Reference Bulk Density Research Articles

Exploring the Influencing Factors of Net Ecosystem Productivity (NEP) Based on Random Forest and SHAP

As global climate change intensifies, Net Ecosystem Productivity (NEP) serves as a crucial indicator for measuring the carbon absorption and release of ecosystems, playing a vital role in understanding the carbon cycle and shaping climate policy. The Northwestern region of China, characterized as a typical inland arid and semi-arid area, is particularly sensitive to global changes. Understanding the environmental drivers of NEP in this region is critical for both regional ecological protection and global environmental management. This study utilized environmental data from five provinces in Northwestern China, applying the Random Forest (RF) model and SHapley Additive exPlanation (SHAP) method to analyze the primary environmental factors influencing NEP. The research integrated climatic data (including temperature, precipitation, wind speed, and solar radiation), soil characteristics such as pH, organic carbon content, and soil texture, topographic attributes elevation and slope, and a Human Footprint. The RF model identified significant environmental factors impacting NEP, and SHAP values were used to explain the specific contributions of these factors. Furthermore, multiple linear regression analysis revealed interactions among environmental factors. The results indicate that solar radiation (Srad), precipitation, topsoil reference bulk density (T_REF_BULK), temperature, and the topsoil clay fraction (T_CLAY) significantly influence NEP. Notably, interactions between aspect and T_CLAY, aspect and T_REF_BULK, as well as the human footprint (HFP) and Srad, also show significant impacts on NEP. This study confirms that solar radiation, precipitation, soil characteristics, and human activities are the primary environmental drivers of NEP in the Northwest region of China, with solar radiation playing the most critical promotional role. The findings not only provide a scientific basis for the management of ecosystems in Northwestern China but also offer references for formulating global climate change mitigation strategies and estimating the global carbon budget. Future research should further explore the specific mechanisms and interactions of these drivers across different ecosystems to more comprehensively understand and predict the trends in NEP changes.

Effect of hydrocarbonic pollutants on the stability and soil water repellency intensity: A case study in Bandar Abbas Oil Refinery, Hormozgan province, Iran

Background: The contamination of soil and water with hydrocarbonic pollutants is a major environmental problem. Soil water repellency will interrupt water infiltration, and may decline plant growth and potentially trigger soil erosion. The aim of this study was to investigate the effect of soil and water contamination by oil on soil water repellency, where the soil has been oil-contaminated due to mismanagement of the lands surrounding the refineries, and many of the trees in the area have dried up. Methods: Water drop penetration time test (WDPT) was performed on contaminated soils. To investigate the effect of the surface water contamination on soil, handmade soil samples were collected and successive dry/wet cycles were applied to them by contaminated and non-contaminated waters. Subsequently, soil water repellency tests, including molarity of ethanol droplet (MED), water and ethanol sorptivity were performed on soil samples. The soils were passed through a 2 mm sieve after being air-dried and the soil texture was determined by pipette method. The SWR was measured by WDPT in the area contaminated with petroleum compounds and 7 to 10 replicates were assigned to each location. In order to determine the effect of water contamination on the area soil and to measure water repellency in the laboratory, disturbed soil samples (36 samples) with a bulk density equal to 80% of the reference bulk density were prepared. Results: The results showed that soil oil-contamination causes water repellency, increased WDPT, a significant increase in water repellency index, and a significant decrease in cosθ at the level of 0.001. The effect of water contamination on the indices and cosθ were statistically significant at the 0.001 and 1% levels, respectively. Therefore, contaminated water increased the water repellency of the soil after successive dry/wet cycles. Conclusion: Significant positive correlations between organic and water repellency indices and significant negative correlations between cosθ and organic indices indicate the effect of oil-contamination of water and soil on creating and increasing the intensity of soil water repellency.

