SSS5.9

The wood ash contribution to soils represents a unique and important part of soil organic carbon following fires.  Wood ash imparts chemical and physical changes to the soil, evident in elements other than carbon.  Our case studies are from recent wildfires and experimental burns in mixed hardwood forests in the Pocono Mountains of Pennsylvania, USA.  In these studies, we identified increases in most of the major elements and some minor elements in soils following forest fires, analyzed with ICP-MS. Elements such as Mn, Mg, Na, Ca, Na, K, Cu, and Ba, derive from an infusion of biomass ash, with variable contribution depending on, for instance, tree species. In the case of Ba and Cu, their presence is distinctly different from any mineral parent material contribution to the soil, and therefore unique signatures of fire contribution. Signature post-fire elements persist in some cases over one year following the fire, and are found in both topsoil horizons and into illuvial soil horizons.

In the course of these investigations, we also found a curious depletion of all rare earth elements (REEs) and certain trace elements from the soil following forest fires, and in adjacent stream and wetland sediments. The post-fire difference in REE concentration was statistically significant (p < 0.10, N=51) in all but Eu and U, with light REEs La, Ce and Pr showing the most significant decreases. Among other trace elements, Sc (which behaves similarly to REEs), V, Cr, Ga, and Rb also exhibited statistically significant decreases (though other elements Cu and Sr increase along with the ash input). The reasons for the depletions are unclear. Other authors report that REE dynamics in soils are poorly understood, but may be associated with phosphates, carbonates, and silicates in the soil. These are relatively enriched via post-fire biomass ash, yet the associated REEs are missing. It is unlikely that the elements would have preferentially translocated through and below the soil profile. Erosion is ruled out, otherwise the ash-associated major and trace elements would also be depleted. Two possible causes for post-fire REE loss are 1) volatilization from the soil during the fire, and 2) rapid uptake by post-fire succession plants, notably ferns, which are known to bioaccumulate REEs. Further research is warranted, following the ongoing post-fire vegetation recovery, and the dynamics of REEs within the soil profile.       

How to cite: Pope, G., Callanan, J., Darley, J., Flood, M., Wear, J., Calderon, B., Gorring, M., and Li, X.: The Fate of Rare Earth Elements in Post-Fire Soils in the Pocono Mountains, Pennsylvania (USA), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13604, https://doi.org/10.5194/egusphere-egu21-13604, 2021.

Increasing frequency of wildfire in humid hardwood forests make it necessary to understand the occurrence and origin of soil water repellency in these systems, as wildfire-induced soil water repellency has been observed to severely impact many biophysical processes in other forest types. In this project, we studied two sites in the Appalachian Mountains, United States, (at Mount Pleasant Wildlife Refuge, Virginia, and Chimney Rock State Park, North Carolina) where wildfires occurred in late 2016. In each site, burned and unburned soils were evaluated for actual (in the field) and potential (in the laboratory) water repellency using the water drop penetration time method. In addition, samples were analyzed for organic carbon content (measured using C/N analyzer), hydrophobic functional groups (using Fourier transform infrared, FTIR), and their rank correlations (r_s) based on multiple samples collected one year after the fires. We found that soil water repellency was substantial greater in burned soils in the first months after the fire, and persisted for the entire year in the more severely burned soils. We also determined that potential water repellency was much greater than actual water repellency, and that organic carbon content and hydrophobic functional groups were significantly correlated to potential water repellency (p < 0.0001). Correlations were stronger at Mount Pleasant (0.77 < r_s <0.91) than at Chimney Rock (0.06 < r_s < 0.70). For actual water repellency only had significant correlations with soil organic content at Mount Pleasant (p < 0.0001), and with hydrophobic functional groups (p < 0.0001) at both sites except the unburned soils at Chimney Rock. However, these correlations were weaker than with potential water repellency, likely due to the influence of soil water content. Altogether, this study provides new insight into the influence of soil organic matter and its composition on post-wildfire soil water repellency.

