Effects of biochar and compost addition in potting substrates on growth and volatile compounds profile of basil (Ocimum basilicum L.).

BACKGROUND: Despite the optimal characteristics of peat, more environmental-friendly materials are needed in the nursery sector, although these must guarantee specific quantitative and qualitative commercial standards. In the present study, we evaluated the influence of biochar and compost as peat surrogates on yield and essential oil profile of two different varieties of basil (Ocimum basilicum var. Italiano and Ocimum basilicum var. minimum). In two 50-day pot experiments, we checked the performances of biochar from pruning of urban trees and composted kitchen scraps, both mixed in different proportions with commercial peat (first experiment), and under different nitrogen (N) fertilization regimes (second experiment), in terms of plant growth and volatile compounds profile of basil. RESULTS: Total or high substitution of peat with biochar (100% and 50% v.v.) or compost (100%) resulted in seedling death a few days from transplantation, probably because the pH and electrical conductivity of the growing media were too high. Substrates with lower substitution rates (10-20%) were underperforming in terms of plant growth and color compared to pure commercial peat during the first experiment, whereas better performances were obtained by the nitrogen-fertilized mixed substrates in the second experiment, at least for one variety. We identified a total of 12 and 16 aroma compounds of basil (mainly terpenes) in the two experiments. Partial replacement of peat did not affect basil volatile organic compounds content and composition, whereas N fertilization overall decreased the concentration of these compounds. CONCLUSION: Our results support a moderate use of charred or composted materials as peat surrogates. © 2023 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Recycling pyrolyzed organic waste from plant nurseries, rice production and shrimp industry as peat substitute in potting substrates.

Organic waste from greens of tomato plants, gardening substrate, rice husks and shrimp-derived chitin were pyrolyzed at 400 °C and 500 °C for 3 h, with the aim to elucidate the feasibility of using such products as replacement of peat in soilless gardening substrates. Characterization of the carbonized organic matter (COM) and the gardening substrate indicated that neither the peat nor the COMs provided the recommended levels of nutrients for the cultivation of tomato plants, although improvements could be obtained using COM/substrate mixtures. The toxicity thresholds for Zn were exceeded significantly by the COMs of the tomato greens and high boron levels were found for all the COMs except for those derived from chitin. In a 40-days pot experiment, germination and development of tomato seeds and plants (Solanum lycopersicum L.) were tested on COM/peat mixtures at 30%, 60% and 100% COM substitution rate. The lack of seed germination on the mixtures with COM from tomato greens is best explained with the high salinity of the COM. Best plant growth was obtained with COM from chitin at 60%, most likely because its high N content satisfied best the N-needs of the growing tomato plants without increasing the pH of the growing media. Moreover, an increase of water retention was evidenced for COM/substrate mixtures. Although the use of COM from chitin and rice husks showed promising results, the proposed recycling of organic waste from agriculture or fishery as soilless gardening substrate requires the development of formulations of COM/peat/and added nutrients with ready-to-use characteristics to increase its feasibility.

Influence of inorganic additives on wheat straw composting: Characterization and structural composition of organic matter derived from the process.

Metallic oxides and clay minerals have gained increasing interest as additives of composting due to their influence in greenhouse gas emissions reduction and their effectivity in the stabilization of carbon both in compost and soils, leading to a cleaner compost production and potentially C sequestrant amendments. In this study, wheat straw (WS) was co-composted with iron oxide and allophanic soil and their influence on WS composting and composition of the end-products was evaluated. WS compost and their humic like-substances (HS) fraction were characterized by chemical and spectroscopic analyzes. After 126 days of process, the elemental composition showed slight differences of the N content for compost and HS, where the C/N atomic ratio tended to decrease relative to the initial material (WS; ~130). This trend was more pronounced in the HS from co-composted treatments (<30). The addition of inorganic materials increased the total acidity and phenolic-OH group contents (~15 and 14 mEq g-1 respectively, iron oxide treatment) relative to the treatment without inorganic additives. Nevertheless, the FTIR and solid-state 13CNMR spectroscopy barely support the wet chemical analysis and revealed a similar final composition between all the studied compost treatments. These results suggest that the incorporation of these materials as compost additives had no major effect on the spectroscopic features of the end-products, however, critical changes of the properties such as the extractability, functionality and composition of HS were revealed by traditional methods. In conclusion, the supply of metal oxides and clays could impact the aerobic composting of WS favorizing the stabilization of certain C pools and adsorptive properties of the end-products, that is of importance in production of amendments suitable for being used in degraded and contaminated soils. Nevertheless, under the experimental conditions of our research C stabilization apparently depends of other mechanisms that still need to be elucidate.

