Soil Macropore Characteristics Research Articles

Preferential flow of phosphorus and nitrogen under steady‐state saturated conditions

AbstractRepeated broiler litter application on agricultural lands can cause nutrient enrichment of subsurface effluent, especially with the existence of preferential flow through soil macropores. Previous studies quantifying soil macropores have not attempted to establish a connection of soil macropore characteristics with the subsurface nutrient (nitrogen [N] and phosphorus [P]) losses, across different topographical locations in the field. This study investigated the effect of broiler litter application and preferential flow on subsurface nutrient transport (N and P) at different topographical positions (upslope, midslope, and downslope) in a no‐till pasture field located in Alabama, USA. Twelve intact soil columns (150 mm id and 500 mm length) were used, and the nutrient leaching measurements from laboratory experiments were linked to soil macropore characteristics quantified using X‐ray computed tomography image analysis and solute transport modeling. Treatments included surface broadcast broiler litter (5 Mg ha−1, on dry basis) and unamended control. Leachates were analyzed for dissolved reactive P (DRP), total P (TP), and nitrate + nitrite‐N (NO3− + NO2−–N). The bromide breakthrough curves provided evidence of preferential flow in all columns. Litter application significantly increased leachate P concentrations, and average TP and DRP concentrations were significantly higher in the leachate from upslope columns compared to those at downslope location. The NO3−–N concentrations in leachate exceeded the US EPA drinking water standard of 10 mg L−1 in all the treatment columns. The highest flow‐weighted mean concentrations of TP and DRP, at 2.7 and 2.5 mg L−1, respectively, were recorded in the upslope columns. Soil physicochemical properties and nutrient leaching losses varied substantially across topographical positions, indicating a need for variable litter application rates to reduce P build‐up and subsequent leaching in vulnerable locations within the field. The relevance of the effect of topographic position on nutrient leaching found in this study should be further tested by investigating a wider range of slopes and soil types in pastures.

Appropriateness of introducing earthworms into sustainable agriculture from the perspective of soil carbon emissions

Sustainable agriculture, as an effective method for reducing the risk of soil degradation, is often accompanied by increases in soil fauna populations. However, the effects of soil fauna (such as earthworms) bioturbation on soil carbon (C) emissions remain unclear. We conducted a 31-day incubation experiment to measure the effects of adding straw (1 g) and anecic earthworms on the burrowing characteristics and C emissions using two soils collected from the plow layer and plow pan layers. We also explored the effects of the soil C source (soil organic C (SOC) and exogenous organic matter (OM) inputs) on the soil C emissions induced in the presence of earthworms by meta-analysis. The incubation experiment showed that earthworms increased the loss rate of surface straw (49.4–96.8%; p < 0.05) and the soil C emissions (160.5–455.4%; p < 0.01). Earthworms significantly increased the macroporosity (p < 0.001) and macropore size (p = 0.016), but decreased the macropore number in the plow layer soil. While, only the soil macropore size increased significantly (p = 0.025) under earthworm addition in the plow pan soil. Structural equation models indicated that straw directly influenced the soil macropore size (p = 0.022) and C emissions (p = 0.039) in the plow layer soil, but had no effects on the soil macropore characteristics and C emissions (p > 0.05) in the plow pan soil. Earthworms indirectly affected soil C emissions through effects on the soil macropore size (p < 0.05). Meta-analysis indicated that although earthworms increased C emissions by 51.7% and 25.0% without and with OM inputs (p > 0.224), respectively, the earthworm-induced effects on soil C emissions were not related to OM inputs (p = 0.337). Soil C emissions induced by earthworms decreased as the SOC content increased (from 45.1% to 2.6%), but these effects were not significant at any SOC level (p > 0.154). These findings indicate that the effects of anecic earthworms on soil C emissions can be reflected by the earthworm burrowing characteristics and earthworms increased the soil C emissions in the short term (incubation experiment), but their effects on soil C emissions eventually became non-significant due to reduced anecic-earthworm burrowing activities (most burrows are permanent or semi-permanent) and SOC content increase in the long term (meta-analysis). We therefore recommend that earthworm regulation together with OM inputs could be considered in sustainable agriculture.