How to cite: Chen, J., Strahm, B., and Stewart, R.: Drivers of soil water repellency after wildfires: case study in the south-central Appalachian Mountains, US, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3738, https://doi.org/10.5194/egusphere-egu21-3738, 2021.

Forest fires are increasing their recurrence, intensity and scale in Portugal, increasing also the vulnerability of this region of Southwestern Europe to the impacts of the climate change. In Portugal, several studies have been focusing the dynamic of cork oak forest after fires from tree level to landscape level. However few information is available about the impact of wildfires in the soil quality, namely C stock, and its evolution. This component of the ecosystem and the evolution of its characteristics can be related with the dynamics of the landscape in the post-fire period and its knowledge can help in the management and rehabilitation of plant-soil system.

Serra do Caldeirão is located in Algarve region (S of Portugal). The soils are classified as Leptosols with low fertility. The landscape is characterized by cork oak forests with shrub cover which is dominated by Cistus species. To assess the distribution of total organic C in the study area, several soil sampling were intersected, on two sampling dates (2012 and 2013), with the vector information of the burned area in 2004. This information was intended to assess the temporal evolution of C concentration, depending on its location in a burnt or non-burnt area.

In general, the burnt areas showed greater variability of C concentrations in soils collected in both the years, with maximum values ​​of 33.0 g/kg for 2012 and 36.5 g/kg for 2013. These maximum values ​​exceed those obtained for soils in non-burnt areas. Despite of this scenario, and independently of the year, no statistically significant differences were found in the C concentrations of the burnt plots and the control plots. For other chemical characteristics (e.g. nutrients), the variations depended on the area. Implementation of post-fire recovery measures and forest management of the areas can justify this variation.

Acknowledgment: This work is co-financed by project REMAS (SOE3/P4/E0954) from Interreg SUDOE 2014-2020 program and is also financed by the FEDER Funds through the Operational Competitiveness Factors Program - COMPETE and by National Funds through FCT - Foundation for Science and Technology within the scope of the project  UID/AGR/04129/2020 (LEAF) and  the project UID/BIA/50027/2019 (CEABN/InBIO).

How to cite: Almeida, P., Santos, E. S., Arán, D., Acácio, V., Duarte, I., Lerma, V., and Rego, F.: Temporal evolution of C stock in soils from the cork oak forest in a post-fire scenario, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16106, https://doi.org/10.5194/egusphere-egu21-16106, 2021.

Wildfires play the role of ecosystem shapers in the majority of terrestrial biomes, altering canopy and litter cover and imposing strong modifications on soils. Organic matter (OM) content and composition, mineralogy, pH, aggregate stability and water repellency (WR) are among the main edaphic properties to be affected by heat. Various studies dealt with occurrence, extent and persistence of burning-induced soil WR, but the dynamics at the basis of its formation (and loss) are still widely unclear. In addition, the vast majority of research on this topic has been carried out in the Mediterranean, even if alpine environments are far from being untouched by fires. Our aims were therefore to provide insight into the key mechanisms regulating WR thermal alterations in a relatively understudied environment.

Our sampling design aimed at collecting soils representative of the Western Italian Alps. Charring was simulated in the lab, at increasing temperatures (up to 300° C), on a set of A soil horizons developed under pine and beech forest covers. Water drop penetration time (WDPT) was employed to test WR persistence. Soils were analyzed in terms of organic carbon (OC) and nitrogen contents, pH, texture and iron (Fe) oxides composition (Fe-DCB and Fe-pyrophosphate extracted). Fe-speciation and OM composition of some selected samples were further characterized using Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and Fourier transform-infra red (FT-IR) spectroscopy, respectively.

WR was found to be extremely variable, event at room temperature. For samples exhibiting an increase in WR upon burning intensity, maximum repellency was observed at 200° C. OC abundance (%) and coarse texture were found to be the main drivers of hydrophobicity in soil. WR was drastically lost when samples were exposed to temperatures higher than 200°C. Above this threshold, pH systematically increased and OC (%) sharply decreased. The increasingly negative charge of mineral surfaces, mirrored by pH increase, seems to result in a significant C volatilization by OM desorption, eventually leading to a super-hydrophilic behavior in soil.

