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Clay CEC is one of identification indexes of the LAC-ferric horizon which is the diagnostic horizon of ferrosols in Chinese Soil Taxonomy, and it is defined as soil CEC × 1000/clay content, rather than the measured CEC of the extracted clays; however, such a calculation method would definitely lead to an overestimation of clay CEC because it doesn’t remove the contribution to soil CEC from other soil parameters. In this study, the physiochemical data of the subhorizons from 82 soil series in the tropical and subtropical regions in south China were used, clay CEC was calculated according to the current formula and measured after clays being extracted, the measured and calculated clay CEC were compared, the influencing factors were analyzed for their difference, and the new algorithms were established for clay CEC. The results showed that the measured clay CEC was 21.86% - 99.53% with a mean of 66.88% of the calculated one (significantly lower at p 2O3 (the contribution was 52.51% and 25.36%, respectively). By comparison of established regression models of clay CEC with other soil parameters, two new algorithms were recommended for clay CEC as follows: 1) Clay CEC = 10.32 − 0.14pH − 0.05OM − 0.11Fe2O3 + 0.01Silt − 0.01Clay + 1.17CECsoil, R2 = 0.705, P −3.40 + 0.01Sand + 0.02Silt + 1.05CECsoil, R2 = 0.589, P < 0.01).

Cation exchange capacity (CEC) is one of the most important properties of soils. The NH4OAc (pH = 7.0) exchange method is usually recommended to determine CEC (CEC1) of all soils with different pH values, particularly for studies on soil taxonomy. But comparatively the BaCl2-MgSO4 forced-exchange method is more authentic in determining CEC (CEC2) of tropical and subtropical highly-weathered acid soils. But so far little is known about the difference between CEC1 and CEC2. In this study, the physiochemical data of 114 acid B horizon soils from 112 soil series of tropical and subtropical China were used, CEC1 and CEC2 were determined and compared, the influencing factors were analyzed for the difference between CEC1 and CEC2, and then a regression model was established between CEC1 and CEC2. The results showed that CEC2 was significantly lower than CEC1 (p 2 was 14.76% - 63.31% with a mean of 36.32% of CEC1. In view of the contribution to CEC from other properties, CEC2 was mainly determined by pH (45.92%), followed by silt (21.05%), free Fe2O3 (17.35%) and clay contents (12.76%), CEC1 was mainly decided by free Fe2O3 content (40.38%), followed by pH (28.39%) and silt content (27.29%; and the difference between CEC1 and CEC2 was mainly affected by free Fe2O3 (50.92%), followed by silt content (26.46%) and pH (21.80%). The acceptable optimal regression model between CEC2 and CEC1 was established as CEC2 = 2.3114 × CEC11.1496 (R2 = 0.410, P 2-MgSO4 forced-exchange method is recommended in determining CEC of the highly-weathered acid soils in the tropical and subtropical regions.

The removal of organic matter and iron oxides could increase and decrease soil CEC in tropical and subtropical regions, but the quantitative information is insufficient so far about the change of soil CEC, the influence factors and their contribution. In this study, the subhorizon soils of 24 soil series in the tropical and subtropical China were used, pH, particle size composition, organic matter, iron oxides of these samples were measured, and also CECs were measured and compared for the original soils and after the removal of organic matter and iron oxides. The results showed that, compared with CEC of the original soil, the eliminating organic matter increased soil CEC significantly by 2.28% - 56.50% with a mean of 24.02%, but the further obliterating iron oxides decreased soil CEC significantly by 0.75% - 20.30% with a mean of 7.73%. CEC after the removal of organic matter and iron oxides had positive correlation with iron oxides (p − 0.546pH − 0.024OM + 0.053FexOy − 0.001Silt + 0.007Clay + 0.972CECoriginal (R2 was 0.923, RSME was 1.55 cmol(+)∙kg−1, p − 0.546pH − 0.024OM + 0.053FexOy − 0.001Silt + 0.007Clay + 0.972CECoriginal (R2 was 0.923, RMSE was 1.55 cmol(+)∙kg−1, p < 0.01). Further research is needed in the future as for exploring internal functional mechanism in view of soil electrochemistry and mineralogy.

