Soil Macropores Research Articles

Soil macropore networks derived from X-ray computed tomography in response to typical thaw slumps in Qinghai-Tibetan Plateau, China

Soil macropore networks derived from X-ray computed tomography in response to typical thaw slumps in Qinghai-Tibetan Plateau, China

Water balance for determination of excess water in soybean cultivated in lowland soils

The aim of this study was to derive a methodology for calculating a sequential water balance that accurately estimates the occurrence of excess water in soybeans cultivated in lowlands. We tested four calculation strategies of water balance associated with the simulation of soybean development, which differed on the calculation of rainfall and time of water drainage from the soil macropores. Data of volumetric moisture monitored in three soil layers throughout the soybean cycle in the 2014/15 agricultural year were used as a reference. Microporosity was used as a lower limit for the occurrence of excess water in the area. Excess water was considered to be whenever the daily volumetric soil moisture in the 0-100 mm layer was greater than 0.39 mm3 mm-3. Over the 111 days of measurement, soil moisture indicated the presence of excess water in 38 days. The traditional calculation strategy of water balance underestimated the occurrence of excess water, as well as the other strategies that considered effective precipitation in their formulas. The calculation strategy that considers that all the rainfall infiltrates in the soil and that the water from macropores is removed only by crop evapotranspiration exhibited good performance and indicated 35 days of excess water, being the most appropriate and recommended for determining excess water in lowland soybeans.

Soil infiltration characteristics and pore distribution under freezing–thawing conditions

Abstract. Frozen soil infiltration widely occurs in hydrological processes such as seasonal soil freezing and thawing, snowmelt infiltration, and runoff. Accurate measurement and simulation of parameters related to frozen soil infiltration processes are highly important for agricultural water management, environmental issues, and engineering problems in cold regions. Temperature changes cause soil pore size distribution variations and consequently dynamic infiltration capacity changes during different freeze–thaw periods. To better understand these complex processes and to reveal the freeze–thaw action effects on soil pore distribution and infiltration capacity, black soils, meadow soils, and chernozem were selected as test subjects. These soil types account for the largest arable land area in Heilongjiang Province, China. Laboratory tests of soils at different temperatures were conducted using a tension infiltrometer and ethylene glycol aqueous solution. The stable infiltration rate and hydraulic conductivity were measured, and the soil pore distribution was calculated. The results indicated that for the different soil types, macropores, which constituted approximately 0.1 % to 0.2 % of the soil volume under unfrozen conditions, contributed approximately 50 % of the saturated flow, and after soil freezing, the soil macropore proportion decreased to 0.05 % to 0.1 %, while the saturated flow proportion decreased to approximately 30 %. Soil moisture froze into ice crystals inside relatively large pores, resulting in numerous smaller-sized pores, which reduced the number of macropores but increased the number of smaller-sized mesopores, so that the frozen soil infiltration capacity was no longer solely dependent on the macropores. After the ice crystals had melted, more pores were formed within the soil, enhancing the soil permeability.

The effect of organic and conventional management practices on soil macropore structure in greenhouse vegetable production

AbstractGreenhouse vegetable production (GVP) is developing rapidly and farming practices strongly affect soil quality. However, quantitative evidence of the differences in soil macropore structure between organic and conventional greenhouse vegetable cultivation is still unavailable. In this study, we sampled paired organic and conventional greenhouse vegetable soils from the tilled and plough pan layers at sample sites south of Nanjing, China. Using industrial computed tomography (CT) scanning technology to measure soil macropore characteristics, we examined the effects of different farming practices on soil macropore structure, as well as the main factors that influenced soil macropore characteristics. Organic greenhouse vegetable (OGV) production was associated with a greater quantity and quality of soil macropores compared with conventional greenhouse vegetable (CGV) production. The total macroporosity in the tilled soil layer averaged 16.9% in OGV soils, more than twice that of CGV soils. OGV soils exhibited an improved pore size distribution with a greater relative abundance of transmission pores (50–500 μm) and a decreased relative abundance of large macropores (>1,000 μm). Pore size and shape data suggested that OGV soils had fewer soil cracks but more biopores than CGV soils in the tilled layer, as well as more root biopores in the plough pan layer. Furthermore, macropore morphology was improved in both the tilled and plough pan layers of OGV soils. In particular, macropore connectivity was nearly 10 times greater in the tilled layer of OGV soils. Soil organic matter (SOM) content was the dominant factor influencing most macropore characteristics (total macroporosity, connectivity, and relative porosity of 100–1,000 μm pores, all small macropores and elongated medium macropores). We suggest that greater SOM content and consequently better soil macropore structure in OGV soils was dependent not only on greater organic manure input, but also on the non‐application of chemical fertilizer.Highlights Farm management affects soil macropore structure in greenhouse vegetable production. Compared soil macropore structure in organic and conventional greenhouse vegetable cultivation. OGV had greater quantity and quality of soil macropores than CGV. Greater SOM in organic farming contributed to the better soil macropore structure.

