Variation In Soil pH Research Articles

The Composition of Nitrogen-Fixing Microorganisms Correlates With Soil Nitrogen Content During Reforestation: A Comparison Between Legume and Non-legume Plantations.

Numerous reforestation projects have been conducted to improve soil fertility in degraded forests, often causing alterations to the soil microbial communities. However, it remains unclear whether microbial functional groups are affected and how these groups correlate with an increase in the nutrient contents during reforestation. We investigated the abundance and composition of free-living nitrogen-fixing microorganisms (diazotrophs) by quantifying and sequencing the marker gene nifH in bulk soils from five reforestation approaches, including legumes and non-legumes, in subtropical China. The relationships between diazotrophic community attributes and soil nitrogen (N) content [NO3−, NH4+, and microbial biomass N (MBN)] were examined under various approaches. Abundance of diazotrophs was highest in the native tree plantation (Schima spp. and Michelia macclurei) and Acacia mangium monoculture (AM), and lowest in the Pinus massoniana monoculture. The diazotrophic abundance correlated positively with soil organic matter and water content while there was a negative correlation to pH. The composition of diazotrophic community differed significantly among the five reforestation approaches examined and was closely correlated with variations in soil pH, NH4+ and water content. Diazotrophic community composition was closely related to soil NH4+ content, whereas abundance was not. The AM contained higher NH4+, NO3− and MBN contents than the other reforestation approaches, which may be associated with the indicator species of diazotrophs (Actinobacteria, Proteobacteria, and Firmicutes). However, there were more indicator species of Proteobacteria in the mixed Acacia plantation (Acacia mangium and Acacia crassicarpa) than in AM, which might have contributed to the remarkedly lower N content compared to AM. Overall, the soil N content under reforestation appeared to be more related to the composition of diazotroph community than to the abundance of diazotrophs.

Spatial and temporal changes of farmland soil acidification and their influencing factors in different regions of Guangdong Province, Chin

Based on the three datasets from 1980s, 2010 and 2015 in Guangdong Province, we analyzed the spatial and temporal variations of soil pH in farmlands in different regions of Guangdong Province and analyzed the driving factors for such variations. The results showed that the spatial distribution of soil pH in Guangdong Province changed significantly in different periods. During 1980s to 2010, soil pH showed an acidification trend with a decline of 0.3, and increased by 0.09 from 2010 to 2015, with more uneven trend and more obvious acid base differentiation. From the perspective of each region, there was generally a trend of acidification from the 1980s to 2010. From 2010 to 2015, the average pH value of farmland soil in the Pearl River Delta increased by 0.27, while that on the east wing and west wing decreased by 0.05 and 0.15 respectively, showing a unapparent change of soil pH in the mountainous area. Our results showed that soil acidification in diffe-rent areas of Guangdong Province was affected by natural factors such as soil itself and precipitation. In addition, anthropogenic factors such as acid rain, unreasonable fertilization and the planting structure of high-yielding crops were also the main causes of soil acidification. Industrialization, urbanization, mining development, and the spread of soil testing formula fertilization increased soil pH in local areas. Our results could provide theoretical guidance for controlling and alleviating soil acidification and improving the quality of cultivated land in different areas.

Environmental controls on soil pH in planted forest and its response to nitrogen deposition

