SSS12.3

The interaction between mineral particles and organic matter (OM) is an important and complex process in the course of soil structure formation. For a better understanding it is necessary to disentangle the texture-dependent interplay of individual OM types and mineral particles. We developed an experimental set-up to study early aggregate formation within a controlled lab environment. Artificial soil microcosms with a mineral mixture resembling arable soils of three different textures (clay loam, loam and sandy loam) were used in a short-term, 30-day incubation experiment under constant water-tension. OM was added individually either as plant litter (POM) of two different sizes (0.63-2 mm and < 63 µm, respectively) or bacterial necromass (Bacillus subtilis). The mechanisms of soil structure formation were investigated by isolating water-stable aggregates after the incubation, analyzing their mechanical stability and organic carbon allocation, and measuring the specific surface area and OM covers of the mineral surface, microbial activity, and community structure.

The dry mixing process and incubation of the mineral mixtures led to particle-particle interactions and fine particle coatings of the sand grains as shown by a reduction of the specific surface area. The OM input of all types caused between 3 to 17% of the mineral surfaces to be covered by OM, with larger covered areas in the clay-rich mixtures. The added OM was quickly accessed and degraded by microbes, as shown by the peak in CO₂-release within the first 10 days of the incubation. The POM of both sizes induced the predominant formation of water-stable macroaggregates (0.63-30 mm) with a mass contribution of 72 to 91% (irrespective of texture) and fostered the development of a microbial community with a high relative abundance of fungi. The bacterial necromass induced the formation of macroaggregates, but also microaggregates (63-200 µm), while the microbial community was dominated by bacteria. The mechanical stability analysis showed that very small forces < 4 N were sufficient for aggregate failure and breakdown to 80% of the original aggregate size.

We propose that the microbial degradation of all OM types leads to small, distinct OM clusters consisting of OM substrate, microbes, and extracellular polymeric substances. These interact with mineral particles, resulting in the cross-linking of particles and formation of water-stable aggregates in all textures. The OM can thereby act both as microbial substrate and as structural building block. The initially formed aggregates are a loosely connected scaffold with a very low mechanical stability. Differences in the developed microbial community may lead to additional stabilization mechanisms, like fungal hyphae enmeshing and stabilizing larger aggregates also in sandy texture.

How to cite: Bucka, F. B., Felde, V. J. M. N. L., Peth, S., and Kögel-Knabner, I.: Initial aggregate formation: Disentangling the effects of soil texture, OM properties and microbial community using artificial model soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2310, https://doi.org/10.5194/egusphere-egu21-2310, 2021.

Soil microaggregates are considered to play an important role in soil functioning and soil organic carbon (SOC) is of great importance for the formation and stabilization of these aggregates. The loss of SOC can occur, for example, after a change in land use and may lead to a decreased aggregate stability, which makes soils vulnerable to various threats, such as erosion or compaction. It is therefore important to shed light on the effect of SOC loss on aggregate stability in order to better understand and preserve the functioning of healthy soils.

We sampled two adjacent plots from a loess soil in Selhausen (Germany) and measured aggregate stability and architecture of soil microaggregates. One plot was kept free from vegetation by the application of herbicides and by tillage (to a depth of 5 cm) from 2005 on (organic matter depletion, OMD), while the other plot was used for agriculture using conventional tillage (control). Over the course of 14 years, the SOC concentration in the bulk soil has been reduced from 12.2 to 10.1 g SOC kg-1 soil. It was, however, unclear whether a loss of SOC had also taken place in microaggregates (since they are known to have very long turnover times). We took 10 undisturbed soil cores from two depths of each plot (Ap and Bt horizons).