With increasing urban area and population, global cities are playing a more important role in the alteration of the global biogeochemical cycles. The aims of this study are to understand the concentrations and stoichiometric relations of biogenic elements (C, N, and P) in urban soils, further to reveal the effects of C, N, and P accumulation on the urban environment. We collected 317 surface (0–10 cm) soil samples taken from Nanjing, China, a typical city with more than 2000 years of history. These soil samples were located in different urban zones of Nanjing with different land use histories. The soil C, N, and P concentrations were determined. The stoichiometric relations of soil C, N, and P were investigated in urban soils. Meanwhile, some studies on sources of C, N, and P in diverse urban settings from literature were combined to explore the universal rule of C, N, and P cycling and their ecological and environmental effects in urban area. Compared to rural soils, more C, N, and P are accumulated in the urban soils, which also change their stoichiometric relations. The concentrations of OC, TN, and TP in urban top soils are 17.0 ± 9.69 g kg−1, 1.53 ± 0.92 g kg−1, and 1.31 ± 0.67 g kg−1, respectively. The mean atomic ratio of C:N:P is 37:3:1 in the surface of urban soils that strictly differs from natural soils in China and the whole world. The mean of C:N ratio in urban soils is similar to that of agriculture, grassland, and forest soils. However, the ratios of C:P and N:P in urban soils are much lower than that in agricultural, forest, and grassland soils. This implies that P is extremely enriched in the urban soils. The high C in urban soils are considered coming from natural and anthropogenic sources. The high N and P mainly come from anthropogenic sources. The well-constrained C:N:P ratio in rural soils does not apply for urban soils. The abnormal C:N:P ratio of urban soils is the result of unbalanced accumulation of C, N, and P from human activities. Urban soils are already an important storage of carbon. High N and P in urban soils may bring threat of surface water eutrophication and ground water contamination. These effects are expected to increase with the city development time.

The black soil region in northeastern China is one of the most important grain production areas in the country, and plays a key role in food security. Large lands have been brought into cultivation in the last 100 years. Because these lands have lesser slope gradients compared to those in the loess plateau and other regions in China, soil erosion has been ignored in this area for a long time. Few soil conservation practices have been implemented. Serious soil erosion problems due to long slope lengths have been identified based on recent research and observations in the study area. The government has realized the necessity and urgency of soil conservation. However, to create policies and plans for black soil conservation, questions such as how much of the black soil region, and where in the region, erosion is a problem must be addressed. Currently, the black soil region of northeastern China is regarded as having a total area of 1.09 million square kilometers, including Heilongjiang, Jilin, the north part of Liaoning, and the east part of Inner Mongolia, in which there are also many non-black soils in mountain regions of Daxingan, Xiaoxingan, and Changbaishan. Therefore, it is difficult to determine precisely the conservation priorities for this large area. In this study, the black soils were defined and their concentrated area was delineated in northeastern China. The black soils were defined as soils with the “mollic epipedon”, which were classified as Isohumosols in Chinese Soil Taxonomy or Mollisols in the USDA Soil Taxonomy, and classified as Phaezoems, Chernozems, Kastanozems and Grayzems soils according to the Genetic Soil Classification of China (GSCC). The region dominated by these four soils in GSCC was defined as the black soil region. Because Phaezoems and Chernozems have higher organic matter contents and deeper mollic epipedon depths, and are distributed in the center of the study area, these two soils were defined as the typical black soil, and their dominate area was defined as the typical black soil region. A soil map in the scale of 1:1000000, made from the second national soil survey, was used with ArcGIS 10.2 software to delineate both black soil and typical black soil regions. A method called “central gravitational agglomeration” was developed. All of the soil polygons were selected, and the farthest and the smallest polygon were merged into its closest adjacent one if its radius was larger than the distance between it and its closest one. Otherwise it was abandoned. The same process was repeated until only a few small polygons were left and a regional outline appeared. Four soils were used to determine the exact boundaries of the black soil region, and two soils were used to determine the exact boundaries of the typical black soil region. The results showed that the area of the black soil region was 556000 km2, including 146 counties in which 33.0% was cultivated land. The area of the typical soil region was 333000 km2, including 138 counties in which 50.2% was cultivated land. Due to the long slopes, the soil erosion rate from the cropland with slope greater than 0.25° is estimated to be larger than soil loss tolerance of 200 t/km2. The cultivated lands with slope greater than 0.25° were regarded as sloped cultivated land or sloped cropland, which totaled 89000 km2, or 16.0% of the total land for the black soil region, and 78000 km2, or 23.4% of the total land for the typical black soil region. The sloped cultivated land in both the black soil region and typical black soil region should be targeted as key areas for soil erosion control in northeastern China.