Combined effects of multiple factors on spatiotemporally varied soil moisture in China’s Loess Plateau

Identifying the factors controlling spatiotemporally varied soil moisture (SM) is fundamental for water resources management, but the combined effects of multiple factors across multiple timescales have been rarely considered. The objective of this study is to identify the factor(s) individually or simultaneously controlling SM in China’s Loess Plateau, a region with arid climate experiencing substantial vegetation change because of a revegetation project implemented in 1999. With SM product derived from Global Land Evaporation Amsterdam Model Version 3.0a, we investigated large-scale SM dynamics for surface soil (0–10 cm) and root zone (10–250 cm) for 1982–2015, and considered environmental factors at the regional scale (e.g. precipitation PCP, mean temperature TMP, potential evapotranspiration PET, normalized difference vegetation index NDVI, land use types, elevation, slope gradient, clay fraction, and reference bulk density) and at the global scale (e.g. El Niño/Southern Oscillation ENSO, Pacific Decadal Oscillation PDO, North Atlantic Oscillation NAO, and Arctic Oscillation AO). Spatially, the mean SM in both surface and deep layers decreases with a gradient from the southeast to the northwest. Temporally, SM tends to decrease in the southeast while increases in the northwest, implying a pattern of “wet gets drier, dry gets wetter”. Individually, PET and ENSO are the most important regional and global factors controlling SM, respectively. Simultaneously, PET, PCP, and NDVI have combined effects, while the global factors exhibit no combined effects. The regional and global factors both influence SM at a scale of 8–16 months across the whole study period. In addition, the effects of regional factors exhibit at the scale of 32–64 months but tend to disappear after 1999. The global factors have smaller impacts than regional factors, but they tend to indirectly influence SM through perturbing regional climate. The identified factors highlight the complicated changes and controlling factors of SM, which provides important information for land use management and water resources management. The employed methods are effective in identifying the combined effects of multiple factors and can be extrapolated to other regions.

Modified Water Displacement Method and its Use for Determination of Bulk Density of Porous Materials

In this research work, a modified water displacement method (MWDM) was designed and used in addition to geometry method (GM) to measure the bulk volume and then determine the bulk density values of asbestos ceiling board, cardboard paper, chalk, clay (compacted) and gypsum board that have been sun-dried to constant weight. The mean bulk densities determined by both methods were compared with the reference bulk density values of the same porous materials obtained in this work using standard test procedure in accordance with ASTM D6683-14. It was observed that, for all the tested porous materials, the percentage error in the mean bulk density values ranged from 2.3% to 49.6% when using GM and 0.9% to 5.7% by using the MWDM. Also, at 0.05 level of significance with a degree of freedom of 3, correlation coefficients of 0.7430 and 0.9955 were obtained in the cases of GM and the MWDM respectively. Again, all other analyses performed similarly revealed that the mean bulk densities obtained by the MWDM only were in close agreement with their corresponding reference values, thereby implying that apart from being cost-effective, the MWDM is better than GM in terms of accuracy, reliability, and validity. More importantly, it is noteworthy that even if the glass cylinder available for use is ungraduated, this MWDM can be employed to obtain accurate, reliable and valid bulk density values of porous materials in order to enhance thorough physical characterization, proper selection and suitable applications of such materials.

Predicting the relative density from on-the-go horizontal penetrometer measurements at some arable top soils in Northern Switzerland