BackgroundSoil pH is important for controlling many soil properties. The variation in soil pH can be associated with changes in climate, soil buffering system, nitrogen deposition, and plants. However, there still lacks a comprehensive study exploring the effects of all these factors on soil pH simultaneously. Here we aimed to investigate the environmental controls on the spatial variation of soil pH in planted forests across Northern China and reveal its response to different-forms of nitrogen deposition for different species of tree plantations. MethodsWe sampled 1980 soil profiles from 660 planted forest plots (3 profiles in each plot) in Northern China. We used correlation analyses and structure equation models (SEM) to explore the impacts of multiple environmental factors on soil pH. ResultsClimate (water balance, temperature) and soil inorganic carbon accounted for most variations of soil pH. Specifically, the concentration of hydrogen ions ([H+]) varied almost isometrically with soil inorganic carbon, which was also the major buffering system in this region. Nitrogen deposition affected both soil pH values and soil buffering system. Results from structure equation model indicated that nitrate nitrogen directly decreased soil pH, while ammonium nitrogen mostly affected soil pH indirectly through its impacts on soil inorganic carbon. The responses of soil pH to nitrogen deposition were species-specific, and conifer stands tended to have higher soil acidification rate than stands of other tree species. ConclusionsOur study provides important information for understanding mechanisms controlling the spatial pattern of soil pH in planted forests and highlights the need to develop informed policies for soil resource management under increasing threats from anthropogenic nitrogen deposition.

Monitoring soil pH variation using Polyaniline/SU-8 composite film based conductometric microsensor

Development of Poly-aniline/SU-8 (an epoxy resin) nanocomposite film based conductometric microsensor platform to measure change in soil pH is reported. For this, poly-aniline doped with dodecylbenzene sulfonic acid (PANI-DBSA) was mixed with SU-8, and spun coated on Au-IDE patterned on Si/SiO2. The nanocomposite exhibited excellent response towards change in pH against three different conditions, namely standard buffer solutions with pH ranging from 2 to 10, Red and Bentonite soils, wherein the pH varied with water (soil moisture contents) and pH with different concentrations of calcium chloride (CaCl2) solution. The films exhibited the sensitivities of 455 and 290 μS/pH towards acidic (Lower pH) and alkaline solutions (Higher pH), respectively. The sensitivities of 34 and 89 μS/pH with water for Red and Bentonite soil samples, respectively were obtained. Importantly, for Red and Bentonite soils, the sensitivities of 41 and 52 μS/pH with CaCl2 concentration were achieved. Moreover, the sensor revealed faster response kinetics with response and recovery times of ˜10 and ˜30 s respectively. Overall, the sensor exhibited good sensitivity, excellent long term stability, high repeatability and low fabrication cost. Our results demonstrated that PANI/SU-8 composite films as a potential material for the determination of soil pH variation crucial for precision agriculture.

Spatial variability in soil pH and land use as the main influential factor in the red beds of the Nanxiong Basin, China.

Soil pH is the main factor affecting soil nutrient availability and chemical substances in soil. It is of great significance to study the spatial variability of soil pH for the management of soil nutrients and the prediction of soil pollution. In order to explore the causes of spatial variability in soil pH in red-bed areas, the Nanxiong Basin in south China was selected as an example, and soil pH was measured in the topsoil by nested sampling (0–20 cm depth). The spatial variability characteristics of soil pH were analyzed by geostatistics and classical statistical methods, and the main factors influencing spatial variability in soil pH are discussed. The coefficient of variation in the red-bed areas of Nanxiong Basin was 17.18%, indicating moderate variability. Geostatistical analysis showed that the spherical model is the optimal theoretical model for explaining variability in soil pH, which is influenced by both structural and random factors. Analysis of the spatial distribution and pattern showed that soil pH is relatively high in the northeast and southwest, and is lower in the northwest. These results indicate that land use patterns and topographic factors are the main and secondary influencing factors, respectively.

Distribution of Cd and Cu Fractions in Chinese Soils and Their Relationships with Soil pH: A Meta-Analysis

Soil contamination by potentially toxic metals (PTMs) has become a public concern in China. However, the distribution and controlling factors of soil PTM fractions remain largely unknown, limiting our ability to assess their health risks and thus to make sound controlling polices. Here, we investigate the fraction distribution of cadmium (Cd) and copper (Cu) in Chinese soils and their relationships with soil pH, based on a national meta-analysis of 163 published literatures. Exchangeable Cd in southern China accounted for 19.50 ± 14.97% of total Cd, significantly (p < 0.01) higher than the corresponding 13.42 ± 6.95% in northern China. Potentially available fractions constituted about 60% of total Cd at the national scale. By contrast, about half of soil Cu existed in unavailable residual fraction. Phytoavailable (i.e., exchangeable) fraction accounted for only 2.71 ± 1.65% and 2.54 ± 1.58% of total Cu in northern and southern China, respectively. Percentages of exchangeable Cd and Cu were negatively correlated (p < 0.01) with soil pH, while potentially available fractions increased significantly (p < 0.05) with soil pH. Our results provide the first national assessment of Cd and Cu fraction distribution and their responses to soil pH variations, highlighting the necessity to consider their fraction distribution and soil properties when assessing the health risks of soil PTM contamination in China.