Quantitative forecasting black (pyrogenic) carbon in soils by chemometric analysis of infrared spectra.

A detailed and global quantitative assessment of the distribution of pyrogenic carbon (PyC) in soils remains unaccounted due to the current lack of unbiased methods for its routine quantification in environmental samples. Conventional oxidation with potassium dichromate has been reported as a useful approach for the determination of recalcitrant C in soils. However, its inaccuracy due to the presence of residual non-polar but still non-PyC requires additional analysis by 13C solid-state nuclear magnetic resonance (NMR) spectroscopy, which is expensive and time consuming. The goal of this work is to examine the possibility of applying infrared (IR) spectroscopy as a potential alternative. Different soil type samples (paddy soil, Histic Humaquept, Leptosol and Cambisol) have been used. The soils were digested with potassium dichromate to determine the PyC content in environmental samples. Partial Least Squares (PLS) regression was used to build calibration models to predict PyC from IR spectra. A set of artificially produced samples rich in PyC was used as reference to observe in detail the IR bands derived from aromatic structures resistant to dichromate oxidation, representing black carbon. The results showed successful PLS forecasting of PyC in the different samples by using spectra in the 1800-400 cm-1 range. This lead to significant (P < 0.05) cross-validation coefficients for PyC, determined as the aryl C content of the oxidized residue. The Variable Importance for Projection (VIP) traces for the corresponding PLS regression models plotted in the whole IR range indicates the extent to which each IR band contributes to explain the aryl C and PyC contents. In fact, forecasting PyC in soils requires information from several IR regions. In addition to the expected IR bands corresponding to aryl C, other bands are informing about the patterns of oxygen-containing functional groups and the mineralogical composition characteristic of the soils with greater black carbon storage capacity. The VIP traces of the charred biomass samples confirm that aromatic bands (1620 and 1510 cm-1) are the most important in the prediction model for PyC-rich samples. These facts suggest that the mid-IR spectroscopy could be a potential tool to estimate the black carbon.

Mulching-induced preservation of soil organic matter quality in a burnt eucalypt plantation in central Portugal.

Mulching has amply proven its effectiveness to mitigate post-fire soil erosion but its impacts on soil organic matter (SOM) quality and quantity continue poorly studied. The present study addressed this knowledge gap for a eucalypt plantation in central Portugal that had been burnt and, immediately after the wildfire, mulched with 13.6 Mg ha-1 of eucalypt logging residues some five years before. This was done by performing a range of analytical techniques (elemental and isotope analyses, analytical pyrolysis and 13C NMR spectroscopy) not only on the bulk soil samples but also on their humic acids (HAs) and free organic matter (FOM) fractions. While mulching reduced soil and SOM losses with 91 and 93%, respectively, it also improved SOM quality of the topsoil, in particular in terms of HAs and FOM. At 0-4 cm depth, both HAs and FOM contents were roughly twice as high in the mulched plots as in the control plots. The effects of mulching on the molecular composition of HAs and FOM fractions, however, varied markedly. Analytical pyrolysis (Py-GC/MS) revealed that mulching had led to a noticeable accumulation of labile, aliphatic SOM constituents such as carbohydrate-derived and alkyl compounds (fatty acids and n-alkanes) but that it hardly affected the composition of HAs. Even so, solid-state 13C NMR spectroscopy showed that mulching had resulted in a relative increase in aryl C in the FOM fraction, suggesting an enhanced preservation of the pyrogenic OM. Overall, the combined use of a range of analytical techniques allowed to conclude that, five years after their application, the forest logging residues had led to a greater preservation of the fire-derived pyrogenic OM (mainly aromatic compounds) in the topsoil as well as to higher contents of SOM's most labile molecular constituents (mainly carbohydrates and n-alkyl compounds). The former reflected the reduced erosion rates, while the latter was probably due to a combination of reduced erosion rates with the additional input of fresh organic matter.

Surfactant properties of humic acids extracted from volcanic soils and their applicability in mineral flotation processes.