This study sought to develop empirical models to predict soil relative density (ρrel) from measurements of horizontal penetrometer resistance (PR) and soil water content (θg) in a wide range of soil textures. This permits the comparison of the state of soil compactness in different soil textures. It was hypothesised that model coefficients would be texture-dependent when soil compactness was expressed as bulk density (ρd) and that a model with constant coefficients could be obtained when soil compactness was expressed in terms of ρrel (obtained as the ratio of ρd to reference bulk density (ρref)). Field measurements were conducted in 2014 using a horizontal penetrometer at 0.25m depth in 10 fields in Switzerland with a wide range of soil textures covering sandy loam, silt loam, loam, clay loam and clay (clay concentration, (CC)=153–585gkg−1 and organic matter concentration, (OM)=9–168gkg−1). At selected locations along the penetrometer measurement transects, cylindrical soil cores were sampled for determination of soil texture, OM, θg and ρd. Soil water potential and effective stress (σ') were also estimated for each location. Standard Proctor tests were performed on eight soils with variable textures. Proctor density was well described as a function of CC and OM (R2adj=0.97, RMSE=0.046Mgm−3) and was used as reference density to obtain ρrel. From this we developed a model for prediction of ρrel from PR and σ′ that allows comparisons between soils without changes in model coefficients. However, σ' cannot be obtained from on-the-go measurements and the model is therefore of limited value for soil compaction mapping. A model for estimating ρrel from PR and θg yielded satisfactory predictions (R2adj=0.66, RMSE=3.3%), although θg is a texture-dependent measure of soil water that cannot be compared across soils. Moreover, ρd was well predicted from PR and θg (R2adj=0.93, RMSE=0.05Mgm−3), possibly because all our measurements were carried out at similar soil water potential, which implies that θg carries soil textural information. Future research should test the proposed equations for a wide range of soil water potential values. The findings presented can be of use in developing measurement systems for mapping soil compactness that combine the proposed prediction functions with horizontal penetrometer and water content sensor systems.

Soil Compaction Assessment in Sugarcane Fields under Different Planting Conditions Using Soil Bulk Density, Relative Bulk Density and Cone Index

Considering soil compaction problem in sugarcane fields due to using heavy harvester and haulout equipment under unsuitable moisture conditions, this research aims to assess soil compaction in sugarcane fields located in Da'balKhazaei Plantation unitofSugarcane Development and By-product Company, Ahvaz. Undisturbed soil samples from the furrow (wheel tracks) were collected for measuring soil water content and bulk density. Considering the changes in soil texture of sugarcane fields, for expressing the degree of soilcompactness, in addition to soil bulk density (BD), relative bulk density (BD divided by reference BD) was also determined. The change in soil mechanical resistance with depth was determined by a cone penetrometer. Results showed that most of soil BD values measured in the sugarcane fields were in the range of small root development scale (high limitation). Comparingthe calculated RBD values with optimum value (0.85), it was observed that most of the values were higher than the optimum values recommended for root growth. This shows excessivesoil compaction in the sugarcane fields. The values of cone indices measured in soil profiles indicated that most of the values were higher than either limiting (2 MPa) or critical (3 MPa) values for root growth. Therefore, for improving soil physical fertility and achieving sustainability in crop production, management of farm machinery traffic in sugarcane fields, especially at the harvest time, needs to be reconsidered.

Impacts of land use on soil organic matter and degree of compactness in calcareous soils of central Iran

AbstractThis study was conducted to investigate the impact of land use (dryland farming, grassland and irrigated farming) on bulk density, (ρb) and relative bulk density (ρb‐rel), and to study the relationships between ρb and ρb‐rel, respectively, and soil organic matter content (OM) and soil texture at 100 locations in calcareous soils of central Iran. The ρb–rel was expressed as the ratio of ρb to a reference bulk density, ρbef. By considering ρb‐ref an inherent soil property that is dependent on soil texture but not on OM, the combined effects of OM due to land use and compaction (due to agricultural machinery) on the degree of compactness could be explored. Multiple linear regression was used to derive pedotransfer functions for predicting ρb and ρb‐rel. It was found that ρb‐rel is strongly affected by OM, and a strong correlation was obtained between ρb‐rel and the ratio of OM to clay content. The predictive performance of the multiple regression models was poorest for irrigated farming, which might be explained by intensive soil disturbance by tillage in irrigated farming. The main effect of land use was on OM, and consequently, the degree of compactness was mainly controlled by OM. The greatest OM and least ρb‐rel were measured in irrigated farming. Dryland farming had the least OM and the greatest ρb‐rel.