Long‐term combined subsoiling and straw mulching conserves water and improves agricultural soil properties

AbstractSubsoiling and straw mulching are two techniques that conserve soil water and improve the sustainability of agricultural production. However, the benefits to soil sustainability of combining subsoiling with straw mulching under intensive rotation between wheat and maize remain uncertain. We conducted a field experiment to determine the long‐term impacts of conventional tillage with straw mulching (CS), and subsoiling with straw mulching (SS), on soil macropore characteristics (&gt;160 μm), soil aggregate characteristics, and soil hydraulic parameters at 0–100 cm depths. Results indicate that SS increased the mean macropore number (66.9%), macroporosity (93.5%), pore circularity (3.5%), field moisture capacity (11.8%), saturated moisture content (21.4%), available soil moisture content (24.1%), and the saturated hydraulic conductivity (39.3%) in the shallowest 50 cm of soil, and total organic carbon content (34.5%) and soil labile organic carbon (20.2%) in the shallowest 30 cm of soil, compared to CS. Compared with CS, SS was more effective at increasing the proportion of aggregates larger than 0.25 mm in the top 20 cm of soil and decreasing the proportion of aggregates smaller than 0.25 mm in the top 30 cm of soil. Correlations between soil organic carbon and various soil physical properties under different practices suggest that SS has larger causative effects on soil physical properties than CS. Therefore, SS is recommended over CS.

Soil Macropore and Hydraulic Conductivity Dynamics of Different Land Uses in the Dry–Hot Valley Region of China

Soil macropores and hydraulic conductivity are important indexes used to describe soil hydrology. In the dry-hot valley region of Southwest China, with its dramatic seasonal dry–wet rhythm, soil properties and hydraulic conductivity can reflect unique dynamics as determined by the interaction between land use and the seasonal dry–wet cycle. In this study, the soil macropore characteristics and hydraulic conductivity of five land uses (traditional corn, plum orchard, pine forest, grassland, and abandoned cropland) in a dry–hot valley region were quantified using X-ray computed tomography (CT) and a mini disk infiltrometer in the rainy season (July) and dry season (November), respectively. The results showed that the soil macropore indexes (soil macroporosity, mean diameter of macropores, connectivity, hydraulic radius and compactness) in the rainy season were, on average, 1.26 times higher than those in the dry season. Correspondingly, the hydraulic conductivity of different land uses in the rainy season was significantly higher than those in the dry season (2.10 times higher, on average). Correlation analysis and principal component analysis (PCA) indicated that the hydraulic conductivity was mainly determined by soil macropore parameters rather than by general soil properties, such as organic matter (OM) and bulk density (BD). The hydraulic conductivity for the five land uses followed the order of PF &gt; GL &gt; TC &gt; PO &gt; AC in both the rainy and the dry seasons. This ranking order reflects the protective effect of vegetation in reducing raindrop splash and soil crust formation processes. The above results can help guide soil water conservation and vegetation restoration in the dry-hot valley region of Southwest China.

Characteristics of the soil macropore and root architecture of alpine meadows during the seasonal freezing-thawing process and their impact on water transport in the Qinghai Lake watershed, northeastern Qinghai–Tibet Plateau