Fe EXAFS allowed to evaluate different thermal-dictated pathways of Fe-speciation. The formation of more crystalline Fe-forms (e.g. hematite, meghemite) was observed above 200° C. Even though a reduction in surface area should be expected when observing an increase in crystallization degree (potentially giving rise to greater WR), OM adhesion to mineral surfaces seems to be inhibited by the change induced in their charge. A reduction in the OM-bound Fe pool (pyrophosphate extracted) above 200° C could be appreciated, supporting the interpretation of oxy-hydroxides transformations and OC (%) loss.  

The current investigation has been carried out to capture an in-depth picture of wildfire impacts on alpine soils, targeting factors responsible for WR enhancement and shred. Identifying the mechanisms regulating wildfire-related WR is a key issue, as the formation of hydrophobic layers in soil highly favors its erosion. Addressing such matters is crucial to tackle the issue of ecosystems recovery, considering that climate-change-related alterations in wildfires regimes are already causing the occurrence of more frequent and disruptive fires.

How to cite: Negri, S., Giannetta, B., Said-Pullicino, D., Celi, L., and Bonifacio, E.: How are mineral and organic phases regulating burning-induced soil water repellency? Unravelling the crucial dynamics occurring in the Alps even at moderate fire intensities., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3533, https://doi.org/10.5194/egusphere-egu21-3533, 2021.

In the context of climate change mitigation, technologies for removing the CO₂ from the atmosphere are key challenges. Most recent scenarios from integrated assessment models require large-scale deployment of negative emissions technologies (NETs) to reach the 2 °C target. Among them, technologies for increasing organic carbon content in soils (SOC) have been developed. In the 15^th IPCC special report on Global Warming of 1.5 °C, biochar and pyrogenic carbon capture and storage have been credited as promising negative emission technology. In fact, soil carbon sequestration (SCS) and biochar have a large negative emission potential (each 0.7 GtCeq. yr^-1) and they are expected to have lower impact on land, water use, nutrients, albedo, energy requirement and cost, and thus fewer disadvantages than many other NETs.

SCS can be assessed using soil carbon dynamic models, such as RothC, as suggested by IPCC. However, studies on the inclusion of biochar in RothC are still scarce. Furthermore, most of these studies are based on the results of laboratory experiments and do not account for the effects of biochar on SOC degradation (the priming effect). The use of laboratory data can be problematic, since they may not adequately represent field conditions, especially due to the lack of long-term field studies.

The aim of this work was to assess and predict how biochar influences the soil C dynamics, by modifying the RothC model to simulate the findings of a long-term field experiment on biochar application to a short coppice rotation in Italy. We first modified the model to include two stocks of C input into the soil: the labile and the recalcitrant biochar pools. We also included a parametrized function to account for the priming effect on SOC dynamics in the soil. Secondly, we calibrated the model parameters with the data obtained from the field experiment. Finally, we validated the model results by estimating the remaining biochar amount in the site after 10 years from application, using an isotopic mass balance.

The results confirm that biochar degradation can be faster in field conditions in comparison to laboratory experiments; nevertheless, it can contribute to substantially increase the C stock in the long-term. Moreover, the modified RothC model allowed to assess the SCS potential of biochar application in soils, at least in the specific conditions examined, and could represent a flexible tool to assess the effect biochar as a SCS strategy in the long-term. We are exploring the possibility to use data from other long-term field experiment to move in that direction. The results of this study could be added to the Italian biochar database, providing new knowledge about a topic that needs to be explored.

How to cite: Pulcher, R., Balugani, E., Ventura, M., Greggio, N., and Marazza, D.: Modelling soil carbon sequestration with biochar using RothC, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4741, https://doi.org/10.5194/egusphere-egu21-4741, 2021.