Purple paddy soils cover about 66% of the paddy soils in Sichuan Basin. Complex interactions made the effects of potassium fertilizer application on NH4+ adsorption more complicated and the behavior of NH4+ and K+ interaction in purple paddy soils has rarely been evaluated. We hypothesize that correct management of these soils can enhance the efficiency of ammonium and potassium fertilizers. In order to determine correct management methods, it is important to understand the interaction of main nutrient cations NH4+ and K+ and maturation hydroponics in purple paddy soils with different paddy cultivation times. A purple paddy soil chronosequence with cultivation history from 0 to 300 years was studied to understand the respective effects of SOM and mineralogy on NH4+ and K+ sorption behavior by adsorption experiments using samples both before and after H2O2 oxidation. The study shows that maturation hydroponics influenced NH4+ and K+ adsorption mainly through changing clay mineralogy and enhancing SOM accumulation. SOM and smectite play positive effects for NH4+ adsorption. The K+ adsorption, however, is mainly affected by the smectite changes. The adsorption percentages are obviously higher at 25–50 mg L−1 for each NH4+ solution and higher at 50–100 mg L−1 for each K+ solution. When ammonium and potassium were applied together, NH4+ and K+ adsorption are mutually weakened. In addition, the percentage of K+ adsorption increased from 49.7–57.7% for the 100 mg L−1 K+ solution to 50.9–58.5% for the 200 mg L−1 K+ solution at 200 mg L−1 of added NH4+. SOM enrichments could be a positive way to improve the ammonium fertilizer utilization efficiency in purple paddy soils. Potassium may be more vulnerable to loss at low K+ concentrations given that ammonium and potassium fertilizers were applied together and it should be separately applied to enhance the utilization efficiency of applied fertilizer.

Nitrate accumulated deep (>100 cm) in the regolith (soil and saprolite) threatens groundwater quality, but most studies focus only on nitrate nearer the surface (

Nitrogen (N)-induced soil acidification has received much attention worldwide. Nitrification and soil N mineralization are two key N cycle processes that affect soil acidification. However, the seasonal dynamics of soil pH under their combined influence is unclear. We studied the effect of N fertilization on soil pH and N transformations using 15N tracing in field lysimeters with soils developed from different parent materials (Quaternary red clay, sandstone, and basalt). Maize was planted with 200 kg N ha−1 yr−1 15N-labeled urea addition. During 7–45 days after fertilization, proton (H+) production due to nitrification of fertilizer N, nitrate (NO3−) leaching, and plant uptake exceeded H+ consumption by base cations mobilization and leaching, resulting in a significant soil pH decline. When nitrification activity decreased (after 45 days), due to exhausted ammonium (NH4+) availability, soil pH rose again. During the fallow period, acid neutralization due to base cation mobilization, and ammonification of soil organic N (SON) offset H+ production caused by nitrification of mineralized SON, leading to a sustained rise in soil pH. After the one-year experiment, no significant soil pH decrease was observed in any of the soils. Parent material had little effect on the seasonal dynamics of soil acidification, which appeared to be controlled by fertilization, environmental factors (temperature and moisture), and plant uptake. In subtropical regions, monitoring of soil pH on an annual basis may mask the effect of N fertilization on soil acidification.

The soil organic carbon (SOC) and total nitrogen (TN) of topsoil could be greatly affected by the conversion of grassland to cropland (CGC) due to vegetation harvesting and soil erosion. However, few studies have evaluated the effects of these changes in the subsoil and distinguished its main controls at the regional scale. This paper investigated the total changes in the SOC stock (SOCS) and TN stock (TNS) of soils from 0 to 30 cm and 0–100 cm in depth over the past three decades in the Songnen Plain, China, a typical Mollisol region. The CGC resulted in a moderate loss of topsoil C and N but an increase in subsoil C and N, and the soil mapping results showed that approximately 7.47 × 1011 kg C and 1.51 × 1011 kg N were accumulated during the past three decades. Prediction uncertainty was assessed by 100 model runs with random jackknife partitions, and high uncertainty was found in the areas with rapidly changing SOCS and TNS levels. Our findings indicated that subsoil should be considered in the estimation of the SOCS and TNS at the regional scale. The SOCS and TNS of the CGC areas were anticipated to increase after long-term cultivation. In view of ongoing cropland expansion, up-to-date land use and soil type data are vital for selecting monitoring sites and understanding long-term soil evolution at the regional scale.

