High organic carbon content constricts the potential for stable organic carbon accrual in mineral agricultural soils in Finland.

Sequestering carbon into agricultural soils is considered as a means of mitigating climate change. We used agronomic soil test results representing c. 95% of the farmed land area in Finland to estimate the potential of the uppermost 15 cm soil layer of mineral agricultural soils to sequester organic carbon (OC) and to contribute to the mitigation of climate change. The estimation of the maximum capacity of mineral matter to protect OC in stable mineral-associated form was based on the theory that clay and fine-sized (fines = clay + silt) particles have a limited capacity to protect OC. In addition, we used the clay/OC and fines/OC ratios to identify areas with a risk of erosion and reduced productivity, thus indicating priority areas potentially benefitting from the increased soil OC contents. We found that 32-40% of the mineral agricultural soils in Finland have the potential to further accumulate mineral-associated OC (MOC), while in the majority of soils, the current OC stock in the uppermost 15 cm exceeded the capacity of mineral matter to protect OC. The nationwide soil OC sequestration potential of the uppermost 15 cm in mineral agricultural soils ranged between 0.21 and 0.26 Tg, which corresponds to less than 2% of annual greenhouse gas emissions in Finland. The fields with the highest potential for SOC accrual were found in the southern and southwestern parts of the country, including some of the most intensively cultivated high-clay soils. Although the nationwide potential for additional OC sequestration was estimated to be relatively small, the current OC storage in Finnish arable mineral soils (0-15 cm) is large, 128 Tg. Farming practices enabling maximum OC input into the soil play an important role as a tool for mitigating the loss of carbon from high-OC soils in the changing climate. Furthermore, especially in high-clay areas with potential for MOC accrual, efforts to increase soil OC could help improve soil structural stability and therefore reduce erosion and the loss of nutrients to the aquatic environments.

A Simple Dynamic Model of Soil Test Phosphorus Responses to Phosphorus Balances.

Soil test P (STP) concentration indicates whether annual P applications can be expected to give yield increases and can also indicate an elevated risk of P mobilization and potential for P transfer to surface waters and groundwater from a particular field. Changes in STP with time thus project agronomic benefits and environmental risks of different P use strategies. To predict STP changes with time, we constructed a simple dynamic model for which the input variables are P balance and initial STP. The model parameters (soil type-specific constants) were fitted using data originating from 44 P fertilizer experiments with different P rates. Model performance was evaluated using independent data sets that either had reasonably accurate input values ( = 103) or were obtained from farmers through interviews ( = 638). The simulations were in agreement with measured STP changes for both evaluation data sets when fittings were performed separately for four main soil types (clays, silts, coarse mineral soils, and organic soils). Statistical analysis confirmed that the model captured the trends in STP (NHOAc test) with acceptable accuracy and precision, with of 0.83 and 0.66 for the data with more accurate input and for farmer interview data, respectively; the corresponding model efficiency statistics were 0.88 and 0.66. The model is not restricted to use with one soil test, as fittings for several different types of soil tests can be generated. In this study, we fitted the model for Olsen P data retrieved from the literature. Agronomic use of the model includes evaluation of P use strategies, e.g., when a certain STP level is targeted or when long-term economy of P use is calculated. In an environmental context, the model can be used to predict STP changes with time under variable P balance regimes, which is essential for realistic assessment of changes in the potential for dissolved P losses.

Methodological choices in size and density fractionation of soil carbon reserves - A case study on wood fiber sludge amended soils.

Soil organic carbon (SOC) is in the focus of research due to its central role in regulating climate and maintaining fertility and resilience of soils. Methodologically, shifting from whole soil C measurements to specific SOC fractions increases possibility to detect small changes in the vast SOC storage, and enhances estimation of SOC stability. However, SOC fractionation schemes are numerous and variable. In this study, deionized water and sodium hexametaphosphate (SHMP) were compared in soil dispersion by separating soils into coarse (0.25-2 mm), medium (0.063-0.25 mm) and fine (<0.063 mm) size fractions. The first two fractions were further separated by density (1.8 g cm-3) to obtain free particulate organic C (POC) and mineral associated organic C (MOC). The approach was applied to a clay and a silt loam soil with and without wood fiber sludge amendment to follow the added C. Aggregate disruption was enhanced with SHMP in comparison to water, but the effect was small and the use of SHMP decreased recovery of SOC, wherefore water was preferred. In both soils, 5-10 % of SOC occurred as coarse POC, 1-3% as coarse MOC, 5 % as medium POC, 10 % as medium MOC, and 70-85 % as fine MOC. The added C resided in the POC fractions with an indication of minor accumulation to the fine MOC in the clay soil. Longer time frame with repeated C additions would be needed to increase the stable MOC storages though saturation of the MOC reserve may hinder accumulation in the silt loam low in fines.

