High Energy Moisture Characteristic Research Articles

Sugarcane bagasse derived biochar potential to improve soil structure and water availability in texturally different soils

Low organic matter content is one of the main constraints in arid and semiarid regions. This constraint and its negative influences on soils and plant growth may be alleviated by biochar (BC). Furthermore, improving soil physical and hydraulic attributes by application of biochar has received increased attention. Therefore, in the present study, the effects of sugarcane bagasse-derived biochar on the structural stability, water availability, and pore-size distribution (PSD) of three texturally different calcareous soils collected from different agro-climatologically regions were examined during a long-term experiment. Low and high-temperature biochars, produced in a muffle furnace by the traditional slow pyrolysis method at 300 °C (BC300) and 600 °C (BC600) were evaluated. Pots (15 kg) were filled with three different silty-clay Inceptisols (SCInc), silty-clay-loam Alfisols (SCLAlf), and loam Aridisols (LArid) soils mixed with 0 (control), 1, 2, and 3 w/w% of BC300 and BC600 during 540 days of incubation. The high energy moisture characteristic (HEMC) data was modeled using a modified van Genuchten function to quantify aggregate stability through stability ratio (SR) and structural stability index (SSI). The plant available water (PAW), least limiting water range (LLWR), and integral water capacity (IWC) were calculated with two matric suctions (h) of 330 cm for field capacity (FC) and 15,000 cm for permanent wilting point (PWP). Then the integral energy (EI) values were calculated (EIIWC). Results indicated that the incorporation of 3 w/w% biochar significantly (p < 0.01) increased SR (35 to 100%) and SSI (21 to 28%) indices in all three soils. Biochar significantly increased modal suction (MS) in LArid soils (5 to 158%); whereas, decreased MS of the other soils (3 to 43%). MS, SR, and SSI of BC300 and BC600-treated soils were not significantly different. PAW, LLWR, and IWC significantly decreased in the SCInc (18 to 61%, 8 to 44%, and 6 to 35%) and SCLAlf (8 to 44%, 18 to 35%, and 20 to 47%) soils and increased in LArid (4 to 54%, 3 to 61%, and 24 to 111%) soil with increasing biochar doses. There were no changes in EIIWC in biochar-treated LArid soil where PAW, LLWR, and IWC increased. Biochar increased EIIWC across the studied soil from 1% to 3.38 folds, thereby increasing the gradient of water potential to absorb the available water. Soil and soil-biochar mixtures exhibited heterogeneous and multimodal pore-size distribution (PSD). Biochar promoted the PSD peaks related to water-transmitting pores in SCInc and SCLAlf soils while decreased in LArid soil. In conclusion, results indicated that among the applied levels of biochar, the application of 3 w/w% biochar is suggested as a suitable way to improve soil physical behavior and structural stability.

Impacts of soil organic carbon and tillage systems on structural stability as quantified by the high energy moisture characteristic (HEMC) method

1. Abdollahi L. and Munkholm L.J., 2014. Tillage system and cover crop effects on soil quality: I. Chemical, mechanical, and biological properties. Soil Sci. Soc. Am. J., 78(1), 262-270, https://doi.org/10.2136/sssaj2.... CrossRef Google Scholar

High-energy moisture characteristics of various low organic matter sandy soils in different land uses