The stability of aggregates against hydraulic and mechanical stresses was tested using wet sieving  (mesh sizes of 0.25 to 8 mm) and a crushing test in a load frame adapted to the microaggregate scale. For the latter test, microaggregates were isolated from the bulk soil using a newly developed dry crushing approach. To shed light on the effect of a decreased SOC content on microaggregate structure, we scanned several microaggregates with a computed tomography scanner at sub-micron resolution and analysed the features of their pore systems. SOC losses had also occurred in large  microaggregates (250-53 µm) in the Ap horizon: SOC contents in this fraction were 16.3 g SOC kg⁻¹ (control) and 12.8 g SOC kg⁻¹ (OMD). While wet sieving indicated a lower stability of macroaggregates from the Ap horizon in the OMD plot (geometric mean diameter: 1.54 mm (control) vs 0.43 mm (OMD)), an effect on the tensile strength of large microaggregates could not be found. Total porosity and pore connectivity, derived from Euler characteristic, as well as several pore skeleton traits (number of branches, junctions, etc.) were lower in aggregates from the OMD treatment. However, the difference was also present or even stronger in the Bt horizon than in the Ap horizon, so the supposed treatment effect might have been due to other effects like spatial heterogeneity of texture. Thus, the observed SOC losses may not have been large enough to substantially influence struture or stability of large microaggregates.

How to cite: Roosch, S., Felde, V., Uteau, D., and Peth, S.: Effect of soil organic carbon loss on the stability and structure of microaggregates: First insights from an organic carbon depletion field trial in a loess soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9266, https://doi.org/10.5194/egusphere-egu21-9266, 2021.

Soil organisms (plants, invertebrates, and microorganisms) are involved in soil structuring and are key factors of aggregation through bioturbation, organic matter (OM) decomposition, and secretion of biogenic OM (e.g., root exudates, mucus and extracellular polymeric substances). At the field scale, soil quality, functions, as well as nutrient cycling usually benefit from the activity of soil organisms that frequently cause substantial changes to soil properties by the formation of aggregates. The biogenic formation pathway of soil aggregates reflects a cascade of small-scale sub-processes (e.g., OM supply, OM adsorption, organo-mineral association formation, their transport, immobilization, and involvement into aggregate structure) that are often portrayed solitarily in literature and demand for a comprehensive framework that consistently describes their synergies and dependencies. Particularly, the role of complexly composed biogenic OM as bridging/aggregation agent is controversially discussed in literature, as they may promote as well as inhibit aggregation at the same time. This non-uniform behavior is controlled by the complex interplay of milieu parameters (e.g., ionic strength, temperature, pH and redox-potential) and the physicochemical properties of biogenic OM (e.g., protein-to-polysaccharide-ratio, molecular weight of biopolymers, functional groups, and biopolymer structure). Hence, we discuss biogenic OM with respect to the three different roles in aggregation which can be identified from literature: (I) as bridging agent which permits the aggregation due to attraction and surface modifications, (II) as separation agent which favors the formation, mobility and transport of organo-mineral associations and inhibits their further involvement into aggregates, and (III) as gluing agent which mediates aggregate stability, after an external force provokes a close approach of soil particles. In natural systems, OM may take these roles simultaneously and with varying degree across spatiotemporal scales. Considering this for the discussion of the role of biogenic OM in soil aggregate formation, we will achieve a more detailed and interdisciplinary understanding of its pathways into soil aggregates, which can help to draw comprehensive conclusions from lab and field-scale studies, prospectively.

How to cite: Guhra, T., Stolze, K., and Totsche, K. U.: The contribution of biogenic organic matter to aggregation in soil - a review, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14927, https://doi.org/10.5194/egusphere-egu21-14927, 2021.

Soil structure, lately referred to as the ''architecture'' is a key to explain and understand all soil functions. The development of sophisticated imaging techniques over the last decades has led to significant progress in the description of this architecture and in particular of the geometry of the hierarchically-branched pore space in which transport of water, gases, solutes and particles occurs and where myriads of organisms live. Moreover, there are sophisticated tools available today to also visualize the spatial structure of the solid phase including mineral grains and organic matter. Hence, we do have access to virtually all components of soil architecture.

