Variability in soil characteristics in the field–bund transition area increases water loss potential in paddy fields

Biochar as a soil amendment can influence the physical and solute transport properties of soils and, thus, provide a means to improve soil fertility. However, the use of different sized biochar particles down to the nano‐scale has revealed inconsistent results, with the mechanisms poorly understood owing to a lack of experimental data. This study investigated the effects and mechanisms of nano‐sized biochar particles (NBC) applied to a sandy loam soil with regard to impacts on the hydraulic characteristics and soil structure. Column experiments were carried out with NBC application rates of 0.0%, 0.3%, 0.5%, 0.7% and 1.0% (by weight). It was found that the NBC changed the distribution of the soil pore structure and affected the degrees of anisotropy, fractal dimension, soil porosity and stability of soil aggregates. NBC applications also increased the water repellency of the soil and reduced the surface energy of soil particles. Regarding soil hydraulic properties, NBC applications increased the saturated hydraulic conductivity of the soil and decreased soil water retention. Increasing the amount of NBC applied to the mixing‐layer decreased cumulative infiltration values (reduced by 3.75%–13.75%). The above results reveal how NBC affects the soil pore structure and soil hydraulic characteristics, which provides a theoretical basis for the systematic evaluation of soil improvement and environmental effects of nano‐sized biochar.

Three-dimensional fractal characteristics of soil pore structure and their relationships with hydraulic parameters in biochar-amended saline soil

Effects of biochar addition on soil hydraulic properties before and after freezing-thawing

Effects of cultivation history in paddy rice on vertical water flows and related soil properties

Indirect methods for prediction of soil hydraulic properties play an important role in understanding site‐specific unsaturated water flow and transport processes, usually via numerical simulation models. Specifically, pedotransfer functions (PTFs) to predict soil‐water retention have been widely developed. However, few datasets that include unsaturated hydraulic conductivity data are available for prediction purposes. Moreover, those available employ a variety of measurement techniques. We show that prediction of soil‐water retention and unsaturated hydraulic conductivity curves from basic soil properties can be improved if hydraulic data are determined by a single measurement method that is consistently applied to all soil samples. Here, we present a unique dataset that consists of 310 soil‐water retention and unsaturated hydraulic conductivity functions, all of which were obtained from the multistep outflow method. With this dataset, neural networks coupled with bootstrap aggregation were used to predict the soil‐water retention and hydraulic conductivity characteristics from basic soil properties, that is, sand, silt, and clay content, bulk density (ρ b ), saturated water content, and saturated hydraulic conductivity. The prediction errors of water content were about 3 to 4% by volume. Unsaturated hydraulic conductivity predictions improved significantly when a so‐called performance‐based algorithm was utilized to minimize residuals of soil hydraulic data rather than hydraulic parameters. The root mean squared of residuals for predicted values of water content and unsaturated hydraulic conductivity were reduced by about 50% when compared with predicted hydraulic functions using a published neural networks program Rosetta Results from a sensitivity analysis suggest that the hydraulic parameters are mostly sensitive to sand content and saturated water content.

Indirect methods for prediction of soil hydraulic properties play an important role in understanding site-specific unsaturated water flow and transport processes, usually via numerical simulation models. Specifically, pedotransfer functions (PTFs) to predict soil-water retention have been widely developed. However, few datasets that include unsaturated hydraulic conductivity data are available for prediction purposes. Moreover, those available employ a variety of measurement techniques. We show that prediction of soil-water retention and unsaturated hydraulic conductivity curves from basic soil properties can be improved if hydraulic data are determined by a single measurement method that is consistently applied to all soil samples. Here, we present a unique dataset that consists of 310 soil-water retention and unsaturated hydraulic conductivity functions, all of which were obtained from the multistep outflow method. With this dataset, neural networks coupled with bootstrap aggregation were used to predict the soil-water retention and hydraulic conductivity characteristics from basic soil properties, that is, sand, silt, and clay content, bulk density (ρb), saturated water content, and saturated hydraulic conductivity. The prediction errors of water content were about 3 to 4% by volume. Unsaturated hydraulic conductivity predictions improved significantly when a so-called performance-based algorithm was utilized to minimize residuals of soil hydraulic data rather than hydraulic parameters. The root mean squared of residuals for predicted values of water content and unsaturated hydraulic conductivity were reduced by about 50% when compared with predicted hydraulic functions using a published neural networks program Rosetta Results from a sensitivity analysis suggest that the hydraulic parameters are mostly sensitive to sand content and saturated water content.

