Soil organic matter (SOM) is known to play vital roles in the maintenance and improvement of many soil properties and processes. These roles, which largely influence soil functions, are a pool of ...specific contributions of different components of SOM. The soil functions, in turn, normally define the level of soil degradation, viewed as quantifiable temporal changes in a soil that impairs its quality. This paper aims at providing a generalized assessment of the current state of knowledge on the usefulness of SOM in monitoring soil degradation, based on its influence on the physical, chemical and biological properties and processes of soils. Emphasis is placed particularly on the effect of SOM on soil structure and availability of plant nutrients. Although these properties are discussed separately, the soil system is of dynamic and interactive nature, and changes in one property will likely affect other soil properties as well. Thus, functions of SOM almost always affect various soil properties and processes and engage in multiple reactions. In view of its role in soil aggregation and erosion control, in availability of plant nutrients and in ameliorating other forms of soil degradation than erosion, SOM has proven to be an important indicator of soil degradation. It has been suggested, however, that rather than the absolute amount, temporal change and potential amount of SOM be considered in its use as indicator of soil degradation, and that SOM may not be an all-purpose indicator. Whilst SOM remains a candidate without substitute as long as a one-parameter indicator of soil degradation is needed, narrowing down to the use of its labile and microbial components could be more appropriate, since early detection is important in the control and management of soil degradation.
Research gaps in understanding flood changes at the catchment scale caused by changes in forest management, agricultural practices, artificial drainage, and terracing are identified. Potential ...strategies in addressing these gaps are proposed, such as complex systems approaches to link processes across time scales, long‐term experiments on physical‐chemical‐biological process interactions, and a focus on connectivity and patterns across spatial scales. It is suggested that these strategies will stimulate new research that coherently addresses the issues across hydrology, soil and agricultural sciences, forest engineering, forest ecology, and geomorphology.
Plain Language Summary
This commentary explores research gaps in the field of land use change impacts on floods at the catchment scale and proposes possible ways forward for addressing these gaps. Specifically the impacts of forest management, agricultural practices, artificial drainage, and terracing on flood generation at the catchment scale are explored. Potential strategies in addressing research gaps in these fields are complex systems approaches to link processes across time scales, long‐term experiments on physical‐chemical‐biological process interactions, and a focus on connectivity and patterns across spatial scales. It is suggested that these strategies will stimulate new research that coherently addresses the issues across hydrology, soil and agricultural sciences, forest engineering, forest ecology, and geomorphology.
Key Points
Land use change impacts on floods are poorly understood at the catchment scale
Numerous synergies are identified in exploring the effects of changed agricultural practices, artif. drainage, terracing, and forest cover
A cross‐disciplinary systems approach aided by long‐term field studies and a focus on flow connectivity are needed to make major advances
Purpose
Soil structure evolving from physical and biological processes is closely related to soil mechanical characteristics and texture. We studied the influence of substrate and genotype on the ...initial development of mechanical traits, differences between depths, and changes over the course of two years in the field.
Methods
Plots were homogeneously filled with a loam and a sand and planted with two maize (
Zea mays
L.) genotypes (wild type (WT) and
rth3
mutant) with contrasting root hair attributes. Undisturbed soil cores were taken in 2019 and 2020 at 14 and 34 cm depth. Confined uniaxial compression tests were performed to determine pre-compression stress (σ
pc
), compressibility (C
c
, C
s
) and elasticity index (EI). Mechanical energy was calculated based on penetration resistance (PR) tests with a penetrometer needle resembling root geometries.
Results
σ
pc
, C
c
and C
s
were significantly higher in loam as compared to sand, whereas the factor genotype proved to be negligible. Over time, σ
pc
increased and C
c
decreased in loam from 2019 to 2020 and C
s
declined in both substrates. Higher mechanical energies were observed in loam and partially in WT. Required energy was higher at 14 cm than at 34 cm depth and decreased from 2019 to 2020 in sand. Airdry sand samples required four times as much energy than those at matric potential (Ψ
m
) of -50 kPa.
Conclusion
For the development of the mechanical traits examined texture proved to be the dominating factor and changes in soil stability could be observed within a short period of time.