Immediate effects of the application of various fungal strains with urea fertiliser on microbiome structure and functions and their relationships with the physicochemical parameters of two different soil types

The incessant increase in the demand for food from the world population, which is occurring due to its continuous growth has, as a consequence, led to an increase in the use of fertilisers. Our work aims to analyse the application effect of various fungal strains with urea fertiliser on the structure and functions of bacterial communities, including ammonia-oxidising microorganisms (AOM) in different soil types. By analysing the abundance of ammonia-oxidising archaea (AOA) and bacteria (AOB), we observed that the application of urea fertiliser (UC) changed the soil microbial community, causing a disturbance in soil microorganisms diversity, whereas the addition of fungal strains to urea fertiliser (UA100; UA60) did not have a disordering effect compared to the Control soil (C). This indicates that the addition of fungal strains mitigated the adverse effects of urea on the soil environment. A similar effect was observed for the genes involved in the carbon cycle (bacterial carbon fixation, chitinase, and methanotrophic pathway genes). It was observed that the addition of fungi to the urea fertiliser had a positive effect, as the abundance of functional genes recorded in these treatments was the same as in the Control soil. From the diversity point of view, it is worth emphasizing that the diversity index changed after fungal organisms' addition. We may also observe that the addition of fungal strains to urea protected or even improved the state of the soil macropores which are important for microbial communication in the soil environment. Furthermore, it may be concluded that to avoid a strong negative effect from the application of urea and the mitigation of its influence by fungal strains, we must consider the soil texture, pH, total carbon content, and soil organic matter including humic and fulvic acids.

Spatially-distributed microbial enzyme activities at intact, coated macropore surfaces in Luvisol Bt-horizons

Soil macropores serve as preferential pathways for water and solute transport as well as for root growth. They are often coated with organic material and known as "hotspots" of nutrient and C turnover. Differences in the SOM composition between macropores and soil matrix as well as between macropore types (biopores, cracks, pinhole fillings) imply potential differences in the microbial community composition and enzymatic activities. The objective of this work was to detect and assess the spatial distribution of enzyme activities related to C turnover, xylanase (XYL) and phenol oxidase (POX), and the composition of microbial communities in structural components of Luvisol Bt-horizons, developed from loess and glacial till. We applied conventional enzyme assays and phospholipid fatty acids (PLFA) analysis to study materials separated from different types of macropores and soil components as well as bulk soil samples. The spatial distribution of enzyme activities on surfaces of large soil core slices (20 cm in diameter) was quantified by soil zymography. Higher XYL activities were detected in separated burrow wall materials, clay-organic coatings, and pinhole fillings from both sites, as compared to the respective soil matrix or bulk soil samples. The XYL activities correlated with bacteria-specific PLFAs. POX activities were solely found increased for earthworm burrow walls from the loess-derived Bt-horizon, but not for burrows from till samples. Zymograms revealed particularly increased XYL activities at rooted earthworm burrows, emphasising evidence for hotspots of enzyme activity and C turnover. The zymography of POX was hampered by methodological restrictions.

