Mouldboard ploughing has been associated with a loss in soil quality, while reduced tillage is often reported to result in crop yield declines. Occasional ploughing every few years could be an attractive compromise, but it remains unclear whether such a system results in a combined benefit of improved crop yield and soil quality. Here, we present data from a 43-year old field experiment on a clay soil in central Sweden. We analysed how three different tillage systems – annual mouldboard ploughing, continuous shallow tillage, and shallow tillage with occasional mouldboard ploughing – affect crop yields and soil properties with the objective of assessing joint response patterns between crop production and soil quality indicators reflecting soil structure, soil organic carbon storage, and habitat for soil biota. Crop yields were not affected by the tillage systems. Characteristics of the soil pore system and soil biota were similar in the occasional ploughing and annually mouldboard ploughed systems. Shallow tillage significantly altered microbial community composition relative to ploughed treatments, increasing bacterial and fungal abundances and reducing diversity in the upper topsoil layer, likely due to stratified soil carbon stocks. There was also a higher proportion of biopores in shallow tillage, which could be advantageous during dry spells that are expected to become more frequent and severe with climate change. Nevertheless, there were only small effects on water retention, biomass of nematodes and earthworms, and most of the abiotic soil properties. We conclude that occasional ploughing did not offer advantages at our site, and our results show that no system improved yield and all soil quality indicators simultaneously.

Earthworms contribute to soil functioning by burrowing through soil, but quantitative knowledge of their burrowing activities and how environmental factors influence it under field conditions is still limited. The objectives of this study were to test a method for quantification of the temporal dynamics of in-situ earthworm burrowing rates, and to investigate links between variations in burrowing rates and soil moisture and temperature. Specially designed perforated cylinders repacked with soil were installed in an arable field with a known earthworm community in Uppsala, Sweden, and repeatedly removed, X-ray scanned, and re-installed, during May-October 2023. Earthworm burrow volumes and characteristics of burrow networks were derived from the X-ray images. Burrows were classified into burrows likely originating from endogeic earthworms (burrow diameter <= 4 mm) and anecic earthworms (> 4 mm). We tested two methodological approaches, using either new repacked soil after each scan or re-installing the same soil cylinder. We also employed two soil moisture treatments (with and without rainout shelters). In-situ soil moisture and temperature were monitored using sensors. The results showed that burrowing rates in newly repacked soils were higher than in the soil columns with the same soil. Depending on treatment and the time of the year, mean burrowing rates varied between 0.05 and 0.64 cm(3) day(-1). On average, we estimated that topsoil turnover rate by earthworm burrowing is about 21-42 years in this field where the earthworm abundance was 81 individuals per m(2). The data from the "same soil" treatment indicates that a steady state in earthworm burrow volume was achieved after five months, indicated by the balance between burrow creation and disappearance. In our study, soil moisture appeared to be a more important driver for earthworm burrowing than soil temperature. We show that about 40% of the earthworm burrow volume was classified as anecic earthworm burrows in the topsoil. We conclude that the proposed method using perforated cylinders with repeated scans is suitable for seasonal monitoring and to quantify earthworm burrowing rates under field conditions.

Previous research has shown the importance of oxalate-extractable aluminium (Alox) for predicting soil organic carbon (SOC) contents across diverse geographical regions. However, studies using data from humid continental climates are scarce, and the data used in these studies have not been statistically representative for larger scales. Our study aimed to 1) evaluate the influence of soil physical and geochemical properties (specifically Alox), farm management, and climate on the spatial distribution and storage potential of SOC in Swedish agricultural soils and 2) to assess whether estimates of aggregation, assumed to influence the protection of soil organic matter, could improve predictions. We analyzed a statistically representative subset of mineral soils with pH < 7 from the Swedish soil and crop monitoring program, which covers the country's agricultural land. We identified the most important predictors for topsoil SOC contents using a random forest model. We employed partial dependence plots to visualize and interpret the interactions between key variables and SOC contents. Results showed that Alox was the most important predictor for SOC contents, as evidenced by its high relative importance score and the increased out-of-bag error when removed from the model. Notably, SOC content reached a plateau at Alox contents of about 3.5 g kg(-1), suggesting the possibility of SOC under-saturation. Climatic variables were of secondary importance, while farm management did not emerge as a significant predictor. Surprisingly, silt-sized aggregation was not identified as an important variable for predicting SOC content. Our findings emphasize the importance of incorporating geochemical properties, particularly Alox, in addition to soil texture, in predictive modelling and monitoring efforts for enhanced soil carbon management in humid climates.