Evaluating the effects of elevated concentrations of urea, phosphogypsum and paper mill sludge on soil chemical, microbial and exoenzyme dynamics

Urea, phosphogypsum (PG) and paper mill sludge (PMS) are commonly used to address soil nutrient deficiency. In this study, these chemicals were used at recommended agricultural dose (RAD) along with low and high concentrations to assess their effects on soil chemical and biological quality. Urea was applied at 0.5, 1.0, 1.5 g/kg soil, PG at 50, 100, 150 g/kg soil and PMS at 5, 10, 15 g/kg soil. Significant variation in soil pH, electrical conductivity and organic carbon (OC) were noticed in response to chemical concentration and treatment types. The percent OC reduction was high in PG and PMS treated soils. Percent nitrogen and phosphorous in soil increased with the concentrations of urea and PG. Invariably, lower bacterial and fungal population were observed in treated soils relative to control. Activities of amylase, cellulase, invertase, dehydrogenase and protease were inhibited at high concentration of all test chemicals with minor deviations. The study indicated that bacterial population and exoenzymes could be useful markers to evaluate soil toxicity due to hyper concentrations of urea, PG and PMS in soil.

Evaluating the effects of elevated concentrations of urea, phosphogypsum and paper mill sludge on soil chemical, microbial and exoenzyme dynamics

Urea, phosphogypsum (PG) and paper mill sludge (PMS) are commonly used to address soil nutrient deficiency. In this study, these chemicals were used at recommended agricultural dose (RAD) along with low and high concentrations to assess their effects on soil chemical and biological quality. Urea was applied at 0.5, 1.0, 1.5 g/kg soil, PG at 50, 100, 150 g/kg soil and PMS at 5, 10, 15 g/kg soil. Significant variation in soil pH, electrical conductivity and organic carbon (OC) were noticed in response to chemical concentration and treatment types. The percent OC reduction was high in PG and PMS treated soils. Percent nitrogen and phosphorous in soil increased with the concentrations of urea and PG. Invariably, lower bacterial and fungal population were observed in treated soils relative to control. Activities of amylase, cellulase, invertase, dehydrogenase and protease were inhibited at high concentration of all test chemicals with minor deviations. The study indicated that bacterial population and exoenzymes could be useful markers to evaluate soil toxicity due to hyper concentrations of urea, PG and PMS in soil.

Effects of Pretreatment Methods of Wheat Straw on Adsorption of Cd(II) from Waterlogged Paddy Soil.

Two types of pretreatment categories, namely microwave-assisted alkalization and microwave-assisted acid oxidation, were used to synthesize novel wheat straw adsorbents for the effective removal of Cd(II) in simulated waterlogged paddy soil. A systematic adsorption behavior study, including adsorption kinetics and adsorption isotherms was conducted. Results showed that wheat straw pretreated by microwave-assisted soaking of NaOH and ethanol solution obtained the highest Cd(II) removal efficiency of 96.4% at a reaction temperature of 25 ℃, pH of 7.0, initial Cd(II) concentration of 50 mg/L, and adsorbent/adsorbate ratio of 10 g/L. Sequential extraction experiment was carried out to analyze the changes of different of Cd(II) in soil, the aim of which was to study the mobility of Cd(II) and then evaluate the toxicity that Cd(II) might bring to plants. A 60-day incubation was performed to investigate the dynamic variations of soil pH and dissolved organic carbon content over incubation time. Characterization analyses revealed the morphological changes of wheat straw adsorbents, which suggested that those pretreatment methods were of significance. This study provided an environmentally friendly way to reuse agricultural wastes and remedy Cd(II) contaminated soil.