Surface Tension (ST) of water solutions of humic acids extracted from volcanic ash derived soils (soil humic acids, S-HA), were measured under controlled conditions of pH (13.0), temperature (25 °C) and ionic strength (NaOH 0.1M) to establish the Critical Micellar Concentration (CMC). All S-HA were characterized by elemental analysis, acid-base titration, Transmission Electronic Microscopy (TEM) micrographs, isoelectric point (IEP) and solid state 13C-NMR. After that, these humic acids were evaluated as potential biomaterials to be used in mineral flotation processes, where a series of experiments were conducted at different S-HA and molybdenite ratio (from 0.2 to 50 g ton-1) establishing the IEP of all resultant materials. The use of solid state 13C-NMR enabled the following sequence of intensity distribution areas of S-HA to be established: O/N Alkyl>Alkyl C>Aromatic C>Carboxyl. The experimental values of ST and the calculated CMC (ranging from 0.8 to 3.3 g L-1) revealed that for S-HA no relationship between the abundance groups and their behavior as surfactant materials was observed. In relation with IEP determined for all materials, the highest surface charge, which can be useful for flotation processes, was obtained with 0.2 g of S-HA per ton of molybdenite. Additionally, TEM studies confirm the formation of pseudoaggregates for all the S-HA considered. Finally, the S-HA could be considered as an alternative to chemical products and commercial humic acids materials in mineral flotation processes.

Hydrothermal carbonization of biomass residues: mass spectrometric characterization for ecological effects in the soil-plant system.

Hydrochars, technically manufactured by hydrothermal carbonization (HTC) of biomass residues, are recently tested in high numbers for their suitability as feedstock for bioenergy production, the bioproduct industry, and as long-term carbon storage in soil, but ecological effects in the soil-plant system are not sufficiently known. Therefore, we investigated the influence of different biomass residues and process duration on the molecular composition of hydrochars, and how hydrochar addition to soils affected the germination of spring barley ( L.) seeds. Samples from biomass residues and the corresponding hydrochars were analyzed by pyrolysis-field ionization mass spectrometry (Py-FIMS) and gaseous emissions from the germination experiments with different soil-hydrochar mixtures by gas chromatography/mass spectrometry (GC/MS). The molecular-level characterization of various hydrochars by Py-FIMS clearly showed that the kind of biomass residue influenced the chemical composition of the corresponding hydrochars more strongly than the process duration. In addition to various detected possible toxic substances, two independent mass spectrometric methods (Py-FIMS and GC/MS) indicated long C-chain aliphatic compounds which are typically degraded to the C-unit ethylene that can evoke phytotoxic effects in high concentrations. This showed for the first time possible chemical compounds to explain toxic effects of hydrochars on plant growth. It is concluded that the HTC process did not result in a consistent product with defined chemical composition. Furthermore, possible toxic effects urgently need to be investigated for each individual hydrochar to assess effects on the soil organic matter composition and the soil biota before hydrochar applications as an amendment on agricultural soils.

Soil Organic Matter Molecular Composition Shifts Driven by Forest Regrowth or Pasture after Slash-and-Burn of Amazon Forest.

Slash-and-burn of Amazon Forest (AF) for pasture establishment has increased the occurrence of AF wildfires. Recent studies emphasize soil organic matter (SOM) molecular composition as a principal driver of post-fire forest regrowth and restoration of AF anti-wildfire ambience. Nevertheless, SOM chemical shifts caused by AF fires and post-fire vegetation are rarely investigated at a molecular level. We employed pyrolysis-gas chromatography-mass spectrometry to reveal molecular changes in SOM (0-10, 40-50 cm depth) of a slash-burn-and-20-month-regrowth AF (BAF) and a 23-year Brachiaria pasture post-AF fire (BRA) site compared to native AF (NAF). In BAF (0-10 cm), increased abundance of unspecific aromatic compounds (UACs), polycyclic aromatic hydrocarbons (PAHs) and lipids (Lip) coupled with a depletion of polysaccharides (Pol) revealed strong lingering effects of fire on SOM. This occurs despite fresh litter deposition on soil, suggesting SOM minimal recovery and toxicity to microorganisms. Accumulation of recalcitrant compounds and slow decomposition of fresh forest material may explain the higher carbon content in BAF (0-5 cm). In BRA, SOM was dominated by Brachiaria contributions. At 40-50 cm, alkyl and hydroaromatic compounds accumulated in BRA, whereas UACs accumulated in BAF. UACs and PAH compounds were abundant in NAF, possibly air-transported from BAF.

