Global environmental changes are threatening the productivity of agroecosystems. Floods or droughts, together with long-term decline in soil organic matter stocks are pointing to the necessity of finding solutions for sustainable performance of agroecosystems.

Deep soil horizons store significant amounts of water, soil organic carbon and nutrients, and thus subsoil management is being increasingly considered as an option to sustain crop productivity under unfavorable conditions.

We used samples from several field experiments in Germany designed to investigate the potential benefits of deep ploughing together with deep placement of organic fertilizers on soil organic matter stocks. We recorded hyperspectral images of 1-metre soil cores in the Vis-NIR range and modelled the C distribution at a very high spatial resolution (53×53 μm²). Using approaches for GIS analyses, we quantified the changes in C and N stocks and we will present their spatial distribution resulting from the incorporation of different types of organic fertilizer (compost vs green manure) in subsoils. The organic matter stocks and C:N stoichiometry are both impacted by the agricultural management and the imaging technique allows us to distinguish between increased amount of organic matter in hotspots or in soil mineral matrice and to discuss the mechanisms controlling the observed changes.

How to cite: Guigue, J., Bauke, S. L., Seidel, S. J., Athmann, M., Schmittmann, O., Kögel-Knabner, I., and Amelung, W.: Subsoil management in agriculture and changes in organic matter stocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15356, https://doi.org/10.5194/egusphere-egu23-15356, 2023.

Soil fertility refers to the capacity of soil to support plant growth and development. Cation exchange capacity (CEC) is a fundamental measure of soil fertility quantifying soil's ability to retain essential cation nutrients such as potassium, ammonium, calcium, and magnesium, serving as a reservoir for soil native cations and artificially applied ones. However, soils are a finite resource and therefore subjected to unprecedented pressure due to rapid human population growth, agricultural activity, and food consumption, resulting in unsustainable soil degradation. For this reason, the application of exogenous organic matters (EOMs) such as biosolid, compost, manure, and biochar, is regarded as one of the most sustainable approaches for enhancing soil fertility, plant growth, and yield, soil carbon content, microbial biomass, and activity as well as preventing desertification by improving soil structure stability. Given the diverse origins of organic amendments from agriculture, forestry, industry, or wastewater-derived biosolids, their physical and chemical properties are different and may differently affect the adsorption and affinity of nutrient cations such as NH₄⁺ and K⁺. Understanding the intrinsic properties of these EOMs in conjunction with the K-NH₄ cation exchange behavior would improve our understanding of K-NH₄ fertility management in organic matter-amended soil. The motivation for this study is the recognized knowledge gaps regarding cations exchange in organic materials, in particular, exchange reversibility. The main objective of this study is to quantify the cation exchange reversibility of K-NH₄ in EOMs and to assess the hysteretic desorption behavior. Binary exchange experiments were conducted in which soils or organic materials were pre-saturated with NH₄⁺. The results were analyzed based on the Gapon equation and the Freundlich model. The results demonstrated significant differences between soil and organic materials in adsorption capacity and selectivity of K⁺ and NH₄⁺. In the desorption phase, the hysteretic desorption of K⁺ and NH₄⁺ was observed and found to be concentration dependent. In our presentation, the results of exchange isotherm and adsorption-desorption will be presented and discussed.    

Keywords: soil organic matter, cation exchange, selectivity coefficient, adsorption, desorption, potassium, ammonium.

How to cite: Nguyen, B.-T. and Arye, G.: Cation exchange reversibility of potassium-ammonium binary solution: effect of organic matter, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5668, https://doi.org/10.5194/egusphere-egu23-5668, 2023.

The cultivated dryland soils of North Africa present low fertility and productivity due to low organic matter content (Brahim et al., 2021). Date palm residues are an abundant resource in these regions and only a minor part is recovered in oasian agroecosystems. The ISFERALDA project – Improving Soil FERtility in Arid and semi-arid lands using Local organic DAte palm residues – aims at developing the use of organic amendments based on traditional production (composting and slow pyrolysis) as a key tool to improve soil fertility and soil properties.
The objective of this study was to quantify the effects of compost and biochar based on date palm residues on soil water retention. Two soils with properties similar to North Africa soils (sandy loam texture, alkaline pH, low organic matter content) were collected in a semi-arid Mediterranean area of southeast Spain. In addition, and in order to test further the influence of soil texture, soil sand content was artificially increased by supplementing the natural soils with washed quartz sand. The different types of organic amendments were tested at a dose of 60 t/ha (Edeh et al., 2020): compost alone, biochar alone and mixture of compost and biochar (50:50 in weight). Water content was measured using pressure membrane apparatus at nine different matric potential (pF), ranging from the saturation to the permanent wilting point.
The results showed that water retention was higher in soil with organic amendments regardless of the pF and the soil type. For a specific soil, the addition of biochar alone or in combination with compost to the soil resulted in higher values than compost alone. The improvement in water retention properties was more pronounced for soils amended with sand. Thus, composting and/or pyrolysis of date palm residues is a viable alternative to improve the water retention properties of sandy and loamy soils. 