Low temperatures, freezing-thawing cycles, and short growing seasons characterize alpine soils. The mattic epipedon, a special diagnostic surface horizon with an intensive root network, is widely distributed in alpine ecosystems. Studies on the soil macropores and roots of the mattic epipedon layer in response to seasonal freezing-thawing processes on the Qinghai–Tibet Plateau are lacking. This study characterized the soil macropores and roots of alpine meadows during the seasonal freezing-thawing process using X-ray computed tomography and revealed the influence of soil macropores and roots structure on water transport in the mattic epipedon layer of the alpine meadows. The results showed that the soil pore distribution was more uniform during the unstable freezing stage (UFP) and the unstable thawing stage (UTP), whereas there was a clear mattic epipedon layer during the completely thawed stage (TP) and the completely frozen stage (FP). Soils in the TP stage had a higher total surface area density (0.1898 mm² mm⁻³), length density (225.28 mm mm⁻³), node density (1,592 no. mm⁻³), and connectivity (0.3144) of soil macropores than those in the UFP, UTP, and FP stages. In the TP stages, the density, surface area density, branch density, length density, and node density of roots had significant correlations with the macroporosity, surface area density, length density, node density, and tortuosity of soil macropores. In the FP stage, there were no correlations between the root and soil macropore characteristics. Vertical water is expected to move more readily through the mattic epipedon in the TP stage than in the UFP and UTP stages. Roots were the preferential pathways for water transport into the soil layer of the alpine meadow. Therefore, the mattic epipedon is a key layer for water and nutrient storage and plays an important role in the water-holding function of the Tibetan Plateau due to its greater root development.

Soil macropore characteristics and aggregate stability with poly‐γ‐glutamic acid amendment under wetting–drying cycles

AbstractSoils are typically subjected to multiple wetting–drying (WD) cycles due to irrigation and seasonal climate cycles, which directly impact soil pore structure and soil aggregate stability. Poly‐γ‐glutamic acid (γ‐PGA) is a polymer used to improve soil water holding capacity and plant growth. However, the impact of γ‐PGA on soil pore structure requires further research, particularly under WD cycles. Therefore, we investigated the different amounts of γ‐PGA on soil structure, including soil aggregate stability, macropore (&gt;100 μm) structure characteristics and the relationship between macropore characteristics (equivalent pore diameter, pore shape factor, soil porosity, fractal dimension (FD), soil connectivity and the percentage of aggregate content with particle size larger than 0.25 mm) and soil aggregate stability by structural equation modelling (SEM) under WD cycles. A sandy soil and a loam soil were studied, and amended with γ‐PGA at three different concentrations: 0 (P0), 0.4% (P4) and 0.8% (P8) (w/w, %). Results showed that γ‐PGA amendment increased the mean weight diameter (MWD) and the percentage of aggregate content with particle size larger than 0.25 mm (R0.25) under WD cycles in both sandy and loam soils, while the MWD between P4 and P8 did not differ significantly. As the number of WD cycles increased, soil porosity (TP) increased due to an increase in pores of 100–500 μm. With γ‐PGA added to soil, large microporosity (&gt;1000 μm) increased in sandy soil, but decreased in loam soil. In addition, 8WD cycles also increased the FD (2.6%–4.2%) and pore connectivity (Con) compared with 4WD. Structural equation modelling (SEM) revealed that soil pore characteristics accounted for 74% and 98% of the variation in sandy and loam soils, respectively. TP, FD, Con and R0.25 directly contributed to MWD, according to the SEM. These findings improve our understanding of pore characteristics and aggregate stability, which are key factors influencing soil quality when amended with γ‐PGA during the seasonal WD period.

Effect of soil compaction on hydraulic properties and macropore structure: Evidence from opencast mines in the Loess Plateau of China