Roots are a major pathway for carbon (C) input into agricultural soils, yet field-scale measurements of belowground C inputs and associated root traits remain limited. Consequently, many soil carbon models rely on fixed root-to-shoot ratios, and root trait variability is rarely considered. In this study, we quantified within-field variation in root-to-shoot ratios and root traits (root diameter, root length density and root tissue density) in spring barley (Hordeum vulgare L.) grown in southwestern Sweden in soil classified as Stagnic Eutric Cambisol, Eutric Stagnosol or Haplic Phaeozem according to the World Reference Base system. Roots (0-40 cm) and shoots were sampled during early to mid-reproductive stage, i.e. milking/early dough development stage, in a 50 x 50 cm grid at 11 sampling locations in the same field in two consecutive years. Shoot and root biomass were not correlated, resulting in variable root-to-shoot ratios (quartile coefficients of variation 7-18 %) and no consistent spatial pattern between years. Root traits displayed clear between year and depth variation, with coarser roots in the topsoil and root tissue densities and root length densities shifting across the profile, reflecting the highly plastic nature of root systems. The spatial variation in root properties in the field could not be explained by basic soil properties. Our findings call for a more mechanistic understanding of the drivers for rootto-shoot ratios and the plastic response of root traits to improve field-scale estimates of root-derived C inputs and SOC modelling accuracy.

Examining soil carbon losses across various time scales is essential for understanding the potential of soil carbon stabilization. It allows for considering these losses in estimating the net changes in carbon stocks. The soil's intrinsic physical and chemical properties, particularly those associated with the mineral phase, have been suggested to regulate soil organic carbon (SOC) losses. However, the relationship between these properties and SOC losses remains to be determined in long-term experiments. A 600-day incubation experiment was conducted on sieved soil samples (

The rapid flow of water through soil macropores significantly affects the partitioning of precipitation between surface runoff and infiltration and also the rate of solute transport in soil, both of which have an impact on the risk of contamination of surface water and groundwater. The kinematic wave equation is often employed as a model of gravity-driven water flow through soil macropores. The exponent in this simple model influences the pore water velocity attained in the macropores at any given input rate and is usually estimated by inverse modelling against measured flow rates or water contents. In theory, the exponent in the kinematic wave equation should depend on the geometry and topology of the conducting macropore networks, although these relationships have not so far been investigated. In this study, we related metrics of soil structure derived from X-ray images to values of the kinematic exponent estimated from drainage experiments on twenty-two columns sampled at three different field sites under two contrasting land uses and at three different depths. We found that smaller values of the exponent in the kinematic wave equation, which would equate to more rapid flow of water through soil macropores, were found in plough pan and subsoil columns of smaller macroporosity, for which biopores comprised a significant fraction. The macroporosity in these columns was more vertically oriented and poorly inter-connected, though still continuous across the sample. In contrast, topsoil columns from both arable land and grassland had better connected, denser and more isotropically-distributed macropore networks and larger values of the kinematic exponent. Our results suggest that for predictive modelling at large scales, it may be feasible to estimate the kinematic exponent using class pedotransfer functions based on pedological information such as land use and horizon type.