Keywords:
Date palm – arid and semi-arid lands – organic amendments – soil water retention

References :

Brahim, N., Karbout, N., Latifa, D., & Bouajila, A. (2021). Global Landscape of Organic Carbon and Total Nitrogen in the Soils of Oasis Ecosystems in Southern Tunisia. Agronomy, 11, 1‑17.

Edeh, I. G., Mašek, O., & Buss, W. (2020). A meta-analysis on biochar’s effects on soil water properties—New insights and future research challenges. The Science of the Total Environment, 714, 136857. https://doi.org/10.1016/j.scitotenv.2020.136857

How to cite: Le Guyader, E., Morvan, X., Gommeaux, M., Miconnet, V., Marin, B., Moussa, M., Karbout, N., Zoghlami, R. I., Delgado-Iniesta, M. J., and Intrigliolo, D. S.: Impact of organic amendments from date palm residues on water retention properties of two coarse texture soils, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7294, https://doi.org/10.5194/egusphere-egu23-7294, 2023.

Average soil temperatures in winter in Germany are frequently between 0 and +10℃, usually at high soil moisture levels. Therefore, the decomposition of soil organic matter and soil nitrogen (N) cycling are still active and could be substantial under these conditions, potentially leading to high N losses both in the form of nitrate to the groundwater and nitrous oxide to the atmosphere. High carbon soil amendments (HCA) have the potential to immobilize excess mineral N in the soil due to stimulation of microbial biomass growth. However, to date, it is not sufficiently known how well this N immobilization works at lower temperatures, and how long the effect will last over winter. In order to elucidate how the application of different HCA affects N immobilization in soil under winter conditions, we conducted a 7-month laboratory incubation experiment with silty clay soil low in soil organic carbon from a recultivation area after open-cast lignite mining near Jülich, Germany. Each soil column contained about 500 g of recultivation soil sieved at 2 mm. A scenario of a typical mineral N content after harvest was created by adding 50 kg NH₄⁺-N ha^-1 to the soil before application of the different HCA, which were then added at a rate of 4 t C ha^-1. Eight different treatments were implemented: application of NH₄⁺ only (B), and then NH₄⁺ applied with wheat straw (WS), biochar (BIO), spruce sawdust (SS), lignite (LIG), cellulose (CEL), a combination of wheat straw (2 t C ha^-1) and spruce sawdust (2 t C ha^-1) (CWS), and a combination of wheat straw (2 t C ha^-1) and biochar (2 t C ha^-1) (CWB), respectively. In the three straw treatments, carbon dioxide (CO₂) emissions peaked 14 days after the start of the experiment. In all treatments, CO₂ emissions decreased with time. In the end, the CEL treatment had the highest cumulative CO₂ emission during the entire incubation period. In contrast, the CEL treatment had a significantly lower soil nitrate content than all other treatments over the whole duration of 7 months, indicating that cellulose was most effective and long-lasting in stimulating microbial N immobilization under temperate winter conditions in silty clay soil.

How to cite: Zhao, K., Reichel, R., Wissel, H., and Brüggemann, N.: The effect of different high carbon soil amendments on N retention capacity under winter conditions in silty clay soil with low organic carbon content: An incubation study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12342, https://doi.org/10.5194/egusphere-egu23-12342, 2023.

Recent studies have shown that mineral-associated organic N is an important source of bioavailable N, and organic N sorption to/desorption from clay minerals may be a key factor of N dynamics in soils. This study aims to elucidate the importance of sorption/desorption to the mineralization of amino acids in volcanic soils. We hypothesized that in volcanic soils, sorption of amino acid to minerals reduces its mineralization and that desorption of amino acid differs reflecting soil properties.