Background Plant-soil interaction is central to human food production and ecosystem function. Thus, it is essential to not only understand, but also to develop predictive mathematical models which ...can be used to assess how climate and soil management practices will affect these interactions. Scope In this paper we review the current developments in structural and chemical imaging of rhizosphere processes within the context of multiscale mathematical image based modeling. We outline areas that need more research and areas which would benefit from more detailed understanding. Conclusions We conclude that the combination of structural and chemical imaging with modeling is an incredibly powerful tool which is fundamental for understanding how plant roots interact with soil. We emphasize the need for more researchers to be attracted to this area that is so fertile for future discoveries. Finally, model building must go hand in hand with experiments. In particular, there is a real need to integrate rhizosphere structural and chemical imaging with modeling for better understanding of the rhizosphere processes leading to models which explicitly account for pore scale processes.
► Restoration improved aggregate microstructure, SOM and aggregate water stability. ► Aggregates from restored land and bare land have different aggregation mechanisms. ► Aggregate microstructure ...showed multifractality at limited scales.
Vegetation restoration is expected to improve soil microstructure and therefore enhance soil stability and reduce soil erosion. The objective of this study was to evaluate the effect of long-term vegetation restoration on the modification of aggregate microstructure with synchrotron-based high resolution X-ray micro-computed tomography (SR-μCT). Triplicate aggregates (5-mm diameter) from (a) severely eroded bare land (BL) and (b) two decades of vegetation restoration land (RL) from Ultisol, Southern China, were collected and scanned with 9μm voxel-resolution at SSRF (Shanghai Synchrotron Radiation Facility). ImageJ software and multifractal theory were used to analyze and quantify aggregate pore structure. Aggregate water stability, mechanical stability, and basic soil properties were also evaluated. Results showed that aggregate water stability and SOM content significantly increased in the RL treatment, while aggregate mechanical stability showed an inverse trend. The microstructure of aggregates had evolved from a very dense massive microstructure to a more porous hierarchical microstructure after two decades of vegetation restoration. Porosity, macro-porosity, fraction of elongated pores, and specific surface area were significantly higher in the RL aggregates as compared to the BL aggregates. Multifractal scaling was observed for the pore structure of the studied aggregates. Generalized dimensions (Dq) were significantly higher in the RL treatment as compared to BL treatment, indicating improved pore system after vegetation restoration. This improved microstructure of RL aggregates was attributed to the increased SOM that prompted soil aggregation. This study showed the positive effects of vegetation restoration on soil microstructure and water stability, which was beneficial to the reduction of soil erosion and to the improvement of soil quality in this region.
Soil deformation is a perpetual process in the pedosphere where besides physicochemical stresses primarily alternating hydraulic and mechanical stresses continuously re-arrange the configuration of ...solid particles. In this study we present a local strain analysis and changes in soil structure resulting from hydraulic and mechanical stresses based on X-ray microtomography data. Digital image reconstructions were used to quantify local structural pore space characteristics and local soil deformation by 3D morphological and correlation analysis of grayscale tomograms. Swelling and shrinkage resulted in a complex heterogeneous soil structure which proofed to be very stable when mechanical loads were applied. The mechanism of soil deformation for both structure formation by internal hydraulic stresses and structure degradation by external mechanical stresses were in both cases controlled by pre-existing (micro)-structures. Especially during wetting such structures served as a nucleus for subsequent structure evolution. The results demonstrate the potential of more detailed non-invasive micromechanical analysis of soil deformation processes which could improve the conceptual understanding of the physical behavior of soil systems.
Spatial inaccessibility of soil organic carbon (SOC) for microbial decay within soil aggregates is an important stabilization mechanism. However, little is known about the stability of aggregates in ...semiarid grasslands and their sensitivity to intensive grazing. In this study, a combined approach using soil chemical and physical analytical methods was applied to investigate the effect of grazing and grazing exclusion on the amount and stability of soil aggregates and the associated physical protection of SOC. Topsoils from continuously grazed (CG) and ungrazed sites where grazing was excluded from 1979 onwards (UG79) were sampled for two steppe types in Inner Mongolia, northern China. All samples were analysed for basic soil properties and separated into free and aggregate‐occluded light fractions (fLF, oLF) and mineral‐associated fractions. Tensile strength of soil aggregates was measured by crushing tests. Undisturbed as well as artificially compacted samples, where aggregates were destroyed mechanically by compression, were incubated and the mineralization of SOC was measured. For undisturbed samples, the cumulative release of CO2‐C was greater for CG compared with UG79 for both steppe types. A considerably greater amount of oLF was found in UG79 than in CG soils, but the stabilities of 10–20‐mm aggregates were less for ungrazed sites. Compacted samples showed only a slightly larger carbon release with CG but a considerably enhanced mineralization with UG79. We assume that the continuous trampling of grazing animals together with a smaller input of organic matter leads to the formation of mechanically compacted stable ‘clods’, which do not provide an effective physical protection for SOC in the grazed plots. In UG79 sites, a greater input of organic matter acting as binding agents in combination with an exclusion of animal trampling enhances the formation of soil aggregates. Thus, grazing exclusion promotes the physical protection of SOC by increasing soil aggregation and is hence a management option to enhance the C sequestration potential of degraded steppe soils.
