Earthworm Burrows Research Articles

Using the ecosystem engineer concept to test the functional effects of a decrease in earthworm abundance due to an historic metal pollution gradient

In a companion study, carried out in a fallow meadow close to a lead recycling factory, we showed that earthworms were absent in the first 20 m and then gradually increased in abundance from 30 to 110 m from the factory. Here we assessed in the same meadow whether these differences in earthworm abundance were associated with the loss of physical soil properties. Soil cores were sampled and infiltration measured in situ at five distances from the factory (10, 30, 50, 80 and 110 m). X-ray tomography was used to characterize the earthworm burrow systems within cores. The burrow systems were minimal at the first two distances, with the only macropores observed probably produced by insects such as ants. Typical earthworm burrows were seen at 50 m but most of their characteristics (volume, diameter, continuity) were similar to those observed at 10 or 30 m. Dense and well-developed burrow systems were observed at 80 and 110 m from the factory with significantly larger volume and continuity. Burrow diameter at 80 m was significantly higher than at closer distances but it significantly decreased at 110 m associated with the higher abundance of endogeics earthworm species. Water infiltration followed the same trend with significantly lower rates at the first two distances compared to those further from the factory, where rates increased from 70 to 250%. This study emphasizes the need to develop a more functional approach to study the spatial effects of contamination on soil ecological processes and fertility, beyond modifications in earthworm communities.

Earthworm Burrowing Activity and Its Effects on Soil Hydraulic Properties under Different Soil Moisture Conditions from the Loess Plateau, China

Earthworm activity has become more important in the Loess Plateau, where hydrological processes are crucial for ecosystem sustainability. In this study, we conducted a laboratory microcosm experiment to determine the various burrowing activities of Eisenia fetida and their impact on the soil hydraulic properties in response to different levels of soil moisture (50%, 70%, 90% of field capacity) in two common soil types (loessial and Lou soil) obtained from the Loess Plateau. Burrowing activity of E. fetida increased with higher soil moisture and was greater in loessial than in Lou soil. Most burrowing activities occurred within the top 5 cm and decreased with increasing soil depth. Macropores and burrow branching, which are highly related to the earthworm burrowing, were more prevalent in wetter soil. Earthworms significantly altered the formation of large soil aggregates (AGL, diameter >2 mm) under different soil moistures and depths. Distinct earthworm burrowing activities, controlled by soil moisture, altered soil hydraulic properties. However, soil saturated hydraulic conductivity (Ks) showed little differences between different treatments due to the horizontal and high–branched burrows of E. fetida, although higher burrowing activities were found in wetter soil. Soil field capacity was highest in drier soil due to the less macropores and burrowing activities.

Effects of microplastics and earthworm burrows on soil macropore water flow within a laboratory soil column setup

AbstractSeveral earlier studies reported that microplastics (MP) accumulated on soil surfaces could be transported into the subsoil and ingested by soil biota, such as earthworms. The present study explores how networks of earthworm burrows and MP (low‐density polyethylene, LDPE) in subsoil affect the soil hydraulic properties and saturated water flow. A repacked and saturated sandy soil column experiment was conducted in an environment‐controlled laboratory with earthworms (anecic, Lumbricus terrestris) inoculated into the soil columns to form networks of macropore. The macropore network parameters (i.e., number, length, volume, diameter, soil saturated conductivity, and tracer breakthrough curves of soil columns) have been determined. The relative arrival times of the tracer mass (i.e. T5%, T25%, and T50%) were determined in order to describe the shapes of the breakthrough curves. The results show that in some breakthrough curves for the treatments with earthworms, there are two peaks. This is an indication that water was flowing faster in the macropores than in the soil matrix. There is a significant correlation between 5% arrival time and the median burrow volume, and the correlation coefficient was .571 (at the level of p < .05). The formation of macropores due to the burrowing activities of earthworms is considered the main cause of nonequilibrium water flow in the present study. The MP did not show any significant effect on the saturated water flow. This may be attribute to the low concentrations of MP used in the present study.