Soils sampled from the O, A, and B horizons of three volcanic soils were used. Incubation experiments using the tracer method were carried out to assess the mineralization of alanine, which was used as a representative amino acid in the soil. Soils were placed in glass jars and were amended with ¹³C labeled alanine at a rate of 1% of total N to investigate the effects of alanine sorption on its mineralization. Similarly, separate soils were amended with ¹³C labeled alanine sorbed to iron oxides to examine the desorption of alanine. The percentage of mineralization of the added alanine over 7 days of incubation was determined. Acid oxalate extractable Al and Fe (Al_o and Fe_o) of soil were measured as representative clay mineral components contributing to sorption. Sorption isotherm experiments were carried out to understand each soil’s sorption characteristics, and the results were fitted to Freundlich’s sorption equation.

The percentage of mineralization of free alanine was the highest in O horizon soils at 40% and showed a decreasing trend going down the soil profile. The mineralization of alanine sorbed to iron oxides was approximately 56% of that of free alanine, regardless of the soil properties. Our results suggest that approximately 44% of the sorbed alanine was strongly sorbed to the iron oxides, but the remaining alanine was easily desorbed and mineralized similarly to free alanine. Freundlich-k constants were correlated with both Al_o and Fe_o content and mineralization of alanine (r = 0. 70, P < 0.05, r = −0.63, P < 0.05, respectively). Furthermore, based on the fitted Freundlich’s equation, the amount of alanine that was sorbed of the added alanine in the incubation experiment was calculated, and the results indicated that alanine mineralization was strongly correlated with the ratio of sorbed to added alanine (r = −0.95, P < 0.001).

We conclude that the desorption of sorbed alanine was constant regardless of soil properties and that sorption reduced alanine mineralization rather than delaying it.

How to cite: Kitagawa, N., Watanabe, T., Sawada, K., Kunito, T., and Funakawa, S.: Sorption and desorption controls on alanine bioavailability in volcanic soils, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11195, https://doi.org/10.5194/egusphere-egu23-11195, 2023.

Soil organic matter (SOM) is an essential fraction of soil and contributes to its fertility. Land use and cultivation may affect SOM. This study investigates whether SOM concentration and composition differ in soil pools or are changed by soil management (tillage, fertilisations, and crop covers). Soil samples were collected in April 2019 from cropland and nearby grassland. The study sites were part of a long-term experiment in Martonvasar (Hungary), established in 1958 and characterised by Chernozem soils. Total organic carbon (TOC) contents and compounds were studied in three soil pools (bulk soil, fast pool, and slow pool). Both TOC and total N concentrations were high in the slow pool, with higher stored C contents in grassland than in cropland. Tillage effects reduced aggregate stability in cropland, which explains a lower aliphatic content than grassland. Insufficient physical protection due to the tillage practice may enhance OM loss in cropland even under fertiliser inputs. Neither fertiliser nor crop covers affected SOM compositions, while they were different in soil pools. More complex OM in the slow pool than in the fast pool. It indicates that the slow pool is the main protecting path for SOM, possibly referred to older or decayed organic compounds. To understand the bottom-up process, microorganisms’ community role in SOM stabilisation needs to be studied.

How to cite: Al-Graiti, T., Jakab, G., Ujházy, N., Márialigeti, K., Árendás, T., Karlik, M., and Szalai, Z.: Studying Soil Organic Matter Composition in Arable land: Can Soil Management Impact Carbon Pools?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15258, https://doi.org/10.5194/egusphere-egu23-15258, 2023.