Pore network geometries of intra‐aggregate pore spaces are of great importance for water and ion flux rates controlling C sequestration and bioremediation. Advances in non‐invasive three‐dimensional ...imaging techniques such as synchrotron‐radiation‐based x‐ray microtomography (SR‐μCT), offer excellent opportunities to study the interrelationships between pore network geometry and physical processes at spatial resolutions of a few micrometers. In this paper we present quantitative three‐dimensional pore‐space geometry analyses of small scale (∼5 mm across) soil aggregates from different soil management systems (conventionally tilled vs. grassland). Reconstructed three‐dimensional microtomography images at approximate isotropic voxel resolutions between 3.2 and 5.4 μm were analyzed for pore‐space morphologies using a suite of image processing algorithms associated with the software published by Lindquist et al. Among the features quantified were pore‐size distributions (PSDs), throat‐area distributions, effective throat/pore radii ratios as well as frequency distributions of pore channel lengths, widths, and flow path tortuosities. We observed differences in storage and transport relevant pore‐space morphological features between the two aggregates. Nodal pore volumes and throat surface areas were significantly smaller for the conventionally tilled (Conv.T.) aggregate (mode ≈ 7.9 × 10−7 mm3/≈ 63 μm2) than for the grassland aggregate (mode ≈ 5.0 × 10−6 mm3/≈ 400 μm2), respectively. Path lengths were shorter for the Conv.T. aggregate (maximum lengths < 200 μm) compared with the grassland aggregate (maximum lengths > 600 μm). In summary, the soil aggregate from the Conv.T site showed more gas and water transport limiting micromorphological features compared with the aggregate from the grassland management system.
Ecological as well as economical consequences of mechanized harvesting procedures are of great importance in forestry, not only because of an intense increase in machine mass during tree cutting and ...transportation, but also because of a drastic increase in the stress application due to more pronounced vibration energy created from the harvesting machines themselves.
Working procedures of typical forest harvesting vehicles were analysed in southern Black Forest area, 30
km east of Freiburg/Breisgau, Germany. The field site location was situated on top of a hill at an altitude of 1000 m above sea level (a.s.l.). The parent material was acid gneiss. The soils are classified as Cambisols, Spodo-Dystric Cambisols (according to WRB, 1998). The actual soil stresses and displacements in soil profiles were analysed to give an overview on the range of possible effects of harvesting/forwarding systems with respect to the changes in soil physical properties of the forest soils. Soil stresses were determined by stress state transducer systems (SST) and displacement transducer system (DTS) at two depths: 20
cm and 40
cm. Complete harvesting and trunk logging processes with a total duration of 9
min were observed with a resolution of 20 readings per second.
The maximum vertical stresses for all experiments exceeded always 200
kPa and reached at the 20
cm depth for some vehicles and sequences of harvesting operations 500
kPa or more. Furthermore, it could be detected that the loading procedure itself doubled the short term mean normal stress as well as the octahedral shear forces and resulted in a more pronounced shear deformation and a corresponding homogenisation of the soil material.
To evaluate the impact of soil stresses on soil structure, the internal soil strength was determined by the precompression stress method. The precompression stress values of forest soils at the field sites ranged from 20 to 50
kPa at the 20
cm depth up to 25–60
kPa at the 40
cm depth at a pore water pressure of −60
hPa.
Generally, the data obtained for the measured soil stresses and the natural bearing capacity prove that sustainable wheeling is impossible, irrespective of the vehicle type and the working process. Top and subsoil compaction, an increase in precompression stress values in the various soil horizons, deep rut depth and vertical and horizontal soil displacement associated with shearing effects take place and affect the mechanical strength of forest soils.
In order to sustain the present “unwheeled” situation, no increase in mass but a decrease of the machine mass is required to prevent or at least to keep the compacted forest soils small. This can be achieved amongst others by a permanent skid trail system.
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