Soil structure serves as one of the most important soil physical properties influencing on water retention, aeration, plant growth as well as environmental conditions. In the current study, the effect of four land-use types on some soil structural stability indices including high-energy moisture characteristic (HEMC) at both fast and slow wetting modes, volume of the drainable pores (VDP), structural index (SI), stability ratio (SR), fractal dimensions (D) and mean weight diameter (MWD) was investigated. Jiroft County, is located in southeastern part of Kerman Province, Iran (28ο40׳ N, 57ο44׳ E) and characterized by soils generally high in sand and low in organic matter (OM) was selected as the study area. Land-use types included disturbed and undisturbed rangelands, protected natural forest and an artificial forest plantation. The HEMC results showed that in the slow wetting mode, undisturbed rangeland (49.6%) and protected natural forest (42.6%) indicated the highest and lowest values for near-saturated water contents, respectively. However, in fast wetting modes no significant differences was observed in case of near-saturated water content between undisturbed rangeland and disturbed rangeland and artificial forest plantation land-uses. Furthermore, the near-saturated water contents in the protected natural forest did not changed for both fast and slow wetting modes. The lowest MWD (0.05 mm) and the highest fractal dimension values (2.97) were observed for protected natural forest. The lowest values for VDP, VDP ratio (VDPR), and SI were found for undisturbed rangeland and disturbed rangeland, with the highest values obtained for protected natural forest. No significant differences were found among the land-use types in terms of SR. Furthermore, soil water retention was also low in the land use types with low aggregate formation agents like clay, OM, and calcium carbonate equivalent (CCE). Among the HEMC indices, near-saturated water content and soil water retention only indicated the structural condition. While other HEMC indices such as VDP, SI, SiR, and VDPR did not show the differences between structural and textural characteristics. This study revealed that, techniques such as measuring MWD and fractal dimension may better show the status of the soil aggregates in the studied region.

Effects of land use and reducing conditions of paddy fields on soil quality and high energy moisture characteristic structural stability indices in North of Iran

The present paper is aimed at investigating the effects of land use (particularly reducing conditions of paddy fields) on soil structural stability as characterized by the high energy moisture characteristic (HEMC) method and finding the relationship between soil structural stability and available water (AW) in Amol city in Mazandaran province, Iran. Land use conversion led to a change in the soil organic matter (OM) content. The highest OM was found in the forest and pasture land uses, being significantly different from those of citrus garden and paddy field land uses. The high values of bulk density in the citrus garden and paddy fields can be attributed to intensive machinery traffic and puddling, respectively. However, bulk density was low in pasture and forest land uses because of high OM and macropores. Forest and the paddy fields had higher soil structural stability due to high OM/Clay ratio. The HEMC indices including volume of drainable pores ratio (VDPR) and stability ratio (SR) were calculated using the modified van Genuchten model, and the ratio of slopes at the inflection point of HEMCs of fast-wetted to slow-wetted soil samples (SiR) was determined using the van Genuchten model. The HEMC indices showed positive and significant correlations with OM and OM/Clay ratio. By increasing the OM/Clay ratio, greater clay became complexed with organic matter, and thus soil structure stability increased. A strong correlation was observed between VDPR and OM/Clay in citrus garden and paddy fields. Furthermore, there was a significant correlation between VDPR and AW. The highest field capacity and AW were observed in the forest and paddy fields due to high OM and the presence of smectitic clays under reducing conditions, respectively.

Structure Stability of Cultivated Soils from Semi-Arid Region: Comparing the Effects of Land Use and Anionic Polyacrylamide Application

The Sustainable Development Goals of the United Nations call for applying soil management practices that contribute land degradation neutrality. Our objectives were to investigate the effect of (i) soil management—conventional tillage (CT under crop) and no-tillage (NT under grass)—and (ii) an amendment (polyacrylamide (PAM)) application on the structure stability indices of soils from a semi-arid region. Two sets of experiments were conducted using the high-energy moisture characteristic (HEMC) method for the assessment of (i) land-use type (CT vs. NT) in soils (30 samples) varying in texture, and (ii) the effect of six PAM concentrations (0, 10, 25, 50, 100, and 200 mg L−1) on three typical soils (sandy clay loam, clay loam, and clay) under CT management; then, the contributions of PAM concentration (CT) and NT were compared. Water retention curves of samples were obtained at a matric potential from 0 to −5.0 J kg−1 and characterized by a modified van Genuchten model that yields (i) model parameters α and n, and (ii) a soil structure stability index (SI). The treatments affected the shape of the water retention curves. Change of land use from CT to NT and PAM application to CT soil increased the SI and ɑ, and decreased n compared to CT-managed soils. The magnitude of the NT and PAM effect was inversely related to soil clay content. CT-managed soils treated with a low PAM rate (10–25 mg L−1) gave SI comparable to that obtained for the NT-managed soils, while CT-managed soils treated with a high PAM rate (50–200 mg L−1) yielded 1.3–2.0 and 2–4 times higher SI than that for NT and CT-managed soils, respectively. Our findings suggest that both the change of land use to NT or the addition of small amounts of PAM are viable alternatives for stabilizing CT-managed weakly alkaline semi-arid soils, whose soil structure stability is a priori limited.

