Structure turnover times of grassland soils under different moisture regimes

Soil structure is a dynamic property of soils, which refers to temporal changes in the spatial arrangement of pores, organic matter, and minerals. Its turnover, i.e. the irreversible redistribution of soil constituents, determines essential soil functions including carbon storage. Structure turnover times and its response to biotic versus abiotic drivers have never been quantified directly under natural conditions. We used a novel combination of structure labelling with inert garnet particles and X-ray Computed Tomography to determine structure turnover times of two grassland topsoils with either access or exclusion of roots and fauna > 30 mu m. Both, abiotic and biotic factors developed soil structure at a site-specific rate towards a dynamic equilibrium, at which bulk properties remain constant because creation and destruction of structural properties are in balance. Its turnover, however, was mainly determined by macrofaunal activity which varied with environmental conditions. Under dry conditions less favorable for bioturbation, the extrapolated structure turnover time was 33 +/- 3 years, while being 16 +/- 1 years under moist conditions. Previous studies on organic matter turnover determined in the vicinity of the experimental site showed similar turnover times for particulate organic matter. The similar turnover times suggest that the accessibility of particulate organic matter to decomposers is closely linked to structure turnover, thus highlighting the intimate nexus between structure evolution and carbon persistence in soil

Effects of freeze-thaw cycles on soil structure under different tillage and plant cover management practices

Soil structure plays a central role in many soil processes that are environmentally relevant. Intermittent freezing of the soil over winter is an important abiotic disturbance in temperate climates and its effects on soil structure depend on the soil's preexistent structural strength and cohesion. Management choices such as tillage and plant cover after harvest strongly influence soil structure, and therefore the soil's response to freeze-thaw. We examined the effects of 5 freeze-thaw cycles (FT) on the mu CT-detectable structure of intact topsoil cores (o=100 mm, h=80 mm) from a long-term rotation and tillage experiment in Denmark. The cores were divided among two tillage treatments and two plant cover treatments, corresponding to a gradient of structural strength: CTB1020 mu m) and analyzed the macroporosity (Vt), mean macropore diameter (dm) and mean Euclidian distance to the nearest macropore (EDm). Additionally, we analyzed the effects of tillage and plant cover on several mu CTderived geometric parameters in Full Range. Overall, NT-B and NT-V resulted in lower macroporosity than in CTB and CT-V. Similarly, we found fewer, less branched macropores with shorter mean branch length in NT compared to CT for both plant cover treatments. However, we propose that mu CT-derived geometric parameters might be confounded by the overlapping influence of relatively few, complex and voluminous coarse macropores and the more abundant, less complex very fine macropores. Freeze-thaw, in turn, caused crumbling of soil around coarse macropores, reducing Vt and dm in Full Range and reducing Vt in the > 1020 mu m range. Additionally, FT caused significant increases in Vt and reductions in dm and EDm in the < 300 mu m range, indicating creation of new very fine macropores and expansion of previously indiscernible macropores. Overall, the effects of FT were reduced in NT (for equal plant cover treatments) and V (for equal tillage treatments), indicating greater resilience against FT in both cases

Evaporation Study for Real Soils Based on HYPROP Hydraulic Functions and Micro-CT-Measured Pore-Size Distribution

Evaporation—a key process for water exchange between soil and atmosphere—is controlled by convective and diffusive surface fluxes that determine the functional time dependence of the evaporation rate (). Recent studies demonstrated that only a pore-scale surface flux model can capture the correct () curve. These studies also showed that a realistic estimate of the hydraulically connected region (HCR) of the pore-size distribution (PSD) is crucial for coupling surface flux to internal water flux. Since previous studies were often based on natural sands and glass beads, the main focus of our study was to test these conclusions for real soils. Therefore, we investigated the evaporation process within undisturbed soil columns of a sandy soil and loamy sand and measured the hydraulic functions via HYPROP experiments (a system to measure hydraulic properties using the evaporation method). Based on the isolated pore evaporation (IPE) model using a discretized form of the PSD, we developed a continuous IPE model and applied it to our experiments. Because the PSD plays a central role in the IPE model, we determined the PSD of the loamy sand soil via X-ray microtomography (μCT) for pores >19 μm. The consistency of the experimental data, i.e., (i) the retention curve for deriving the HCR of the pore size distribution, (ii) the unsaturated hydraulic conductivity for calculating the characteristic lengths of the evaporation process, and (iii) the high accuracy of the mass loss data strongly support the HYPROP method for this kind of complex evaporation experiment. The continuous IPE model describes the characteristic Stage 1 behavior well (functional form of the evaporation rate and length of Stage 1) for both soil types if a realistic HCR estimate is used that (i) is derived from a characteristic length analysis estimating the lower boundary of the HCR and (ii) the upper range of the HCR is based on the true PSD derived from μCT data