Volume‐related quantification of organic carbon content and cation exchange capacity of macropore surfaces in Bt horizons

AbstractIn structured soils, earthworm burrows, root channels, shrinkage cracks, and interaggregate spaces form complex macropore networks relevant for preferential transport, turnover processes, and root growth. Macropore walls are often coated with organomineral material, which determine physicochemical properties such as wettability, sorption, and the cation exchange capacity (CEC). The objective here was to identify volume‐averaged mean macropore coating properties of larger intact soil cores (∼7,500 cm3) from Bt horizons of Luvisols developed from loess and glacial till. The quantification of organic C (OC) content and CEC of macropore surfaces was based on three‐dimensional images of X‐ray computed tomography (XRCT) of 231‐μm voxel resolution and a vesselness procedure to distinguish between biopores and cracks. Macropore surface areas were combined with millimeter‐scaled data of OC contents and CEC of macropore coating material. The surface of macropores that accounted for 5.6 % (loess‐Bt) and 4.6 % (till‐Bt) of the samples’ volumes represented approximately one‐third of the OC content and CEC of the bulk soil. Among the macropores, surfaces of larger biopores contributed most to OC content of the soil cores. The contribution of coated cracks and pinhole fillings to OC content was larger for the till‐Bt than for the loess‐Bt. Locally higher OC contents and CEC values emphasize the role of macropore surfaces in Bt horizons of Luvisols as geochemical hotspots and for mass exchange, especially during preferential flow and transport. Volume‐based coating properties may help improving macroscopic‐scale two‐domain flow and transport models.

Variations in near-saturated hydraulic conductivity of arable mineral topsoils in south-western and central-eastern Finland

Soil hydraulic properties play an important role in determining the level of crop productivity and the extent of environmental loading. Consequently, measurement of near-saturated soil hydraulic conductivity (K(h)) by tension infiltrometer (TI) is an interesting technique. To provide reference values for near-saturated K(h) in arable mineral topsoils of Finland and to investigate variability in these values caused by tillage, antecedent soil moisture content and earthworm burrows, field measurements were conducted in south-western and central-eastern Finland using the TI technique, at supply pressure head –6, –3, and –1 cm. The range of near-saturated K(h) values obtained was 0.02–12.6 mm h-1 at –6 cm, 0.30–85.9 mm h-1 at –3 cm, and 2.55–250 mm h-1 at –1 cm. At –6 cm pressure head, clay soils tended to show lower values than coarser-structured soils, but the order was reversed at –1 cm. The spatial variation in near-saturated K(h) was moderate and was exceeded by the temporal variation.

Bioturbation of Ag2S-NPs in soil columns by earthworms

Sewage sludge contains Ag2S-NPs causing NP exposure of soil fauna when sludge is applied as soil amendment. Earthworm bioturbation is an important process affecting many soil functions. Bioturbation may be affected by the presence of Ag2S-NPs, but the earthworm activity itself may also influence the displacement of these NPs that otherwise show little transport in the soil. The aim of this study was to determine effects of Ag2S-NPs on earthworm bioturbation and effect of this bioturbation on the vertical distribution of Ag2S-NPs. Columns (12 cm) of a sandy loamy soil with and without Lumbricus rubellus were prepared with and without 10 mg Ag kg−1, applied as Ag2S-NPs in the top 2 cm of the soil, while artificial rainwater was applied at ∼1.2 mm day−1. The soil columns were sampled at three depths weekly for 28 days and leachate collected from the bottom. Total Ag measurements showed more displacement of Ag to deeper soil layers in the columns with earthworms. The application of rain only did not significantly affect Ag transport in the soil. No Ag was detected in column leachates. X-ray tomography showed that changes in macro porosity and pore size distribution as a result of bioturbation were not different between columns with and without Ag2S-NPs. Earthworm activity was therefore not affected by Ag2S-NPs at the used exposure concentration. Ag concentrations along the columns and the earthworm density allowed the calculation of the bioturbation rate. The effect on the Ag transport in the soil shows that earthworm burrowing activity is a relevant process that must be taken into account when studying the fate of nanoparticles in soils.

Distinct communities of Cercozoa at different soil depths in a temperate agricultural field.

Protists are the most important predators of soil microbes like bacteria and fungi and are highly diverse in terrestrial ecosystems. However, the structure of protistan communities throughout the soil profile is still poorly explored. Here, we used Illumina sequencing to track differences in the relative abundance and diversity of Cercozoa, a major group of protists, at two depths; 10-30 cm (topsoil) and 60-75 cm (subsoil) in an agricultural field in Germany. At the two depths, we also distinguished among three soil compartments: rhizosphere, drilosphere (earthworm burrows) and bulk soil. With increasing depth, we found an overall decline in richness, but we were able to detect subsoil specific phylotypes and contrasting relative abundance patterns between topsoil and subsoil for different clades. We also found that the compartment effect disappeared in the subsoil when compared to the topsoil. More studies are now needed to describe and isolate these possibly subsoil specific phylotypes and better understand their ecology and function.