Characterization of macropore structure of remolded loess and analysis of hydraulic conductivity anisotropy using X-ray computed tomography technology

The pore structure of loess is crucial for understanding the transport of water in soil and affects soil permeability. In this study, X-ray computed tomography (X-ray CT) technology was used to establish the macropore structure of remolded loess, and analyze the influence of macropore on permeability. A new specific and efficient workflow for constructing macropore structure of loess samples was established. The workflow improves the precision of image segmentation and provides a new method for creating 3D soil macropore structure. The macropore structure showed that the macro-porosity variation of loess sample was clearly anisotropic. Based on the macropore structure model, a modified Kozeny–Carman equation was suggested to investigate the hydraulic conductivity anisotropy of the loess sample. Subsequently, permeability test was used to verify the results calculated by equation. Both the results showed that hydraulic conductivity was positively correlated with the uniformity of the porosity changes in each direction. It provides a new method to estimate the anisotropy of hydraulic conductivity based on macropore structure model.

Can yield, soil C and aggregation be improved under long-term conservation agriculture in the eastern Indo-Gangetic plain of India?

Deteriorating soil health, diminishing soil organic carbon (SOC), development of subsurface hard compact layer and declining system productivity are barriers to achieving sustainable production in the traditional rice–wheat cropping system (TA) in the eastern Indo‐Gangetic Plain of India. Conservation agriculture (CA), which favours minimum soil disturbance, crop residue retention and crop diversification could be a viable alternative to the TA to address most of those major problems. With that in mind, a long‐term experiment is being implemented at ICAR‐RCER, Patna, Bihar, India, with four treatments: (a) TA, (b) full CA (fCA) and (c and d) partial CA (pCA1 and pCA2), differing in crop establishment methods, cropping system and crop residue management in a randomized complete block design. Measurement of soil health parameters was carried out in the 11th year of the experiment. The results revealed a beneficial effect of CA and 46 and 40% increase in SOC concentration and stock, respectively, under fCA over TA in the 0–7.5‐cm soil layer. The effect of partial CA (pCA1 and pCA2) was variable, but an increasing trend was always observed under pCA compared to TA. There was an enrichment in SOC content of aggregates under CA irrespective of size class; however, no relation was found between SOC content and aggregate diameter. The contribution of macroaggregates to SOC stock was larger (36–66%) under CA in the 0–7.5‐cm soil layer. Adoption of CA improved the macroaggregate content, MWD and GMD of aggregates, and aggregation ratio. Soil macropore content was greater under fCA, whereas other parameters were similar among treatments. The impact of CA was mostly limited to 0–7.5 cm soil layer and a maximum up to 15 cm soil depth while evaluation until 60 cm soil depth was realized. The yield of rice in CA was comparable to or higher than in TA, whereas the system rice equivalent yield was always higher (38–53%) under CA than under the conventional practices. Therefore, a CA‐based cropping system must be encouraged, to increase SOC status, improve aggregation stability and, consequently, sustain or increase system productivity, in order to achieve food and nutritional security in the eastern Indo‐Gangetic Plain of India.Highlights Effects of long‐term conservation agriculture (CA) on soil C, aggregation and yield were evaluated.CA improved SOC concentration and stock by 46 and 40%, as well as macroaggregate SOC stock by 36–66%.Macro‐aggregation and mean weight diameter improved in CA but was mostly limited to a shallow soil depth.CA can be promoted for sustainability of a rice–wheat system due to higher productivity (38–53%).

Earthworm burrow number and vertical distribution are affected by the crop sequence of a grass-clover rotation system

Grass-clover is an important element in crop rotations due to its beneficial agronomic properties including nitrogen build-up, stimulation of earthworms and their burrowing and maintenance of soil macropores. We studied the relationship between important elements of grass-clover management in a crop rotation and earthworm diversity and macropore depth distribution, diameter and density from the ploughing layer to a depth of 1 m. Treatments included winter rye, 1st year cutting + slurry, 3rd year cattle grazing + slurry, 3rd year cutting + slurry and 3rd year cutting without slurry. The four-species earthworm assemblages and their burrow spatial structure responded distinctly to the temporal sequence of farming system management elements. The relative density of the endo-anecic and epi-anecic earthworm species, Aporrectodea longa and Lumbricus herculeus, was higher in the grass-clover phase of the rotation than under the annual crop, and increased with the age of grass-clover. The same was true for Aporrectodea rosea, but density and biomass of Aporrectodea caliginosa showed the opposite trend. Cattle grazing favours the occurrence of burrows (>5 mm ⌀) made by the anecics, A. longa and L. herculeus, while otherwise decreasing the number of fine-medium macropores (<5 mm ⌀) below the topsoil. Our study reveals the intimate interdependence between the cropping sequence and the earthworm burrow structure with low number of detectable burrows under annual crops, then increasing in number during the grass-clover over 3 years. The highest number of burrows was found at 0.5 m depth in 3rd year cutting ± slurry, presumably because there is no soil disturbance by tillage in this horizon. The density of the endogeic A. caliginosa was negatively correlated with the presence of burrows and sensitive to grazing cattle compacting the topsoil. Positive correlations between burrows and density of L. herculeus indicate that this species was responsible for the burrows at one m depth while the density of A. longa was positively correlated with the burrows in the Ap horizon. We discuss the species composition in the grass-clover rotation system and its influence on soil ecosystem functioning.