A paddy soil chronosequence consisting of four profiles derived from Quaternary red clay (QRC) in Southern China with cultivation history from 0 to 300 years was studied to understand the underlying mechanisms and processes controlling the centennial scale Fe evolution during anthropedogenesis. We evaluated the chronosequencial changes in depth distribution of Fe oxides and Fe isotopic compositions. Results showed that paddy soil development on QRC over a centennial time scale caused increasing profile differentiation of Fe oxides and measurable Fe isotope fractionation. Total Fe and oxide bound Fe as well as the maximum Fe illuviation depth decreased as paddy soils age, leading to significant differences in morphological properties between paddy and non-paddy soils with the same parent material. Selective extractions showed that the weakly-bound, oxide-bound and silicate bound Fe corresponded to 4–18%, 49–81%, and 11–40% of the total Fe, respectively, and these proportions varied with both time and depth due to the redox-related Fe transformation and redistribution. δ56Fe values in the studied paddy soil chronosequence ranged from 0.05‰ to 0.26‰ and exhibited a strong negative correlation with the logarithm of total Fe concentrations, suggesting mass-dependent Fe isotope fractionation occurred as a result of the preferential removal of lighter Fe isotopes during Fe leaching loss under the predominant reducing conditions. However, the Fe isotopic ratio of a specific paddy soil horizon was a result of a complex interaction of different processes, which were summarized and interpreted in our proposed conceptual model of Fe evolution in terraced paddy soils. Further investigations of Fe isotope signatures in the soil-plant-water system with a combination of laboratory simulation and prediction of Fe isotope fractionation under different pedogenic processes are needed to evaluate the relative contribution of multiple processes to Fe isotope fractionation during anthropedogenesis of paddy soils.

Soil’s hydrological and ecological functions depend largely on soil water retention, which is affected by both structural and compositional soil properties. Studies have shown that alpine soils derived from sediments with different origins in the northeast Qinghai-Tibetan Plateau differ significantly in terms of particle size distribution, bulk density and soil organic carbon content. However, the variation pattern and its drivers of soil water retention in this region remain unclear. In this study, soil water retention characteristics (saturated soil water capacity, field capacity, wilting point and plant available water capacity) from different geomorphic settings (i.e., hillslopes, alluvial fans and floodplains) of an alpine catchment in the western part of the Qilian Mountains were analyzed in relation to sediment types and soil properties. Results show that soil water retention varies significantly with sediment types. Soils with an aeolian origin have a higher capacity to retain water compared with soils developed on fluvial sediments. Variations in soil water retention are mainly controlled by particle size distribution for fluvial sediments and by soil organic carbon content for aeolian sediments. The accumulation of soil organic carbon enhances plant available water capacity for sandy fluvial sediments but not for silty fluvial and aeolian sediments. The critical eco-hydrological functions of the mattic surface soil horizon in relation to water retention, runoff regulation and erosion control were highlighted for the alpine ecosystems. By deciphering sedimentological and pedological controls of soil water retention, this study improves the understanding of the evolution and functioning of alpine soils, which has further implications for other sedimentary ecosystems.

A paddy soil chronosequence consisting of five profiles derived from calcareous marine sediments with cultivation history from 0 to 1000 years was studied to understand the underlying mechanisms and processes controlling the millennial scale Fe evolution. We evaluated the chronosequencial changes in depth distribution of Fe oxide contents and Fe isotopic compositions. Results showed that paddy soil evolution under the influence of periodic flooding and groundwater fluctuation resulted with time in variations of soil moisture regime and redox condition that control Fe mobilization, translocation and redistribution, leading to enhanced profile differentiation of Fe oxides and measurable Fe isotope fractionation. Total Fe and oxide bound Fe as well as their differentiation between surface and subsurface horizons increased as paddy soils age, leading to the formation of diagnostic horizons and features characterizing Fe distribution and redistribution. Selective extractions showed that the weakly-bound, oxide-bound and silicate bound Fe corresponded to 1–16%, 8–46%, and 52–91% of the total Fe, respectively, and these proportions varied with both time and depth due to the redox-related Fe transformation and translocation. δ56Fe values in the studied paddy soil chronosequence ranged from − 0.01‰ to 0.18‰ and exhibited a strong negative correlation with the logarithm of total Fe concentrations, suggesting mass-dependent Fe isotope fractionation occurred as a result of the preferential removal of lighter Fe isotopes during long-term paddy soil evolution under the predominant reducing conditions. However, the Fe isotopic ratio of a specific paddy soil horizon was a result of a complex interaction of different processes, which were summarized and interpreted in our proposed conceptual model. Comparison of Fe isotopic compositions in the worldwide soils demonstrated that Fe isotopes can evidence Fe transfer and pinpoint the factors and processes that control Fe mobilization and redistribution particularly in soils with changing moisture regimes and redox conditions. Our findings provide new insights into the behavior and geochemical cycle of Fe at the Earth's surface strongly affected by human activities and contributes to an improved understanding of how anthropedogenesis affects Fe evolution in the Earth's Critical Zone.