Affected by heavy machinery, the soil compaction in the opencast coal mine area is serious, which will change the characteristics of soil macropores, then affect the soil hydraulic properties and damage the ecological environment in mining area. In this study, the disturbed soil samples from the dumping area of Pingshuo opencast coal mine of China were collected. The soil samples were processed in bulk density from 1.30 g·cm−3 to 1.70 g·cm−3, and the soil hydraulic properties were measured, and the soil macropore characteristics were obtained using computerized tomography scanning in laboratory. The path analysis method was applied to analyze the effect of macropore characteristics caused by compaction on soil hydraulic properties. Soil compaction resulted in severe effects on soil hydraulic characteristics and macropore properties. Under same soil suction, the water contents remained in soil decreased with increasing compaction. The explanatory variables (average Euler number (AEN), macropore throat number (MTN), and macropore fractal dimension (MFD)) were main determinants of soil-water characteristic curve (SWCC), and the indirect path coefficient of macroporosity. The effects of AEN and MFD on soil infiltration properties were relatively high. The bulk density of 1.4 g·cm−3 was a critical point of affecting macropore properties then affecting soil hydraulic properties, and some soil reclamation measures should be conducted to make bulk density < 1.4 g·cm−3 for compacted soil in opencast coal mining area. This study provides a basis for selecting technology of land reclamation and the subsequent monitoring and maintenance of soil quality in mining areas.

Distinct effects of biochar addition on soil macropore characteristics at different depths in a double-rice paddy field.

Soil macropores largely control the water and nutrients transport as well as runoff processes in the soil. Biochar is frequently applied to soils to improve the macropore structure, but the effects remain controversial. To clarify depth-dependent soil macropore characteristics affected by biochar addition, the intact soil cores with a depth of 200 mm were collected from biochar-amended paddy field at addition rates of 0, 24, and 48 t ha-1 (CK, BC1, and BC2, respectively). The two biochar treatments did not change the overall soil pore indices (e.g., macroporosity, pore number, fractal dimension, and circularity), but showed distinct effects at different soil depths. At a soil depth of 0-50 mm, the biochar treatments had higher macroporosity (8.59-8.85 %) than CK (4.94 %) (p < 0.05), but relatively lower pore circularity (0.83-0.84) than CK (0.88) (p < 0.05). The connectivity of biochar treatments (88-97) was 9.5-10.4 times higher than that of CK (9.3). At a soil depth of 100-200 mm, the biochar treatments exhibited lower macroporosity, macropore number, connectivity, and fractal dimension than CK (p < 0.05). The macropore indices (except circularity) of BC1 were relatively higher than those of BC2 in the most soil depths. Whether biochar altered the soil macropore indices depended on the addition rate of biochar and soil depth. The expansion and occupying effects of biochar were dominant at soil depths of 0-50 and 100-200 mm, respectively; and the two effects were stronger in BC1 than in BC2. A combination of the expansion and occupying effects occurred at a soil depth of 50-100 mm. The distinct effects of biochar on soil pore structure at different depths could mitigate methane emission and nutrient runoff loss from the double-rice paddy. Therefore, soil depth-dependent macropore structure should be considered when assessing the influence of biochar on soil properties and the associated environmental effects.

Evaluation of soil macro-aggregate characteristics in response to soil macropore characteristics investigated by X-ray computed tomography under freeze-thaw effects

As two complementary aspects of soil structure, the importance of macropore and macro-aggregate characteristics in maintaining soil functions and productivity is paramount. However, the changes in macropores and macro-aggregates and their interactions induced by the effects of freeze–thaw action have rarely been evaluated. This study aimed to evaluate the freeze-thaw effects on soil macropore and macro-aggregate characteristics and their relationships using X-ray computer tomography and dry sieving. Thirty-nine undisturbed soil columns were derived from a depth of 0–15 cm in the Mollisol area of Northeast China. Seven freeze-thaw treatments, including 0 (CK), 1 (FT.1), 3 (FT.3), 5 (FT.5), 7 (FT.7), 10 (FT.10), and 15 (FT.15) freeze-thaw cycles, and two initial soil mass water contents (30% and 40% the initial soil water content (ISWC)) were selected under controlled laboratory conditions. Our results showed that freeze-thaw effects significantly increased the total image porosity, mean pore diameter, porosity with branches, and connectivity density (p < 0.05). However, the total number of pores showed a “first increase – then decrease” trend with an increasing number of freeze-thaw cycles. During the freeze-thaw cycles, the macropore and macro-aggregate size distributions exhibited dynamic processes. Compared to the CK treatment, soils for 30% and 40% ISWC under the FT.15 treatment exhibited a lower > 7 mm macro-aggregates by 27.9% and 43.1%, but a higher < 3 mm macro-aggregates by 78.0% and 88.4%, respectively. The total image porosity, porosity with branches, and connectivity density were significantly positively and negatively correlated (p < 0.05) with the mean weight diameter and fractal dimension of macro-aggregates, respectively. The increase in macroporosity and pore branches easily induced the breakdown of the macro-aggregates. This study can help improve the understanding of the effects of changes in macropore characteristics and the processes of macro-aggregate breakdown and further enable the utilisation of soil structure as an indicator for assessing the sustainability and erodibility of soils.