Biochar increases pesticide-detoxifying carboxylesterases along earthworm burrows

Herein, we examined whether synergistic effects of earthworms (Lumbricus terrestris) and pine needle-derived biochar result in biochar-coated burrows with enhanced carboxylesterase (CE) activity (a pesticide-detoxifying enzyme). Biochar was placed at the top of soil columns at two doses (2.5 and 5% w/w dry mass), with an additional biochar-free treatment as control. Carboxylesterase and dehydrogenase activities were measured in the burrow walls sampled at three depths (0–4, 8–12, and 18–22 cm). Biochar was recovered from these samples to confirm its vertical transport and enzymatic activation. We tested whether biochar protected CE activity against desiccation stress of burrow wall samples. Likewise, the role of earthworm mucus in stabilizing CE onto biochar surface was also investigated by measuring the enzyme activity in fresh biochar particles previously incubated in the presence of earthworm mucus and purified esterase. Finally, we checked for the sensitivity of biochar-bound CE activity against selected organophosphorus pesticides. The main results were: i) co-application of earthworms and biochar caused a significant increase of CE activity in the first 12 cm of the soil column, ii) recovered biochar particles displayed CE activity which was significantly higher in the particles collected from the bottom of columns, iii) soil desiccation decreased the enzymatic activity, although such effect depended on biochar treatment and column depth (22–77% inhibition). Nevertheless, CE activity still was higher in the samples from the 5%-biochar treatment than activity in control and 2.5%-biochar treatments, iv) earthworm mucus favored the retention of CE onto the biochar surface, and v) the activity of biochar-bound CE was sensitive to inhibition by chlorpyrifos-oxon. These results suggest that the joint application of L. terrestris and biochar may be a suitable vermiremediation strategy to inactivate OP pesticides.

Epi-anecic rather than strict-anecic earthworms enhance soil enzymatic activities

Earthworms in interaction with soil microorganisms play a key role in litter decomposition. Moreover, as soil engineers, earthworms modify microbial communities and their enzymatic activities. Most studies focusing on earthworms and soil enzymatic activities compare distinct ecological categories of earthworms whereas their contributions and interactions within a given ecological category remain largely unknown. In this context, the aims of the present study were to determine and compare the contribution of (1) three strict-anecic earthworm species, (2) three epi-anecic earthworm species and (3) the pairwise interactions between these different species on Lolium perenne leaf litter decomposition and soil microbial activity. After 30 days of incubation, the surface litter mass loss and five soil enzymatic activities (FDAse, β-D-glucosidase, cellobiohydrolase, leucine amino-peptidase and acid phosphatase) were measured in both earthworm burrows and middens. In mono-specific assemblages, leaf litter mass loss and enzymatic activities were significantly higher in the presence of epi-anecic compared to strict-anecic species, whatever the species identity. These differences were higher for the β-D-glucosidase, leucine amino-peptidase and FDAse (+78%, +57% and +34%, respectively). Earthworm species interactions at both intra- and inter-ecological sub-categories did not enhance either leaf litter mass loss or enzymatic activities. Interestingly, FDAse activity was higher in earthworm burrows whereas acid phosphatase activity was higher in earthworm middens. These results indicate that the two anecic ecological sub-categories have different impacts on soil functioning and each of them regroups earthworm species with similar behaviour. This functional distinction highlights the key role of epi-anecic earthworms in fresh surface litter burial and decomposition, featuring their importance on nutrient cycling in soil and for microbial activities stimulation through resource availability.

Separation of Soil Macropore Types in Three‐Dimensional X‐Ray Computed Tomography Images Based on Pore Geometry Characteristics