Unfortunately, it has so far proven very challenging to study the dynamics of soil architecture over time, which is of critical importance for soil as habitat and the turnover of organic matter. Several largely conflicting theories have been proposed to account for this dynamics, especially the formation of aggregates. We review these theories, and we propose a conceptual approach to reconcile them based on a consistent interpretation of experimental observations and by integrating known physical and biogeochemical processes. A key conclusion is that rather than concentrating on aggregate formation in the sense of how particles and organic matter reorganize to form aggregates as distinct functional units we should focus on biophysical processes that produce a porous, heterogeneous organo-mineral soil matrix that breaks into fragments of different size and stability when exposed to mechanical stress.  The unified vision we propose for soil architecture and the mechanisms that determine its temporal evolution, should pave the way towards a better understanding of soil processes and functions.

How to cite: Vogel, H.-J., Balseiro-Romero, M., Baveye, P. C., Kravchenko, A., Otten, W., Pot, V., Schlüter, S., and Weller, U.: Obvious and less obvious processes of aggregate formation in soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10597, https://doi.org/10.5194/egusphere-egu21-10597, 2021.

It is well known that some fractions of soil organic matter (SOM) can resist to physical and (bio)chemical degradation which can be attributed to factors ranging from molecular properties to the preference for digesting other molecular species by microorganisms. Some mechanisms, by which organic matter is protected, are often referred to as: physical stabilization through microaggregation, chemical stabilization by formation of SOM-mineral aggregates, and biochemical stabilization through the formation of recalcitrant SOM.

Protection mechanisms are responsible for the accumulation process of organic carbon, reducing the exposure of organic matter and making it less vulnerable to microbial, enzymatic or chemical attacks. In these mechanisms, water molecular bridges and metal cation bridges play a key role. Cation bridges serve as aggregation sites on humic substances, forming dense matter, in comparison to systems where bridges are missing. This effect is enhanced in systems with cations at higher oxidation states.

By using the modeler tool developed in our group (Vienna Soil–Organic–Matter Modeler, VSOMM2) (Escalona et al., 2021), we generated aggregate models of humic substances at atomistic scale reflecting the diversity in composition, size and conformations of the constituting molecules. Further, we built models of organo-clay aggregates using kaolinite and montmorillonite as typical soil minerals. This allowed a systematic study to understand the effect of the surrounding environment at microscopic scale, not fully accessible experimentally.

Molecular simulations of the adsorption process of SOM aggregates on the reactive surfaces of led to two observations: 1) the humic substances aggregates were able to interact with the reactive surfaces mainly via hydrogen bonds forming stable organic matter-clay complexes and 2) the aggregates subsequently lost rigidity and stability after metal cations removing, consequently leading to a gradual loss of humic substance molecules, evidencing the role of metal cations in the protection mechanism of soil organic matter aggregates and possibly explaining its recalcitrance (Galicia-Andrés et al., 2021).

References

• Escalona, Y., Petrov, D., & Oostenbrink, C. (2021). Vienna soil organic matter modeler 2 (VSOMM2). Journal of Molecular Graphics and Modelling, 103, 107817. https://doi.org/10.1016/j.jmgm.2020.107817
• Galicia-Andrés, E., Grančič, P., Gerzabek, M. H., Oostenbrink, C., & Tunega, D. (2021). Modeling of interactions in natural and synthetic organoclays. In I. C. Sainz Diaz (Ed.), Computational modeling in clay mineralogy.

How to cite: Galicia-Andrés, E., Escalona, Y., Grančič, P., Oostenbrink, C., Tunega, D., and Gerzabek, M. H.: A molecular dynamic study of Soil Organic Matter stabilization mechanisms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2486, https://doi.org/10.5194/egusphere-egu21-2486, 2021.

