominately influenced by residence time. • Feedstock and pyrolysis temperature conditioned water holding capacity and ash content. Biochar is a pyrogenous organic material resulting from the pyrolysis of organic residues, which is attracting the interest from researchers and farmers for its potential to sequester carbon and its use as soil ameliorant. Pyrolysis conditions and feedstock determine the properties of the biochars produced. In order to understand the relationship between these variables we analysed in detail the physical, chemical and surface characteristics of biochars produced from three contrasting agronomic residues abundantly generated in South Spain, such as rice husk (RH), olive pit (OP) and pruning remains of olive trees (mainly composed of olive branches and leaves; OB), using a temperature range from 350 to 600 °C and residence times from 0.5 to 4 h. High pyrolysis temperature (600 °C) and time resulted in the greatest pH and C content in the biochars. In general, elemental composition and ash content were dependent on the type of organic waste used as feedstock. 13C Nuclear Magnetic Resonance Spectroscopy and thermal (TG-DSC) analyses showed that temperatures ≥500 °C are needed to achieve a high degree of aromatization of the chars. Micro-computed tomography and field emission scanning electron microscopy revealed that the structure of RH was preserved during the pyrolysis process, favouring a greater porosity for these biochars. These data are very useful for the production of stable biochars obtained from residual biomass, maximising the value of residual biomass resources. These biochars show physical and chemical properties, such as adequate pH, high water retention capacity or high porosity, of interest for their use as soil amendments. [ABSTRACT FROM AUTHOR]

Abstract: Current environmental research is paying increasing attention to reliable analytical surrogates of soil quality. In this work a series of molecular features of soil organic matter were studied in different soil types from Central Spain with the purpose of identifying the soil functions most closely correlated with specific pools of soil organic matter and their structural characteristics. Soil physical variables—including bulk density, total porosity, aggregate stability, available water capacity and water infiltration parameters (Kostiakov's equation coefficients)—were determined. The major soil organic fractions (lipids, particulate free organic matter, fulvic acids, humic acids and humin) were quantified using standard procedures and the soil organic matter was characterised by spectroscopic techniques. Statistical data treatments including simple regression, canonical correlation models and multidimensional scaling suggested two well-defined groups of physical properties in the studied soils: (i) those associated with organic matter of predominantly aromatic character (e.g., water infiltration descriptors), and (ii) soil physical variables related to organic matter with marked aliphatic character and comparatively low degree of humification (e.g., porosity, aggregate stability, available water capacity and field air capacity). From the practical viewpoint, the results support the idea that the detailed structural study of the soil organic matter is useful for accurately monitoring soil physical status. The only determination of total soil organic carbon ought to be complemented with qualitative analyses of the organic matter fractions, at least at the spectroscopic level, which to large extent help to explain the origin of the variability in soil physical properties and can be used for the early diagnosis of possible degradation processes. [Copyright &y& Elsevier]

Wildfire is a recurrent phenomenon in Mediterranean ecosystems and contributes to soil degradation and desertification, which are partially caused by alterations to soil organic matter (SOM). The SOM composition from a Cambisol under a Mediterranean forest affected by a wildfire is studied in detail in order to assess soil health status and better understand of soil recovery after the fire event. The soil was sampled one month and twenty-five months after the wildfire. A nearby unburnt site was taken as control soil. Soil rehabilitation actions involving heavy machinery to remove burnt vegetation were conducted sixteen months after the wildfire. Immediately after fire the SOM increased in topsoil due to inputs from charred vegetation, whereas a decrease was observed in the underlying soil layer. Twenty-five months after fire soil-pH increased in fire-affected topsoil due to the presence of ashes, a decrease in SOM content was recorded for the burnt topsoil and similar trend was observed for the water holding capacity. The pyro-chromatograms of burned soils revealed the formation of additional aromatic compounds. The thermal cracking of long-chain n -alkanes was also detected. Solid-state 13 C NMR spectroscopy supported the increase of aromatic compounds in the fire-affected topsoil due to the accumulation of charcoal, whereas the deeper soil sections were not affected by the fire. Two years later, soil parameters for the unburnt and burnt sites showed comparable values. The reduction of the relative intensity in the aromatic C region of the NMR spectra indicated a decrease in the charcoal content of the topsoil. Due to the negligible slope in the sampling site, the loss of charcoal was explained by the post-fire restoration activity, degradation, leaching of pyrogenic SOM into deeper soil horizons or wind erosion. Our results support that in the Mediterranean region, fire-induced alteration of the SOM is not lasting in the long-term. [ABSTRACT FROM AUTHOR]

Several studies have shown that biochars have a high potential to stabilize trace elements in soils [1]. However, different feedstocks and pyrolysis conditions result in biochars with contrasting properties [2, 3]. This study aimed to evaluate the effect of two different biochars used as amendments of trace-element contaminated soils.Biochars were produced from rice husk (RH) and olive pit (OP) by pyrolysis at 400 and 500 ºC for 1 and 4 hours at a steel batch reactor. The remediation potential of these biochars was tested in a contaminated Typic Xerofluvent soil from Aznalcóllar (SW, Spain), which was affected by the breaching of the tailings dam in 1998, flooding 4,286 ha of land located downriver. The soils used for this study have two different levels of contamination: a) moderately polluted (MPS), with pH 6.5 and a Water Holding Capacity (WHC) of 51.5 %, and b) highly polluted (HPS), with pH 3.6 and a WHC of 32.7 %. Each soil was mixed with 0, 2, 5 and 10 % of the two biochars and incubated for 65 days at 25 ºC, 60 % of WHC and 12h light/day. Subsequently, seeds of Brassica rapa spp. pekinensis were sowed. Plant germination rates, dehydrogenase enzymatic activity and the bioavailability of selected trace elements (As, Ba, Cd, Cu, Fe, Ni, Pb, Sr and Zn) were determined. Soil pH slightly increased with the addition of biochars. The application of 10 % of RH biochar produced at 500 ºC during 4h significantly increased soil pH in HPS and MPS. After the incubation period, all biochars significantly increased the soil carbon contents when compared with the untreated soils (control samples). The relative abundance of trace elements in the least available fraction of the biochar amended soils increased, showing a lower toxicological risk than the untreated soils. Dehydrogenase activity was higher in the biochar amended soils than in the un-amended ones, and about 10 times greater in the MPS than in the HPS. Biochar increased the germination rates of Brassica rapa spp. pekinensis in the HPS. The pots amended with RH biochar obtained a greater plant growth and root development than the OP amended soils. This difference may be related to the sharp increase of WHC in the soils amended with RH biochar.Our data showed that RH biochar produced at 500 °C is a promising amendment for the recovery of soils moderately contaminated with trace elements.References:[1] Uchimiya, M.; Klasson, K.T.; Wartelle, L.H.; Lima, I.M.; 2011. Chemosphere 82, 1438-1447. [2] De la Rosa, J.M.; Paneque, M.; Miller, A.Z.; Knicker, H.; 2014. Science of the Total Environment 499, 175-184. [3] Zhao, L.; Cao, X.; Masek, O.; Zimmerman, A.; 2013. Journal of Hazardous Materials 256-257, 1-9.Acknowledgements:The Spanish Ministry of Economy, Industry and Competitiveness (MINEICO) and AEI/FEDER are thanked for funding the projects CGL2016-76498-R and CGL2015-64811-P. P. Campos thanks the "Fundación Tatiana Pérez de Guzmán el Bueno" for funding her PhD. [ABSTRACT FROM AUTHOR]