Core Ideas For preferential flow modeling, different macropore types must be considered. A method was developed for separating biopores and cracks in 3D images from XRCT. The method enabled a more objective determination of structuring element sizes. The voxel‐based approach was found useful for quantification of macropore types. In structured soils, earthworm burrows, root channels, shrinkage cracks, and interaggregate spaces form complex macropore networks. Depending on the type and morphological properties, each macropore surface type is coated with specific organo‐mineral compounds, differently affecting sorption and mass exchange during preferential flow and turnover processes. For a quantitative, macropore type–specific analysis using X‐ray computed tomography (XRCT) with subsequent three‐dimensional (3D) image analysis, a discrimination of biopores from cracks and interaggregate spaces is necessary. We developed a method that allows separating biopores from other larger macropores in 3D images from XRCT of intact soil cores. An image‐processing workflow using the MAVI (Modular Algorithms for Volume Images) software framework ToolIP (Tool for Image Processing) was created to handle XRCT 3D images. Masking steps enabled to retain the surface roughness in the resulting two images of separated biopores and cracks. As a key point, the sizes of the structuring elements used in the spherical opening and dilation were objectively determined. For this purpose, maximum differences in the pore shapes between the 3D images of cylindrical biopores vs. more flat cracks and unregularly interaggregate spaces were focused. At the given resolution of 231‐μm voxel edge length, an optimum size of 2.5 voxels was found for both processing steps. The voxel‐based approach is applicable to XRCT 3D images of different spatial resolution and appears useful for the quantification of physicochemical surface properties of different macropore types for soil volumes, enabling a more precise description of preferential flow and transport.

Determining Millimeter‐Scale Maps of Cation Exchange Capacity at Macropore Surfaces in Bt Horizons

Core Ideas Coatings determine the cation exchange capacity (CEC) of macropore surfaces. We predicted the millimeter‐scale, 2D spatial distribution of CEC at intact macropores. The approach combined infrared spectroscopy and CEC measurements of small samples. The CEC distribution in clay–organic coatings was similar for two different Bt horizons. During preferential flow in structured soils, solute transport is largely restricted to a complex network of macropores. Clay–organic coatings of macropore surfaces determine soil physicochemical properties relevant for mass transport and carbon and nutrient turnover, such as the cation exchange capacity (CEC). However, due to the lack of an appropriate measurement approach, the small‐scale spatial distributions of the CEC and its quantities are unknown to date. The objective of this work was to develop a method for predicting the millimeter‐ to centimeter‐scale, two‐dimensional spatial distribution of the CEC at intact macropore surfaces. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was used to analyze bulk soil and separated coating material and for intact macropore surfaces as DRIFT mapping. To determine effective CEC (CECeff), a reduction of soil mass down to 0.5 g for use in the standard barium chloride batch method was tested to account for the limited amount of soil material that can be separated from thin macropore coatings. Linear and partial least squares regression analyses were applied to predict the CECeff distribution at intact macropore surfaces for samples from Luvisol Bt horizons from loess (L) and glacial till (T) using DRIFT spectral data. The highest CECeff values were found for coatings and pinhole fillings rich of clay–organic material (L: 38 cmol kg−1; T: 29 cmol kg−1) compared with low CECeff values of uncoated cracks and earthworm burrows that were similar to those of bulk soil (L: 21 cmol kg−1; T: 14 cmol kg−1). The location of millimeter‐ to centimeter‐sized regions with increased CECeff levels at intact macropore surfaces corresponded with the location of clay–organic coatings. The proposed method allows determining the CEC at macropore surfaces to quantify their effect on nutrient transport by preferential flow as well as on plant nutrient supply in macropores that may serve as preferential growth paths for plant roots.

Leaching of microplastics by preferential flow in earthworm (Lumbricus terrestris) burrows

Environmental contextMicroplastics found in soil pose several potential environmental risks. This study shows that microplastics on the soil surface can be ingested by earthworms and transported to the lower soil layers. In this way, microplastics may enter the food chain and find their way into groundwater systems, especially in cases where the water table is shallow. AbstractIn the current study, we examine how the activities of earthworms (Lumbricus terrestris) affect microplastic (MP) distribution and concentration in soil, with a focus on low density polyethylene (LDPE). We also want to determine if MPs can be flushed out with water. We used a laboratory sandy soil column (polyvinyl chloride tube) experimental set-up and tested five different treatments: (1) treatment with just soil (control) to check if the saturated conductivity (Ksat) could be impacted by MP, (2) treatment with MP, (3) treatment with MP and litter, (4) treatment with earthworms and litter as a second control for treatment 5 and (5) treatment with MPs, earthworms and litter. Each treatment consisted of eight replicates. For the treatments with MP, the concentration of MP added at the start of the experiment was 7% by weight (3.97g, polyethylene, 50% 1mm–250µm, 30% 250µm–150µm and 20% <150µm) based on 52.78g of dry litter from Populus nigra. In the treatments using earthworms, two adult earthworms, with an initial average weight of (7.14±0.26) g, were placed in each column. Results showed that LDPE particles could be introduced into the soil by the earthworms. MP particles were detected in each soil sample and within different soil layers for the earthworm treatments. Earthworms showed a tendency to transport the smaller MP particles and that the amount of MPs in size class <250μm increased in soil samples with increasing soil depth in comparison to the other size classes. After leaching, MPs were only detected in the leachate from the treatments with the earthworms, and the MP had similar size distributions as the soil samples in the 40–50cm layer of the treatment with MP, earthworms and litter. The results of this study clearly show that biogenic activities can mobilise MP transport from the surface into the soil and even be leached into drainage. It is highly likely that biogenic activities constitute a potential pathway for MPs to be transported into soil and groundwater.