Soil colloids < 220 nm including nanoparticles (1-100 nm), mainly composed of mineral particles and organic matter (OM) as well as their associations, have been gradually recognized as primary building units of the hierarchically organized soil aggregate system. As these colloidal building units are normally occluded inside soil aggregates, we refer to them as occluded colloids. Meanwhile, a large proportion of soil colloids is free from aggregate occlusion and mobile in the soil matrix. These free colloids can potentially serve as carriers for adsorbed nutrients and contaminants, mediating their translocation in the subsurface. However, the differences between free and occluded colloids remain unclear.

Here, both occluded and free colloids were isolated from soil samples of an arable field with different clay contents. The occluded colloids were released from soil macroaggregates (>250 µm) with ultrasonic treatments. The free and occluded colloids were sequentially characterized for their size-resolved elemental composition using flow field-flow fractionation inductively coupled plasma mass spectrometry and organic carbon detector (FFF-ICP-MS/OCD). Besides, selected samples were also subjected to transmission electron microscopy (TEM) and pyrolysis field ionization mass spectrometry (Py-FIMS).

Both free and occluded colloids mainly consisted of three size fractions: the first size fraction (0.6–60 nm), the second sized fraction (60–170 nm), and the third size fraction (>170 nm). The first size fraction was dominated by organic carbon and Ca, which were likely to be present as Ca-bridged OM associations. The elemental composition of the second and third size fractions was similar, which mainly consisted of organic carbon, Al, Si, and Fe, indicating the presence of mineral-mineral or mineral-organic associations. However, the ratios of organic to inorganic components in each size fractions varied among colloidal samples. TEM-EDX revealed that particles from free colloids were mainly present as mineral-mineral associations, while particles from occluded colloids tended to be mineral-organic associations. The C and N analysis showed higher N contents and narrower C/N ratios in free colloids when compared with occluded ones, suggesting different OM compositions in free and occluded colloids. The Py-FIMS results suggested that alkyl aromatics, phenols, lignin monomers, and lipids were the major OM compound classes in both free and occluded colloids. The relative abundance of carbohydrates, amides, heterocyclic nitrogen, and nitriles were higher in occluded colloids, whereas suberin and free fatty acids were relatively abundant in free colloids. Moreover, thermograms of OM compounds showed that occluded colloids possessed a higher proportion of thermal stable fractions of OM compounds, while the proportion of thermal liable fractions of OM compounds was relatively higher in free colloids. Overall, shedding light on the differences between free and occluded colloids may help us to gain insight into soil aggregate formation.

How to cite: Tang, N., Siebers, N., and Klumpp, E.: Free soil colloids and colloidal building units of soil aggregates, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8532, https://doi.org/10.5194/egusphere-egu21-8532, 2021.

Trophic regulation of microbial communities is receiving growing interest in soil ecology. Most studies investigated the effect of higher trophic levels on microbial communities at the bulk soil level. However, microbes are not equally accessible to consumers. They may be hidden in small pores and thus protected from consumers, suggesting that trophic regulation may depend on the localization of microbes within the soil matrix. As microaggregates (< 250 µm) usually are more stable than macroaggregates (> 250 µm) and embedded in the latter, we posit that they will be less affected by trophic regulations than larger aggregates. We quantified the effect of four contrasting species of collembolans (Ceratophysella denticulata, Protaphorura fimata, Folsomia candida, Sinella curviseta) on the microbial community composition in macro- (250 µm – 2mm) and microaggregates (50 – 250 µm). To do so, we re-built consumer-prey systems comprising remaining microbial background (post-autoclaving), fungal prey (Chaetomium globosum), and collembolan species (added as single species or combined). After three months, we quantified microbial community composition using phospholipid fatty acid markers (PLFAs). We found that the microbial communities in macroaggregates were more affected by the addition of collembolans than the communities in microaggregates. In particular, the fungal-to-bacterial (F:B) ratio significantly decreased in soil macroaggregates in the presence of collembolans. In the microaggregates, the F:B ratio remained lower and unaffected by collembolan inoculation. Presumably, fungal hyphae were more abundant in macroaggregates because they offered more habitat space for them, and the collembolans reduced fungal abundance because they consumed them. On the contrary, microaggregates presumably contained microbial communities protected from consumers. In addition, collembolans increased the formation of macroaggregates but did not influence their stability, despite their negative effect on fungal abundance, a well-known stabilizing agent. Overall, we show that trophic interactions between microbial communities and collembolans depend on the aggregate size class considered and, in return, soil macroaggregation is affected by these trophic interactions.