Biochar type and pyrolysis temperature effects on soil quality indicators and structural stability

Quality of soils of the arid zones with low organic matter can be improved through the application of natural amendments especially biochar from various available feedstocks. The objective of this study was to evaluate the impacts of corn residue and poultry manure and their biochars on soil organic carbon (SOC), hot-water soluble carbohydrates (HWSC), basal soil respiration (BSR),and structural stability determined by HEMC (high-energy moisture characteristic). A sandy loam soil in pots were thoroughly mixed with 1, 2 and 4% w/w of corn residues (CR) and poultry manure (PM) feedstock and their biochars prepared at 350 and 650 °C of slow pyrolysis. Maize seeds were planted in pots and grown until physiological maturity when soil characteristics were measured. Treatments considerably altered the means of studied soil quality indicators, and increased SOC (1.5–10 times) and HWSC (1–7 times), and HEMC indices: volume of drainable pores ratio (VDPR, 1.5– 3.5 times), and stability ratio (SR, 1–3 times). Increasing pyrolysis temperature, regardless of the type and rate of feedstock, significantly decreased the SOC, BSR and percent of water-stable aggregates, and consequently structural stability indices. Contribution of both PM feedstock and its biochars was less effective than the CR ones (particularly the biochar produced at higher pyrolysis temperature), due to elevated sodium adsorption ratio (SAR) associated with higher slaking, physico-chemical dispersion and lower aggregate and structural stability.

Deforestation effects on soil aggregate stability quantified by the high energy moisture characteristic method

Soil aggregate size distribution and structure stability are important soil physical quality indices. The objective of this study was to quantify soil structure stability as a result of land-use conversion from forest to tea farm within Guilan Province of northern Iran over the past 50 years. Soil aggregate or structure stability metrics, along with more common soil chemical and physical properties, were determined on triplicate soil samples (0–30 cm depth), randomly collected from six sites. Dexter's S-index (slope of soil water retention curve at its inflection point), tensile strength (TS), aggregate mean weight diameter (MWD), and water stable aggregate (WSA) were also used to assess stability. Lastly, the high energy moisture characteristic (HEMC) method measured the destructive force used to break the aggregates through controlled wetting. Structure stability using HEMC method and S-index was determined based on the soil water retention curve but using different ranges of matric potentials. The results showed that despite an insignificant decrease in TS, WSA and MWD between land use types there was significant correlation (P < 0.05) between tensile strength with HEMC method's, stability ratio (SR) and Dexter's S-index. The volume of drainable pores after fast wetting of aggregates (fast-VDP) in HEMC method reflected a significant (P < 0.01) decrease in aggregate size between forest and tea farms with means of 0.347 and 0.297 g g−1, respectively. In only one of six locations, the soil water retention curves and the aggregate stability indexes (SI and SR) of HEMC method decreased significantly after land use change. Our results indicated that macro aggregates and structure stability in HEMC method could distinguish differences between two land uses and better than previous methods like MWD, WSA, and TS. The results of S-index and stability ratio of HEMC method were different. This difference can be related to: 1) the soil initial aggregate size that is between 0.5 and 1 mm in HEMC method and about 4 mm in Dexter's S-index and 2) the range of matric potential in each method that is 0 to 50 and 0 to 15,000 hPa in HEMC and S-index, respectively. Although both methods are based on soil water retention curve to estimate soil structure stability and physical quality, but HEMC results show macrostructure stability (macro aggregates) whereas S-index is inflection point of soil water retention curve between textural and structural porosity and estimate microstructure stability.