The interest in biochar, the persistent product of pyrolysis as a soil amendment, began in the first decade of the 2000s and rooted in research on fertile, anthropogenic Terra preta soils in the Amazon region. Research paper numbers started to rise exponentially from 2007/2008 onwards, resulting in more than 27,434 published papers with the keyword “biochar” to date (January 2022) according to Easy Web of Science. Its persistence [1] makes properly produced biochar an interesting approach for carbon dioxide removal (CDR) with added benefits for soil fertility [2]. For the overall soil C sequestration that comes with biochar use, three points are crucial: (1) the persistence of biochar itself with depends largely on the pyrolysis temperature and duration, the soil and climatic conditions; (2) the effect that biochar application may have on the already existing soil organic carbon, where an initial short-lived positive priming seems to switch towards a negative priming after 0,5 – 2 years when soil-biochar only mixtures are investigated [3]. However, the least well-known item regarding the overall CDR potential of biochar use in soils is the question (3) if and under what circumstances biochar may cause an additional soil organic carbon build-up, above that observed in a control soil/agricultural ecosystem without biochar application when soils receive permanently new C, e.g. via a green cover, crops plus intercropping and other practices. For example Blanco-Canqui et al. [4] observed a significantly higher SOC build-up over 6 years after biochar application on average in three field experiments in the mid-west US of about 7 vs. 2 tons per ha in the biochar versus control plots. Weng et al. [5] demonstrated that, indeed, the maximum soil C concentration ceiling could be lifted by (repeated) biochar applications to a subtropical grassland in Australia. In this contribution, I examine the available experimental evidence and mechanistical understanding with regard to such “humus-return of biochar investment” effects, if and under what conditions they can be obtained and what methods are available to investigate this effect in long-term field experiments. The contribution aims to stimulate discussion on a joint methodical framework to investigate such a potentially free “SOC interest return” effect of biochar use in agriculture which may be as important as the C sink generated by biochar application itself.

• Lehmann, J., et al., Biochar in climate change mitigation. Nature Geoscience, 2021. 14(12): p. 883-892.
• Schmidt, H.P., et al., Biochar in agriculture - A systematic review of 26 global meta-analyses. Global Change Biology Bioenergy, 2021. 13(11): p. 1708-1730.
• Wang, J., Z. Xiong, and Y. Kuzyakov, Biochar stability in soil: meta-analysis of decomposition and priming effects. GCB Bioenergy, 2016. 8(3): p. 512-523.
• Blanco-Canqui, H., et al., Soil carbon increased by twice the amount of biochar carbon applied after 6 years: Field evidence of negative priming. GCB Bioenergy, 2020. 12(4): p. 240-251.
• Weng, Z., et al., Microspectroscopic visualization of how biochar lifts the soil organic carbon ceiling. Nature Communications, 2022. 13(1): p. 5177.

How to cite: Kammann, C.: Evidence and research needs to identify potential SOC stock increases after biochar application: A literature study and roadmap to understand long-term effects, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6678, https://doi.org/10.5194/egusphere-egu23-6678, 2023.

Fusain is a macroscopic component, or lithotype, of coal. Microscopic fragments of fusain are referred to as “inertinite” maceral, which is commonly found in all carbonaceous and most sedimentary rocks. Fusain/inertinite is formed by carbonization of biomass in oxygen deficient paleo-wildfires and subsequent transportation into peatbogs and sedimentary basins. Substantial quantity of fusain/inertinite is commonly found in most post-Devonian coal and sedimentary rocks. Inertinite fragments are often microscopically characterized by their intricate vacuole structures inherited from the original cell lumens, which attests to their remarkable high preservability during geological processes.

Fusain/Inertinite is generally believed by geologists to be chemically highly inert due to its intense degree of aromatization and ordering of carbon molecular structure and cannot be degraded by shallow surface processes including oxidation and biodegradation. The “selective diagenesis” processes continue to preferentially degrade the more labile organic carbon fractions while preserving the most refractory fractions that are thermodynamically least favored to breakdown. The long-term, geological evolution of organic carbon in the Earth’s crust through three main stages of diagenesis, catagenesis, and metagenesis, is studied by organic petrology and organic geochemistry methods. This presentation provides results of these methods for a set of 20 synthetic biochars produced from different feedstocks to compare their geochemistry and optical characteristics with the commonly preserved geological fusain/inertinite in coal and other carbonaceous rocks.

The results show that biochars that have been produced at maximum pyrolysis temperature of over 600°C, resemble properties of the most refractory fusain/inertinite in the Earth’s crust and would not be degraded as long as other more thermodynamically labile organic carbon compounds are in existence. Any degradation of these biochars can only be perceived under intense geological burial heat in the Earth’s mantle or contact metamorphism by igneous intrusions and hydrothermal processes. The claim of short-term carbon permanence for biochars contradicts co-existence of fusain/inertinite with labile organic carbon commonly observed in carbonaceous rocks. Degradation of refractory fusain/inertinite would not be thermodynamically favored while large quantity of labile organic carbon is readily and commonly available in carbonaceous rocks. The results of this study highlight the need for re-thinking carbon permanence of biochars within the context of the deep geological carbon cycle.