Impact of Biochar Amendment on Soil Properties and Organic Matter Composition in Trace Element-Contaminated Soil.

The application of biochar as an organic amendment in polluted soils can facilitate their recovery by reducing the availability of contaminants. In the present work, the effect of biochar application to acid soils contaminated by heavy metal spillage is studied to assess its effect on the quantity and composition of soil organic matter (SOM), with special attention given to soil humic acids (HAs). This effect is poorly known and of great importance, as HA is one of the most active components of SOM. The field experiment was carried out in 12 field plots of fluvisols, with moderate and high contamination by trace elements (called MAS and AS, respectively), that are located in the Guadiamar Green Corridor (SW Spain), which were amended with 8 Mg·ha-1 of olive pit biochar (OB) and rice husk biochar (RB). The results indicate that 22 months after biochar application, a noticeable increase in soil water holding capacity, total organic carbon content, and soil pH were observed. The amounts of oxidisable carbon (C) and extracted HAs in the soils were not altered due to biochar addition. Thermogravimetric analyses of HAs showed an increase in the abundance of the most thermostable OM fraction of the MAS (375-650 °C), whereas the HAs of AS were enriched in the intermediate fraction (200-375 °C). Spectroscopic and chromatographic analyses indicate that the addition of biochar did not alter the composition of the organic fraction of HAs, while Cu, Fe, and as were considerably accumulated at HAs.

Fractionation of organic matter due to reaction with ferrihydrite: coprecipitation versus adsorption.

In soil and water, ferrihydrite frequently forms in the presence of dissolved organic matter. This disturbs crystal growth and gives rise to coprecipitation of ferrihydrite and organic matter. To compare the chemical fractionation of organic matter during coprecipitation with the fractionation involved in adsorption onto pristine ferrihydrite surfaces we prepared ferrihydrite-organic matter associations by adsorption and coprecipitation using (i) a forest-floor extract or (ii) a sulfonated lignin. The reaction products were studied by (13)C CPMAS NMR, FTIR, and analysis of hydrolyzable neutral polysaccharides. Relative to the original forest-floor extract, the ferrihydrite-associated organic matter was enriched in polysaccharides, especially when adsorption took place. Moreover, mannose and glucose were bound preferentially to ferrihydrite, while fucose, arabinose, xylose, and galactose accumulated in the supernatant. This fractionation of sugar monomers was more pronounced during coprecipitation and led to an enhanced ratio of (galactose + mannose)/(arabinose + xylose). Experiments with lignin revealed that the ferrihydrite-associated material was enriched in its aromatic components but had a lower ratio of phenolic C to aromatic C than the original lignin. A compositional difference between the adsorbed and coprecipitated lignin is obvious from a higher contribution of methoxy C in the coprecipitated material. Coprecipitated organic matter may thus differ in amount and composition from adsorbed organic matter.

Ammonoxidised lignins as slow nitrogen-releasing soil amendments and CO2-binding matrix.

Nitrogen (N) is a major nutrient element controlling the cycling of organic matter in the biosphere. Its availability in soils is closely related to biological productivity. In order to reduce the negative environmental impact, associated with the application of mineral N-fertilizers, the use of ammonoxidised technical lignins is suggested. They can act as potential slow N-release fertilisers which concomitantly may increase C sequestration of soils by its potential to bind CO2. The idea of our study was to combine an improved chemical characterisation of ammonoxidised ligneous matter as well as their CO2-binding potential, with laboratory pot experiments, performed to enable an evaluation of their behaviour and stability during the biochemical reworking occurring in active soils.

Biochar ageing in polluted soils and trace elements immobilisation in a 2-year field experiment.

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 (CaCl2 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.

An examination of the role of biochar and biochar water-extractable substances on the sorption of ionizable herbicides in rice paddy soils.