Spatial distribution of microbial biomass and residues across soil aggregate fractions at different elevations in the Central Austrian Alps

The distribution of microbial biomass and residues and their role in carbon (C) dynamics within soil aggregate fractions in the Alps are still poorly explored. We assessed the distribution of bacterial and fungal residues and their impact on the contribution of the microbial community to C dynamics within soil aggregate size fractions at different elevations. We sampled three sites located at different elevations (1300, 1600 and 2100 m a.s.l.) and subject to different temperature and precipitation regimes in the Central Austrian Alps. Fungal biomass and residues were higher in macro-aggregate fractions compared to micro-aggregates, as indicated by the ratios ergosterol/microbial biomass C and fungal C/bacterial C, suggesting a combined but discrete contribution of fungi and bacterial residues to soil aggregate formation. A greater contribution of microbial residues to soil organic C (SOC) in macro-aggregates demonstrates the importance of microbial residues for the soil organic matter pool's capacity to function as an active nutrient sink and source reservoir. The mid- and low-elevation spruce sites were dominated by fungi and higher aggregate stability, whereas the high-elevation grassland site exhibited a stronger bacterial community as indicated by fungal C/bacterial C ratio. In addition, greater proportions of fungal and bacterial residue in SOC at mid-elevation spruce and high-elevation grassland sites respectively, indicate differential role of fungi and bacteria in SOC sequestration. However, the observed changes in microbial community and residue contribution were mainly driven by variations in soil pH, C/N ratio and fine root biomass. Our findings indicate that fungi played a key role in the internal cycling of SOC and aggregate stability at the mid-elevation spruce site, while bacterial residues were successively activated and contributed increasingly to SOC sequestration at the high-elevation grassland site.

Urbanization and biodiversity of arbuscular mycorrhizal fungi- The case study of Delhi, India

Increasing urbanisation is widely associated with decline in biodiversity of all forms. The aim of the present study was to answer two questions: (i) Does rapid urbanization in Delhi (India) affect biodiversity of arbuscular mycorrhizal (AM) fungi? (ii) If so, how? We measured the AM fungal diversity at nine sites located in Delhi forests, which had different types of urban usage in terms of heavy vehicular traffic pollution, littering, defecation and recreational activities. The study revealed a significant decrease in AM fungal diversity (alpha diversity) and abundance measured as spore density, biovolume, mean infection percentage (MIP) in roots, soil hyphal length and easily extractable glomalin related soluble proteins (EE-GRSP) at polluted sites. Non-metric multidimensional scaling (NMDS) and nested PERMANOVA, revealed significant differences in AM fungal community structure which could be correlated with variations in soil moisture, temperature, pH, carbon, and nitrogen and phosphorus levels. BEST (biota and environmental matching) analysis of biological and environmental samples revealed that soil temperature and moisture accounted for 47.6 % of the total variations in the samples. The study demonstrated how different forms of human activities in urban ecosystems of Delhi are detrimental to the diversity and abundance of AM fungi.

Effects of grazing exclusion on soil–vegetation relationships in a semiarid grassland on the Loess Plateau, China

AbstractGrazing exclusion (GE) is regarded as an effective practice to restore degraded grasslands. However, the patterns of vegetation recovery and related regulating factors in response to fencing have not been fully recognized. Hence, quantitative analysis of vegetation–soil relationships in response to GE was conducted in a semiarid grassland located on the Loess Plateau, North China. The results revealed that enclosure establishment significantly increased vegetation cover, height, and productivity but reduced plant diversity. Soil bulk density (0–20 cm) and pH (0–50 cm) clearly decreased after GE, whereas soil water (0–20 cm), organic matter (0–10 cm and 30–40 cm), and nutrient concentration (0–20 cm) increased significantly. Redundancy analysis of vegetation and environmental variables suggested that edaphic properties, including soil water, soil pH, total N, bulk density, and organic matter, was associated with plant community composition. Subsequent canonical correlation analysis indicated that soil bulk density, organic matter, and total N played an important role in shaping vegetation patterns in response to fencing whereas variations in soil pH and total N were the major contributors to variations in grazing rangeland. This work emphasized that fencing is a positive grassland management approach and suitable changes in grazing stock and soil variability are required to quantify vegetation recovery in response to grazing exclusion.