The structure of soil is critical for the ecosystem services it provides since it regulates many key soil processes, including water, air and solute movement, root growth and the activity of soil biota. Soil structure is dynamic, driven by external factors such as land management and climate and mediated by a wide range of biological agents and physical processes operating at strongly contrasting time-scales, from seconds (e.g., tillage) to many decades (e.g., faunal activity and soil aggregation). In this respect, positive feedbacks in the soil-plant system may lead in the longer term to soil physical degradation or to the recovery of structurally poor soils. As far as we are aware, no existing soil-crop model can account for such processes. In this paper, we describe a new soil-crop model (USSF, Uppsala model of Soil Structure and Function) that accounts for the effects of soil structure dynamics on water and organic matter cycling at the soil profile scale. Soil structure dynamics are expressed as time-varying physical (bulk density, porosity) and hydraulic properties (water retention, hydraulic conductivity) responding to the activity of biological agents (i.e., earthworms, plant roots) and physical processes (i.e., tillage, soil swell-shrink) at seasonal to decadal time-scales. In this first application of the model, we present the results of 30-year scenario simulations that illustrate the potential role and importance of soil structure dynamics for the soil water balance, carbon storage in soil, root growth, and winter wheat yields on two soils (loam and clay) in the climate of central Sweden. A sensitivity analysis was also performed for these two scenarios using the Morris method of elementary effects, which revealed that the most sensitive parameters controlling soil structure dynamics in the USSF model are those determining aggregation induced by organic matter turnover and swell/shrink. We suggest that the USSF model is a promising new tool to investigate a wide range of processes and phenomena triggered by land use and climate change. Results from this study show that feedback in the soil-crop system mediated by the dynamics of soil physical and hydraulic properties are potentially of central importance for long-term predictions of soil water balance, crop production, and carbon sequestration under global change.

Soil mixing by earthworms can have a large impact on the fate of nutrients and pollutants and on the soil's ability to sequester carbon. Nevertheless, methods to quantify earthworm ingestion and egestion under field conditions are largely lacking. Soils of the Fennoscandian tundra offer a special possibility for such quantifications, as these soils commonly lack burrowing macrofauna and exhibit a well-defined O horizon with low bulk density on top of a mineral soil with higher density. Since ingestion-egestion mixes the two soil layers, the temporal changes in the bulk density profile of such soils may be useful for estimating field ingestion rates. In this study, we applied a model for earthworm burrowing through soil ingestion to observed changes in soil densities occurring in a mesocosm experiment carried out in the arctic during four summers with intact soil. The earthworms present in the mesocosms were Aporrectodea trapezoides, Aporrectodea tuberculata, Aporrectodea rosea, Lumbricus rubellus and Lumbricus Terrestris (fourth season only). We show that changes in soil density profiles can indeed be used to infer earthworm ingestion rates that are realistic in comparison to literature values. Although uncertainties in parameter values were sometimes large, the results from this study suggest that soil turnover rates and endogeic earthworm soil ingestion rates in tundra heath and meadow soils may be as high as those reported for temperate conditions. Such large ingestion rates can explain observed large morphological changes in arctic soils where dispersing earthworms have resulted in complete inmixing of the organic layer into the mineral soil. Our approach is applicable to soil profiles with marked vertical differences in bulk density such as the soils of the Fennoscandian tundra where earthworms are currently dispersing into new areas and to layered repacked soil samples that are incubated in the field.

Earthworm burrowing is essential for soil functioning in temperate climates. It is known that soil compaction hampers earthworm burrowing, but there is a lack of knowledge on how it affects the energy costs of earthworms. In the present study, we used respirometry and isothermal calorimetry to quantify earthworm respiration rates and heat dissipation in two endogeic species, Aporrectodea caliginosa and Aporrectodea tuberculata, in compacted and non-compacted soils. We put the measured respiration rates and heat dissipation in relation to the burrow volume and cast volume produced by the earthworms. We found that at higher compaction levels, respiration rates and dissipated heat increased for both studied species. The energy costs associated with burrowing were a significant fraction of the total energy costs. Our results indicate that energy costs per burrow volume increase due to compaction, and that the specific energy costs for burrowing (i.e., per gram earthworm) were lower for A. tuberculata than for A. caliginosa. Further studies are needed to confirm our results. We discuss the potential and current limitations of isothermal calorimetry as a method for direct quantification of energy costs of earthworms. There is a need for further studies that quantify how energy costs of burrowing are affected by various soil conditions, to better predict the implications of land use and soil management on soil processes and functions mediated by earthworm burrowing.