Soil properties are often assumed to be static over time in hydrological studies, especially in hydrological modelling. Although it is well appreciated that soil structure and its impact on hydraulic properties are time-variable, particularly on cultivated land, very few studies have focused on quantifying the influence of such changes on soil hydrology, especially at the short term (i.e. seasonal). This study explored the value of incorporating such short-term time-variable soil properties in hydrological models. It is based on soil hydraulic properties from temporal field data under no-till done by direct seeding and under conventional cultivation done by ploughing to 0.2 m and harrowing. It uses a controlled tillage experiment in Scotland, on a soil with very good structural stability that experiences gentle rainfall in a temperate oceanic climate (Koppen Cfb). Water retention data were collected from intact soil cores sampled at 0.025, 0.095 and 0.275 m depth at three times between April and August 2013; (i) immediately following tillage, (ii) at barley crop establishment 1 month later and (iii) after harvest. Soil structure varied over time, with no-till soils gaining porosity and ploughed soils losing porosity. We hypothesised that no-till soils would have less seasonal temporal variability, but found it to be comparable to ploughed soils, albeit with pore structure changes following different trends. These changes were reflected in Van Genuchten fitting parameters, which if accounted for in 1-D HYDRUS modelling, had a marked impact on modelled soil water content over time if contrasted to predictions assuming a static pore structure. Using data from multiple sampling events, as opposed to one sampling event, resulted in up to a 44 % difference in soil water content predictions and increased the temporal variability by a factor of 1.5. Hence, our results have demonstrated that it is important to account for short-term temporal variability in soil physical properties in soil water modelling studies, and should not be ignored as a default, particularly on cultivated agricultural soils.

Calcareous soils are those soils containing layers of carbonate accumulation formed by either secondary accumulation or inheritance of CaCO 3 from calcareous parent materials. These soils are widely distributed in arid, semi-arid and even some humid climate environments. However, in the Chinese Soil Taxonomy (CST, 3rd version), the soils that contain large amounts of CaCO 3 but do not meet requirements of Aridosols are classified as Hapli-Ustic Cambosols. This group also includes many non-calcareous soils and those soils strongly affected by secondary carbonate accumulation together with quite dissimilar soils in terms of their morphology and properties. This study was conducted to determine calcification patterns and calcic processes occurring in representative pedons of calcareous soils in northwestern Henan Province, China and to classify these soils in CST. A Calcic subgroup was proposed to add within the Hapli-Ustic Cambosols. A diagnostic key was also provided to separate this subgroup from the Typic subgroup. In this way the different calcification degrees of soils were better reflected in the classification, which should lead to more uniform interpretive groups for better soil management.

Soil organic phosphorus transformation during ecosystem development exerts a crucial influence on soil fertility and ecosystem properties. This paper reviews the use of solution 31P NMR spectroscopy for characterizing organic phosphorus speciation in soil chronosequence and long-term field experiments in order to improve our understanding of the temporal changes, fundamental processes, and associated natural and anthropogenic controls of organic phosphorus transformation during long-term ecosystem evolution. Published soil chronosequence studies show that organic phosphorus compounds under aerobic conditions are dominated by phosphate monoesters (occurred mainly as inositol phosphates) followed by phosphate diesters (occurred mainly as DNA) and phosphonates, irrespective of the different parent materials, vegetation covers and climatic conditions. This contrasted markedly with wetland soils in which phosphate monoesters and diesters maintained approximately equal proportions, which is attributed to the limited reactive clay surfaces for stabilization and/or decomposition of myo-inositol hexakisphosphate under frequent anaerobic conditions. Most organic phosphorus compounds in soil chronosequences increase with age to reach a maximum and then decline with time, although the apex varies significantly among different organic phosphorus compounds and chronosequences. Variations of the potential for phosphorus stabilization resulting from mineralogical transformation, changes in phosphorus sources due to shifts in plant and microbial communities, and differences in the biological utilization of various phosphorus compounds have been suggested as three main mechanisms controlling the temporal changes in organic phosphorus species, abundance and availability during natural ecosystem development. In agricultural soils, the amounts, forms, and dynamics of organic phosphorus are determined by internal soil properties, external environmental conditions and managements, including the history and intensity of land use, different tillage practices and fertilizer treatments. These mechanisms are interlinked and more research is required to isolate both internal and external factors that regulate organic phosphorus transformation in agricultural ecosystems. Given the universal dependence on organic phosphorus for life and its critical roles in biogeochemical cycling, we put forward several open questions that need to be resolved in the future studies by emphasizing the multidisciplinary collaborations, the use of multiple analytical techniques and the establishment of quantitative organic phosphorus transformation models.