How to cite: Sanei, H. and Petersen, H. I.: Carbon permanence of biochar; a lesson learned from the geologically preserved charcoal in carbonaceous rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10913, https://doi.org/10.5194/egusphere-egu23-10913, 2023.

The efficiency of biochar application in soil carbon sequestration (SCS) is highly sensitive to biochar longevity. To predict biochar longevity at extended timescales, modelling is essential. It is often expressed that data from field experiments can support this understanding, especially long-term field experiments (LTEs). Our work tests this assertion, using the existing evidence base. A literature search for LTEs of greater than 3-year duration and other criteria was conducted, with the null assumption that biochar C is inert in soil. Observations of soil organic carbon (SOC) after biochar additions from selected LTEs were made against predictions from the Roth C model, assuming no biochar decay.

At the end of July 2022, we found 982 articles describing field experiments that concerned biochar longevity and SCS and initiated within the last two decades. Among them, only 17 reported LTEs matching our screening criteria and providing long-term data suitable for modelling. In these LTEs, a range of SOC dynamics were observed where an acceptable level of fit could not be achieved using a plausible range in parameters. A range of potential reasons for the deviation between measured observations and model predictions were considered, including priming effects on native SOC, migration of biochar particles, sampling and measurement issues, etc. Such factors could not be isolated with sufficient confidence to adjust observational data or model parameters, confounded by the inability to distinguish the dynamics of biochar C and non-biochar C.

Current field data do not enable us to reject the null hypothesis. Reliable parameterization for biochar longevity solely based on field experiments may not be possible on timescales relevant to mitigation of climate change. Instead, alternative strategies for assessing biochar longevity are required, that can be verified in real-time alongside a set of permanent benchmark sites, across different agro-ecological zones with uniform experimental standards, not least in sampling strategy and biochar C and non-biochar C distinction. That will support the incremental adoption of biochar, while providing a robust method for post-hoc adjustment of mitigation benefit.

How to cite: Xu, C., Sohi, S., Hillier, J., and Baggs, E.: Do published field experiments inform the longevity of biochar in soil?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-303, https://doi.org/10.5194/egusphere-egu23-303, 2023.

Biochar is considered a promising amendment to store carbon and improve degraded soil properties. However, this additive could have disadvantageous effects on plants and soil organisms due to its charring process toxic by-products and its ability to retain nutrients. To mitigate these negative effects, co-amendments with an organic additive such as compost have been proposed, but comparative studies are scarce. In this study, we investigated the influence of biochars applied (i) solely, (ii) mixed with matured compost or (iii) co-composted biochar on soil properties and plant growth. To this aim, three different types of biochar derived from various feedstocks were tested in two soil with different agronomic properties (Luvisol and Fluvisol). After three months of greenhouse experiment with grown lettuce (Lactuca sativa var. capitata L.), the shoot and root biomasses were quantified and the soil physicochemical properties were measured (pH, CEC, total N, organic C and soil available P). Solely applied biochar did not influence plant yield and maintained alkaline soil pH. Contrariwise, biochar mixed with matured compost maintained an average increase of lettuce growth by three times with risen soil nutrient content and kept alkaline pH. Whereas treatments with co-composted biochar and solely added compost promoted plant growth by almost six times, kept pH on neutral levels and nutrients on an average level with CEC equally enhanced, regardless of the biochar origin or the soil type. These results suggest that co-composted biochar addition to the soil is a convincing way to maintain soil fertility in a long term.

How to cite: Mikajlo, I., Louvel, B., Záhora, J., Lerch, T. Z., and Pourrut, B.: Co-composted biochar enhances soil fertility more than individual additives, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2978, https://doi.org/10.5194/egusphere-egu23-2978, 2023.