Temporal and spatial variability in 3D soil macropore characteristics determined using X-ray computed tomography

Temporal and spatial variability in 3D soil macropore characteristics determined using X-ray computed tomography

Impact of land use and tillage practice on soil macropore characteristics inferred from X-ray computed tomography

Quantification of soil macropores is important to enhance our understanding of preferential flow behavior in soils. However, knowledge of 3D soil architecture for different land uses in a continental climate region is limited. X-ray computed tomography is a novel technique that can provide valuable information about the 3D features of the soil, such as connectivity, length density, and tortuosity, that largely control water flow through the macropores. In this study, X-ray computed tomography and image analysis were employed to quantify the soil macropore characteristics from four combinations of different land uses and soil tillage practices - native grassland (Nat), alfalfa (Alf), conventional till-corn (CT), and no-till corn (NT) systems. A total of 15 undisturbed soil columns, 150 mm in diameter and 500 mm in length, were collected from different fields. The macropore characteristics were distinctively different for different land use and tillage treatments, especially near the surface depth (top 100 mm). The soil macropores under CT were highly connected and concentrated mostly in the surface soil layer. In contrast, macropores under NT were larger and had well-developed pore networks. The Alf field had relatively larger and more vertically oriented macropores as compared to the Nat field. Relatively larger macroporosity and a higher number of macropores were observed in the surface of soils collected from Nat field. Macropore characteristics were mostly similar between Alf and NT treatments. The results of this study facilitate quantitative evaluation of 3D soil macropore features with significant implications for non-equilibrium flow prediction and contaminant transport modeling in soils.

Using X-ray computed tomography to quantify variability in soil macropore characteristics in pastures

Soil macropores largely control the transport of water and solutes in subsurface flows. Preferential flow via soil macropores can substantially affect water quality. Hence, it is important to quantify soil macropore characteristics and link this information with the preferential flow behavior in soils. However, whether macropore structure at one slope position within a field is different than that at another is unclear. With differences in the macropore characteristics, each slope position can contribute differently to the runoff and subsurface flows. The objective of this study was to use X-ray computed tomography (CT) and image analysis to characterize soil pore structure (> 0.75 mm diameter) at upslope, midslope, and downslope positions within a 0.40 ha pasture field. A total of 18 undisturbed soil columns (150 mm diameter and 500 mm depth) were collected from a pasture field located at the Sand Mountain Research and Extension Center, Alabama, during May 2019. The results indicated that both the macropore number and macroporosity values were lowest at the downslope position in the 0–100 mm soil layer. In contrast, a large number of macropores was observed at the downslope soils for depths below 200 mm. The lowest macroporosity values in the surface layer at the downslope position can be attributed to higher degree of trampling induced compaction due to higher soil moisture content at the downslope location, via runoff and seepage losses from the upper slopes. Likewise, it was found that using smaller diameter soil cores for sampling can cause bias in the soil macropore measurements due to considerably high coefficient of variation compared to the larger diameter soil cores. Macropore interconnectivity at the subsurface layer (100–500 mm) increased from the upslope to the downslope position, whereas at the soil surface (0–100 mm), the interconnectivity was lowest at the downslope position as compared to the upslope and midslope locations. The results of this study provide quantitative information of different soil macropore characteristics under varying topographical locations and depths in a pasture field. The relevance of topographical variation of different macropore characteristics found in this study should be tested by investigating a wider range of soil types and slopes in pastures.