Effects of earthworm (Metaphire guillelmi) density on soil macropore and soil water content in typical Anthrosol soil

The burrowing activities of earthworms can change the soil macropore structure in terrestrial ecosystems. Studying the burrows structure and their effects on soil water movement may be crucial for exploring earthworm ecological functions and improving water use efficiency in farmlands in semi-humid areas. It was an experiment taken to assess the impact of anecic earthworm Metaphire guillelmi on the characteristics of soil macropore and soil water content (SWC). Earthworms with different densities (0, 3 and 10 in. in each column) were introduced in soil columns (height 38 cm, diameter 23 cm) for 33 days. The macropore characteristics were determined by X-ray computed tomography (CT). Results showed that increasing earthworm density increased the macroporosity and macropore number. The macropore number in the high-density earthworm treatment (HDE) was 1.3 times larger than that in columns with low-density earthworm (LDE). The diameter of soil macropores ranged from 2.12 mm to 7.30 mm. The mean macroporosities in HDE and LDE treatments were 1.35 % and 0.61 %, respectively. The soil pore morphology (mean macropore circularity, branch, junction and connectivity) was also improved by earthworms. The high number of macropores (45.30/N) and connectivity (0.05 cm−3) were observed in the column with high density of earthworms. Earthworms improved the soil macropores and macroporosity. The SWC linearly decreased with increasing earthworm density. A significant difference (P < 0.05) of SWC was observed between columns with earthworm (LDE and HDE) and those without earthworm (CK), especially at the 10–30 cm soil depth. The burrows generally open to the soil surface and increased water vapour transfer channels, thus accelerated soil evaporation. In areas with uneven rainfall, high earthworm density could aggravate the risk of seasonal drought at the plough layer, which is an important threaten for crop growth and yield.

Prediction of the Slope Solute Loss Based on BP Neural Network

The existence of soil macropores is a common phenomenon. Due to the existence of soil macropores, the amount of solute loss carried by water is deeply modified, which affects watershed hydrologic response. In this study, a new improved BP (Back Propagation) neural network method, using Levenberg–Marquand training algorithm, was used to analyze the solute loss on slopes taking into account the soil macropores. The rainfall intensity, duration, the slope, the characteristic scale of macropores and the adsorption coefficient of ions, are used as the variables of network input layer. The network middle layer is used as hidden layer, the number of hidden nodes is five, and a tangent transfer function is used as its neurons transfer function. The cumulative solute loss on the slope is used as the variable of network output layer. A linear transfer function is used as its neurons transfer function. Artificial rainfall simulation experiments are conducted in indoor experimental tanks in order to verify this model. The error analysis and the performance comparison between the proposed method and traditional gradient descent method are done. The results show that the convergence rate and the prediction accuracy of the proposed method are obviously higher than that of traditional gradient descent method. In addition, using the experimental data, the influence of soil macropores on slope solute loss has been further confirmed before the simulation.

Evaluation of hydromechanical behavior and macro-structure of an expansive soil in the city of Paulista-PE Brazil

The characterization of the structural arrangement of an unsaturated soil is important for the understanding of its behavior. For expansive soils, obtaining this information and combining it with an understanding of their intrinsic and extrinsic properties, makes it possible to predict their performance. The experimental procedure for this study was developed using the expansive soil of the Maria Farinha formation in the city of Paulista, in Pernambuco, Brazil. Physical and chemical characterization tests, edometric tests with wetting, and structural analysis through X-Ray computed tomography were performed for both undisturbed and compacted samples, before and after wetting and swelling. The compaction process was performed without any prior air drying and without loosening, beginning with the field moisture level. Based on its liquid limit and plasticity index, the soil can be classified as CH according to the USCS, moderately acidic, with a swelling potential ranging from high to very high, depending on the initial suction of the sample. The analysis of the soil macro-structure revealed a smaller number of voids in the compacted samples. In addition, it was found that the wetting process caused a reduction of the soil macro pores, in both cases.