The application of biochar as a soil amendment can increase concentrations of soil organic matter, especially water-extractable organic substances. Due to their mobility and reactivity, more studies are needed to address the potential impact of biochar water-extractable substances (BWES) on the sorption of herbicides in agricultural soils that are periodically flooded. Two paddy soils (100 and 700 years of paddy soil development), unamended or amended with raw (BC) or washed biochar (BCW), were used to test the influence of BWES on the sorption behavior of the herbicides azimsulfuron (AZ) and penoxsulam (PE). The adsorption of AZ to biochar was much stronger than that to the soils, and it was adsorbed to a much larger extent to BC than to BCW. The depletion of polar groups in the BWES from the washed biochar reduced AZ adsorption but had no effect on PE adsorption. The adsorption of AZ increased when the younger soil (P100) was amended with BC and decreased when it was amended with BCW. In P700, which has lower dissolved organic carbon (DOC) content than P100, the adsorption of AZ increased regardless of whether biochar was raw or washed. The adsorption of PE slightly decreased when P100 was amended with BC or BCW and slightly increased when P700 was amended with BC or BCW. In order to evaluate compositional differences in the biochar and BWES before and after the washing treatment, we performed solid-state 13C NMR spectroscopy of BC and BCW, and high resolution mass spectrometry of BWES. Our observations stress the importance of proper consideration of soil and biochar properties before their incorporation into paddy soils, since biochar may reduce or increase the mobility of AZ and PE depending on soil properties and time of application.

Chemical, physical and morphological properties of biochars produced from agricultural residues: Implications for their use as soil amendment.

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.

Impact of wildfires on subsurface volcanic environments: New insights into speleothem chemistry.

Siliceous speleothems frequently reported in volcanic caves have been traditionally interpreted as resulting from basalt weathering combined with the activity of microbial communities. A characteristic feature in lava tubes from Hawaii, Azores and Canary Islands is the occurrence of black jelly-like speleothems. Here we describe the formation process of siliceous black speleothems found in a lava tube from La Palma, Canary Islands, Spain, based on mineralogy, microscopy, light stable isotopes, analytical pyrolysis, NMR spectroscopy and chemometric analyses. The data indicate that the black speleothems are composed of a hydrated gel matrix of amorphous aluminum silicate materials containing charred vegetation and thermally degraded resins from pines or triterpenoids from Erica arborea, characteristic of the overlying laurel forest. This is the first observation of a connection between fire and speleothem chemistry from volcanic caves. We conclude that wildfires and organic matter from the surface area overlying caves may play an important role in the formation of speleothems found in La Palma and demonstrate that siliceous speleothems are potential archives for past fires.

Abundance and diversity of CO2-fixing bacteria in grassland soils close to natural carbon dioxide springs.

Gaseous conditions at natural CO2 springs (mofettes) affect many processes in these unique ecosystems. While the response of plants to extreme and fluctuating CO2 concentrations ([CO2]) is relatively well documented, little is known on microbial life in mofette soil. Therefore, it was the aim of this study to investigate the abundance and diversity of CO2-fixing bacteria in grassland soils in different distances to a natural carbon dioxide spring. Samples of the same soil type were collected from the Stavesinci mofette, a natural CO2 spring which is known for very pure CO2 emissions, at different distances from the CO2 releasing vents, at locations that clearly differed in soil CO2 efflux (from 12.5 to over 200 micromol CO2 m(-2) s(-1) yearly average). Bulk and rhizospheric soil samples were included into analyses. The microbial response was followed by a molecular analysis of cbbL genes, encoding for the large subunit of RubisCO, a carboxylase which is of crucial importance for C assimilation in chemolitoautotrophic microbes. In all samples analyzed, the "red-like" type of cbbL genes could be detected. In contrast, the "green-like" type of cbbL could not be measured by the applied technique. Surprisingly, a reduction of "red-like" cbbL genes copies was observed in bulk soil and rhizosphere samples from the sites with the highest CO2 concentrations. Furthermore, the diversity pattern of "red-like" cbbL genes changed depending on the CO(2) regime. This indicates that only a part of the autotrophic CO2-fixing microbes could adapt to the very high CO2 concentrations and adverse life conditions that are governed by mofette gaseous regime.

Distribution of black carbon and black nitrogen in physical soil fractions from soils seven years after an intense forest fire and their role as C sink.