The physical properties of biochar have been shown to dramatically influence its performance as a soil amendment. Biochar particle size is one of key parameters, as it controls its specific surface area, shape, and pore distribution. Therefore, this study assessed the role of biochar particle size and hydrophobicity in controlling soil water movement and retention. Softwood pellet biochar in five particle size ranges (>2 mm, 2 – 0.5 mm, 0.5 – 0.25 mm, 0.25 – 0.063mm and <0.063 mm) was used for the experiment. These particle sizes were tested on 2 soil types (sandy loam and loamy sand) at four different application rates (1, 2, 4 and 8%).  Our results showed that biochar hydrophobicity increased with decreasing biochar particle size, leading to a reduction in its water retention capacity. The effect of biochar on soil hydraulic properties varied with different rate of application and particle sizes. With increasing rate of application, water retention increased while hydraulic conductivity decreased. Water content at field capacity, permanent wilting point, and the available water content increased with increasing biochar particle size. The soil hydraulic conductivity increased with decreasing particle sizes apart from biochar particles <0.063mm which showed a significant (p≤0.05) decrease compared to the larger particle sizes. The results clearly showed that both biochar intra-porosity and inter-porosity are important factors affecting soil hydraulic properties. Biochar interpores affected mainly hydraulic conductivity, both interpores and intrapores controlled soil water retention properties. Our results suggest that for a more effective increase in soil water retention in sandy loam and loamy sand, the use of hydrophilic biochar with high intra-porosity is recommended.

How to cite: Edeh, I. and Mašek, O.: The role of biochar particle size and hydrophobicity in improving soil hydraulic properties, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4117, https://doi.org/10.5194/egusphere-egu21-4117, 2021.

Biochar application has been reported for improving the physical, chemical, and hydrological properties of soil. However, biochar can be produced from different feedstocks and at different conditions having a direct impact on its properties. Furthermore, the overall effect of improvement depends on the type of soil. That makes biochar amendment difficult to optimize and creates the need for extensive studies of this issue for its better understanding. In these studies, we show that water holding capacity (by means of Available Water Content - AWC) can be significantly improved in arable sandy soil using fine-sized biochar particles.

For our studies, we have used sunflower husk biochar (pyrolyzed at 650^oC). Biochar samples were characterized using an elemental analyzer for C, H, N content studies, mercury porosimeter for porosity and specific pore volumes, and vibratory shaker with a stack of sieves for particle size distribution. The examined biochar was sieved in order to obtain four diameter size fractions: <50 µm, 50–100 µm, 100–250 µm and <2000 µm and mixed with arable sandy soil for 0.95, 2.24, 4.76 and 9.52 wt.%. The unamended soil sample served as a reference. At first, we have measured the bulk density of the air-dried samples. After then the pressure plate method was used to determine the water retention curves. The results were fitted using the van Genuchten equation. Finally, the AWC for all the measured samples was calculated from a difference between soil water contents for pF=2.2 and pF=4.2. 

The bulk density studies have shown a nonlinear behavior as a function of dose for all fractions of the biochar. The clearest effect is observed for fractions below 100 µm for which the density vs dose characteristics of the samples revealed a maximum for 0.95 wt.% and a decreasing trend for higher biochar contents. The AWC studies shown that the particle size fractions of biochar below 100 µm in diameter cause also the most significant improvement in the water retention, almost doubling the reference level (0.078 m³ m^-3) to approximately 0.155 m³ m^-3 after biochar amendment. The results are explained by the filling of the free volume in the sandy soil matrix by small biochar particles. That leads to a shift of the pore size distribution to smaller radiuses, which in consequence promotes an increase in AWC. 

The research was conducted under the project  "Water in soil  -  satellite monitoring and improving the retention using biochar" No. BIOSTRATEG3/345940/7/NCBR/2017 which was financed by the Polish National Centre for Research and Development in the framework of “Environment, agriculture and forestry" -BIOSTRATEG strategic R&D programme.