New Proximal Sensing Approach to Measure Soil Aggregate Stability

New Proximal Sensing Approach to Measure Soil Aggregate Stability

Effect of biochar on the physical and structural properties of a sandy soil

Biochar application to soil can be considered as a means to improve soil quality, thereby optimizing irrigation management and reducing irrigation needs, especially in dryland regions. This paper is aimed at investigating the effect of biochar (BC) on a desert sandy soil (Al Foah, United Arab Emirates) in terms of bulk density, porosity, water retention, plant available water (AWmax), aggregate stability (AS) and specific surface area (BET-SSA). BC was produced from forest biomass (Italy). Soil water retention was measured by the High Energy Moisture Characteristic (HEMC) and by pressure plate measurements, by using BC fractions equal to 0 (soil only), 0.014, 0.091, 0.23, 0.33 and 1 (BC only). The results indicated that BC significantly (p = 0.05) increased soil porosity and the amount of storage pores. As a consequence, water retention was enhanced and AWmax from 19.3 mm/m for the sandy soil (fbc = 0), to 60.8, 102.3 and 107.6 at fbc = 0.091, 0.23, and 0.33, respectively, was raised. No significant effects of BC were detected on the soil water retention curve of the BC-amended soil at fbc = 0.014, compared to the original condition. The significant increase in the structural index (SI) at fbc, ≥0.091, indicated that BC improved soil aggregate stability. The BET-SSA was measured by nitrogen adsorption measurements and indicated that the connected porosity was significantly increased by the 0.091 BC fraction. Both AWmax and SI were found to be correlated with BET-SSA. This showed an outcome that has been so far little investigated, namely that the BET-SSA played a significant role in the mechanism affecting the soil response to BC addition. The results showed that application of BC can improve soil structure and water retention and therefore optimize irrigation management by reducing irrigation needs, in the Al-Foah area and/or in similar dryland regions.

A Novel Shortwave Infrared Proximal Sensing Approach to Quantify the Water Stability of Soil Aggregates

A novel proximal sensing approach to determine soil aggregate stability is demonstrated. Change in pre‐ vs. post‐wetted aggregate reflectance yields aggregate stability (AS) from surface structure alteration. Results were well‐correlated with those of the high energy moisture characteristic (HEMC) method, which involves rapid and slow wetting of aggregates. The new method only employs rapid wetting of aggregates, saving time for sample processing and evaluation. Soil structure and aggregate stability (AS) are critical soil properties affecting water infiltration, root growth, and resistance to soil and wind erosion. Changes in AS may be early indicators of soil degradation, pointing to low organic matter (OM) content, reduced biological activity, or poor nutrient cycling. Hence, efficient and reliable AS measurement techniques are essential for detection, management, and remediation of degraded soil resources. Here we quantify soil AS by developing a novel proximal sensing technique based on shortwave infrared (SWIR) reflectance measurements. The novel approach is similar to the well‐documented high energy moisture characteristic (HEMC) method, which yields a stability ratio (SR) derived from comparison of hydraulic and structural characteristics of slowly‐ and rapidly‐wetted soil samples near‐saturation. We rapidly wetted aggregated soil samples (i.e., high energy input) and hypothesized that an AS index can be derived from SWIR surface reflectance spectra due to differences in post‐wetting surface roughness that is intimately linked to AS. To test this hypothesis, surface reflectance spectra from a wide range of structured soil textures under both slowly‐ and rapidly‐wetted samples, were measured with a SWIR spectroradiometer (350–2500 nm). The ratio between pre‐ and post‐wetting spectra was determined and compared with the HEMC method's volume of drainable pore ratio (VDPR). We found a strong correlation (R 2 = 0.87) between the VDPR and the SWIR‐derived reflectance index (RI) and also between the SR (R 2 = 0.90) and the RI for all soils. These results point to the feasibility and appeal of quantifying AS using the newly proposed and more time‐efficient proximal sensing method.

Effect of temperature on soil structural stability as characterized by high energy moisture characteristic method