Model representation and quantitative analysis of pore three‐dimensional morphological structure based on soil computed tomography images

AbstractComputed tomography technology has contributed greatly to the visualization of soil internal structure. Complex topological structure and spatial distribution of pores have become a focal area for studying the spatiotemporal evolution of soil structure. This is significant for understanding soil functions and ecological processes. Image‐processing software, such as ImageJ and Image Pro‐Plus, is widely used to visualize and quantify soil pores. However, it does not account for the irregularity and heterogeneity of pore structure, leading to low segmentation accuracy and inaccurate description of pore morphology. Therefore, we used the simplified convolutional network (SCN) method to segment the pore structure and proposed a parallel thinning method to extract the pore skeleton structure. Thereafter, using three‐dimensional modelling and quantitative analysis, we initially examined the spatiotemporal evolution of soil pores during freeze–thaw cycles. Based on the custom rule model and soil pore real model, we evaluated the performance of the distance transform method and parallel thinning method in extracting the pore skeleton structure. The parallel thinning method more effectively described the topological characteristics of pores, and the pore skeleton model had better centrality, refinement and connectivity. Quantitative analysis of pore characteristics illustrates that, although freeze–thaw cycles can affect pore structure at different scales, they do not regularly affect pore macroporosity and the three‐dimensional fractal dimension. These results illustrate that the soil pore structure of the viscous layer is sensitive to freeze–thaw cycles and provides preliminary evidence that seasonal freeze–thaw cycles can cause a soil water retention effect and weaken resistance to erosion. This study presents a novel technique for examining soil function at the pore scale and a theoretical foundation for future research on soil structure and hydrology.Highlights Freeze–thaw cycles have important effects on soil pore characteristics and thus on soil ecology. We provide novel evidence of pore structural changes in response to freeze–thaw cycles. Soil macropore skeleton was extracted using the parallel thinning method. Seasonal freeze–thaw cycles reconfigured the soil macropore model.

Bio-improved hydraulic properties of sand treated by soybean urease induced carbonate precipitation and its application Part 2: Sand-geotextile capillary barrier effect

Nonwoven-geotextile is used to construct soil-geotextile capillary barrier for minimizing water infiltration. However, the sand-geotextile capillary barrier breakthrough can occur due to the low water retention ability and the high saturated permeability coefficient of sand. To address this, enzyme induced carbonate precipitation (EICP) technology was applied to the overlying sand layer to improve its hydraulic properties and enhance the sand-geotextile capillary barrier. The precipitated calcium carbonate has the potential to enhance the capillary barrier effect by coating soil particles and filling the soil macro-pores. To investigate the effect of EICP treatment on the sand-geotextile capillary barrier, three column tests were conducted, including one untreated sand-geotextile column and two sand-geotextile columns treated with 0.5 and 1 mol/L urea-calcium chloride solutions and soybean urease liquid, respectively. Transient seepage analysis was carried out based on the column tests to calibrate the hydraulic parameters. A numerical parametric analysis was conducted to assess the effect of EICP-enhancement on the hydraulic behavior and slope stability of a sand-geotextile capillary barrier retaining wall during exposure to rainfall with a 100-year return period. The results of the column infiltration tests show that the occurrence of breakthrough in the sand-geotextile capillary barrier is significantly delayed with increasing concentration of the urea-calcium chloride solutions. The enhanced capillary barrier effect is attributed to the increased water retention ability and reduced saturated permeability coefficient. The results of the parametric analysis show that the matric suction within the backfill is maintained and the factor of safety remains almost unchanged due to the EICP-enhanced sand-geotextile capillary barrier effect.