Accurately predicting the stability of a biochar sample placed in the environment is important for guiding climate policies and the emerging voluntary market for carbon removal. The stability of biochar in soils varies with feedstock type, pyrolysis parameters, and environmental conditions. Previous assessments have correlated biochar stability estimates to single features – like pyrolysis temperature, elemental molar ratios, or incubation duration – but these assessments used different datasets and methodologies and reached different conclusions. Therefore, our aim was to develop an open dataset of biochar decomposition experiments, and to develop a transparent data preparation and processing toolchain, enabling reproducibility of scientific results. We first made an inventory of all published biochar incubation experiments, and then collected the incubation data and an extensive set of associated metadata (i.e., biomass and biochar properties, pyrolysis and incubation conditions). In a second step, we developed a data analysis toolchain, including functions for extrapolation of the incubation data to longer times and models for correlation between metadata and estimated biochar stability. In the extrapolation step, the incubation data was fitted to decay functions. Care was taken to explore the effect of using different fitting algorithms and constraints, and to apply a recalibration of the incubation temperature. For the correlation step, several strategies were applied, including both single-feature linear regressions to reproduce previous results and multi-feature regressions based on decision trees. So far, a dataset of 135 observations with more than 8000 data elements was collected making it one of the largest biochar stability datasets available. For the first time, raw biochar decomposition data is also compiled for 111 observations (mostly laboratory incubations). The initial data exploration revealed that although pyrolysis temperatures in the range 350 to 700°C are well represented, there is a data gap at higher temperatures with only a few data points at 1200°C. Likewise, only two observations are available with a molar H/C ratio below 0.2. These gaps can guide design of future incubation studies, as these parameters are often seen as indicators of stability. During curve fitting with single, double, or triple exponential models, we noted that the choice of initial conditions was important for finding a good fit, but we also noted that in many cases fitting uncertainties were high, residuals were not necessarily randomly distributed, and that some observations did not fit well to any type of exponential model (likely due to experimental conditions). Nevertheless, we were able to approximately reproduce the fitting results reported in Woolf et al. (2021). Finally, linear correlations were established between predicted stability and pyrolysis temperature, molar H/C ratio, but also other features available in the dataset, yielding similar correlation coefficients as previously reported (0.1 to 0.4). Attempts to understand the variability in predicted stability (using principal component analysis) and to develop multi-variate and non-linear models have so far not significantly improved model performance without overfitting. Opportunities remain to use the compiled data for other types of modelling, e.g. in soil carbon models.

How to cite: Azzi, E. and Sundberg, C.: Revisiting biochar decomposition data and long-term stability estimates: a transparent and reproducible analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5625, https://doi.org/10.5194/egusphere-egu23-5625, 2023.

Biochar is an aspirational strategy for long-term carbon sequestration in soil, emergent in guidelines by the Intergovernmental Panel on Climate Change (IPCC). Yet, the rate and pathways of biochar mineralization remain uncertain, and information is scarce on the role of soil temperature. Recent studies predicting the 100-yr stability of biochar in soil use a profile of temperature sensitivity (Q₁₀) for biochar mineralization that deviates markedly from common biochemical temperature relationships, especially at mean annual temperatures of 0-10°C, which prevail in many temperate soils. Here, we compared estimates of biochar stability using (i) empirical Q₁₀ data and (ii) Arrhenius activation energies for biochar mineralization similar to those for other recalcitrant biomolecules. The results indicate that empirical Q₁₀ data used so far overestimate the long-term stability of biochar in soils at 0-10°C, but underestimate the stability at >10°C. The size of these effects increases with higher molar ratio of hydrogen to organic carbon (H/C_org) in the biochar, meaning that predictions for labile biochars are more uncertain. We conclude that care should be taken when normalizing biochar stability data to prevailing soil temperatures and call for further studies to document the temperature sensitivity of biochar mineralization

How to cite: Elsgaard, L. and Eriksen, R. L.: Temperature control on biochar decomposition in soil - implications for long-term carbon sequestration, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9326, https://doi.org/10.5194/egusphere-egu23-9326, 2023.

