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, C NMR spectroscopy and CO-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 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.

In the European Union (EU), the major N and P sources in agriculture are synthetic fertilizers although inputs from animal manure remain important, especially in regions of high livestock density. The use of synthetic fertilizers has dramatically increased food production worldwide, but the unintended costs to the environment and human health due to surplus and inefficient application have also been substantial. Thus, alternatives are needed. One of those represents the valorization of organic waste, the amount of which increased exponentially during the last decades. Its estimated annual increase is around 25 million tons [1].A major source of organic waste is sewage sludge (SS). Containing high amounts of N, with a median of 3.3% [2], P and OM, this material offers an excellent feedstock for the production of soil amendments that can reduce the need of synthetic P and N fertilizer [3,4]. At the same time it may increase the C-sequestration potential of cropped soils. In addition, sludge recycling as fertilizer or organic amendment helps to reduce the amount of organic waste by returning it into the bio-cycle while at the same time soil physical and chemical properties are restored [5,6]. Indeed, the Sewage Sludge Directive [7] and the working document on sludge [8] describe the use of SS on soils as beneficial but also seek to encourage a safe use of this material in agriculture and to regulate its application to prevent harmful effects on soil, vegetation, animals and humans [9]. Thus, in order to enable the use of SS as soil application, it first has to be decontaminated by removing organic pollutants and pathogen bacteria. Different treatments can be used for the latter: thermal drying, anaerobic digestion, conditioning with lime, etc. In general, the process of removing pathogen bacteria of SS is called hygienization. Heavy metals content also need to be taken into account. In cases their concentrations are above the allowed thresholds, strategies have to be developed to avoid secondary contamination due to SS application.Hydrothermal carbonization and pyrolysis of sewage sludges: wWhat happen to carbon and nitrogen? M. Paneque a, ⁎

Three biochars (B1: pine wood, B2: paper-sludge, B3: sewage-sludge) produced under controlled pyrolysis conditions and one produced in kilns (B4: grapevine wood) were used as organic ameliorants in a Calcic Cambisol, which represents a typical agricultural soil of the Mediterranean region. This field study was performed with plants of sunflower (Helianthus annuus L.) at the experimental station "La Hampa", located in the Guadalquivir river valley (SW Spain). The soil was amended with doses equivalent to 1.5 and 15 t ha −1 of the four biochars in two independent plantations. In addition, un-amended plots were prepared in both experiments for comparison purposes. The major goal of this study was the assessment of the effect of biochar amendment on the physiology and development of sunflower plants at field conditions. During most of the growing period plants of un-amended and amended plots showed no stress symptoms either by their appearance or by stress-sensitive biochemical parameters such as the stability of the photosystem II (QY). Biochar addition had no effect on seed germination. Addition of 1.5 t ha −1 biochar did not significantly change the pH of the soil, its electrical conductivity (EC) or its water holding capacity (WHC). Concomitantly the plant development and plant biomass production remained unaltered. Amendments with 15 t ha −1 slightly increased the WHC of the soil but showed no lasting impact on the soil pH. It stimulated plant growth and led to a greater leaf area, larger plant stems and wider inflorescences of the sunflower plants than those cultivated on the un-amended soil. At the end of the experiment, biochar amendment of soil caused no significant increase of the total biomass production excepting B4, the biochar with the lowest capacity of water retention, which exhibited the highest vegetative growth and seed production. The lack of rain during the last weeks caused a water shortage in the culture that produced greater QY loss in non-amended plants. Interestingly, better growth of amended plants during the drought period correlated with higher reduction of stomatal conductance, indicating that the greater water use efficiency is at the origin of the better crop performance of biochar-amended plants. This finding points to the agronomic relevance of biochar amendment of Mediterranean rain fed crops.

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.

The pyrolysis and hydrothermal carbonization (HTC) of sewage sludge (SS) resulted in products free of pathogens, with the potential for being used as soil amendment. With this work, we evaluated the impact of dry pyrolysis-treated (600 °C, 1 h) and HTC-treated (200 °C, 260 °C; 0.5 h, 3 h) SS on the germination, survival, and growth of Lolium perenne during an 80 day greenhouse experiment. Therefore, the hydrochars and pyrochars were amended to a Calcic Cambisol at doses of 5 and 25 t ha−1. The addition of sludge pyrochars to the Cambisol did not affect Lolium germination, survival rates or plant yields. However, the use 25 t ha−1 of wood biochar reduced germination and survival rates, which may be related to the low N availability of this sample. In comparison to the control, higher or equal plant biomass was produced in the hydrochar-amended pots, even though some hydrochars decreased plant germination and survival rates. Among all the evaluated char properties, only the organic and inorganic N contents of the chars, along with their organic C values, positively correlated with total and shoot biomass production. Our work demonstrates the N fertilization potential of the hydrochar produced at low temperature, whereas the hydrochar produced at 260 °C and the pyrochars were less efficient with respect to plant yields.