Temperature is a key factor that can affect soil properties and processes. Surface tension-viscous flow (STVF) theory is widely used to explain the temperature effects on soil hydraulic properties. We hypothesized that one of the reasons for deviation of measured soil water retention data from the STVF theory predictions is due to the temperature effects on soil structural stability (i.e., near-saturated pore size distribution [PSD]). Therefore, the objective of this study was to evaluate the effect of temperature on soil structural stability as characterized using the high energy moisture characteristic (HEMC) method in a wide range of arid and semi-arid soils of Iran. Aggregates of 28 soil samples were fast-wetted to mimic the natural conditions of soils during rainfall and irrigation. Four ambient temperatures (5, 10, 15 and 30 °C) were applied using an incubator during the wetting of aggregates, and after that HEMCs in the matric suction (h) range of 2 to 50 hPa were measured. Both van Genuchten (VG) and modified van Genuchten (MVG) models were fitted to the HEMC data and structural stability indices were calculated. The results generally showed that ambient temperature significantly altered soil PSD and structural stability, and as temperature increased: (i) an expansion of entrapped air initiated by the formation of microbubbles, was enhanced, (ii) water content at near full saturation h = 2 hPa (θ2hPa) decreased, and at h = 50 hPa (θ50hPa) it increased, leading to a reduction in the volume of drainable pores [VDP], and thus (iii) the soil structural index [SI] and slope at the inflection point of HEMC [Si] decreased, and (iv) amount of macropores (h, 2–12 hPa) was markedly decreased, however, micropores (h, 12–50 hPa) were not notably affected or were increased at high temperature due to intensive shifting of macropores into micropores. These findings imply that soil structure was damaged due to loosening effects of high temperatures on inter-particles bonds and increased repulsive forces between clay particles. Cluster analysis showed that the effect of ambient temperature on the structural stability was greatest in carbonate-rich soils with low clay and organic carbon (OC) contents and was least in the soils rich in OC. The results of this study could be important for projecting the effect of global warming and climate change on soil structure and erosion.

Effects of endophyte-infected and non-infected tall fescue residues on aggregate stability in four texturally different soils

Fungal endophyte (Epichloë coenophiala) usually infects cool-season perennial forage grasses and enhances host plant resistance to biotic and abiotic stresses and, by potentially altering the litter quality and decomposition rate, can affect soil properties. Aggregate stability, as an important determinant of plant root growth and soil quality, may be affected by the endophyte status of plant residues. In this study, the effects of endophyte-infected (E+) and endophyte-free (E−) tall fescue residues (0%, 1% and 2%) on soil organic carbon (SOC), basal soil respiration (BSR) and aggregate stability were investigated in four texturally different soils in the laboratory. The aggregate stability indices were determined by the high energy moisture characteristic (HEMC) method. Moist soil samples were thoroughly mixed with either E+ or E− tall fescue residues before being incubated at 25°C. During the 2-month incubation period, the amended soil samples were subjected to ten wetting and drying cycles, after which soil properties were measured. The results indicated that SOC and aggregate stability were higher and that BSR was lower in the finer-textured soils. Furthermore, an increase in the application rate of plant residues significantly increased SOC, BSR and aggregate stability. Treating the soils with E+ tall fescue residues increased SOC and aggregate stability (i.e., stability ratio, SR, a dimensionless index defined as the ratio of the structural index for the fast wetting to the structural index for the slow wetting), and significantly decreased BSR due to the toxic effects of phenolic compounds on soil microbial communities and changes in the litter chemical quality. The interactive effect of soil type and endophyte status on the SOC and BSR values was significant but no clear trend was observed. Our findings indicated that tall fescue residues (especially E+ ones) can improve soil physical quality due to increased SOC storage and greater aggregate stability. Therefore, these plants have great potential for use in grassland reclamation and soil conservation plans in semi-arid pasture and agricultural lands.

Relationship between soil water retention model parameters and structure stability

Studying and modeling the effects of soil properties and management on soil structure and near saturation water retention is vital for the development of effective soil and water conservation practices. The contribution of soil intrinsic properties and extrinsic conditions to structure stability was inferred, in quantitative terms, from changes in water retention curves near saturation (low matric potential, 0-50 cm, macropores > 60 µm) that were obtained by the high energy moisture characteristic (HEMC) method. The S-shaped water retention curves were characterized by the modified van Genuchten model that provided: (i) the model parameters α and n, which represent the location of the inflection point and the steepness of the water retention curve, respectively; and (ii) the soil structure index, SI=VDP/MS, where VDP is the volume of drainable pores, and MS is the modal suction. Model parameters, claculated by the soil-HEMC model, were related to soil properties and hence soil water retention properties were linked to measured characteristics in several field and laboratory experiments. Soil SI increased exponentially with the increase in α and the decrease in n, while the relationship between SI and α/n was linear. An improved description of the water retention and its link to pore and apparent aggregate size distribution, by using the model parameters α and n, could potentially assist in the selection of management practices for obtaining the most suitable type of soil structure depending on the desired soil function.