Contribution of soil macropores to water infiltration across different land use types in a desert–oasis ecoregion

AbstractSurface soil hydraulic properties play critical roles in controlling water infiltration, evaporation, and soil water storage, in‐turn affecting the evolution of soil pore structure in cultivated desert soils of arid areas. The variability in soil pores and hydraulic properties caused by conversion of native semidesert soils to agroforestry use alters soil water storage and water budget in the desert–oasis ecotone. The objective of this study was to investigate that land use conversion alters surface soil hydraulic properties and enhances the contribution of soil macropores to water flow in the Linze desert–oasis ecoregion. Water infiltration and soil pores characteristics with nine replicates were measured using a disc tension infiltrometer across four habitat types (Haloxylon ammodendron shrublands, Populus gansuensis forests, Medicago sativa Linn grasslands, and Zea mays croplands) during the growing season. Soil properties and hydraulic conductivity characteristics clearly varied across the four habitat types. Saturated hydraulic conductivity was 0.177, 0.126, 0.118, and 0.031cm min−1 in the forest, grassland, shrubland, and cropland, respectively, with the corresponding average soil moisture being 7.6, 6.7, 0.9, and 19.2%. Clay content, bulk density, and initial soil moisture were the key soil physicochemical properties affecting saturated hydraulic conductivity (Ks). The macropores (&gt; 0.5 mm in radius) accounted for 15–60% of total water flow, while 0.25–0.5 mm pores contributed to 12–24% across the four habitat types. The contribution of &gt;0.5 mm pores to water flow in the forest was 3.3–4.0‐times greater than in shrubland and cropland, due to greater soil macroporosity (58 cm−3 cm3 ) and undisturbed soil pore structure. Land use conversion has a positive impact on soil properties, structure features, and soil hydraulic properties, and as a consequence, enhance the complexity of water exchange in the arid areas.

Permeability and setting time of bio-mediated soil under various medium concentrations

The bio-clogging using bacteria can be an eco-friendly and sustainable alternative to conventional grouting methods for seepage control. However, it remains unclear to date how the dilute concentration of bacterium and medium during field installation can affect the setting time of bacterium and its correlation with permeability reduction. In this study, the setting time of bacterium and its effectiveness in permeability reduction were addressed through experimental and theoretical investigations. A series of sand column was cultivated using different concentrations of Leuconostoc mesenteroides and culture medium. The distribution and composition of the bacterial product (i.e. dextran) were observed by refractometer, scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDS). Soil permeability was recorded using a constant head test. The results revealed that bacterium was effective to produce dextran at the setting time of about 5 d after installation. This dextran can reduce the permeability of bio-mediated soil by two orders of magnitude, even without culture medium supply. In general, the dextran production decreased proportionally with increase of bacterium and medium concentration. However, at 50% bacterium and medium concentration by weight, it still has a significant influence on permeability reduction with similar setting time, compared to 100% concentration. • Bacteria became effective to produce dextran with setting time after 5 d of installation. • The bacteria and medium concentration seems to have little influence on the setting time. • Dextran can reduce the permeability of bio-mediated soil by two orders of magnitude, even without culture medium supply. • With 50% of bacteria and medium concentration, the dextran can be accumulated in soil macro-pores and thus remain effective to reduce permeability.

Freeze-thaw impact on macropore structure of clay by 3D X-ray computed tomography

The change of soil macropores, especially fissures, due to freeze-thaw (F-T) has important implications for soil engineering properties such as hydraulic conductivity and compressibility. This paper describes a quantitative study of the soil macropore changes due to F-T by using three-dimensional (3D) X-ray Computed Tomography (CT). Saturated lean clay specimens were subjected to unidirectional F-T under different water-supply conditions for acquiring 3D X-ray CT images before and after F-T. The CT imagery data were filtered and processed for obtaining parameters such as transverse-sectional macro-porosity, macropore size, transverse-sectional fissure porosity, and longitudinal-sectional fissure orientation frequency distribution. It was found that F-T alters the transverse-sectional macro-porosity and macropore size significantly, the changes are different across the specimen height. Moreover, the changes in the transverse-sectional fissure porosity and longitudinal-sectional fissure orientation frequency distribution vary along with the specimen height. More horizontal fissures form in the unfrozen zone or near the freezing front, depending on water-supply conditions. Pore water pressure data confirm that, besides water migration, soil particles also migrate from the unfrozen zone to the freezing front during freezing with water-supply due to piping. The macropore structural changes help understand the F-T impact on soil engineering properties such as hydraulic permeability and compressibility.