Biogenic transport of glyphosate in the presence of LDPE microplastics: A mesocosm experiment

The accumulation of plastic debris and herbicide residues has become a huge challenge and poses many potential risks to environmental health and soil quality. In the present study, we investigated the transport of glyphosate and its main metabolite, aminomethylphosphonic acid (AMPA) via earthworms in the presence of different concentrations of light density polyethylene microplastics in the litter layer during a 14-day mesocosm experiment. The results showed earthworm gallery weight was negatively affected by the combination of glyphosate and microplastics. Glyphosate and AMPA concentrated in the first centimetre of the top soil layer and the downward transport of glyphosate and AMPA was only detected in the earthworm burrows, ranging from 0.04 to 4.25 μg g−1 for glyphosate and from 0.01 (less than limit of detection) to 0.76 μg g−1 for AMPA. The transport rate of glyphosate (including AMPA) from the litter layer into earthworm burrows ranged from 6.6 ± 4.6% to 18.3 ± 2.4%, depending on synergetic effects of microplastics and glyphosate application. The findings imply that earthworm activities strongly influence pollutant movement into the soil, which potentially affects soil ecosystems. Further studies focused on the fate of pollutants in the microenvironment of earthworm burrows are needed.

Impact of Small-Scaled Differences in Micro-Aggregation on Physico-Chemical Parameters of Macroscopic Biopore Walls

Macroscopic biopores, like earthworm burrows or channels which remain after a root decayed, act as preferential flow paths for water, gas, and heat transport processes, and viewing at agricultural production, as preferential elongation paths for plant roots. These processes result in intense alterations of the soil volume and its composition that surrounds the pores. The effects of these processes were analyzed at small-scale and physico-chemical soil parameters, i.e. relative oxygen diffusion coefficient (Ds/DO), oxygen partial pressure pO2, Eh and pH of biopore walls, were measured. The analyses were carried out on undisturbed soil samples with different colonization history, excavated from a haplic Luvisol derived from loess. Soil resistance to penetration was determined simultaneously with Ds/DO and pO2 using a coupled, self-developed approach, and four matric potentials (namely, ᴪ_m = -1 kPa; -3 kPa; -6 kPa or -30 kPa) were considered. We hypothesized that physico-chemical soil parameters in biopore walls were altered due to differing influences on the soil aggregation. Aggregation was visualized with scanning electron microscopy, classified and used to explain differences in - soil properties. Plant roots and earthworms altered aggregation next to biopore surfaces in a contrasted way, either by enhancing aggregates diversity or homogenizing it. Roots led to the formation of subpolyeders while earthworms formed subplates. Pore functions of microaggregates were comparable to those of larger scale, and subpolyeders showed much more favorable soil properties in terms of soil aeration (Ds/Do, pO2). Replicates of all parameters scattered intensely and showed deviations up to several orders of magnitude in case of Ds/DO underlining the large variability of soil properties in biopore walls.