Influence of tall fescue endophyte infection on structural stability as quantified by high energy moisture characteristic in a range of soils

Plant microbial symbionts could play a significant role on aggregate stability, an important index for soil physical quality. So far there is little information available linking the presence of endophyte in the aboveground parts of plants, with aggregate stability of rhizosphere soil. We hypothesized that the presence of Epichloë coenophiala in the aboveground parts of tall fescue (Festuca arundinacea Schreb.=Schedonorus arundinaceus and Lolium arundinaceum) may alter the structural properties of rhizospheric soil. A greenhouse pot experiment was performed to evaluate the evidence for a relationship between endophyte–tall fescue symbiosis and rhizosphere aggregate stability for a range of soils. Traditional methods for measuring aggregate stability would not be able to distinguish the impacts of these biotic factors because they usually rely on rapid immersion of aggregates in water which is very disruptive. In this study, the aggregate stability was quantified by high energy moisture characteristic (HEMC) because HEMC is capable to accurately characterize small differences in aggregate stability among the treatments and soils. Clonally propagated tillers of tall fescue (genotype 75C) naturally infected with E. coenophiala (E+) and endophyte-free ones (E−) were used. Plants were grown under optimal water, light and temperature conditions. After eight months, when plants reached the maximum vegetative growth, intact soil aggregates adherent to plant roots (i.e. rhizosphere soil) and those free of root (i.e. bulk soil) were collected for HEMC determination. The HEMCs of slow and fast wetted aggregates were determined at high matric suctions of 0 to 50hPa. HEMC data was modeled using a modified van Genuchten function. Then, various indices including volume of drainable pores (VDP), relative VDP (RVDP), VDP ratio (VDPR), modal suction (hmodal), relative hmodal (Rhmodal), stability index (SI), relative SI (RSI), stability ratio (SR), relative SR (RSR), slope at the inflection point (Si), and inflection point slope ratio (SiR) were derived to characterize soil structural stability. Endophyte presence was correlated with increased soil organic carbon (SOC) pools especially for the fine-textured soils. Changes in the SOC storage of rhizosphere soil altered the structural stability and increased the SR. The effect of endophyte infection on SR of the rhizosphere soil was also more evident in medium- and fine-textured soils due to physical protection of SOC. Lower values of hmodal and higher values of VDP of fast wetted aggregates revealed more stable macropores and greater aggregate stability in the E+ rhizosphere soil vs. E− one. VDPR, SR and SiR values of rhizosphere soil were greater in E+ plants associated soils. Fine-textured soils had lower values of SiR because of their greater values of Si in the slow wetting treatment. Plant root presence could improve the soil structure compared to the bulk soil due to exudation of cementing materials; values greater than 1 for RVDP, RSI and RSR indicate that plant root enhanced the structural stability of rhizosphere soil. In contrast to stability ratio, the effects of Epichloë endophyte–tall fescue symbiosis on relative structural stability indices such as RVDP, Rhmodal, RSI, and RSR were more pronounced in coarse- and medium-textured soils. Overall, endophyte–tall fescue symbiosis could increase aggregate stability of rhizosphere soil and thus its physical quality.

Aggregate stability by the "high energy moisture characteristic" method in an oxisol under differentiated management