Degree of compactness, soil physical properties and yield of soybean in six soils under no-tillage

The ‘degree of compactness’ is a useful parameter to study soil compaction and represents the current bulk density in relation to the bulk density of the same soil in a reference state. The objectives of this study were to: (i) determine the best compression stress to establish the reference bulk density in the uniaxial compression test using undisturbed samples; (ii) quantify the effect of texture on degree of compactness, and (iii) evaluate the influence of degree of compactness on selected soil physical properties and crop yield. Six soils under no-tillage from southern Brazil were used and the reference bulk density was evaluated on soil samples equilibrated to the matric suction of 33 kPa and subjected to uniaxial compression test. Soil macroporosity, mechanical penetration resistance, root growth, and yield of soybean were also evaluated. For undisturbed soil samples, stresses ≥800 kPa (particularly the stress of 1600 kPa) are appropriate to determine the reference bulk density. Degree of compactness is independent of clay content and is associated with changes in soil physical properties. A degree of compactness ~100% restricted root growth of soybean, whereas the highest soybean yield was obtained with a DC of 82% for Alfisols and Ultisol, and 85% for Oxisols.

Estimation of reference bulk density from soil particle size distribution and soil organic matter content

Abstract Crop growth and yield are affected by soil compactness. Rather than using bulk density, it has been suggested that soil compactness be described by relative bulk density, e.g. the degree of compactness, previously defined as the ratio of bulk density ( ρ ) to reference bulk density ( ρ ref ). This study investigated relationships between ρ ref as defined by Hakansson (1990) and soil particle size distribution (PSD) by analysing soil data from 171 experimental sites in Sweden, two in Poland and three in Finland. PSD was characterised either by the common size fractions (i.e. clay, silt and sand content) or by fitting a (continuous) mathematical function to the experimental PSD data. The Rosin–Rammler equation was used for the latter, and the PSD was characterised by the Rosin–Rammler parameters α and β . We present equations for estimation of ρ ref from either soil textural classes or from α and β in combination with soil organic matter content (OM). It was shown that ρ ref is largely controlled by OM. The best model (i.e. the model with the smallest value of Akaike Information Criterion) was found to be one that estimates ρ ref from α , β and OM. The regression models for calculation of ρ ref presented here could be incorporated into models for calculation of crop yield losses due to soil compaction. Furthermore, we found good agreement between ρ ref and values for critical bulk density for root growth reported in the literature, indicating that ρ ref is a critical bulk density for root growth.

Reference bulk density and critical degree-of-compactness for no-till crop production in subtropical highly weathered soils

The concept of degree of compactness (DC), referred to as field bulk density (BD) as a percentage of a reference bulk density (BD ref), was developed to characterize compactness of soil frequently disturbed, but for undisturbed soil such as under no-tillage critical degree of compactness values have not been tested. The objective of this study was to compare methods to determine BD ref and limits of DC and BD for plant growth under no-tillage in subtropical soils. Data from the literature and other databases were used to establish relationships between BD and clay or clay plus silt content, and between DC and macroporosity and yield of crops under no-tillage in subtropical Brazil. Data of BD ref reached by the soil Proctor test on disturbed soil samples, by uniaxial compression with loads of 200 kPa on disturbed and undisturbed soil samples, and 400, 800 and 1600 kPa on undisturbed soil samples, were used. Also, comparisons were made with critical bulk density based on the least limiting water range (BDc LLWR) and on observed root and/or yield restriction in the field (BDc Rest). Using vertical uniaxial compression with a load of 200 kPa on disturbed or undisturbed samples generates low BD ref and high DC-values. The standard Proctor test generates higher BD ref-values, which are similar to those in a uniaxial test with a load of 1600 kPa for soils with low clay content but lower for soils with high clay content. The BDc LLWR does not necessarily restrict root growth or crop yield under no-tillage, since field investigations led to higher BDc Rest-values. A uniaxial load greater than 800 kPa is promising to determine BD ref for no-tillage soils. The BD ref is highly correlated to the clay content and thus pedotransfer functions may be established to estimate the former based on the latter. Soil ecological properties are affected before compaction restricts plant growth and yield. The DC is an efficient parameter to identify soil compaction affecting crops. The effect of compaction on ecological properties must also be further considered.