How to cite: Gluba, Ł., Rafalska-Przysucha, A., Szewczak, K., Łukowski, M., Szlązak, R., Vitková, J., Kobyłecki, R., Bis, Z., Wichliński, M., Zarzycki, R., Kacprzak, A., and Usowicz, B.: Studies on the water retention in arable sandy soil amended with fine size-fractionated sunflower husk biochar, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15549, https://doi.org/10.5194/egusphere-egu21-15549, 2021.

The unique chemical and physical properties of biochars typically include low bulk density, high porosity, high surface area, reactive surface functional groups, and variable particle size distribution. These attributes make biochar a promising material for amendment to agricultural soils, as biochar may help improve soil water holding capacity, hydraulic conductivity (K_sat), and nutrient retention through chemical or physical means. Despite increased interest and investigation, there remains uncertainty regarding the ability of biochar to alter soils to deliver these agronomic benefits, due to differences in biochar feedstock, production method, production temperature and soil texture. In this project, a suite of experiments was carried out using biochars of diverse feedstocks and production temperature, in order to determine the biochar parameters which may optimize agricultural benefits. Sorption experiments [SJP1] were performed with seven distinct, commercially available biochars to determine sorption efficiencies for ammonium and nitrate. Only one biochar effectively retained nitrate, while all biochars bound ammonium. The three biochars with the highest nitrate and/or ammonium binding capacity (produced from almond shell at 500 and 800 °C (AS500 and AS800) and softwood at 500 °C (SW500)), were chosen for a series of column experiments. These biochars were amended to a sandy loam and a silt loam at 0 and 2% (w/w) and saturated hydraulic conductivity (K_sat) was measured. The biochars reduced K_sat in both soils by 64-80%, with the exception of AS800, which increased K_sat by 98% in the silt loam. Breakthrough curves for nitrate and ammonium, as well as concentrations of nutrients in the leachate, were also measured in the sandy loam columns.  Biochars significantly reduced the quantity of ammonium in the leachate, and significantly slowed its movement through the soil profile. Biochars had little to no effect on the timing and quantity of nitrate release. In this presentation, we present results from each experiment, and show images from our current work using x-ray micro-computed tomography on these soils and biochars to quantify porosity, pore size, and pore connectivity. Together, this work sheds new light on the chemical and physical means by which biochar alters soils to impact nutrient leaching and hydraulic conductivity.

How to cite: Gelardi, D. L., Rippner, D. R., Ainuddin, I., McElrone, A. J., Abou Najm, M., and Parikh, S. J.: The influence of biochar on nutrient leaching and hydraulic conductivity in two California soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-178, https://doi.org/10.5194/egusphere-egu21-178, 2021.

Although research on biochar use in soils has increased, its influence on soil compaction has been reported relatively little. The primary objective of this study was to measure the effects of biochar amendment on soil compaction, including infiltration capacity and aggregation, of two contrasting soils: a low soil organic carbon and hydrophilic degraded vineyard soil and a wildfire-degraded high soil organic carbon and hydrophobic forest soil.

We conducted a controlled laboratory soil column study (6 replicates), with PVC tubes filled with control soils and soil-biochar mixtures at a range of moisture contents. The mixtures were compacted under a falling load height that mimicked the standard proctor test. After the compaction procedure, infiltration capacity was determined with a mini disk infiltrometer, and bulk density and mean weight diameter were determined for the upper and lower halves of the soil column.

How to cite: Ul Haq, S., Verheijen, F. G. A., Möerz, T., and Gonzalves Pelayo, O.: Biochar Effects on Soil Compaction in Two Contrasting Soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9085, https://doi.org/10.5194/egusphere-egu21-9085, 2021.