Microbial processing of plant residues in the subsoil – The role of biopores

Most subsoil carbon (C) turnover occurs in biopore hotspots such as root channels and earthworm burrows. Biopores allocate large C amounts into the subsoil, where a vast capacity for long-term C sequestration is predicted. We hypothesise that organic matter (OM) cycling in biopores depends on their origin.Earthworm and root biopores were induced under field conditions and were sampled from the subsoil (45–75 and 75–105 cm) after two years of biopore formation. The effects of biopore formation on OM decomposition were studied by biomarkers: neutral sugars, cutin and suberin-derived lipids, lignin-derived phenols and free lipids. The degradation stage of OM was biopore type-specific but was only governed by the soil depth in root biopores. Degradation of OM increased from earthworm biopores to root biopores and bulk soil. Hemicelluloses (GM/AX ratio) were more strongly degraded than lignin side-chains (relative change from initial values). Two years of microbial processing during biopore formation increased the GM/AX ratio in earthworm biopores from 0.65 to 1.05 and in root biopores from 0.15 to 1.35 (both relative to source biomasses). Root biopores and bulk soil had the highest GM/AX ratios (1.2–1.3), hinting to rapid processing of plant residues and accumulation of microbial residues. The regular, frequent OM inputs by earthworms stimulated microbial growth and processing of mostly bioavailable OM and, thus, relatively enriched more persistent OM (e.g. lignin). Syringyl subunits of lignin underwent low (ratio changed from 0.35 to 0.55 relative to initial input) and vanillyl subunits underwent almost no processing in earthworm biopores indicating the preferential microbial utilisation of the easily available compounds frequently replenished by earthworm activity. After two years of decomposition of the root detritus, mainly structural plant material was enriched in root biopores. Short periods (6 months) of earthworm activity effectively recharged the highly processed OM in root biopores with fresh OM.In total, deep-rooting catch crops and short-term earthworm activities promote C accumulation in the subsoil followed by biopore-specific microbial processing predominantly governed by the C input frequency. As root biopores are up to 40 times more common than earthworm biopores, they dominate the OM input into subsoils. Such C inputs create several years lasting hotspots for preferential root growth and nutrient mobilisation in the subsoil. We conclude that root- and earthworm-derived biopores are vertical pathways for plant C from the soil surface into the subsoil and for intensive processing of litter C and sequestration of microbial necromass.

Feeding behaviour of epi-anecic earthworm species and their impacts on soil microbial communities

Earthworms contribute to numerous ecosystem services provided by soils. Most of the studies focusing on the contributions of earthworms on leaf litter decomposition were conducted by comparing distinct ecological categories (epigeic, epi-anecic, anecic strict and endogeic), whereas their specific contributions within a given ecological category remains largely unknown. In this context, the aim of this study was to determine the contribution of four epi-anecic earthworm species (Lumbricus rubellus, Lumbricus festivus, Lumbricus centralis and Lumbricus terrestris) to the leaf litter decomposition of three plant species (Lolium perenne, Holcus lanatus and Corylus avellana) with contrasted litter traits located at both the soil surface and at a depth of 10 cm. Fungal and bacterial communities inhabiting epi-anecic earthworm burrows were also assessed using T-RFLP analysis. Epi-anecic earthworms improved the leaf litter mass loss solely at the soil surface, while leaf litter mass loss was mainly due to microbial activity at 10 cm deep. Leaf litter mass loss was positively correlated to the initial biomass of the epi-anecic earthworms and the intensity of this relationship was dependent on litter type. Interestingly, L. festivus seemed to have a higher contribution to surface leaf litter mass loss that was linked to a stimulation of the fungal communities in its burrows. Fungal communities were thus impacted by both the litter type and the epi-anecic earthworm identity whereas soil bacterial diversity and richness were stimulated in the earthworm burrows whatever the epi-anecic earthworm species considered. Overall, epi-anecic earthworms contributed to enhance the diversity of the drilospheric microbiota.

Listening to earthworms burrowing and roots growing - acoustic signatures of soil biological activity

We report observations of acoustic emissions (AE) from growing plant roots and burrowing earthworms in soil, as a noninvasive method for monitoring biophysical processes that modify soil structure. AE emanating from earthworm and plants root activity were linked with time-lapse imaging in glass cells. Acoustic waveguides where installed in soil columns to monitor root growth in real time (mimicking field application). The cumulative AE events were in correlation with earthworm burrow lengths and with root growth. The number of AE events recorded from the soil columns with growing maize roots were several orders of magnitude larger than AE emanating from bare soil under similar conditions. The results suggest that AE monitoring may offer a window into largely unobservable dynamics of soil biomechanical processes such as root growth or patterns of earthworm activity - both important soil structure forming processes.

Biomechanical limits to soil penetration by earthworms: direct measurements of hydroskeletal pressures and peristaltic motions.