Quantifying hydro-sedimentary transfers in a lowland tile-drained agricultural catchment

Soil erosion, runoff and sediment connectivity are strongly impacted by anthropogenic features in lowland agricultural catchments. Among these landscape features, the role played by tile drainage on water and sediment transfers and hillslope-to-river connectivity in drained catchments remains poorly understood. This study quantified water and sediment transfers in a tile drained catchment of central France by combining high frequency rainfall, discharge and sediment concentration measurements at the outlet of a set of 10 tile drained plots (34 ha) and at the medium-sized (120 km2) catchment scale. Over the monitoring period, including a dry and a wet year compared to average conditions (one year with 112% of the mean annual rainfall and one year with 64% of the mean annual rainfall), 36 rainfall-flood events were recorded and analyzed. The high frequency analysis of water and sediment transfers in tile-drained plots showed a high seasonal variability and the occurrence of two transfer pathways in the soil column including the slow drainage of saturated soils and the occurrence of preferential flow pathways through the soil column. Indeed, 13 of the 36 recorded flood events showed hydrographs with two components, reflecting these two pathways: slow transfers in the soil columns and fast transfers through soil macropores and/or cracks. Indeed, at the beginning of the flood event, a high-magnitude peak overlaid on the hydrograph. On average, this fast peak contributed 15% of the water and sediment fluxes. The sediment dynamics in tile drains was suggested to depend on sediment storage and exhaustion processes occurring in the tile drain network.

Enhancing the Noah‐MP Ecosystem Response to Droughts With an Explicit Representation of Plant Water Storage Supplied by Dynamic Root Water Uptake

AbstractPlants are able to adapt to changing environments and thus survive droughts. However, most land surface models produce unrealistically low ecosystem resiliency to droughts, degrading the credibility of the model‐predicted ecohydrological responses to climate change. We aim to enhance the Noah‐MP modeled ecosystem resilience to droughts with an explicit representation of plant water storage supplied by dynamic root water uptake through hydrotropic root growth to meet the transpiration demand. The new model represents plant stomatal water stress factor as a function of the plant water storage and relates the rate of root water uptake to the profile of model‐predicted root surface area. Through optimization of major leaf, root, and soil parameters, the new model improves the prediction of leaf area index, ecosystem productivity, evapotranspiration, and terrestrial water storage variations over most basins in the contiguous United States. Sensitivity experiments suggest that both dynamic root water uptake and groundwater capillary rise extend the plants' “memory” of antecedent rainfall. The modeled plants enhance their efficiency to use antecedent rain water stored in shallow soils mainly through more efficient root water uptake over the U.S. Southwest drylands while use that stored in deep soils and aquifers with the aid of groundwater capillary rise in the Central United States. Future plant hydraulic models should not ignore soil water retention model uncertainties and the soil macropore effects on soil water potential and infiltration.

The interaction between wheat roots and soil pores in structured field soil.

Wheat (Triticum aestivum L.) root growth in the subsoil is usually constrained by soil strength, although roots can use macropores to elongate to deeper layers. The quantitative relationship between the elongation of wheat roots and the soil pore system, however, is still to be determined. We studied the depth distribution of roots of six wheat varieties and explored their relationship with soil macroporosity from samples with the field structure preserved. Undisturbed soil cores (to a depth of 100 cm) were collected from the field and then non-destructively imaged using X-ray computed tomography (at a spatial resolution of 90 µm) to quantify soil macropore structure and root number density (the number of roots cm–2 within a horizontal cross-section of a soil core). Soil macroporosity changed significantly with depth but not between the different wheat lines. There was no significant difference in root number density between wheat varieties. In the subsoil, wheat roots used macropores, especially biopores (i.e. former root or earthworm channels) to grow into deeper layers. Soil macroporosity explained 59% of the variance in root number density. Our data suggested that the development of the wheat root system in the field was more affected by the soil macropore system than by genotype. On this basis, management practices which enhance the porosity of the subsoil may therefore be an effective strategy to improve deep rooting of wheat.