The fate of silver nanoparticles in riverbank filtration systems — The role of biological components and flow velocity

Riverbank filtration is a natural process that may ensure the cleaning of surface water for producing drinking water. For silver nanoparticles (AgNP), physico-chemical interaction with sediment surfaces is one major retention mechanism. However, the effect of flow velocity and the importance of biological retention, such as AgNP attachment to biomass, are not well understood, yet. We investigated AgNP (c = 0.6 mg L−1) transport at different spatial and temporal scales in pristine and previously pond water-aged sediment columns. Transport of AgNP under near-natural conditions was studied in a long-term riverbank filtration experiment over the course of one month with changing flow scenarios (i.e. transport at 0.7 m d−1, stagnation, and remobilization at 1.7 m d−1). To elucidate retention processes, we conducted small-scale lab column experiments at low (0.2 m d−1) and high (0.7 m d−1) flow rate using pristine and aged sediments. Overall, AgNP accumulated in the upper centimeters of the sediment both in lab and outdoor experiments. In the lab study, retention of AgNP by attachment to biological components was very effective under high and low flow rate with nearly complete NP accumulation in the upper 2 mm. When organic material was absent, abiotic filtration mechanisms led to NP retention in the upper 5 to 7 cm of the column. In the long-term study, AgNP were transported up to a depth of 25 cm. For the pristine sediment in the lab study and the outdoor experiments only erratic particle breakthrough was detected in a depth of 15 cm. We conclude that physico-chemical interactions of AgNP with sediment surfaces are efficient in retaining AgNP. The presence of organic material provides additional retention sites which increase the filtration capacity of the system. Nevertheless, erratic breakthrough events might transport NP into deeper sediment layers.DFG, 172114680, FOR 1536: INTERNANO: Mobility, aging and functioning of engineered inorganic nanoparticles at the aquatic-terrestrial interfac

Microscale carbon distribution around pores and particulate organic matter varies with soil moisture regime

Soil carbon sequestration arises from the interplay of carbon input and stabilization, which vary in space and time. Assessing the resulting microscale carbon distribution in an intact pore space, however, has so far eluded methodological accessibility. Here, we explore the role of soil moisture regimes in shaping microscale carbon gradients by a novel mapping protocol for particulate organic matter and carbon in the soil matrix based on a combination of Osmium staining, X-ray computed tomography, and machine learning. With three different soil types we show that the moisture regime governs C losses from particulate organic matter and the microscale carbon redistribution and stabilization patterns in the soil matrix. Carbon depletion around pores (aperture > 10 µm) occurs in a much larger soil volume (19–74%) than carbon enrichment around particulate organic matter (1%). Thus, interacting microscale processes shaped by the moisture regime are a decisive factor for overall soil carbon persistence

Soil water retention and hydraulic conductivity measured in a wide saturation range