Effect of manure co-digestion on methane production, carbon retention, and fertilizer value of digestate.

Anaerobic digestion can provide benefits not only from the perspective of renewable energy production but also in the form of fertilization effect and increased retention of C in soils after digestate application. This study consisted of two phases, where the first phase assessed the suitability of carbon-rich co-feedstocks for methane production via laboratory testing. The second phase assessed the balance and stability of C before and after anaerobic digestion by systematic digestate characterization, and by evaluating its carbon retention potential using a modeling approach. The results indicated that pyrolysis chars had a negligible effect on the methane production potential of cattle manure, while wheat straw expectedly increased methane production. Thus, a mixture of cattle manure and wheat straw was digested in pilot-scale leach-bed reactors and compared with undigested manure and straw. Although the total amount of C in the digestate was lower than in the untreated feedstocks, the digestion process stabilized C and was modeled to be more effective in retaining C in the soil than untreated cattle manure and wheat straw. In addition, digestion converted 23-27 % of the C into valuable methane, increasing the valorization of the total C in the feedstock. Considering anaerobic digestion processes as a strategy to optimize both carbon and nutrient valorization provides a more holistic approach to addressing climate change and improving soil health.

Industrial pilot scale slow pyrolysis reduces the content of organic contaminants in sewage sludge.

Pyrolysis has been gaining global interest as a viable option for reducing organic contaminant levels in waste materials such as sewage sludge (SS) for their subsequent use as a soil amendment. However, publicly available knowledge on the capacity of pyrolysis to reduce the levels in SSs is mostly based on laboratory or bench scale studies. The aim of this study was to examine the effects of industrial pilot scale slow pyrolysis at two temperatures and retention times (450 °C, 1 h and 500 °C, 1.5 h) on a wide range of organic and inorganic contaminants in SSs. Pyrolysis at 500 °C decreased the concentrations of the detected per- and polyfluoroalkyl substances (PFASs, by 30-93 %), brominated diphenyl ethers (BDEs; by 97-98 %) and most endocrine disrupting compounds (EDCs, by 82-96 %) more efficiently than pyrolysis at 450 °C. Estrone and pharmaceuticals, with the exception of paracetamol, were removed to below quantification limits. Non-volatile inorganic contaminants concentrated to the chars (22-46 % increase). These results confirm that slow pyrolysis has the capacity to significantly reduce organic contaminant levels in SSs at an industrial scale, while content of inorganic contaminants depends mainly on the feedstock properties. Pyrolysis temperature of over 500 °C is advised to secure efficient removal of organic contaminants. However, it is anticipated that reactor design with good heat transfer and volatile removal could further improve the removal of organic contaminants from SSs. The results are especially valuable for sludge management operators planning to procure a pyrolysis plant.

Slow pyrolysis liquid in reducing NH3 emissions from cattle slurry - Impacts on plant growth and soil organisms.

A substantial percentage of manure nitrogen (N) can be lost as gaseous ammonia (NH3) during storage and field spreading. Lowering slurry pH is a simple and accepted method for preserving its N. Efficiency of slow pyrolysis liquid (PL) produced from birch (Betula sp.) as an acidifying agent, and its ability to reduce NH3 emissions following surface application of cattle slurry, was studied in a field experiment. Untreated slurry (US) and slurries acidified with PL and sulfuric acid (SA) were applied to the second harvest of a grass ley. Immediate NH3 emissions, grass biomass, N-yield and possible toxic impacts on soil nematodes and enchytraeids were examined. Furthermore, the effects on soil respiration, nitrogen dynamics and seed germination were studied in subsequent laboratory experiments. In the field, over one third of the water-extractable ammonium-N (NH4-N) applied was lost through NH3 volatilization from US. SA and PL acidified slurries reduced NH3-N emission rate equally from 3.4 to <0.04 kg ha-1 h-1. Acidification with SA resulted in the highest and that with PL in the lowest grass dry matter (DM) and N yield. Neither SA nor PL acidification had negative effects on soil enchytraeids or nematodes. Reduced yield production, seed germination and delayed microbial activity after PL slurry application were most probably caused by the PL containing organic compounds. However, later increase in carbon dioxide (CO2) production and improved seed germination suggest that these compounds were rapidly volatilized and/or degraded by soil microbes. Though PL efficiently cut NH3 emission from surface-spread slurry, further studies on appropriate application methods and possible phytotoxicity are needed.