How to cite: Erktan, A., Haque, M. E., Cortet, J., Henning Krogh, P., and Scheu, S.: Soil microbial communities in microaggregates are less affected by top-down effects of collembolans than those in macroaggregates, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10345, https://doi.org/10.5194/egusphere-egu21-10345, 2021.

Selenium (Se) is an essential yet toxic trace element with one of the narrowest nutritional optimums of all elements. Se speciation plays a crucial role in its mobility, bioavailability, bioaccumulation, and toxicity. The current perception of Se environmental cycling encompasses a linear series of successive, bi-directional redox processes. Elemental Se is seen as a central species thermodynamically favored in redox conditions found in most environments. Most studies on Se environmental transformations focused on systems characterized by high Se concentrations. In nature though, sulfur (S) concentrations are in general orders of magnitude higher than those of Se. This work investigated elemental selenium reactivity in sulfur dominated environments. A set of laboratory experiments were conducted to determine the reaction rates of elemental selenium with sulfur in various environmental conditions. Our data clearly indicates that an abiotic reaction was occurring between elemental Se and S at neutral to alkaline conditions under anaerobic conditions, solubilizing elemental Se. At neutral pH (pH = 7), the reaction rates were low, whereas at high pH (pH = 12), the reaction was fast and all elemental Se was consumed by the reaction within 12 h. We present for the first time the detailed kinetics of reaction at various environmental conditions and discuss the control exerted by sulfur on selenium cycling.

How to cite: Martinez, M. and Lenz, M.: Fate of elemental Selenium in Sulfur dominated environments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7438, https://doi.org/10.5194/egusphere-egu21-7438, 2021.

Soil phosphorus (P) is one of the main factors affecting ecosystem productivity. With progressing soil weathering, P can be increasingly immobilized and become the limiting nutrient in ecosystems. Volcanic soils are known for their exceptionally high phosphate (PO₄) retention capacity. However, the changes in PO₄ sorption behavior as their mineralogy evolves during pedogenic development, are still not fully understood. Short-term and longer-term PO₄ sorption-desorption behavior was studied in six volcanic topsoils (0 – 10 cm) from four Galápagos Islands along an age gradient (chronosequence, 1.5 – 1070 ka) under humid climate. Labile P (Mehlich-3 P, resin P), PO₄ sorption kinetics (4 h – 62 days), PO₄ sorption capacity (sorption isotherms, equilibration time = 72 h) and longer-term desorption (resin P after 1 and 6 month incubation, respectively) were analyzed. Soils developed very high PO₄ sorption capacity within 4.3 ka of soil weathering (Langmuir Q_max = 18.2 g P kg^-1) due to the development of amorphous soil constituents. As the colloidal fraction changed to 2:1-type crystalline clays after 26 ka of soil weathering, PO₄ sorption capacity declined rapidly while the labile P fraction reached a maximum. In older soils (≥ 165 ka), acidification and prevalence of Al und Fe (hydr)oxides led to increased P sorption again. Overall, soil PO₄ retention capacity was closely related to amorphous Si, Al and Fe phases; however, it did not predict P availability.