Changes in the pH of paddy soils after flooding and drainage: Modeling and validation

The precise determination and characterization of soil acidity was the basis for a robust and realistic assessment of many biogeochemical processes. Samples of twenty paddy soils with varying soil properties were subjected to a successive flooding and drainage period in this work. The soil pH was measured in situ at intervals, and soil samples were collected after each pH measurement during drainage for an analysis of the moisture content. During the flooding period, the pH for soils with an initial pH < 6.5 increased to approximately 7.0, and for soils with an initial pH > 6.5, the pH first decreased and then increased to approximately 7.0. The changes were reversed during the drainage period, as the pH of acidic soils decreased linearly with the decreasing soil moisture content, while neutral-to-alkaline soils showed the opposite pattern. The developed predictive models indicated that the initial soil pH, cation exchange capacity, content of organic matter and flooding or drainage time were the main factors that controlled the change of soil pH after flooding and drainage. The models explained 82% and 67% of the soil pH variability after flooding and drainage, respectively. The predictive model of the soil pH change after flooding was further validated and was found to be reliable on the basis of a number of independent data points for which the predicted soil pH after flooding was within the 95% prediction intervals of the observations. The results suggested that the soil pH, which was determined in the laboratory using air-dried samples, could be corrected to the in situ pH through the developed predictive models.

Digital soil assessment for quantifying soil constraints to crop production: a case study for rice in Punjab, India

AbstractAssessments of land capability for particular functions such as food production need to allow for uncertainties both in the criteria used to specify the function and in information on relevant soil properties. In this paper, we evaluate the use of digital soil assessment (DSA) for dynamic assessment of soil capability allowing for both uncertainties and spatial variability in soil properties and flexibility in the values of assessment criteria. We do this for soil constraints to rice production in the state of Punjab, India, where soil salinity and alkalinity are potentially important constraints to cropping. In DSA, spatial predictions of soil properties and associated uncertainties made with digital soil mapping (DSM) are used to assess soil functions. We use a combination of DSM and Monte Carlo simulation methods to estimate the spatial variation in soil electrical conductivity (ECe) and pH to 20 cm depth in soils across Punjab. We then use the estimates and associated uncertainties to assess the likelihood that soil salinity or alkalinity or both could constrain rice production. Results show that allowing for prediction uncertainties of soil attributes results in far smaller areas affected by salinity (1.2 vs. 2.0 Mha) and alkalinity (3.0 vs. 3.2 Mha). Results also show the importance of correctly setting threshold values for constraint criteria and the flexibility of the DSA approach for setting thresholds.

Geographical pattern of methanogenesis in paddy and wetland soils across eastern China

Large variation of CH4 emissions from paddy and wetland ecosystems exists across different geographical locations in China. To obtain mechanistic understanding of this variation, we investigated the dynamics of methanogenesis over the course of glucose degradation in fourteen paddy field soils and five wetland soils collected from different regions of China. The results revealed that the maximal rate (2–3 mM per day) and the total amount (25–30 mM) of CH4 produced were similar across soil samples. The lag phase of methanogenesis, however, differed substantially with the shortest lag phase of 4 days in a paddy soil from north China and the longest of 32 days in a soil from south China, and this difference reflected a general geographical trend among all soils tested. Nitrate was reduced completely within 4 days in all soils. The reduction of Fe(III) and sulfate was completed after 21 days and 29 days, respectively. The depletion time of Fe(III) and sulfate were positively correlated with the lag phase of methanogenesis. Competition for common substrates between methanogens and iron and sulfate reducers, however, does not explain this coincidence because a slow production of CH4 was detected at the very beginning. It appears that the geographical variations in methanogenesis and the reduction of ferric iron and sulfate are related to the variation in soil pH but not to temperature, soil organic C and nutrient conditions in paddy and wetland soils across eastern China.