Studies testing the High Energy Moisture Characteristic (HEMC) technique in tropical soils are still incipient. By this method, the effects of different management systems can be evaluated. This study investigated the aggregation state of an Oxisol under coffee with Brachiaria between crop rows and surface-applied gypsum rates using HEMC. Soil in an experimental area in the Upper São Francisco region, Minas Gerais, was studied at depths of 0.05 and 0.20 m in coffee rows. The treatments consisted of 0, 7, and 28 Mg ha-1 of agricultural gypsum rates distributed on the soil surface of the coffee rows, between which Brachiaria was grown and periodically cut, and compared with a treatment without Brachiaria between coffee rows and no gypsum application. To determine the aggregation state using the HEMC method, soil aggregates were placed in a Büchner funnel (500 mL) and wetted using a peristaltic pump with a volumetric syringe. The wetting was applied increasingly at two pre-set speeds: slow (2 mm h-1) and fast (100 mm h-1). Once saturated, the aggregates were exposed to a gradually increasing tension by the displacement of a water column (varying from 0 to 30 cm) to obtain the moisture retention curve [M = f (Ψ) ], underlying the calculation of the stability parameters: modal suction, volume of drainable pores (VDP), stability index (slow and fast), VDP ratio, and stability ratio. The HEMC method conferred sensitivity in quantifying the aggregate stability parameters, and independent of whether gypsum was used, the soil managed with Brachiaria between the coffee rows, with regular cuts discharged in the crop row direction, exhibited a decreased susceptibility to disaggregation.

Structure alteration of a sandy-clay soil by biochar amendments

The aim of the present study was to investigate structure alterations of a sandy-clay soil upon addition of different amounts of biochar (f bc ). All the f bc samples were analyzed by high energy moisture characteristic (HEMC) technique and 1H nuclear magnetic resonance (NMR) relaxometry. HEMC was applied in order to evaluate aggregate stability of biochar-amended soil samples. 1H NMR relaxometry experiments were conducted for the evaluation of the pore distributions through the investigation of water dynamics of the same samples. The HEMC technique revealed improvement in aggregate stability through measurements of the amount of drainable pores and the stability ratio. The latter increased as the amount of biochar was raised up. The 1H NMR relaxometry revealed a unimodal T 1 distribution for both the sole sandy-clay soil and the biochar. Conversely, a bimodal T 1 distribution was acquired for all the different f bc samples. Improvement in aggregate stability was obtained as biochar was progressively added to the sandy-clay soil. A dual mechanism of water retention has been hypothesized. In particular, intra-aggregate porosity was indicated as the main responsible for molecular water diffusion when f bc comprised between 0 and 0.33. Conversely, inter-aggregate porosity resulted predominant, through swelling processes, when f bc overcame 0.33.

Soil water retention and structure stability as affected by water quality

In arid and semi-arid zones with a short water resources studying the effects of water quality on soil water retention and structure is important for the development of effective soil and water conservation and management practices. Three water qualities (electrical conductivity, EC ~ 2, 100 and 500 μS cm -1 with a low SAR representing rain, canal-runoff and irrigation water respectively) and semi-arid loam and clay soils were tested to evaluate an effect of soil texture and water quality on water retention, and aggregate and structure stability using the high energy moisture characteristic (HEMC) method. The water retention curves obtained by the HEMC method were characterized by the modified van Genuchten (1980) model that provides (i) model parameters α and n, which represent the location (of the inflection point) and the steepness of the S-shaped water retention curve respectively, and (ii) a volume of drainable pores (VDP), which is an indicator for the quantity of water released by the tested sample over the range of suction studied, and modal suction (MS), which corresponds to the most frequent pore sizes, and soil structure index, SI =VDP/MS. Generally (i) treatments significantly affected the shape of the water retention curves (α and n) and (ii) contribution of soil type, water EC, and wetting rate and their interaction had considerable effect on the stability induces and model parameters. Most of changes due to the water quality and wetting condition were in the range of matric potential (ψ), 1.2-2.4; and 2.4-5.0 J kg -1 (pore size 125-250 μm and 60-125 μm). The VDP, SI and α increased, and MS and n decreased with the increase in clay content, water EC and the decrease in rate of aggregate wetting. The SI increased exponentially with the increase in VDP, and with the decrease in MS. Contribution of water EC on stability indices and model parameters was not linear and was soil dependent, and could be more valuable at medium water EC. Effect of wetting rate was more pronounced at low water EC. Results indicate that effectiveness of water EC in the field condition has no simple outcome on water retention and soil structure, and that its application should consider and be adjusted to soil properties and condition, such as soil texture, and moisture content and solution EC. Detailed contribution of treatments on structure induces and model parameters are discussed in the paper.