The goal of this study was to determine if the application of four biochars produced from different feedstock leads to soil contamination with the 16 US Environmental Protection Agency (EPA) polycyclic aromatic hydrocarbons (PAHs) in biochar-amended pots incubated under greenhouse conditions. Materials and methods The P AHs were extracted by Soxhlet and quantified by gas chromatography mas s spectrometry in the biochars and in biochar-amended Calcic Cambisols on which Lolium perenne was grown under greenhouse conditions during 79 days. Three of the used biochars were produced by pyrolysis at 620 oC from wood chips, paper sludge, and sewage sludge, respectively. The fourth biochar was produced from old grapevine wood by the traditional kiln method. Results and discussion The Soxhlet extraction of the ~16PAHs of pure biochars yielded between 959 ± 62 (sewage sludge biochar) and 2613 ± 1380 ~g kg-1 (pine wood biochar) dry mass. The lowest abundance ofPAHs ofsewage sludge biochar is consistent with its very low content of organic carbon and the abundance of ashes. The PAH concentration of the wood biochar produced by kiln was six times higher than the wood biochar produced by fast pyrolysis, indicating that the biochar production process significantly affected the PAH levels. The unamended soil showed PAH levels typically for non-polluted soils (~16PAHs=59±4 ~g kg-1). Addition of 10 t biochar ha-1 soil resulted in a lower increase of the 16 PAHs than expected for wood and paper sludge biochars, whereas for the soils amended with kiln wood biochar, the content ofPAHs increased up to a factor of 10. Amendments of 20 and 40 t ha-1 increased PAH concentrations to values on the same order of magnitude than the predicted ones, except for pine wood biochar, where the concentrations were ten times greater than expected. The major divergences were found for high-molecular-weight P AHs. Conclusions The production process of biochar affected significantly their PAR levels. The material carbonized in traditional kilns contained the greatest amounts of PARs. Sampling procedures for biochar-amended soils should be carefully described for future legislation guidelines to guarantee quality biochar application. Keywords Pyrolysis Pot experiments Kiln Polycyclic aromatic hydrocarbons Soxhlet extraction Soil amendment Responsible editor: Rongliang Qiu composition ofproduced biochars (Zhao et al. 2013). Thus, bearing in mind that biochar composition and characteristics can largely vary, the focus has to be oriented on guaranteeing the safe agricultural use ofbiochar. However, biochar policy demands are at an early stage of development, and their regulatory and testing requirements are still being defined by international initiatives such as the European Biochar Certificate (EBC 2012) or the International Biochar Initiative (lBI 2013). Both pay particular attention to the formation and accumulation ofpolycyclic aromatic hydrocarbons (PAHs) during pyrolysis (Fagernas et al. 2012) These persistent organic pollutants are highly condensed ar...

Hydrothermal carbonization and dry pyrolysis transform sewage sludge (SS) into nitrogen‐ (N) and phosphorus‐ (P) rich hydrochars (Hyd) and pyrochars (Py), respectively, which may act as slow‐release fertilizers with carbon (C) sequestration potential. Whereas this has been mostly studied with short‐term experiments, this study focused on the cycling of char‐derived N, P and C after ageing during a second grass cropping cycle. Lolium perenne was grown for 3 months in pots on soil mixed with 13C and 15N‐enriched SS, Hyd or Py and allowed to age during a first cropping period of 10‐month incubation. The δ15N of the plants confirmed that even during the second cropping, N derived from the amendments was plant accessible. Higher uptake of N from Hyd than from Py is explained by the lower biodegradability of the latter. Plant growth during the second cropping period was associated with a decrease of total P in all treatments, but only the soils with Hyd and Py evidenced an increase of Olsen P. Thus, during the second cropping, more insoluble P was mobilized from the carbonized residues than P needed for plant growth. Compared to control soils prepared with and without KNO3, higher biomass production was yielded with the amended soils. Hyd proved to have the highest longer‐term N mobilization potential. Following the change in δ13C of the soil, we observed that during the second incubation, independently of their aromaticity, all amendments and the native soil organic matter had comparable turnover rates, although the amount of organic matter with slower turnover added with the amendment increased with aromaticity. A rough estimation of the impact of thermal treatment of SS on its C‐sequestration potential revealed no major differences between char types. The higher fertilization capacity of Hyd, however, indicates that it is a good candidate for soil amendment as long‐term fertilization is combined with a long‐term increase of the SOC pool. Highlights Medium‐term fertilization and C‐sequestration potential of chars from sewage sludge were tested. Hydrochar (Hyd) and pyrochar (Py) provide N and P for plant growth during a second cropping period. Compared to soils with and without KNO3, Hyd and Py increase plant productivity of the second crop. On a long‐term scale, native SOM, amended SS, its aged Hyd and Py show comparable turnover rates.

This study was conducted to evaluate the effect of compost produced with agricultural residues and oat-based biochar, iron oxide and halloysite nanoparticles as additives of the process of composting on soil chemical properties, nutrient status and growth of ryegrass Lolium perenne L. For this, a 90-day mesocosm experiment was carried out under greenhouse conditions. Bare soil and a basal fertilization treatment were compared to soils amended with nonadditive compost (NA compost), compost supplied with oat-based biochar (Bioch compost), iron oxide nanoparticles (Fe compost), and halloysite nanoparticles (Ha compost). Compost supplied with nanoparticles and biochar combined were also considered. The incorporation of compost with or without additives increased the content of total C and N in soil, with N diminishing (total and mineral forms) and C/N modifications after 90 days. The addition of compost and co-composted treatments also increased the total contents of main nutrients such as Ca, K, P and S. Furthermore, the supply of additives into composting did not increase the concentration of trace toxic elements. At the end of the experiment, plant biomass increased by the addition of the different organic amendments, with the highest shoot biomass in soils amended with compost supplied with nanoparticles. These results suggest that the addition of compost based on agricultural residues with additives such as halloysite or biochar improves chemical properties and nutritional status of soil that favor and increase plant growth of Lollium perenne stablished in soils from the Mediterranean Region.