Pyroligneous Acids of Differently Pretreated Hybrid Aspen Biomass: Herbicide and Fungicide Performance.

The pyroligneous acids (PAs) of woody biomass produced by torrefaction have pesticidal properties. Thus, PAs are potential alternatives to synthetic plant protection chemicals. Although woody biomass is a renewable feedstock, its use must be efficient. The efficiency of biomass utilization can be improved by applying a cascading use principle. This study is novel because we evaluate for the first time the pesticidal potential of PAs derived from the bark of hybrid aspen (Populus tremula L. × Populus tremuloides Michx.) and examine simultaneously how the production of the PAs can be interlinked with the cascade processing of hybrid aspen biomass. Hybrid aspen bark contains valuable extractives that can be separated before the hemicellulose is thermochemically converted into plant protection chemicals. We developed a cascade processing scheme, where these extractives were first extracted from the bark with hot water (HWE) or with hot water and alkaline alcohol (HWE+AAE) prior to their conversion into PAs by torrefaction. The herbicidal performance of PAs was tested using Brassica rapa as the test species, and the fungicidal performance was proven using Fusarium culmorum. The pesticidal activities were compared to those of the PAs of debarked wood and of commercial pesticides. According to the results, extractives can be separated from the bark without overtly diminishing the weed and fungal growth inhibitor performance of the produced PAs. The HWE of the bark before its conversion into PAs appeared to have an enhancing effect on the herbicidal activity. In contrast, HWE+AAE lowered the growth inhibition performance of PAs against both the weeds and fungi. This study shows that hybrid aspen is a viable feedstock for the production of herbicidal and fungicidal active chemicals, and it is possible to utilize biomass according to the cascading use principle.

Pulp and paper mill sludges decrease soil erodibility.

Declining carbon (C) content in agricultural soils threatens soil fertility and makes soil prone to erosion, which could be rectified with organic soil amendments. In a 4-yr field trial, we made a single application of three different organic sludges from the pulp and paper industry and studied their effects on cereal yield, soil C content, and fungal and bacterial composition. In laboratory rainfall simulations, we also studied the effects of the soil amendments on susceptibility to erosion and nutrient mobilization of a clay-textured soil by measuring the quality of percolation water passing through 40-cm intact soil monoliths during 2-d rainfall simulations over four consecutive years after application. A nutrient-poor fiber sludge reduced wheat yield in the first growing season, but there were no other significant effects on cereal yield or grain quality. An input of ∼8 Mg ha-1 C with the soil amendments had only minor effects on soil C content after 4 yr, likely because of fast microbe-mediated turnover. The amendments clearly changed the fungal and bacterial community composition. All amendments significantly reduced suspended solids (SS) and total phosphorus (TP) concentrations in percolation water. The effect declined with time, but the reduction in SS and TP was still >25% 4 yr after application. We attributed the lower tendency for particle detachment in rain simulations to direct interactions of soil minerals with the added particulate organic matter and microbe-derived compounds that stabilize soil aggregates. In soils with low organic matter content, pulp and paper industry by-products can be a viable measure for erosion mitigation.

Are there environmental or agricultural benefits in using forest residue biochar in boreal agricultural clay soil?

Short-term agronomic and environmental benefits are fundamental factors in encouraging farmers to use biochar on a broad scale. The short-term impacts of forest residue biochar (BC) on the productivity and carbon (C) storage of arable boreal clay soil were studied in a field experiment. In addition, rain simulations and aggregate stability tests were carried out to investigate the potential of BC to reduce nutrient export to surface waters. A BC addition of 30 t ha-1 increased soil test phosphorus and decreased bulk density in the surface soil but did not significantly change pH or water retention properties, and most importantly, did not increase the yield. There were no changes in the bacterial or fungal communities, or biomasses. Soil basal respiration was higher in BC-amended plots in the spring, but no differences in respiration rates were detected in the fall two years after the application. Rain simulation experiments did not support the use of BC in reducing erosion or the export of nutrients from the field. Of the C added, on average 80% was discovered in the 0-45 cm soil layer one year after the application. Amendment of boreal clay soil with a high rate of BC characterized by a moderately alkaline pH, low surface functionalities, and a recalcitrant nature, did not induce such positive impacts that would unambiguously motivate farmers to invest in BC. BC use seems unviable from the farmer's perspective but could play a role in climate change mitigation, as it will likely serve as long-term C storage.