How to cite: Rechberger, M. V., Gerzabek, M. H., and Zehetner, F.: Phosphate sorption-desorption behavior in volcanic topsoils along a chronosequence (1.5 ka – 1070 ka) in the humid zone, Galápagos Islands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8799, https://doi.org/10.5194/egusphere-egu21-8799, 2021.

Forest soils are a major contributor to soil organic carbon (C) storage in terrestrial ecosystems and play a key-role in climate change mitigation. Mineral weathering in soils is expected to promote chemical and physical interactions between soil organic matter and mineral phases. These interactions are known to enhance the protection of organic matter from decomposition. The investigation of the mineral-organic associations (MOA) formation mechanisms during weathering is therefore crucial to understand carbon storage processes in soils. Until now studies have been mainly conducted through laboratory experiments in simplified and controlled conditions or over very long-term time scales using pedosequences. But knowledge about MOA formation processes occurring in situ is lacking, notably during the first stage of mineral weathering.

To fill this gap, we performed a mesh bag incubation of large Na-saturated vermiculite particles (100-200 µm in size) in a Typic Dystrochrept soil of a Douglas-fir forest, in the Beaujolais area (France). The incubated particles were deposited at the interface under the forest floor. After 20 years, the weathered vermiculite particles were collected and characterized at the macro-scale (XRD and physico-chemical analysis), at the micro-scale (Scanning Electron Microscopy – SEM, imaging and element mapping) and at the nano-scale (Transmission Electron Microscopy - TEM imaging, element mapping and speciation).

Cation exchange capacity, exchangeable cations and elemental analysis showed significant differences between the mineral structures of the initial (V0) and 20 year incubated (V20) vermiculite particles. The exchangeable Na was completely depleted. Cation exchange capacity strongly decreased in V20 (49.2 cmol_c kg^-1) compared to V0 (173.6 cmol_c kg^-1). The V20 lost its specific interlayer collapsing property (≈1.4 -> ≈1.0 nm) with K saturation. V20 interlayer collapsing was only observed with a 330°C heating treatment, suggesting the interlayer hydroxylation of vermiculite. High sheet dissolution, around 10%, was also observed. All these changes were attributed to chemical weathering, during which total C analysis showed significant enrichment in V20 (5.7 mg g^-1) compared to V0 (0.8 mg g^-1).

Macro, micro and nano-scale images and elemental mapping of V0 particles showed a highly flat, smooth surface morphology with no detected C. In contrast, V20 particles showed irregular outer and inner surfaces marked by multiple cracks of chemical dissolution. We also observed internal nano-sized exfoliation spaces filled with C and enriched in Ca, and micro-sized exfoliation spaces filled with C entrapped in nano-crystalline Mn oxides or K-rich aluminosilicates precipitates. The nature of the organic matter found strongly differed between small and large exfoliation spaces. It was characterized by alcohol, carboxyl functional groups and C=C bonds in small exfoliation spaces, while the obtained EELS spectra were more difficult to interpret in large exfoliations spaces. These results provide new evidence that over 20 years in situ weathering induces a significant dissolution, among other physical and chemical changes in large vermiculite particles. They reveal that the mineral weathering processes are responsible for the organic matter entrapment (i) in the newly formed mineral nano-sized spaces, possibly mediated by Ca, and (ii) in association with secondary minerals deposits in micro-sized spaces.

How to cite: Jesus Van Der Kellen, I., Derrien, D., Ghanbaja, J., and Turpault, M.-P.: Mineral Weathering Promotes Carbon Storage in Forest Soils Macro to Nanoscale Characterizations of 20-year in situ Weathered Vermiculite Particles, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9867, https://doi.org/10.5194/egusphere-egu21-9867, 2021.