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 (&gt;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.

Three-dimensional quantification of macropore networks of different compacted soils from opencast coal mine area using X-ray computed tomography

Heavy machinery compaction in mining area is widespread, and seriously jeopardizes soil restoration. Characterization of soil macropores with three-dimensional methods is essential for understanding the reclaimed minesoil (RMS) development. The aim of this study was to demonstrate the application of X-ray computed tomographic (CT) scanning for analyzing soil macropore structure and to explore how soil compaction affects soil macropore properties. To achieve this, soil samples collected from Pingshuo opencast coalmine were tested through a laboratory scheme, which was conducted to simulate the real soil pore situation under increasing soil bulk density treatments of 1.3, 1.4, 1.5, 1.6, 1.7, and 1.8 g cm−3 (T1.3-T1.8). Three-dimensional (3D) macropore networks were observed via CT, and macroporosity, macropore number, area, volume and its connectivity were characterized. Soil compaction had a major impact on soil macropore characteristics. The macropores of the 1.3 g cm-3 treatment were well connected, whereas those of the 1.8 g cm-3 treatment were poorly connected and isolated from each other. Macropore number and macroporosity decreased significantly as the soil compaction increased. Moreover, a 3D visualization of soil pore connectivity results suggested that the macropore connectivity markedly decreased with increasing compaction level, with average Euler number increasing from 0.677 in the 1.3 g cm-3 treatment to 0.999 in the 1.8 g cm-3 treatment. Soil bulk density was negatively correlated with macropore throat number and thickness, indicating that the soil compaction causes a loss of pore connectivity. Despite using a coarse resolution, the combination of laboratory simulation tests and 3D visualization of soil macropores provides valuable data for identifying the differences in soil pore structure among compacted RMSs and monitoring the persistent effects of soil compaction caused by heavy machinery in the mining area. Furthermore, we recommend a new method combining 3D visualization techniques and hydraulic analysis in the study of compacted soils, and the establishment of soil restoration measurements to avoid further compaction in mining areas.

Changes in soil macropores: Superposition of the roles of organic nutrient amendments and the greenhouse pattern in vegetable plantations

AbstractOrganic farming can potentially mitigate soil compaction, which commonly occurs in vegetable plantations, particularly in greenhouses when compared with open‐air systems. Although several studies have addressed the effect of planting patterns on soil pore characteristics, few studies have focused on changes in pore attributes under organic fertilization. We used adjacent fields during green manure planting as a control to compare the differences in soil physicochemical and macropore parameters between 14‐year‐old greenhouse plots and open‐air plots under organic fertilization. Pearson's correlation analysis was performed to analyse the drivers of soil macropore characteristics in vegetable plantations in Jiangsu Province, China. A significant increase in connectivity, fractal dimensions and total macroporosity and a decrease in bulk density after 14 years of organic farming, particularly in the surface layer in the greenhouse, were observed. However, the difference in pores in the plough pan layer was not significant between the treatments. The volume of small pores (50–500 μm) and medium pores (500–1,000 μm) increased significantly (p &lt; 0.05) in both the greenhouse and open‐air systems, showing significantly positive relationships with soil organic matter (SOM) and total nitrogen (TN) content. Large pores (&gt;1,000 μm) showed a clear decrease, especially in open‐air fields, possibly due to the disappearance of the original straw residues in the surface layer. Small and medium pores in the plough layer increased in greenhouse fields, whereas the opposite occurred in open‐air fields. Overall, compared with open‐air systems, long‐term organic greenhouse patterns had a significantly positive effect on soil pore attributes.