Biomass pyrolysis and a non-oxidative use of the produced biochar is widely acknowledged as a negative emission technology and part of Pyrogenic Carbon Capture and Storage (PyCCS). Biochar with a molar H/C ratio < 0.4, which is usually achieved by pyrolysis at 550°C or above, is highly persistent when applied to the soil. Still, the exact residence time remains subject to debate. Practical assessment tools and reliable models for carbon accounting are needed. Persistence of soil applied biochar is often assessed using soil incubation trials of rather short time horizons, lasting several month or years, with consecutive extrapolation of the observed degradation rate. Within such experiments, the decomposition rate of the biochar continues to decrease over time indicating that biochar consists of a broad range of carbonaceous compounds of different recalcitrance. Hydrogen pyrolysis, an analytical method used to determine the degree of aromatisation, suggests that 75% of the carbon in biochar with an overall H/C ratio <0.4 consists of persistent aromatic carbon (PAC), which will persist after soil application for > 1000 years, (Bowring et al., 2022; Howell et al., 2022), independent of the soil type and climate. The remaining 25% of the biochar carbon (heteroaromatic, aliphatic, etc) are considered semi-persistent carbon (SPC), presenting a mean residence time (MRT) in soil of 50 to 100 years, depending on soil type and climate. Based on this, up to 99% of the PyC loss quantified in an incubation trial may be attributed to the spectrum of SPC compounds, while the occurring PAC decay is very small and a neglectable loss in the context of carbon sink accounting. To validate the PAC residence time, data on long-term dynamics of the global, natural PyC cycle provides new insights. Natural PyC is produced from incomplete combustion in e.g. forest fires and is introduced to ecosystems globally at a scale of 0.114-0.383 Gt year^-1. Given the global deposits of natural PyC of 550-1,650 Gt a MRT of 1,440 to 14,500 years can be calculated (Bird et al., 2015; Santín et al., 2016; Bowring et al., 2022 ). Natural PyC is produced in an uncontrolled manner, thus achieving a lower degree of aromatisation, a smaller PAC pool and lower MRT compared to optimized PyC produced by controlled pyrolysis. Therefore, observations from the natural PyC cycle render the assumed PAC residence time conservative. Further research is necessary to enable empirical quantification of the PAC content of biochar across a broad range of feedstock material and pyrolysis conditions. Hydrogen pyrolysis is an elaborate, yet expensive tool not suitable for routine analysis e.g. in biochar and biochar C-sink certification. Thus, further methods for PAC quantification must be developed and standardized.

• Bird et al. (2015). Annual Review of Earth and Planetary Sciences, 43(1), 273–298. https://doi.org/10.1146/annurev-earth-060614-105038
• Bowring et al. (2022). Nature Geoscience 15:2, 15(2), 135–142. https://doi.org/10.1038/S41561-021-00892-0
• Howell et al. (2022). Science of The Total Environment, 849, 157610. https://doi.org/10.1016/J.SCITOTENV.2022.157610
• Santín et al. (2016). Global Change Biology, 22(1), 76–91. https://doi.org/10.1111/gcb.12985

How to cite: Meyer zu Drewer, J., Abiven, S., Hagemann, N., and Schmidt, H.-P.: Permanence of soil applied biochar: Conclusions from the natural pyrogenic carbon cycle validate carbon sink accounting., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11054, https://doi.org/10.5194/egusphere-egu23-11054, 2023.

Quantification of the time-dependent carbon sequestration by biochar remains a challenge. Recently, hydrogen pyrolysis was suggested to identify the content of stable polycyclic aromatic carbon (SPAC) or persistent aromatic carbon (PAC), which would not degrade to a relevant extent for centennial timescales due to their high degree of aromaticity and condensation (>7 rings). However, hydrogen pyrolysis is too expensive and laborious for broad application or even use in routine analysis.

Here, we test a suite of analytical methods in order to identify alternatives to hydrogen pyrolysis for the quantification of SPAC/PAC in routine analysis of biochar or other pyrogenic carbons. We use 34 experimental biochars out of two precursors, wood and straw, produced at different highest treatment temperatures (HTT = 400-800 °C) and eight industrial biochars obtained from different feedstocks and different pyrolysis technology with HTT of 550-1200 °C. The methods include elemental analysis to obtain molar ratios of H/C and O/C, electrical conductivity of the solid as a proxy for the degree of condensation, thermogravimetric analysis coupled to differential scanning calorimetry (TG-DSC) to assess thermal stability (R50) and Raman spectroscopy. Raman spectra of the D- & G band provide information on the nanostructural development and should allow relative quantification of the semi-persistent and persistent carbon fractions. An incubation experiment (biochar + sand + compost microbial consortium) under laboratory conditions will provide direct data on biochar mineralization to quantify the semi-persistent fraction (SPC).

Using a broad range of HTT and two different precursors as well as industrial biochars out of existing commercial pyrolysis reactors, we aim to cover a relevant parameter space to identify the possible range of SPAC/PAC and SPC content in industrial biochars. In an ideal case, this work will enable the use of low-cost technology such as Raman and/or electrical conductivity of the solid to quantify or estimate SPAC/PAC with a sufficient accuracy and precision.