Acidified rivers may have increased CO₂ emissions because their low pH transforms inorganic carbon in the form of bicarbonate anions to CO₂, which can evade to the atmosphere, thus interrupting the delivery inorganic carbon to the oceans, a key flux in the long-term carbonate silicate cycle. Enhanced weathering (EW) is a carbon dioxide removal (CDR) strategy aiming to increase drawdown of atmospheric CO₂ through accelerated carbonation weathering of crushed minerals with targeted carbonate sequestration in oceanic stores. To date, EW research has been focused on terrestrial application of crushed minerals, and the CDR capability of enhancing weathering via addition of crushed minerals to rivers from lime dosers is essentially unexplored. Lime dosers have been used for decades to directly deposit crushed carbonate rock to rivers as a function of river flow in Norway and Nova Scotia, Canada, yet their potential as a CDR tool has yet to be verified in the field. In this study, we adapt CO₂ flux sensors (eosFD) designed for soils to be deployed in rivers. We conducted field trials on the Killag River, Nova Scotia, upstream and downstream of a lime doser over a period of six weeks in the autumn of 2020. Preliminary analysis shows elevated CO₂ evasion rates upstream of the lime doser and decreased evasion rates downstream. Aside from flood waves, CO₂ evasion at the downstream (treated) site is reduced to almost zero for extended periods of time. Next steps are to identify whether the reduced CO₂ evasion is due to CO₂ drawdown via increased carbonation weathering of the crushed dolomite or through reduced CO₂ evasion due to increased pH, or from a combination of the two processes. The results of this study may have implications for carbon credit programs for acidification mitigation and may encourage more widespread use of enhanced weathering as a CDR tool in rivers.

How to cite: Sterling, S., Nickerson, N., Halfyard, E., Hart, K., Mallyon, D., McCavour, C., and Rotteveel, L.: Direct Aquatic Application of Crushed Dolomite Reduces CO2 Evasion in an Acidified River, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13633, https://doi.org/10.5194/egusphere-egu21-13633, 2021.

Decades of acid deposition across northeastern North America has caused excess leaching of soil base cations (Ca²⁺, Mg²⁺, K⁺) and increases in bioavailable aluminum (Al³⁺) that, in combination, have resulted in widespread decreases in potential forest productivity. Despite major reductions in SO₂ and NO_x emissions since the 1990s, forest soils across the region have shown few signs of recovery from acid deposition impacts and it could take decades or centuries for natural recovery to occur. As a result, affected forests are stressed, less productive, and more prone to climate change-induced damage. Helicopter liming of upland forests may be an effective way to jump-start the soil recovery process. Here we report on early results (one-year) from a helicopter liming trial in Nova Scotia, Canada where 10 tonnes/ha of dolomitic limestone was applied to approximately 8 ha of mature red spruce (Picea rubens) and mature tolerant hardwood (Acer spp. and Betula spp.) forest. Data are presented on (i) the effectiveness of helicopter liming in forests; (ii) the initial chemical response of forest floor organic and mineral soil horizons; and (iii) the initial chemical response of red spruce foliage, maple foliage, and ground vegetation. Preliminary results showed that despite non-uniform lime distribution, there were significant increases (P < 0.05) in Ca²⁺, Mg²⁺, pH, and base saturation (BS), and significant decreases in total acidity in forest floor organic horizons in both the mature red spruce and tolerant hardwood stands; however, there were no significant changes in Al³⁺. The initial chemical response in sugar maple and red spruce foliage showed significant increases in the Ca/Al molar ratio .  The initial response in ground vegetation (Schreber’s moss; Pleurozium schreberi and wood fern; Dryopteris intermedia) showed significant increases in Ca²⁺ and decreases in K⁺ for both species; however, Schreber’s moss also showed significant increases in Mg²⁺ and Al³⁺ while wood fern did not. These early chemical results are promising and further support the use of helicopter liming as an effective tool to combat lingering effects from acid deposition in acidified forests.

How to cite: McCavour, C., Sterling, S., Keys, K., and Halfyard, E.: Helicopter Liming to Help Restore Acidified Forest Soil Productivity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13660, https://doi.org/10.5194/egusphere-egu21-13660, 2021.