Cascade processing of softwood bark with hot water extraction, pyrolysis and anaerobic digestion.

A process model based on hot water extraction (HWE), slow pyrolysis and anaerobic digestion (AD) were used for pine and spruce bark utilisation. First tannins (32 mg/g and 11.8 mg/g, respectively) and polyphenols were recovered via HWE. Then, the residue was pyrolysed to produce biochar (marketable quality), gas (energy source) and liquid fractions. The liquid fraction was further separated into aqueous acidic fraction and to tar fraction. Bark, extracted bark residue and acidic liquid fraction from pyrolysis were treated in AD to produce biomethane and digestate. The methane yields from pine and spruce bark and extracted bark residue were low (from 42 to 96 mLCH4/gVSadded) and showed only small differences. In conclusion, cascade processing can improve the performance of subsequent single processes and utilise biomass sources with higher efficiency. The best processing chain may vary in different cases and the overall energy balance of processing needs further research.

Quantitative characterization of pore structure of several biochars with 3D imaging.

Pore space characteristics of biochars may vary depending on the used raw material and processing technology. Pore structure has significant effects on the water retention properties of biochar amended soils. In this work, several biochars were characterized with three-dimensional imaging and image analysis. X-ray computed microtomography was used to image biochars at resolution of 1.14 µm and the obtained images were analysed for porosity, pore size distribution, specific surface area and structural anisotropy. In addition, random walk simulations were used to relate structural anisotropy to diffusive transport. Image analysis showed that considerable part of the biochar volume consist of pores in size range relevant to hydrological processes and storage of plant available water. Porosity and pore size distribution were found to depend on the biochar type and the structural anisotopy analysis showed that used raw material considerably affects the pore characteristics at micrometre scale. Therefore, attention should be paid to raw material selection and quality in applications requiring optimized pore structure.

Effects of de-icing chemicals sodium chloride and potassium formate on cadmium solubility in a coarse mineral soil.

Excessive use of sodium chloride (NaCl) as de-icing chemical causes environmental problems, such as elevated chloride concentrations in groundwater. On vulnerable sites, this can be avoided by using alternative organic de-icing chemicals, such as potassium formate (KHCOO). The environmental impacts of KCHOO are, however, not well known. This study reports the potential effects of NaCl and KCHOO on mechanisms controlling the mobility of cadmium (Cd) in roadside soils as a result of vehicular traffic. Changes in the solubility of Cd in a coarse mineral soil treated with these two de-icing chemicals were studied in a 50-day incubation experiment under four different moisture and temperature combinations and an initial soil Cd concentration of 3 mg kg(-1). After incubation, the distribution of soil Cd into different fractions was analyzed using a sequential extraction method. Soil pH and soil redox potential were recorded and the occurrence of Cd-Cl complexes in the soil was estimated using published stability constants. During incubation, KCHOO lowered the soil redox potential, but this was not accompanied by a decrease in the sorption capacity of oxides and the release of oxide-bound Cd into soil solution. On the other hand, elevated pH (from 4.3 to 6.7-8.5) in the formate treatments increased the sorption of Cd onto the oxide surfaces (up to 80% of total sorbed Cd). In the NaCl treatments, cation competition and formation of Cd-Cl complexes increased the water-soluble Cd fraction. Consequently, the amount of bioavailable Cd was 3.5 times smaller in the KCHOO than in the NaCl treatments.

Conversion of dissolved phosphorus in runoff by ferric sulfate to a form less available to algae: Field performance and cost assessment.

Conversion of dissolved P by ferric sulfate into a particulate form sparingly available to algae was studied in 15 ditches in Finland using stand-alone dispensers for ferric sulfate administration. Ferric sulfate typically converted 60-70 % of dissolved P into iron-associated form, a process which required 250-650 kg per kg dissolved P. Mean cost was 160 EUR per kg P converted (range 20-400 EUR kg(-1)). The costs were lowest at sites characterized by high dissolved P concentrations and small catchment area. At best, the treatment was efficient and cost-effective, but to limit the costs and the risks, ferric sulfate dispensers should only be installed in small critical source areas.