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Biochar application to soils has become a focus of research during the last decade due to its high potential for C sequestration. Nevertheless, there is no exhaustive information on the long-term effects of biochar application in soils contaminated with trace elements. In this work, a 2-year field experiment was conducted comprising the application of different types of biochar to acidic and moderately acidic soils with high concentrations of As, Cu, Pb, Ba and Zn. In addition, representative samples of each biochar were buried in permeable bags that allowed the flow of water and microorganisms but not their physical interaction with soil aggregates. The biochars significantly adsorbed trace elements from polluted soils. However, given the high total concentration of these persistent trace elements in the soils, the application of biochars did not succeed in reducing the concentration of available metals (CaCl 2 extractable fraction). After 2 years of ageing under field conditions, some degradation of the biochars from olive pit, rice husk and wood were observed. This study provides novel information concerning the biochar alterations during ageing in polluted soils, as the decrease of aryl C signal observed by 13C nuclear magnetic resonance (NMR) spectroscopy and the presence of O-containing groups shown by Fourier Transform mid-Infrared Spectroscopy (FT-IR) in aged biochar which enhanced trace elements adsorption. Scanning electron microscopy (SEM) revealed slight changes on surface morphology of aged biochar particles. [Display omitted] • Metal immobilisation by biochar was determined by burying and used as amendment. • Field-aged biochars exhibited significant adsorption of trace element contents. • > 8 t ha−1 of biochar are needed to reduce soil-available trace element contents. • Biochar amendment increased total C and aryl C in soils at 0–10 cm depth. • Biochar aromatic C was degraded under 2-year field ageing. [ABSTRACT FROM AUTHOR]

Application of biochar has been widely suggested as a remediation tool for trace element-polluted soils, but the impact of biochar on microbial communities and on native plants remain largely unknown. To overcome this knowledge gap, biochar produced from rice husk and olive pit were applied at a rate of 8 t ha−1 into a soil with two contrasting levels of trace elements (high and moderate) to study their effects on soil microbial community composition, vegetation cover and soil properties after 1, 6, 12 and 20 months under field conditions. Differences in bacterial community composition were studied using the Illumina Miseq technology of the 16S rRNA gene. Although variations in soil properties and ecological function were seasonal and soil-type dependent, biochar application enhanced soil properties and vegetation cover in the moderately polluted soil (MPS), and increased microbial diversity as well as vegetation cover in the highly polluted soil (HPS). Enzymatic activities and soil respiration rates were not modified with the application of biochar, but increased total carbon content of soils. The application of biochar from crop residues to trace-element contaminated soils provided environmental benefits, including plant diversity and growth, as well as the increase of bacterial diversity and carbon sequestration. • Biochar was applied in moderately and highly polluted soils under field conditions. • Biochar application increased bacterial and plant diversity in highly polluted soil. • Bacterial diversity slightly changed in moderately polluted soil with biochar. • Soil pH was key factor of bacterial community structure. • Seasonal changes affected microbial community, soil respiration and vegetation. [ABSTRACT FROM AUTHOR]

Three pyrolysis biochars (B1: wood, B2: paper-sludge, B3: sewage-sludge) and one kiln-biochar (B4: grapevine wood) were characterized by determining different chemical and physical properties which were related to the germination rates and to the plant biomass production during a pot experiment of 79 days in which a Calcic Cambisol from SW Spain was amended with 10, 20 and 40 t ha − 1 of the four biochars. Biochar 1, B2 and B4 revealed comparable elemental composition, pH, water holding capacity and ash content. The H/C and O/C atomic ratios suggested high aromaticity of all biochars, which was confirmed by 13 C solid-state NMR spectroscopy. The FT-IR spectra confirmed the aromaticity of all the biochars as well as several specific differences in their composition. The FESEM-EDS distinguished compositional and structural differences of the studied biochars such as macropores on the surface of B1, collapsed structures in B2, high amount of mineral deposits (rich in Al, Si, Ca and Fe) and organic phases in B3 and vessel structures for B4. Biochar amendment improved germination rates and soil fertility (excepting for B4), and had no negative pH impact on the already alkaline soil. Application of B3, the richest in minerals and nitrogen, resulted in the highest soil fertility. In this case, increase of the dose went along with an enhancement of plant production. Considering costs due to production and transport of biochar, for all used chars with the exception of B3, the application of 10 t ha − 1 turned out as the most efficient for the crop and soil used in the present incubation experiment. [ABSTRACT FROM AUTHOR]

Abstract The interest of using biochar, the solid byproduct from organic waste pyrolysis, as soil conditioner is significantly increasing. Nevertheless, persistent organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), are formed during pyrolysis due to the incomplete combustion of organic matter. Consequently, these pollutants may enter the environment when biochar is incorporated into soil and cause adverse ecological effects. In this study, we examined the content of the 16 United States Environmental Protection Agency (USEPA) PAHs in biochars produced from rice husk, wood, wheat and sewage sludge residues using three different pyrolytic reactors and temperatures (400, 500 and 600 °C). The total concentration of PAHs (∑PAH) ranged from 799 to 6364 μg kg−1, being naphthalene, phenanthrene and anthracene the most abundant PAHs in all the biochars. The maximum amount of PAHs was observed for the rice husk biochar produced in the batch reactor at 400 °C, which decreased with increasing temperature. The ∑PAH value of the wood biochar produced via traditional kilns doubled compared with the wood biochar produced using the other pyrolytic reactors (5330 μg kg−1 in Kiln; 2737 μg kg−1 in batch and 1942 μg kg−1 in the rotary reactor). Looking for a more reliable risk assessment of the potential exposure of PAHs in biochar, the total toxic equivalent concentrations (TTEC) of the 14 produced biochars were calculated. When comparing the same feedstock and temperature, TTEC values indicated that the rotary reactor produced the safest biochars. In contrast, the biochars produced using the batch reactor at 400 and 500 °C have the greatest hazard potential. Our results provide valuable information on the potential risk of biochar application for human and animal health, as well as for the environment due to PAHs contamination. Graphical abstract Unlabelled Image Highlights • Risk assessment of biochar due to PAHs contamination • Pyrolysis conditions and reactors are evaluated in terms of the total toxic equivalent concentrations (TTEC) in biochars. • Greatest PAHs levels in the biochar produced in the batch reactor at 400 °C • Rotary reactor produced the most secure biochars regardless of the feedstock or temperature. • TTEC is proposed for evaluating the potential hazard of biochars instead of total PAHs. [ABSTRACT FROM AUTHOR]