How to cite: Voßwinkel, S., Ulbricht, A., Hagemann, N., Schmidt, H.-P., Herdegen, V., and Braeuer, A. S.: Quantification of the persistent aromatic carbon content in biochar, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17065, https://doi.org/10.5194/egusphere-egu23-17065, 2023.

How to cite: Hagemann, N., Conte, P., Leifeld, J., Giger, R., Bucheli, T. D., Schmidt, H.-P., and Grafmüller, J.: Impact of biomass ash content on biochar carbon speciation and stability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17040, https://doi.org/10.5194/egusphere-egu23-17040, 2023.

We investigated the effects of biochar (BC) at 0 and 20 t ha^-1, combined with two organic fertilizers (municipal solid waste compost, MC, and sewage sludge, SS), on soil organic matter (SOM) in a 9-year field experiment. To capture the protection by soil minerals and iron (Fe) against microbial decomposition, we fractionated SOM into particulate (POM) and mineral-associated organic matter (MAOM), and analyzed the fractions by iron (Fe) K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy.

BC and the two organic amendments increased soil organic carbon (OC) content, but only the organic fertilizers significantly increased total nitrogen (N) content. BC increased particulate OC and total N contents, while the organic fertilizers only increased particulate total N content. BC significantly increased mineral-associated OC content, while the organic fertilizers increased both mineral-associated OC and total N contents. We found no interaction between BC and organic fertilizers on mineral-associated OC and total N contents. The Fe EXAFS data fitting showed that the Fe(III)-SOM content of the Fe phases in POM and MAOM in unamended soils were noticeably different. Hematite represented the main Fe oxide phase in the POM fractions from all the amended soils, and Fe(III)-SOM averaged around 15%. In the amended soils, besides hematite (also present in the unamended soil), ferrihydrite occurred in all MAOM fractions, although at a different proportion.

How to cite: Giannetta, B., Plaza, C., Galluzzi, G., Benavente-Ferraces, I., García-Gil, J. C., Panettieri, M., Gascó, G., and Zaccone, C.: Response of soil organic matter fractions to biochar and organic fertilizers – Results from a nine-year field experiment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8354, https://doi.org/10.5194/egusphere-egu23-8354, 2023.

Increasing soil carbon (C) sequestration in paddy fields is significant for ensuring food security and achieving C neutrality in China. Biochar has been widely used as a soil amendment; still, long-term effects on the mechanisms of biochar's effect on soil C accumulation and the mediating role of microorganisms are poorly understood. To address this issue, three field experiments on paddies were chosen (Changsha, Nanjing, and Jiaxing), where biochar was applied for 7 to 8 years. The treatments included control (no addition), N (N fertilizer, 120 kg ha^-1), N+B1 (N and low amount of biochar, 15-24 t ha^-1), and NB2 (N and high amount of biochar, 22.5-48 t ha^-1), effects on soil organic C (SOC) mineralization, dissolved organic C (DOC), activities of enzymes, microbial biomass C (MBC) and community composition (based on phospholipid fatty acids (PLFAs), and C utilization efficiency (CUE) were studied. Biochar reduced cumulative CO₂ emissions in Changsha (by 32-34 %), Jiaxing (3.0-27 %) (p<0.05), and in Nanjing with under NB2 treatment (by 36 %) compared to N treatment. Biochar increased soil pH (0.03-0.38 units) in Changsha and Nanjing but did not affect Jiaxing plots. Biochar increased SOC, total N, chitinase activity, MBC (by 18-28 %,) and CUE (by 24-65 %, except in Jiaxing) but decreased DOC content (by 3-14 %) and peroxidase activity. Biochar addition increased the total and bacterial PLFA contents and decreased the bacteria:fungi ratio at the three sites (except for total PLFA in Nanjing) compared to the N treatment. The correlation analysis revealed that cumulative CO₂ emission was reduced under the increase of pH, MBC, SOC, and CUE, bacterial PLFA, and stimulated by DOC content and the rise of bacterial:fungi ratio. These indicated that long-term biochar amendments mainly increased the amount of C that bacteria can assimilate; the increase of MBC content and CUE could point to the stimulation of microbial C sequestration.

How to cite: Zhang, Q., Ge, T., Dippold, M., and Gunina, A.: Long-term action of biochar in paddy soils: effect on organic carbon and functioning of microbial communities, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5811, https://doi.org/10.5194/egusphere-egu23-5811, 2023.