After vegetation fires, incorporation of pyrogenic organic matter (PyOM) into soil organic matter (SOM) shifts its composition toward higher aromaticity and to an increase of N-heterocyclic constituents, formerly introduced as black nitrogen (BN). To investigate the medium-term impact of these shifts on the quality of SOM and its role as an important C sink, the A horizon from soils of the fire-prone Sierra de Aznalcóllar (Southern Spain) were sampled 4 weeks and 7 years after a severe fire. The solid-state 13C and 15N nuclear magnetic resonance (NMR) spectra of the samples obtained 4 weeks after the fire indicated quick incorporation of PyOM into SOM. Correspondingly, pyrrole-type N dominated the organic N fraction. Seven years after the fire, the aryl C contribution decreased from 46% to 23% of organic C, although it was still higher at the burnt sites than in the unburnt reference soil (16%). This fast loss of PyOM may be due to erosion, transport into deeper soils or microbial decomposition. The contribution of the latter is in line with former incubation experiments with burnt soils from the same area. Comparably, in the recovered soil, BN was almost completely substituted by amide N. Studying the partitioning of PyOM among the density and particle size fractions of the soils, indicated that after medium-term recuperation, most PyOM occurred in the free and occluded particulate OM fractions (fPOM and oPOM). The low protection against microbial degradation and the low density of these fractions may explain the high PyOM losses from the studied soils either by decomposition or by transport. We suggested that formation of PyOM-soil mineral associates represents an important step for reducing losses of fire-derived charcoal due to biochemical mineralization and thus to its sequestration in soils.

Soil-borne fungi challenge the concept of long-term biochemical recalcitrance of pyrochar.

Pyrogenic organic matter (PyOM) is assumed to be biochemically recalcitrant, but recent studies indicated a quick decrease of PyOM in post-fire soils. Regardless erosion and abiotic degradation, microbial decomposition has been the explanation for this response, but no direct proof has been provided up to now. In the present study, we were able to demonstrate for the first time that the soil-borne fungus Fusarium oxysporum is not only colonizing the pore system of pyrochar (PyC) but is also involved in the degradation of its aromatic network. We showed that PyC not only stimulates microbial degradation of soil organic matter (SOM), but is also attacked and decomposed by microorganisms. Our observations are based on the chemical and morphological alterations of a sewage-sludge derived PyC produced at 600 °C after its amendment to a Calcic Cambisol by solid-state 13C nuclear magnetic resonance spectroscopy, analytical pyrolysis, elemental analysis, field emission scanning electron microscopy and DNA-based analysis of the isolated fungi. We showed that biofilms detected in the PyC play an essential role in the degradation process. These results are indispensable for a reliable assessment of the carbon sequestration potential of PyC in soils but also for improving global C cycling models.

Effects of aging under field conditions on biochar structure and composition: Implications for biochar stability in soils.

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, 13C NMR spectroscopy and CO2-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 13C 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.

Effect of biochar amendment on compost organic matter composition following aerobic composting of manure.

Biochar, a material defined as charred organic matter applied in agriculture, is suggested as a beneficial additive and bulking agent in composting. Biochar addition to the composting feedstock was shown to reduce greenhouse gas emissions and nutrient leaching during the composting process, and to result in a fertilizer and plant growth medium that is superior to non-amended composts. However, the impact of biochar on the quality and carbon speciation of the organic matter in bulk compost has so far not been the focus of systematic analyses, although these parameters are key to determine the long-term stability and carbon sequestration potential of biochar-amended composts in soil. In this study, we used different spectroscopic techniques to compare the organic carbon speciation of manure compost amended with three different biochars. A non-biochar-amended compost served as control. Based on Fourier-transformed infrared (FTIR) and 13C nuclear magnetic resonance (NMR) spectroscopy we did not observe any differences in carbon speciation of the bulk compost independent of biochar type, despite a change in the FTIR absorbance ratio 2925cm-1/1034cm-1, that is suggested as an indicator for compost maturity. Specific UV absorbance (SUVA) and emission-excitation matrixes (EEM) revealed minor differences in the extractable carbon fractions, which only accounted for ~2-3% of total organic carbon. Increased total organic carbon content of biochar-amended composts was only due to the addition of biochar-C and not enhanced preservation of compost feedstock-C. Our results suggest that biochars do not alter the carbon speciation in compost organic matter under conditions optimized for aerobic decomposition of compost feedstock. Considering the effects of biochar on compost nutrient retention, mitigation of greenhouse gas emissions and carbon sequestration, biochar addition during aerobic composting of manure might be an attractive strategy to produce a sustainable, slow release fertilizer.