Aggregate stability in soils cultivated with eucalyptus

The objective of this work was to evaluate the aggregate stability of tropical soils under eucalyptus plantation and native vegetation, and assess the relationships between aggregate stability and some soil chemical and physical properties. Argisols, Cambisol, Latosols and Plinthosol within three eucalyptus-cultivated regions, in the states of Espírito Santo, Rio Grande do Sul and Minas Gerais, Brazil, were studied. For each region, soils under native vegetation were compared to those under minimum tillage with eucalyptus cultivation. The aggregate stability was measured using the high-energy moisture characteristic (HEMC) technique, i.e., the moisture release curve at very low suctions. This method compares the resistance of aggregates to slaking on a relative scale from zero to one. Thus, the aggregate stability from different soils and management practices can be directly compared. The aggregate stability ratio was greater than 50% for all soils, which shows that the aggregate stability index is high, both in eucalyptus and native vegetation areas. This suggests that soil management adopted for eucalyptus cultivation does not substantially modify this property. In these soils, the aggregate stability ratio does not show a good relationship with clay or soil organic matter contents. However, soil organic matter shows a positive relationship with clay content and cation exchange capacity.

Polyacrylamide Effects on Aggregate and Structure Stability of Soils with Different Clay Mineralogy

Adding anionic polyacrylamide (PAM) to soils stabilizes existing aggregates and improves bonding between and aggregation of soil particles. However, the dependence of PAM efficacy as an aggregate stabilizing agent with soils having different clay mineralogy has not been studied. Sixteen soil samples (loam or clay) with predominantly smectitic, illitic, or kaolinitic clay mineralogy were studied. We measured aggregate sensitivity to slaking in soils that were untreated or treated with an anionic high‐molecular‐weight PAM using the high energy moisture characteristic (HEMC) method and deionized water. The index for aggregate susceptibility to slaking, termed stability ratio (SR), was obtained from quantifying differences in the water retention curves at a matric potential range of 0 to −5.0 J kg−1 for the treatments studied. For the untreated soils, the SR ranged widely from 0.24 to 0.80 and generally SR of kaolinitic &gt; illitic &gt; smectitic soils. The SR of PAM‐treated aggregates exhibited a narrow range from 0.70 to 0.94. The efficiency of PAM in improving aggregate and structural stability relative to untreated soils ranged from 1.01 to 3.90 and the relative SR of kaolinitic &lt; illitic &lt; smectitic samples. These results suggest that the less stable the aggregates the greater the effectiveness of PAM in increasing aggregates stability (i.e., smectitic vs. kaolinitic samples). The effectiveness of PAM in improving structure and aggregate stability was directly related to clay activity and to soil conditions affecting PAM adsorption (e.g., electrolyte resources, pH, and exchangeable cations) to the soil particles and inversely to the inherent aggregate stability.

An Improved Method for Measuring Aggregate Stability of a Weakly Aggregated Loessial Soil

AbstractThe original high energy moisture characteristic (HEMC) technique for measuring aggregate stability is shown to have limitations for use with weakly aggregated soils. Improvements were made to expand the applicability of the technique to weakly aggregated soils and reduce its subjectivity to operator bias. The destructive force used to break down the aggregates was controlled by wetting aggregates 1.0 to 0.5 mm in diameter under suction on a sintered glass plate. The moisture characteristic curve for the aggregates was fit using a modified van Genuchten S‐shaped equation, and the specific water capacity curve was analytically calculated. The modal suction and the area under the specific water capacity curve were then computed and used to calculate a structural index value for aggregate stability at a fast and a slow wetting rate. Surface samples of weakly aggregated loessial soil were collected from adjacent long‐term organic and conventional farms in the Palouse region of Washington State. The coefficient of variation for replicate determination of structural indices from a slow wetting treatment using the original HEMC method on samples from both farms was 15%, whereas it was 7% using the improved method. Using the original HEMC technique, there was no difference in the stability ratio of either farm due to complete breakdown of aggregates dropped into standing water. With the improved HEMC method, the aggregate stability ratio and structural index for fast wetting were both significantly higher for soil on the traditional farm than on the organic farm. Thus, the improved method facilitates the measurement of differences in stability of weakly aggregated soils subject to long‐term differences in cropping and management history. In contrast, the original method was inappropriate for these weakly aggregated soils due to improper wetting rates and high variability resulting from operator subjectivity.