The scaling characteristics of soil parameters: From plot scale heterogeneity to subgrid parameterization

This paper examines the potential of soil maps and spatial information on basic soil properties for predicting soil hydraulic properties in the Shepparton irrigation region (SE Australia). For this purpose, the relationship between locally measured soil hydraulic properties and basic soil properties, and soil categories was analyzed. Pedotransfer functions developed for Australian soil were tested. Furthermore, association of field‐scale final infiltration rates with basic soil properties was investigated. Water‐retention properties, and in particular subsoil water‐retention properties, were significantly correlated with readily available basic soil properties. Spearman's rank correlation coefficients were particularly high for clay content, bulk density, and the sum of exchangeable cations Ca2+, Mg2+, Na+, and K+. Water‐retention properties were adequately predicted using Australian pedotransfer functions. Water‐transmission properties such the saturated conductivity and the final infiltration rate were overall poorly correlated with basic physical and chemical properties. Generally, median water‐transmission properties did not significantly change with soil groups and “within‐paddock variability” accounted for over half of the “within‐soil‐type variability” for many soil types. We concluded that it is feasible to regionalize water‐retention properties for the Shepparton irrigation region using basic physical and chemical soil properties, whereas the information on basic soil properties and from soil maps was insufficient to reliably estimate water‐transmission properties. It is demonstrated why field‐scale estimates of final infiltration rates, obtained by fitting a model for surface irrigation to field measurements of advance, depletion, and recession, may be better correlated with basic soil properties.

Effects of collapsing gully erosion on soil qualities of farm fields in the hilly granitic region of South China

Saline soil reduces soil productivity and exacerbates food security problem. Therefore, it is increasingly necessary to find sustainable farming practices for soil and water conservation. In this regard, conditioning the soil with biochar (BC) or attapulgite (ATP) has been proved as a feasible method of improving water conservation and soil hydraulic characteristics. However, the interactive effect of BC and ATP on saline soil is still unclear. The objective of this study was to investigate the effect of BC and/or ATP on water retention, pore size distribution, hydraulic characteristic parameter, and shrinkage strain of saline soil through laboratory trials. For this purpose, BC and/or ATP were added to the loam soil at three levels of 0%, 2%, and 4% by weight, and saturated in the sodium chloride solution with a concentration of 5 g L−1. Results showed that the Van Genuchten model accurately fitted the obtained soil water characteristic curve (R2 > 0.99). BC and/or ATP amendments increased saline soil water retention capacity compared to the unamended soil. Soil water retention capacity increased as the ATP application rate increased. The saturated water content, field water holding capacity, soil microporosity, and soil retention effect of 2%BC combined with 4%ATP were higher than other treatments. BC facilitated the reduction of soil shrinkage, and ATP increased soil shrinkage. Axial shrinkage strain was negatively correlated with the application rate of BC, and positively correlated with ATP. 4%BC combined with 2%ATP had the least shrinkage strain. The present results could serve as a theoretical foundation for soil amendments to improve soil quality.