The use of biochar has increased worldwide in the last years due to its good results for several soil quality indicators. However, restoration potential depends on the type and amount of biochar for each specific soil and land use. In order to investigate this restoration potential differential, we conducted an experiment where we amended two contrasting degraded soils with the same biochar. We installed a controlled and fully randomized percolation lysimeter experiment (3 replicates) with 15 lysimeters on a moderately steep slope angle, monitored for one year. Two types of soil were collected, a low organic matter, hydrophilic vineyard soil and a high organic matter, hydrophobic forest soil. Biochar was applied at 4% for both soils, and an additional treatment at 2% for the forest soil only. Selected soil quality indicators are: soil organic matter, medium weight diameter, aggregate stability, bulk density, pH, electric conductivity, potassium (K), phosphorus (P), soil water repellency, biomass quality. The present study comprises four data collections in different seasons along the year, enabling to compare the development of the biochar effects on different types of soil and its short- and medium-term behaviour.

How to cite: Isaka, S., Hendrychová, M., Campos, I., Bastos, A. C., González-Pelayo, Ó., Caetano, A., Abrantes, N., Martins, M., Jongen, M., Ferreira, C., and Verheijen, F.: Can biochar restore soil quality in a degraded forest and vineyard soil in a one-year percolation lysimeters study, in Portugal?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10400, https://doi.org/10.5194/egusphere-egu21-10400, 2021.

The amendment of biochar to soils is often considered for its potential as a climate change mitigation and adaptation tool through agriculture. Its presence in tropical agroecosystems has been reported to positively impact soil productivity whilst successfully storing C on the short and long-term. In temperate systems, recent research showed limited to no effect on productivity following recent biochar addition to soils. Its long-term effects on productivity and nutrient cycling have, however, been overlooked yet are essential before the use of biochar can be generalized.

Our study was set up in a conventionally cropped field, containing relict charcoal kiln sites used as a model for century old biochar (CoBC, ~220 years old). These sites were compared to soils amended with recently pyrolyzed biochar (YBC) and biochar free soils (REF) to study nutrient dynamics in the soil-water-plant system. Our research focused on soil chemical properties, crop nutrient uptake and soil solution nutrient concentrations. Crop plant samples were collected over three consecutive land occupations (chicory, winter wheat and a cover crop) and soil solutions gathered through the use of suctions cups inserted in different horizons of the studied Luvisol throughout the field.

Our results showed that YBC mainly influenced the soil solution composition whereas CoBC mainly impacted the total and plant available soil nutrient content. In soils with YBC, our results showed lower nitrate and potassium concentrations in subsoil horizons, suggesting a decreased leaching, and higher phosphate concentrations in topsoil horizons. With time and the oxidation of biochar particles, our results reported higher total soil N, available K and Ca in the topsoil horizon when compared to REF, whereas available P was significantly smaller. Although significant changes occurred in terms of plant available nutrient contents and soil solution nutrient concentrations, this did not transcend in variations in crop productivity between soils for neither of the studied crops. Overall, our study highlights that young or aged biochar behave as two distinct products in terms of nutrient cycling in soils. As such the sustainability of these soils differ and their management must therefore evolve with time.

How to cite: Burgeon, V., Fouché, J., Garré, S., Heidarian-Dehkordi, R., Colinet, G., and Cornelis, J.-T.: Nutrient cycling differs between cropped soils with century-old and recently pyrolyzed biochar, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15676, https://doi.org/10.5194/egusphere-egu21-15676, 2021.