Influence of shrub roots on soil macropores using X-ray computed tomography in a shrub-encroached grassland in Northern China

The relationship between soil macropore and plant roots warrants special attention because it influences the behavior of water in soil. However, the influence of shrub roots on soil macropores in shrub-encroached grasslands is poorly understood. The objectives of this study were to quantify soil macropores and root architecture in a shrub-encroached grassland in northern China, and to reveal the relationship between shrub roots and soil macropore. In this study, treatments were performed that corresponded to three successional states of the shrub C. microphylla L. with three different shrub densities. At each site, three undisturbed soil cores were excavated under the shrub canopies, and a Philips medical scanner was used to simultaneously visualize and quantify the soil and root architectures. Strong positive correlations between root volume, length, and surface area and the solid surface/solid volume ratio were found, and greater root growth was noted in more porous soil. The results highlighted that the soil macropore characteristics corresponded well with the root characteristics of the soils for the three treatments. Soil macroporosity and macropore volume increased with increasing shrub root network density. In addition, the influence of plant roots on soil macropores increased with increasing shrub encroachment. The study confirmed that the large number of macropores found in the soils under shrubs was attributed to the great degree of root development there. The greater degree of macroporosity under shrubs was attributed to the larger root network density, which might cause greater amounts of water to be concentrated in the deep soil layer by macropore flow under shrub patches. The influence of plant roots on soil macropores increased with increasing shrub encroachment.

Characterizing the effects of opencast coal-mining and land reclamation on soil macropore distribution characteristics using 3D CT scanning

Opencast coal mining activities can cause severe changes to terrestrial ecosystems, leading to soil degradation. Soil pores play an important role in the transport and storage of water and nutrients. Soil macropore systems may be used as an indicator of soil quality, and soil macropore characteristics can reflect the quality of reclaimed soil in opencast coal mining areas. In this study, unmined, unreclaimed and reclaimed soils from the Antaibao opencast coal mine of the loess area from China were scanned to acquire information regarding soil macropores using a high-resolution, non-destructive computed tomography (CT) technique. The 3D distributions of different soil macropores were reconstructed using the Matlab platform, and the quantitative characteristics of soil macropores were determined by analyzing the number of individual detached soil macropore and the volume of individual detached macropore. The results indicated that 3D reconstruction technique can visually and accurately characterize the macropore system of compacted soils in opencast coal mining area. The number and volume of individual detached soil macropores can be acquired from a 3D soil model. The macroporosity of unreclaimed soils was poor due to the compaction of heavy machinery during mining activities, and it was about 3% based on CT technique. The macroporosity and functions of reclaimed soils significantly improved with increasing reclamation age. The number of individual detached soil macropores had a good linear relationship with macroporosity. With increasing macroporosity, the diversity of macropore sizes decreased and the volume of the largest individual detached macropore increased. To improve the macropore conditions of compacted soils, land reclamation (including land consolidation, surface soil covering, and vegetation planting) should be carried out as soon as possible.

Corn Residue Removal Effects on Hydraulically Effective Macropores

ABSTRACTEnhanced understanding of biomass removal effects on soil quality could be achieved with greater knowledge of how corn residue removal and cover crops interact to affect surface pore structure. This study was conducted to evaluate the effects of corn (Zea mays L.) residue removal on soil macropore characteristics and to assess the effectiveness of cover crops in mitigating the potential negative impacts of corn biomass removal on surface pore structure. Three different corn residue removal rates and the presence or absence of cover crops were evaluated in a no-till corn/soybean (Glycine max L.) rotation near Brookings, SD. Following eight years of residue removal high (HRR) and medium (MRR) rates of residue removal reduced water inflow into the soil surface compared to the low (LRR) residue removal treatment. The representative mean pore radius (λΔψ) for both rotation phases of LRR approached the same value (≈ 235 µm). However, nine months after corn residue removal the λΔψ for HRR in the soybean phase of the rotation was significantly lower than LRR at 161 µm. There was no significant difference in λΔψ between HRR (214 µm) and LRR (236 µm) 21 months after residue removal during the corn phase of the rotation. The initial reduction in λΔψ following corn residue removal in HRR followed by soil surface recovery the following year suggests that inclusion of decaying corn residue is critical in the maintenance of hydraulically functional macropores in this fine textured soil. Cover crops were not observed to mitigate these impacts of crop residue removal on surface soil structure within the time period of the study.