One current challenge in sustainable agriculture is to redesign cropping systems to reduce the use and impacts of pesticides, and by doing so protect the environment, in particular groundwater, and human health. As a large range of systems could be explored and a wide number of pesticides used, field experiments cannot be carried out to study the sustainability of each of them. Thus, the objectives of this work were (1) to measure water flows and pesticide leaching in six contrasted low input cropping systems based on sunflower-wheat rotation, oilseed rape-wheat-barley rotation, and maize monoculture, experimented for three years in three different soil and climatic conditions, and (2) to assess and to compare the ability of three pesticide fate models (MACRO, PEARL, PRZM) to simulate the observed water flows and pesticide concentrations. The systems were designed using various crop rotations, including cover crops and intercrops. The models were parameterized with generic parameter estimation routines as done for regulatory risk assessment, and a method was developed to parameterize intercrops, not represented in the models: the use of average crop factors, maximum LAI, crop height and rooting depth of the crops constituting the intercrop allowed acceptable simulations of cumulative water flows, but not their dynamic. Twelve pesticides of 70 applied were quantified in lysimeter samples (e.g. bentazone, glyphosate, S-metolachlor), and their concentrations exceeded 0.1 μg L−1 in several occasions. The performance of the models to reproduce pesticide concentrations was generally poor illustrating the great challenge and the progress needed to simulate accurately pesticide transfers into the soil. The best fits to measured data were attained using “worst-case” pesticide sorption and degradation parameters. Overall, MACRO performed better than PEARL and PRZM. The method developed to parameterize intercrops could be used for risk assessment of groundwater contamination by pesticides in low input cropping systems, but the use of the three models without any calibration is likely to underestimate pesticide leaching in several situations.

Freezing and thawing affect the pore -space structure in agricultural soils with implications for soil hydraulic properties and water flow. Previous studies have focused on the upper few centimeters of the tilled topsoil, where most freeze -thaw (FT) cycles occur, even though deeper soil layers are also subject to freezing and thawing in cold climates. Thus, little is known about how freezing and thawing affect untilled soil layers, which often show high bulk densities that restrict vertical water movement. Furthermore, it remains unclear how shifts in FT patterns with climate change may change the pore -space structure and water flow through these soil layers. Here we investigated the effects of freezing and thawing on X-ray imaged pore -space characteristics, water retention and near -saturated hydraulic conductivity (K) in untilled soil directly below plough depth. Intact cores were sampled at two sites in central Sweden under the same long-term reduced tillage management. The two soils, a silt loam and a silty clay loam, were subjected to three FT scenarios in a laboratory environment intended to represent FT patterns that are considered likely under current and future winter conditions for this region. The latter scenario was characterised by more FT cycles and a lower freezing temperature. Freezing and thawing increased K in the near -saturated range in both soils, which we attribute to observed small (<0.01 mm(3 )mm( -3)) increases in the volume of pores of diameters close to the X-ray resolution limit. Concomitant increases in pore network connectivity and critical pore diameter, especially in the denser silty clay loam soil, probably contributed to this increase in K. The water retention data suggested that changes in pore -space characteristics below X-ray resolution also occurred in both soils. Furthermore, our results indicate that both soils may show higher drainage rates due to shifts in FT patterns in the future, although longer -term changes in pore -space structure with an increasing number of FT cycles would mostly be limited to soils with relatively high clay contents. These soils are often more compacted below plough depth and, thus, benefits from improvements in soil structure such as improved root growth and plant water supply are also expected to be larger.