Biochar is the solid carbonaceous product of biomass pyrolysis and is suggested as a viable tool to improve soil properties and to build up terrestrial carbon sinks. Since biochar is usually poor in nutrients, it needs to be enriched with nutrients before being applied to agricultural soils. Recently, the production of biochar from biomass with added wood ash has been proposed as a novel strategy. Among other elements, wood ashes are rich in potassium, an important macronutrient for plants. Compared to the direct application of pure wood ash, rapid nutrient leaching may be avoided by incorporating the ash into the pyrogenic carbon. In addition, alkali and alkaline earth metals in the wood ash promote the formation of the solid product during biomass pyrolysis. However, it is necessary to find out to what extent the potassium in the ash-modified biochar is available for plants when introduced into the soil. Based on a greenhouse trial, we investigated the potassium fertilisation effect of ash-modified biochars (2 t·ha^-1) compared to a pure mineral fertilisation and the application of wood ash to the soil. Therefore, softwood sawdust mixed with different concentrations of wood ash was pyrolyzed at 500 °C resulting in ash contents between 2 and 70 wt% in the biochars. Contents of trace elements and organic pollutants (PAH, PCDD/F and PCB) were mostly below the limits of the European Biochar Certificate. Based on CaCl₂ (0.01 M) extractions, between 6 and 10 % of the total potassium content in the ash-modified biochars was plant-available. For a greenhouse experiment, sunflower (Helianthus annuus, Santa Fe variety) was chosen because of its high potassium demand. A lack of potassium in the different treatments is expected to result in reduced plant growth and deficiency symptoms on the leaves. All treatments were fully fertilised, while mineral K was (partially) replaced by ash-modified biochar or wood ash according to the available potassium content of these additives. The evaluation of the fresh and dry biomass yields, as well as other plant vitality parameters, will show whether the potassium in the ash-modified biochars can replace mineral potassium fertiliser and/or whether ash-modified biochar can promote plant growth beyond a nutrient effect. Our results will determine if and how the use of wood ash in biochar production is a viable way to close nutrient cycles, reduces the use of mineral potassium fertiliser in agriculture and at the same time promotes pyrogenic carbon capture and storage.

How to cite: Grafmüller, J., Hagemann, N., Schmidt, H.-P., and Dahmen, N.: Potassium availability in ash-modified biochar and its impact on plant growth, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2177, https://doi.org/10.5194/egusphere-egu21-2177, 2021.

Food processing creates many by-products, and not all of them are used efficiently. Especially animal-based side products are frequently considered as waste with costly disposal requirements. For recycling of the nutrients contained in these residues, also under consideration of the hygienic specifications, pyrolysis can be used to create animal bone-based biochars. A lab-scale pyrolysis reactor (Pyreka 3.0) was used to produce biochars from different bone fractions of cattle and pigs after these bones had originated as waste from abbatoir operations. This study had the objective to investigate the potential of the bone chars to serve as a phosphorus (P) supply for agricultural purposes and to study the ammonium sorption potential of these chars.

The total phosphorus content of bones reached up to 140 mg/g. The water-soluble phosphorus content was in the range of 0.16 – 0.93 mg/g, an increase in pyrolysis temperature from 350 °C to 500 °C or 650 °C increased the water-soluble content by 13.3 or 12.2 % respectively. The citric acid soluble phosphorus content was between 1.75 – 2.19 mg/g. After pyrolysis temperatures of 350 °C, slightly more phosphorus dissolved in the coal products than at 500 °C (+2.7 %) and at 650 °C (+5.5 %).

The ammonium sorption capacity of biochars produced by varying pyrolytic processes was investigated by a series of sorption experiments. The removal of ammonium by the biochars from an aqueous ammonium solution was measured by using colorimetric determination of the ammonium content. The maximum ammonium sorption results were achieved by biochars produced from bovine heads and feet respectively at a temperature of 900°C and activated with H₂O.

When exposed to a solution containing 50 mg/L of ammonium, these biochars adsorbed 1.23 and 1.14 mg ammonium/g biochar, respectively. The possibility to enrich abattoir waste biochars, which are depleted in nitrogen because of the pyrolysis process, with ammonium gained from a nitrogen-enriched biogas slurry produced from animal residues of the meat production process was tested using a substitute slurry made with ammonium sulfate. The highest absorbance rate using the substitute slurry containing 10 g/L ammonium was achieved by biochar made from bovine heads and resulted in 43.1 mg ammonium/g biochar.

This study shows that bone-based biochars enriched with nitrogen from e.g. biogas digestates have significant potential as an NP-fertilizer that supports the strategies of circular economy.

How to cite: Soja, G., Tauber, D., Höllrigl, J., Mayer, A., and Pfeifer, C.: Nitrogen enrichment of animal bone-derived biochars as an agricultural NP-fertilizer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14764, https://doi.org/10.5194/egusphere-egu21-14764, 2021.