Site-specific assessment of spatial and temporal variability of sugarcane yield related to soil attributes

The adoption of information technology (IT) and precision agriculture (PA) has converted agricultural fields into data sources. However, the transformation of data into knowledge for decision making remains a major challenge. In the Brazilian sugarcane industry, the current use of PA technology is very far from its full potential for site-specific management, mainly because yields are not temporally or spatially monitored. The objective of the present study was to investigate the relationship between the physical and chemical properties of soils and sugarcane yield, thereby identifying the soil parameters that determine the final productivity of the field. Two sugarcane fields were monitored from 2011 to 2014. During the crop season, soil samples and yield data were collected annually. A random forest algorithm was applied to investigate the influence of different soil attributes on yield using data that were collected spatially over the study period. The results showed that the amount of available soil organic matter (OM), clay content and cation exchange capacity (CEC) are important factors impacting sugarcane yield variation. Furthermore, it was found that the temporal variability in yield is caused mainly by the variability in soil pH over the study period. The results indicated that when OM increased over time, there was greater phosphorus availability. Large volumes of spatial and temporal data, together with data mining techniques, allowed the extraction of knowledge and the creation of specific management zones in the field to support the decision-making process for producers.

Photosynthesis and growth responses of Fraxinus mandshurica Rupr. seedlings to a gradient of simulated nitrogen deposition

During an N-deposition simulation experiment, we showed that low to medium addition of N had beneficial effects on growth and photosynthetic rates of Fraxinus mandshurica Rupr. seedlings, while beyond a threshold of 80 kg N ha −1 year −1 , performance plateaued and even declined at higher immissions. Temperate forests are shifting from naturally N-limited toward N-saturated status with increasing N deposition. Yet, our knowledge regarding how seedling growth and physiology respond to excessive N input in temperate tree species remains very limited. The objective of this study was to examine growth and photosynthetic responses of F. mandshurica seedlings to a gradient of simulated N deposition. We conducted a 4-year study to investigate growth and photosynthetic responses of F. mandshurica seedlings to a large gradient of simulated N deposition (0, 20, 40, 60, 80, 100, and 120 kg N ha−1 year−1). Biomass accumulation and allocation, photosynthetic gas exchange, expression, and activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in leaves were determined during the fourth growing season. Soil biochemical properties were measured to link them to the alterations in growth and photosynthetic traits across the N addition gradient. Seedling growth and photosynthesis were dependent upon the rates of N deposition. The maximum rate of carboxylation (V c,max) and the net photosynthetic rate under saturating light (A sat) reached a maximum under 60 kg N ha−1 year−1. By contrast, high-level N inputs (100 and 120 kg N ha−1 year−1) resulted in suboptimal values in biomass and photosynthetic activity. Nitrogen deposition also modulated the activity and expression of Rubisco in leaves with a maximum around 80–100 kg N ha−1 year−1. Redundancy analysis (RDA) showed that the changes of seedling growth and photosynthesis along the gradient of N deposition were mostly attributed to the variations of soil pH and total N content. Our data suggest that the threshold of N deposition is about 80 kg N ha−1 year−1 for F. mandshurica seedlings in this region. Excessive N input decreased performance on the seedling growth and photosynthesis.

Will spent mushroom substrate application affect the dissipation and plant uptake of phthalate esters?