An improved understanding of preferential solute transport in soil macropores would enable more reliable predictions of the fate of agrochemicals and the protection of water quality in agricultural landscapes. The objective of this study was to investigate how soil organic carbon (SOC) and soil texture shape soil pore structure and thereby determine the susceptibility to preferential transport under steady-state near-saturated flow conditions. To do so, we took intact topsoil samples from an arable field that has large variations in SOC content (1.1-2.7%) and clay content (8-42%). Soil pore structure was quantified by X-ray tomography and soil water retention measurements. Non-reactive solute transport experiments under steady-state near-saturated conditions were carried out at irrigation rates of 2 and 5 mm h- 1 to quantify the degree of preferential transport. Near- saturated hydraulic conductivities at pressure heads of-1.3 and-6 cm were also measured using a tension disc infiltrometer. The results showed that larger abundances of small macropores (240-720 mu m diameter) and mesopores (5-100 mu m diameter) resulted in weaker preferential transport, due to larger hydraulic conductivities in the soil matrix that prevented the activation of water flow and solute transport in large macropores. In particular, the degree of preferential transport was most strongly and negatively correlated with the mesoporosity in the 30-100 mu m diameter class. In contrast, the degree of preferential transport was not correlated with connectivity measures (e.g. the percolating fraction and critical pore diameter for the macropore network), probably because i.) the pore space of almost all samples was highly connected, being dominated by one percolating cluster, and ii.) only a part of this percolating macroporosity was active under the near-saturated conditions of the experiment. We also found that the degree of preferential transport was strongly and negatively correlated with clay content, whilst the effects of SOC were not significant. Nevertheless, macroporosity in the 240-720 mu m diameter class and mesoporosity were positively correlated with SOC content in our soils and in some previous studies. Therefore, SOC sequestration in arable soils may potentially reduce the risk of preferential transport under near-saturated flow conditions through better developed networks of small macropores and mesopores.

Root circumnutation, the helical movement of growing root tips, is a widely observed behaviour of plants. However, our mechanistic understanding of the impacts of root circumnutation on root growth and soil exploration is limited. Here, we deployed a unique combination of penetrometer measurements, X-ray computed tomography and time-lapse imaging, and cavity expansion modelling to unveil the effects of root circumnutation on the mechanical resistance to soil penetration. To simulate differences in circumnutation amplitude and frequency occurring among plant species, genotypes and environmental conditions, we inserted cone penetrometers with varying bending stiffness into soil samples that were subjected to orbital movement at different velocities. We show that greater circumnutation intensity, determined by a greater circumnutation frequency in conjunction with a larger circumnutation amplitude, decreased the mechanical resistance to soil penetration. Cavity expansion theory and X-ray computed tomography provided evidence that increased circumnutation intensity reduces friction at the cone-soil interface, indicating a link between root circumnutation and the ability of plants to overcome mechanical constraints to root growth. We conclude that circumnutation is a key component of root foraging behaviour and propose that genotypic differences in circumnutation intensity can be leveraged to adapt crops to soils with greater mechanical resistance.

Freeze-thaw (FT) cycles have shown to affect the evolution of the pore space of agricultural soils, thereby affecting their hydraulic properties. In the temperate-boreal zones, FT patterns are projected to shift from relatively long and uniform freezing periods to more frequent fluctuations around 0°C as a result of climate change. To better anticipate potential consequences for water storage and flows in agricultural soils, a thorough evaluation of the importance of FT cycles in this context is required.Here we summarize the findings of studies investigating the effects of FT cycles on various pore-space characteristics (e.g. macroporosity, pore connectivity, percolating pore space) and hydraulic properties (e.g. infiltration capacity, hydraulic conductivity, water retention) of agricultural soils. This includes the results of a laboratory experiment where we simulated different FT scenarios representative for current and future winter conditions in the temperate-boreal zones.Our findings suggest that a shift in FT patterns with climate change indeed has the potential to alter, at least temporarily, water retention properties and (near-)saturated hydraulic conductivities of agricultural soils. We highlight that this is despite most changes in pore-space characteristics seem to occur in pores with a diameter smaller than 50 µm. The persisting increase in pore connectivity of specific soils with an increasing number of FT cycles appears to be decisive in this respect. However, to assess fully the magnitude of changes in soil water functions at the field scale may require modelling. We finally stress that the sensitivity of hydraulic properties to FT patterns questions the transferability of results of some previous studies to the natural environment, applying unrealistic temperatures and rates of freezing and thawing.