Soil hydraulic properties (SHPs), particularly soil water retention capacity and hydraulic conductivity of unsaturated soils, are among the key properties that determine the hydrological functioning of terrestrial systems. Some large collections of SHPs, such as the UNSODA and HYPRES databases, have already existed for more than 2 decades. They have provided an essential basis for many studies related to the critical zone. Today, sample-based SHPs can be determined in a wider saturation range and with higher resolution by combining some recently developed laboratory methods. We provide 572 high-quality SHP data sets from undisturbed, mostly central European samples covering a wide range of soil texture, bulk density and organic carbon content. A consistent and rigorous quality filtering ensures that only trustworthy data sets are included. The data collection contains (i) SHP data, which consist of soil water retention and hydraulic conductivity data, determined by the evaporation method and supplemented by retention data obtained by the dewpoint method and saturated conductivity measurements; (ii) basic soil data, which consist of particle size distribution determined by sedimentation analysis and wet sieving, bulk density and organic carbon content; and (iii) metadata, which include the coordinates of the sampling locations. In addition, for each data set, we provide soil hydraulic parameters for the widely used van Genuchten–Mualem model and for the more advanced Peters–Durner–Iden model. The data were originally collected to develop and test SHP models and associated pedotransfer functions. However, we expect that they will be very valuable for various other purposes such as simulation studies or correlation analyses of different soil properties to study their causal relationships. The data are available at https://doi.org/10.5880/fidgeo.2023.012 (Hohenbrink et al., 2023)

Response of subsoil organic matter contents and physical properties to long‐term, high‐rate farmyard manure application

Application of farmyard manure (FYM) is common practice to improve physical and chemical properties of arable soil and crop yields. However, studies on effects of FYM application mainly focussed on topsoils, whereas subsoils have rarely been addressed so far. We, therefore, investigated the effects of 36‐year FYM application with different rates of annual organic carbon (OC) addition (0, 469, 938 and 1875 g C m−2 a−1) on OC contents of a Chernozem in 0–30 cm (topsoil) and 35–45 cm (subsoil) depth. We also investigated its effects on soil structure and hydraulic properties in subsoil. X‐ray computed tomography was used to analyse the response of the subsoil macropore system (≥19 μm) and the distribution of particulate organic matter (POM) to different FYM applications, which were related to contents in total OC (TOC) and water‐extractable OC (WEOC). We show that FYM‐C application of 469 g C m−2 a−1 caused increases in TOC and WEOC contents only in the topsoil, whereas rates of ≥938 g C m−2 a−1 were necessary for TOC enrichment also in the subsoil. At this depth, the subdivision of TOC into different OC sources shows that most of the increase was due to fresh POM, likely by the stimulation of root growth and bioturbation. The increase in subsoil TOC went along with increases in macroporosity and macropore connectivity. We neither observed increases in plant‐available water capacity nor in unsaturated hydraulic conductivity. In conclusion, only very high application of FYM over long periods can increase OC content of subsoil at our study site, but this increase is largely based on fresh, easily degradable POM and likely accompanied by high C losses when considering the discrepancy between OC addition rate by FYM and TOC response in soil. Highlights A new image processing procedure to distinguish fresh and decomposed POM. The increase of subsoil C stock based to a large extend on fresh, labile POM. Potential of arable subsoils for long‐term C storage by large FYM application rates is limited. The increase in TOC has no effect on hydraulic properties of the subsoil.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165

Transport and Retention of Sulfidized Silver Nanoparticles in Porous Media: The Role of Air-Water Interfaces, Flow Velocity, and Natural Organic Matter

The sulfidation and aging of silver nanoparticles (Ag-NPs) with natural organic matter (NOM) are major transformation processes along their pathway in wastewater treatment plants and surface waters. Although soils appear to be a sink for disposed Ag-NPs, the impact of variable saturation on the transport and retention behavior in porous media is still not fully understood. We studied the behavior of sulfidized silver nanoparticles (S-Ag-NPs, 1 mg L−1) in saturated and unsaturated sand columns regarding the effects of (i) the presence of NOM (5 mg L−1) in the aquatic phase on retention, transport, and remobilization of S-Ag-NPs and (ii) the distribution and quantity of air-water and solid-water interfaces for different flow velocities determined via X-ray microtomography (X-ray μCT). Unsaturated transport experiments were conducted under controlled conditions with unit gradients in water potential and constant water content along the flow direction for each applied flux. It was shown that (i) NOM in S-Ag-NP dispersion highly increased the NP-mobility; (ii) differences between saturated and unsaturated transport were increasing with decreasing flux and, consequently, decreasing water contents; (iii) both, solid-water and air-water interfaces were involved in retention of S-Ag-NPs aged by NOM. Using numerical model simulations and X-ray μCT of flow experiments, the breakthrough of Ag-NP could be explained by a disproportional increase in air-water interfaces and an increasing attachment efficiency with decreasing water content and flow velocity