Surface soil hydraulic conductivity and macro-pore characteristics as affected by four bamboo species in North-Western Himalaya, India

Improving the soil physical properties and relationships between soil properties in arable soils of contrasting texture enhancement using biochar substrates: Case study in Slovakia

Aims Fractal theory, a study tool popular in recent years, offers a new means to quantitatively investigate soil structure. Soil structure is the basis of soil fertility, which is the basic property of soil. It can be comprehensively reflected by soil physical, chemical and organism properties, and change of soil structure will result in changes in other soil properties. Fractal features of soil aggregate structure under natural evergreen broadleaved forest and regeneration of artificial systems is rarely studied. We chose to study natural evergreen broadleaved forest and three artificial plantations of Sassafras tzumu, Cryptomeria fortunei and Metasequoia glyptostroboides in southern Sichuan Province. Our objective was to determine a) effects of artificial regeneration on fractal features of soil aggregate structure, b) effects of different plantations on fractal dimension of soil aggregate structure, c) relationships between fractal dimension and soil physical properties, nutrient content and microbe number, and d) use of fractal dimension of soil aggregate structure for evaluating the water conservation, fertility and microbe activity of soil. Methods Soils were collected from each forest to determine a) fractal dimension of soil aggregate structure using Yang Peiling's approach and b) soil physical and chemical properties and soil microbe number. The relationship between fractal dimension and soil physical properties, nutrient content and microbe number was analyzed with regression analysis. Important findings Natural evergreen broadleaved forest and artificial regeneration resulted in increased fractal dimension of soil aggregate structure and percent of construction damage, poorer soil physical properties and reduced nutrient content and number of microbes. The higher the content of aggregates, water-stable aggregates and water-stable big aggregates in soil, the smaller the fractal dimension of soil aggregate structure. With wet sieving condition and decreased fractal dimension, the percent of construction damage decreased. There were close relationships between fractal dimension of soil aggregate structure and soil natural water content, bulk density, capillary porosity, non- capillary porosity, original infiltration coefficient, stable infiltration coefficient, content of organic matter, total-N, hydrolysis-N, total-P, available-P, total-K, available-K and the number of bacteria, fungi and actinomyces. Different stands had different effects for maintenance of soil structure, which resulted in changes of soil physical, chemical and organism properties under natural evergreen broadleaved forest and artificial regeneration, and changes of soil aggregate structure had an effect on the value of fractal dimension. This indicates that fractal dimension can be used as a comprehensive quantitative index to evaluate water conservation function, fertility states and microbe activity of soil for natural evergreen broadleaved forest and artificial regeneration. It provides a solid foundation for protecting natural evergreen broadleaved forest, choosing appropriate trees for its artificial regeneration, managing soil after artificial regeneration and choosing trees for converting farmland to forest.

Water flow through hillslopes consisting of five soil layers, with varying spatial dependence in hydraulic characteristics in the lateral plane was simulated by solving Richards' equation in three dimensions under varying rainfall intensities and for two complexities of terrain. By concepts of similar media the variability in soil hydraulic characteristics was expressed by a single dimensionless parameter, the scaling factor α. The moments of log normally distributed α were set as: Mean = 1.0 and standard deviation = 1.0. Four cases of spatial dependence of α in the lateral plane were selected for simulation, using exponential variogram functions ranging in spatial structure from radom (no spatial dependence) to large dependence (large correlation lengths). The simulations showed that the rates of subsurface flow from the 30° hillslope, during and following rainfall, were significantly enhanced with an increase in spatial dependence. Subsurface drainage was also increased with increases in rainfall intensity and slope complexity. For hillslopes the relative effect of spatial dependence in soil hydraulic characteristics was smaller with 30° horizontal pitching than without pitching. Hillslopes with a random distribution of hydraulic characteristics provided greater opportunity for soil units with differing water capacities to interact than in cases with spatially correlated distributions. This greater interaction is associated with a greater lag in subsurface flow generation. These studies illustrate some of the expected effects of spatial dependence of soil hydraulic characteristics on the integrated hydrologic response of land areas.

Divergent biotic-abiotic mechanisms of soil organic carbon storage between bulk and rhizosphere soils of rice paddies in the Yangtze River Delta.