&lt;p&gt;Macropores have important beneficial impacts on the hydrological cycle, since they reduce risks of waterlogging, surface runoff, soil erosion and flooding. On the other hand, macropore flow is also associated with significant ecosystem disservices, since it can dramatically accelerate the leaching of contaminants to surface water and groundwater. Several approaches to model preferential macropore flow have been developed. One approach is to use the kinematic wave equation, in which the kinematic exponent should depend on the exponent in a power law relationship between wetted macropore surface area and macropore saturation. Most model applications have relied on calibration of model parameters against measured data on water flow. This makes critical testing of the underlying model concepts difficult and raises the question of whether the model is matching the data for the right reasons or not. In this study, we used X-ray tomography to quantify water and air distributions in macropores at varying steady-state flow rates in two topsoil and two subsoil columns (diameter 9 cm) sampled from a clay soil. We collected sufficient data to derive the kinematic wave exponent from the image data for the two topsoil samples. We found that the wetted macropore surface area and macropore saturation were indeed related by a power law for the first three irrigation intensities, corresponding to kinematic exponents of 1.22 and 1.26, respectively. These promising results need to be verified in future experiments that should be conducted on soil samples with smaller diameters to achieve better image resolutions and signal-to-noise ratios.&lt;/p&gt;

Macropores are known to be important pathways for the rapid transport of water, solutes and colloids in soil. Nevertheless, we still know very little about how the topology and geometry of macropore networks govern water flow configurations and velocities in natural soil. In this study, we aimed at gaining more insight into macropore flow by using X-ray tomography to quantify air–water distributions in the macropore networks of undisturbed topsoil and subsoil columns of a clay soil at varying steady-state flow rates. We observed that while large fractions of the macropore network remained air-filled, the air phase only became entrapped when the irrigation rate was very close to the saturated hydraulic conductivity of the soil. The data enabled us to parameterize a kinematic wave model for water flow following the approach proposed in Jarvis et al. (2017a). Follow-up experiments would be required to evaluate whether these kinematic wave parameters derived by X-ray imaging match with those obtained from outflow measurements. We found that quantitative X-ray imaging of macropore flow through soils still remains a challenging task. We recommend that future experiments are conducted on smaller soil samples to improve image resolution and minimize experimental time spans as well as X-ray image noise and illumination bias. Such experiments could also include 3-D tracer imaging to identify the imaged macropore networks transporting most of the water (i.e. the backbone) at varying steady irrigation rates.

Adopting soil and crop management practices that conserve or enhance soil structure is critical for supporting the sustainable adaptation of agriculture to climate change, as it should help maintain agricultural production in the face of increasing drought or water excess without impairing environmental quality. In this paper, we evaluate the evidence for this assertion by synthesizing the results of 34 published meta-analyses of the effects of such practices on soil physical and hydraulic properties relevant for climate change adaptation in European agriculture. We also review an additional 127 meta-analyses that investigated synergies and trade-offs or help to explain the effects of soil and crop management in terms of the underlying processes and mechanisms. Finally, we identify how responses to alternative soil–crop management systems vary under contrasting agro-environmental conditions across Europe. This information may help practitioners and policymakers to draw context-specific conclusions concerning the efficacy of management practices as climate adaptation tools. Our synthesis demonstrates that organic soil amendments and the adoption of practices that maintain “continuous living cover” result in significant benefits for the water regulation function of soils, mostly arising from the additional carbon inputs to soil and the stimulation of biological processes. These effects are clearly related to improved soil aggregation and enhanced bio-porosity, both of which reduce surface runoff and increase infiltration. One potentially negative consequence of these systems is a reduction in soil water storage and groundwater recharge, which may be problematic in dry climates. Some important synergies are reductions in nitrate leaching to groundwater and greenhouse gas emissions for nonleguminous cover crop systems. The benefits of reducing tillage intensity appear much less clear-cut. Increases in soil bulk density due to traffic compaction are commonly reported. However, biological activity is enhanced under reduced tillage intensity, which should improve soil structure and infiltration capacity and reduce surface runoff and the losses of agro-chemicals to surface water. However, the evidence for these beneficial effects is inconclusive, while significant trade-offs include yield penalties and increases in greenhouse gas emissions and the risks of leaching of pesticides and nitrate. Our synthesis also highlights important knowledge gaps on the effects of management practices on root growth and transpiration. Thus, conclusions related to the impacts of management on the crop water supply and other water regulation functions are necessarily based on inferences derived from proxy variables. Based on these knowledge gaps, we outlined several key avenues for future research on this topic.