Exclosure on CT-measured soil macropore characteristics in the Inner Mongolia grassland of northern China

Grassland exclosure is a widely-used option to prevent from grazing in degraded grasslands for restoration. However, the influence of exclosure on soil macropore of grassland remain scarce. The objective of this study was to quantify the pore architecture of grassland soils under exclosure. Two treatments, 9E (grassland enclosed for 9 years) and 5E (grassland enclosed for 5 years), were designed, with grazing as a control in the experiment. Nine soil columns (0–50 cm deep) were taken at the three sites with three replicates. At each site, three soil columns were from the grassland, and cores were scanned with a Philips Brilliance ICT Medical Scanner. Numbers of macropores, macroporosity, network density, length density, and node density within the 50-cm soil profile were interpreted from X-ray computed tomography to analyze soil pore architecture. The results indicated that exclosure significantly influenced CT-measured soil macroporosity in the Inner Mongolia grassland of northern China. Soils under enclosed grassland had greater macroporosity, length density, total volume, and node density than that of under freely grazed grassland. Macroporosity increased as the enclosure age increased. For soils under enclosed grassland, macropores were concentrated at 0–300-mm soil layers, and macropores were mainly present at 0–100-mm soil depth under freely grazed grassland. The large number of macropores found in soil under enclosed grassland can be attributed to greater root development. Exclosure increases soil macroporosity and improve soil structure.

Soil macropore characteristics following conversion of native desert soils to irrigated croplands in a desert-oasis ecotone, Northwest China

Sustainable use of cultivated desert soils is important for agricultural productivity in desert-oasis ecosystems. However, how soil macropore characteristics may change as a result of agricultural exploitation remains unclear. The objective of this study was to quantify and compare soil properties and macropore characteristics in an old oasis field (>50years of cultivation, OOF), young oasis field (20 years, YOF), and adjacent uncultivated sandy land (0year, USL) in Northwest China. Three replicated soil samples were collected from each site to investigate soil properties. Meanwhile, twelve (four replicates by three sites) intact soil core columns, 15cm in diameter and 30cm in height, were taken to analyze soil structure. Each soil column was scanned with a helical medical X-ray computed tomography (CT) at a voxel resolution of 0.469mm×0.469mm×0.600mm. The results indicated that soil properties and macropore features improved after cultivation. Silt and clay content and dry mean weight diameter (DMWD) of aggregates increased with cultivation time, whereas bulk density decreased. Soil organic carbon and total nitrogen were 7.3 times and 6.7 times greater in soils at the OOF site than USL site, respectively. The increase in silt and clay content and aggregates formation likely resulted from irrigation with silt-laden river water, which in turn impacted soil nutrient accumulation. Soils at the OOF and YOF sites had greater macroporosity compared with soil at USL, and macroporosity also increased with cultivation time. X-ray CT revealed that soil macroporosity was 8.7–18.9 times greater in irrigated croplands than native desert. Soil macropores were mainly distributed at soil depths of 0–200mm at the OOF and YOF sites, while smaller and less continuous macropores were randomly distributed across soil depths at the USL site. The larger number of macropores at the cultivated sites can be attributed to greater soil organic carbon, tillage-induced soil horizons, and alternate wetting and drying processes. Few soil macropores at the USL site may be associated with wind erosion and soil fauna burrows. Conversion of native desert soils to irrigated croplands had a positive effect on soil pore development in the desert-oasis ecotone.