The objective of this study was to investigate whether spent mushroom substrate (SMS) amendment was an appropriate way to reduce di(2-ehylhexyl) phthalate (DEHP) and di-n-butyl phthalate (DnBP) contents in soil and whether SMS could reduce DnBP accumulation in bok choy (Brassica rapa subsp. chinensis). Microcosm and pot experiments were carried out to study the influence of spent Agaricus bisporus substrate application on DnBP and DEHP dissipation in soils and plant uptake of DnBP. Variations in soil pH and enzyme activities were determined. The concentrations of phthalate esters (PAEs) in soils, bok choy, and atmosphere were examined with gas chromatography or gas chromatography–mass spectrometry. Adding sterilized or non-sterilized SMS can increase soil pH and urease activity, and non-sterilized SMS can promote soil laccase activity. The results show that the dissipation of DEHP is accelerated after incubation with SMS for 25 days; however, little effect can be found with continuing incubation due to low DEHP bioavailability. In this research, SMS amendment exhibits no effect on DnBP dissipation in soils and DnBP accumulation in bok choy. It was proposed that atmospheric deposition of DnBP might be the main source of DnBP in bok choy in the study, since equivalent amounts of DnBP were detected in the vegetables grown in soils with or without DnBP spiking. This study indicates that the application of SMS as an organic fertilizer is less likely to affect the fate of PAEs in soils, and proper strategies should be conducted to reduce PAE levels in atmosphere to control PAE contamination in vegetables.

Spatial variability of soil texture fractions and pH in a flood plain (case study from eastern Iran)

Flood plain ecosystems show significant soil spatial variability. Understanding spatial variations of soil texture fractions and pH in flood plains is necessary for ensuring proper management of these plains, because these properties influence soil structure, fertility, hydraulic conductivity, infiltration, and erosion. In the present study, the distribution of soil texture fractions and pH was investigated in a flood plain with intensive wind erosion for an area of ~41,000ha in Zahak county of Sistan and Baluchestan province in eastern Iran. A random forest technique was used to link environmental variables and the studied properties. 460 soil samples were collected from 0 to 30cm depth across a 750m grid. 361 samples were used for training and 99 for independent validation. Results showed that the distance from the river was the most important environmental variable for predicting soil texture fractions and pH in the study area. Natural channel networks, elevation, valley depth, LS factor, NDSI, vertical distance to channel networks, slope, wind effect, NDVI, and brightness were other important variables. The maps produced indicated a higher sand content near Sistan River. Clay, silt, and pH contents increased with distance from Sistan River. Results showed that clay and pH had a similar distribution in the study area. The values of RMSE for the maps of estimated sand, silt, clay, and pH in validation data were respectively 21.40, 17.45, 6.06 and 0.45. These values of RMSE for sand, silt, clay, and pH were respectively 10.3, 10.7, 15.1, and 13.3% lower than a simple model (mean model). Results indicated that using the distance from the river and channel networks as a variable in digital soil mapping can increase the accuracy of the predictive maps of soil properties in flood plains.

Soil pH is a major driver of soil diazotrophic community assembly in Qinghai-Tibet alpine meadows

The biological nitrogen input is essential for nutrient enrichment in natural grassland ecosystems. This process is carried out by diazotrophic microbes. However, the patterns and assembly processes of diazotrophic communities in grassland soil are not fully understood, especially in alpine meadows of high-altitude Qinghai-Tibet Plateau (QTP). In this study, the diversity patterns and community assembly processes of soil diazotrophic communities were investigated in QTP alpine meadows. The results showed that different diazotrophic taxa responded differentially to the increase of soil pH, and α-diversity increased linearly with pH values. Diazotrophic community turnover was enhanced by the increased variation in soil pH, moreover, it also showed strong response to mean annual temperature. Null model analysis showed that deterministic processes were most important in shaping the phylogenetic community structure and assembly of diazotrophs. Diazotrophic communities of each sampling site tended to be phylogenetically clustered, while communities at high pH soils (pH 7.00 to 8.00) showed more clustering than those of low pH (pH 5.02 to 7.00). The soil pH is a main driver in structuring soil diazotrophic community assembly by regulating the relative importance of deterministic and stochastic processes in QTP alpine meadows.