Soil-crop models are potentially useful tools to support analyses of the effects of climate and crop and soil management practices on crop production and the environment (e.g. carbon sequestration and greenhouse gas emissions or the leaching of agro-chemicals). However, it is not clear whether current generation models can be used to simulate long-term trends in crop production and the environmental impacts induced by changes in climate or land use because they do not consider the effects of soil structure dynamics at seasonal to decadal (e.g. root growth, activity of macro-fauna) and centennial time scales (e.g. changes in organic matter content) on soil hydraulic functions, hydrological processes, crop growth and carbon cycling.Here, we present a new soil-crop model that accounts for the interactions between soil structure dynamics, carbon cycling, soil physical and hydraulic properties, soil water balance and crop growth. The importance of soil structure dynamics is illustrated by long-term simulations of soil organic matter storage, soil water balance components and crop yields for a field site in central Sweden under climate change and contrasting management practices (organic amendments and crop varieties with an enhanced allocation of carbon to roots).

Stabilizing soil organic carbon (SOC) is essential for maintaining soil structure and carbon sequestration. Soil aggregation through organic-mineral associations helps to protect SOC from microbial decomposition physically. Recent research has shown that the chemical interactions between SOC and reactive aluminum (Al) phases may be central for this aggregation in acidic soils and, hence, for the protection of SOC. However, the role of reactive Al phases in protecting SOC on a national level in Sweden is unclear. We analyzed 100 topsoil samples from the Swedish national monitoring program ‘Swedish soil and crop monitoring inventory’ taken between 2001 and 2017 in a grid covering all major agricultural areas in Sweden and determined the Al and iron content after chemical extraction using ammonium oxalate acid. We also estimated silt-sized aggregation from the differences in particle size distributions measured with a laser diffraction analyzer (Partica LA-950 V2, Horiba) between mechanically and chemically dispersed samples. Preliminary results show that SOC is indeed positively correlated with oxalate extractable Al on a national scale in Sweden, while correlations with clay and oxalate extractable iron are much weaker. The results derived from this study can help to determine the relationship between reactive Al phases, soil aggregation, structure, and the potential for carbon sequestration in Swedish arable soils.

Nitrous oxide (N2O) emissions from arable soils are predominantly caused by denitrifying microbes, of which fungal denitrifiers are of particular interest, as fungi, in contrast to bacteria, terminate denitrification with N2O. Reduced tillage has been shown to increase gaseous nitrogen losses from soil, but knowledge of how varying tillage regimes and associated soil physical and chemical alterations affect fungal denitrifiers is limited. Based on results from a long-term (>40 years) tillage experiment, we show that non-inversion tillage resulted in increased potential denitrification activity in the upper soil layers, compared to annual or occasional (every 4–5 years) conventional inversion tillage. Using sequence-corrected abundance of the fungal nirK gene, we further identified an increased genetic potential for fungal denitrification, compared to that caused by bacteria, with decreasing tillage intensity. Differences in the composition and diversity of the fungal nirK community imply that different tillage regimes select for distinct fungal denitrifiers with differing functional capabilities and lifestyles, predominantly by altering carbon and nitrogen related niches. Our findings suggest that the creation of organic hotspots through stratification by non-inversion tillage increases the diversity and abundance of fungal denitrifier communities and modifies their composition, and thus their overall relevance for N2O production by denitrification, in arable soils.