The effects of aging on biochar (BC) properties, composition and carbon sequestration are still under debate. This study aimed at illustrating the qualitative alterations of five different BCs aged during a 24-month field experiment located in Southwest Spain. To determine the recalcitrance of each BC, physical fragmentation test, scanning electron microscopy, 13 C NMR spectroscopy and CO 2 -respiration experiments were performed. The physical fragmentation values of all types of BC increased significantly over time at field conditions. FESEM examinations of aged BCs showed collapsed structures and the presence of entrapped soil material and microbial mats into the BC pores. The 13 C NMR spectroscopy demonstrated an increase of the relative abundance of O -alkyl C and alkyl C at expenses of aromatic-C in aged BCs. The C losses of all BCs ranged from 27% to 11% of the initial C. In contrast, the nitrogen (N) content of wood-derived BCs significantly increased probably due to the sorption of nitrogen containing compounds into these highly-porous weathered chars. With the exception of that for the sewage sludge-BC, the pH of all aged BCs decreased from > 9 to the soil pH, indicating a short lasting of the liming effect caused by BC addition. The respiration experiment revealed that BC recalcitrance was much lower than expected and, within the range of decades. Only wood-derived BCs significantly increased the mean residence time of the slow C pool of the Cambisol by factors between 3.4 and 7.7. Mediterranean climate conditions and the characteristics of the Cambisol used probably accelerated the microbial degradation of BCs. [ABSTRACT FROM AUTHOR]

Biochar is the solid material obtained from thermochemical conversion of biomass underoxygen-limited conditions (pyrolysis), which can be applied as soil ameliorant [1]. In general,biochar properties are very heterogeneous due to the diverse pyrolytic conditions and thewide variety of organic residues used as feedstock [2,3]. This study intends to discern therelationship between feedstock, pyrolysis conditions and biochar properties with the goal ofproducing biochar with a high potential for the stabilization of trace elements in contaminatedsoils. For that purpose, biochars were produced using four different feedstock (rice husk,pruned olive trees, olive pit and "alperujo", a byproduct of olive oil production) and twocontrasting pyrolysis systems: a batch reactor (temperature ranged from 350 to 600 ºC;reaction time from 0.5 to 4 h under N2 atmosphere with a heating rate of 20 ºC min−1) and acontinuously feed reactor with a screw conveyor (Pyreka reactor; 500 ºC, residence time 12min and N2 flux). Biochars were characterized by determining their pH, water holding capacity (WHC),elemental composition (C, H, N), ash content, internal structure by micro-computed tomographyand chemical composition by field emission scanning electron microscopy with energydispersive X-ray spectroscopy. To complete the characterization, Brunauer-Emmett-Tellerspecific surface area (SSABET; N2 adsorptive) and solid-state 13C-NMR spectroscopy wereperformed. Biochars produced in the batch reactor showed that pH, WHC, TC, SSABET,ash content and aromaticity increased with temperature and reaction time. Ricehusk biochars showed the highest WHC (> 100%), while olive pit biochars thelowest ones. Rice husk and olive pit biochars had the highest aromaticity (between75 and 91% of aryl carbon). The H/Cat ratio decreased with increasing pyrolysistemperature, which suggests an increase in the condensation degree of the aromaticstructures. SSABET surface area ranged from 20 to 100 m2 g−1 and increased withtemperature. Biochars produced in the batch reactor resulted in greater SSA values thanPyreka biochars. The pyrolysis conditions of 500 ºC and 2 h at the batch reactorresulted in similar biochars than those produced by the Pyreka reactor (500 ºC and 12min). Taking into account the necessity of applying biochar to soil for remediationpurposes, we selected those biochars of expected high stability (ratio H/Cat ≤ 0.7 &high aromaticity by 13C NMR spectroscopy), great capacity for the sorption andstabilization of trace elements(SSABET ≥ 100 m2g−1;pH ≥9) and good potentialto act as soil amendment (high WHC). The pyrolysis conditions finally selectedwere 500 ºC and 2 hours for the steel-batch reactor and 500 ºC and 12 min forthe continuous reactor. At these conditions, rice husk biochars showed the mostappropriate characteristics to be used as soil amendment for trace-elements contaminatedsoils. References: [1] IBI; 2015. IBI-STD-2.1. International Biochar Initiative. [2] De la Rosa, J.M.; Paneque, M.; Miller, A.Z.; Knicker, H.; 2014. Sci. Tot. Env. 499,175-184. [3] Zhao, L.; Cao, X.; Masek, O.; Zimmerman, A.; 2013. J. Haz. Mat. 256-257,1-9. Acknowledgements: The former Spanish Ministry of Economy, Industry and Competitiveness (MINEICO) andAEI/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]