Influences of degree of compactness and matric water tension on some important plant growth factors

The degree of compactness ( D) has been defined earlier as the dry bulk density of a soil in percent of a reference bulk density obtained by a standardized uniaxial compression test on large samples at a stress of 200 kPa. It was primarily aimed for use in annually disturbed soil layers. Field experiments have demonstrated its usefulness for characterizing the state of compactness from a crop production point of view, but knowledge is lacking regarding the relation between D and various plant growth factors. While the bulk density or porosity optimal for crop growth has varied considerably between soils, the optimal D-value has been virtually independent of soil texture. This led to a hypothesis that critical limits of penetration resistance (3 MPa) and air-filled porosity (10%, v/v) are similarly related to the D-value in most soils. With the objective to test this hypothesis, the positions of these limits as functions of the D-value and the matric water tension were studied in four soils with clay content ranging from 70 to 220 g kg −1. In all of them, the positions of the critical limits were similar. The higher the D-value, the more limited was the tension range offering adequate conditions for crop growth, the higher was the water tension (the lower the water content) at which aeration became critical, and the lower was the water tension (the higher the water content) at which penetration resistance became critical. The effects of critical soil conditions were reflected in increased stomatal resistance of plants grown in soil with a high D-value. The results provide basic information for modeling the relationships between soil compactness and plant growth.

Effects of uniaxial stress on the physical properties of four Swedish soils

The effects of uniaxial stress and soil water content at the time of compression on the degree of compactness ( D), defined as the bulk density expressed as a percent of a reference bulk density, and on other soil physical properties were investigated. Stresses of 25, 50, 100, 200 and 400 kPa were applied to large, loose soil samples of light, heavy and humus-rich clays as well as loamy sand in a laboratory experiment. D increased approximately with the log of applied stress over the range of gravimetric water contents normally encountered during field work. Thus, a modified version of the compression model proposed by [Larson, W.E., Gupta, S.C., Useche, R.A., 1980. Compression of agricultural soils from eight soil orders. Soil Sci. Soc. Am. J., 44: 450–457] could be used to compute the stress that gives the optimal degree of compactness for crop growth. The soil compression index, which shows the susceptibility of a soil to compaction, was greatest for the heavy clay and about 30% less for the humus-rich clay, even though they had almost the same clay content. For the loamy sand, D was greatest in the driest sample (water content = 0.01 kg kg −1). In clayey soils, samples at a water content of < 0.20 kg kg −1 were not tested, and the maximum value of D was reached when the soils were compressed at or slightly below field capacity but, as the uniaxial stress increased, the soil water content at which maximum D was reached decreased. In wet clayey soils, soil matric tension was influenced by compaction. Stresses of 200 kPa and above caused the air permeability to fall below a value that is regarded to be critical with respect to crop growth.

A method for characterizing the state of compactness of the plough layer

A method for characterizing the state of compactness of the plough layer is described. The “degree of compactnes” is defined as the ratio (%) of the dry bulk density of the soil and the dry bulk density of the same soil in a compacted reference state. The bulk density in the field is determined by a frame-sampling technique. The reference state is determined by a drained, uniaxial compression test using wet soil samples which are loosely filled in an oedometer and loaded with a pressure of 200 kPa until drainage ceases. In a series of 100 field experiments with spring barley this method was method was used for characterizing the state of compactness of the annually tilled soil layer. On mineral soils, maximum crop yield was obtained at the same degree of compactness irrespective of soil type, which was the main goal of the method. For organic soils a modification of the procedure for determining the reference bulk density is required.

Using bulk density to calculate soil water changes in soils with shrinkage cracks

Apparent bulk �a measured between shrinkage cracks is used to assess vertical shrinkage on the assumption that soil cores shrink isotropically from a fully swollen reference bulk density pr. A fully swollen depth increment �X corresponds to an increment �x = (�t/�a)1/3 �X at any other water content, field bulk density (which includes cracks) is given by and the fractional area of shrinkage cracks is 1 - (�t/�a) 2/3. When gravimetric water contents are known, volumetric water contents can be calculated in terms of the fully swollen reference depth scale and changes in water contents of soil layers obtained. The procedure applies to any soil, from rigid through to one which undergoes normal shrinkage, provided the above assumption is satisfied. A BASIC program to perform the calculations is given.