Burrows resulting from earthworm activity are important for supporting various physical and ecological soil processes. Earthworm burrowing activity is quantified using models for earthworm penetration and cavity expansion that consider soil moisture and mechanical properties. Key parameters in these models are the maximal pressures exerted by the earthworm's hydroskeleton (estimated at 200 kPa). We designed a special pressure chamber that directly measures the pressures exerted by moving earthworms under different confining pressures to delineate the limits of earthworm activity in soils at different mechanical and hydration states. The chamber consists of a Plexiglas prism fitted with inner flexible tubing that hosts the earthworm. The gap around the tubing is pressurized using water, and the earthworm's peristaltic motion and concurrent pressure fluctuations were recorded by a camera and pressure transducer. A model that links the earthworm's kinematics with measured pressure fluctuations was developed. Resulting maximal values of radial pressures for anecic and endogeic earthworms were 130 kPa and 195 kPa, respectively. Mean earthworm peristaltic frequencies were used to quantify burrowing rates that were in agreement with previous results. The study delineates mechanical constraints to soil bioturbation by earthworms by mapping the elastic behaviour in the measurement chamber onto the expected elasto-viscoplastic environment of natural soils.

Relating soil organic matter composition to soil water repellency for soil biopore surfaces different in history from two Bt horizons of a Haplic Luvisol

AbstractThe deposition of organic matter (OM), which is known for its high potential water repellency, on biopore walls can enhance preferential flow through these pores. In this study, OM composition determined with diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was related to soil water repellency (SWR) determined with sessile drop method, Wilhelmy plate method, and sorptivity tests. We hypothesized that the chemical composition (in terms of potential wettability index; a) is related to the physical properties (i.e., contact angle) of biopore walls, (b) depends on the history of the biopore, and (c) differs from the bulk soil matrix. Thus, the main objective was to identify the relation between OM composition determined with DRIFT spectroscopy and SWR in structured soils. The experiments were carried out on biopores and their surrounding soil matrices, excavated from 2 depths of a haplic Luvisol, with 3 different biopore histories (i.e., root channels, earthworm burrows, and root channels that were short‐term colonized by an earthworm). All measurements at intact biopore surfaces indicated a larger SWR at the surface of biopore walls as compared with the surrounding matrices and showed a higher proportion of hydrophobic functional groups. The OM composition determined with DRIFT spectroscopy correlated (R2 > .7) with contact angles (sessile drop method) that is in line with results of both water sorptivity and Wilhelmy plate method for soils with reduced wettability. The surfaces of short‐term colonized earthworm burrows had the most varying hydrophobic to hydrophilic components (A/B)‐ratio of all investigated biopore surfaces depending on soil depth. For biopore surfaces at this depth, contact angles >90° were frequently observed. The results also indicate that earthworms can lower SWR by aggregate disruption.

Two-dimensional distribution of soil organic carbon at intact macropore surfaces in BT-horizons

In structured soils, the surfaces of biopores, shrinkage cracks, and inter-aggregate spaces are often enriched in organic matter (OM) as compared to the matrix. The small-scale distribution of OM at macropore surfaces is affecting preferential flow, mass transfer, and OM turnover in macropores. While the mm-scale spatial distribution of the OM composition at intact surfaces could be observed, the distribution of the organic carbon (OC) content remained limited to date due to a suitable approach. The objective was to develop and test a method for quantifying the two-dimensional (2D) OC distribution at intact macropore surfaces at the mm-to-cm scale. We combined diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and OC analyses of reference samples of few milligrams manually separated from macropore surfaces. DRIFT spectra were measured in a mapping approach at intact surfaces from earthworm burrows, cracks with and without clay-organic coatings, and single pore fillings (‘pinholes’) from Bt-horizons of Luvisols developed on loess and glacial till. The sample’s OC contents were determined with elemental isotope ratio mass spectrometry (EA-IRMS) and related to the DRIFT spectra by partial least squares regression (PLSR). The DRIFT mapping spectra were used to predict the 2D spatial distribution of the OC content at intact macropore surfaces via the PLSR models. The maps revealed mm-scale differences in the OC contents of the macropores which were often related to the 2D distribution of the macropore type surfaces. The validation of the PLSR showed R2 values of 0.77 (loess) and 0.83 (till) between measured and predicted OC contents. The prediction accuracy of the OC values was reduced by the micro-topography of the intact surfaces and by a variable thickness of the clay-organic surface layer. DRIFT mapping spectra from intact soil macropore surfaces were suitable to quantify the 2D OC distribution of the outermost surface layer. The heterogeneous small-scale spatial distribution of the OC contents at macropore surfaces implies a more specific consideration of these macropore walls when studying inter-domain mass transfer and OM turnover in structured soils.