Increasing acid deposition and intense nitrogen (N) fertilization have resulted in severe soil acidification in many regions of the world. However, the soil acidification rate, especially quantitative contributions of different proton (H+) sources remain unclear in various cropping systems. In this study, a two-year field lysimeter experiment was conducted with a set of different N fertilization treatments (0, 100, 200 and 400 kg N ha−1 yr−1) for maize crop in typical red soil in subtropical China. The pathways and budgets of different H+ sources were quantified by calculating the input–output balance of major elements. Results showed that fertilization significantly accelerated soil acidification by increasing the leaching of NO3– and base cations (K+, Ca2+, Na+, and Mg2+) as well as increasing the cation removals by plant uptake and harvest. Unbalanced plant uptake of anions and cations was the primary H+ source (64.4–80.5%) to soils. Plants have an important effect on soil acidification by redistributing cations in soils. Due to cation removals by plant uptake and harvest, there were significant decreases of soil exchangeable base cations under high urea-N treatments (200 and 400 kg N ha−1 yr−1) and significant increases of soil exchangeable H+ and Al3+ under three fertilization treatments. Of course, N transformation also plays an important role (12.1% to 38.8%). However, the effect of direct H+ deposition is minor in this area comparatively (5.78% to 7.34%). Quantification of acidification contribution of different sources is useful for the establishment of further soil remediation practices. The field management is considered very important to alleviate soil acidification, including increasing straw return and controlling fertilizer application.

Nitrate contamination of water bodies is a serious concern in regions with intensive agriculture. Previous studies, which only considered surface water and groundwater but not the regolith, do not provide a comprehensive accounting of nitrate sources and their proportional contributions in the Critical Zone, which extends from the top of the vegetation canopy down to the bottom of the fresh groundwater. In this study, dual nitrate isotopes, hydrochemical compositions and a Bayesian isotope mixing model were used to investigate nitrate sources, processes and individual source contributions across surface water, regolith and groundwater at the Red Soil Critical Zone Observatory of China. Denitrification was found to be weak in the Critical Zone. Dilution processes were important in eliminating nitrate from streamflow and groundwater. More than 90% of the nitrate inventory was stored at depths from 1 m to the bedrock surface in the upland regolith. Nitrate with light nitrogen (14N) isotopes was preferentially adsorbed by the variable charge soils with the help of weak electrostatic attraction. The contribution of soil organic N (SON) to the nitrate in the upland catchment streamflow was found to be highest with a mean of 56%, followed by manure and sewage (MS, 26%), NH4+ in chemical fertilizers (CF, 17%). For the paddy subcatchment streamflow, the contribution of SON (68%) was highest, followed by CF (19%) and MS (13%). In contrast, for the groundwater, the contributions of CF and SON were 60% and 36%, respectively. For the regolith, the contributions of CF and SON were 72% and 25%, respectively. Thus the most important sources of nitrate were SON for the streamflow and CF for the groundwater and regolith. These findings show that the different partitioning and isotopic fractionation within the regolith strongly affect the sources of nitrate reaching groundwater, compared to the sources reaching surface water.

The parental material of soils in the Qilian Mountains of northwest China is mainly aeolian loess containing CaCO3 which may remain in soils under the semiarid-arid climate. To disclose the CaCO3 characteristics change with the altitude and the terrain attributes, we surveyed 18 soil profiles in an altitude sequence from 3076 m to 4510 m in the Hulugou Watershed in the Qilian Mountains, measured CaCO3 contents of all genetic horizon samples, analyzed the densities, illuviation modes and depths of CaCO3 in the profiles, extracted values of the terrain attributes of the profiles including altitude slope, aspect, plane curvature, profile curvature and terrain wetness index (TWI) from the 90 m resolution SRTM3 DEM data on ArcGIS 9.3 platform. We found that CaCO3 weighted content of the profiles ranged from 1.30 g·kg-1 to 93.09 g·kg-1, CaCO3 density from 0.05 kg/m2 to 75.69 kg/m2, CaCO3 illuviation depth from 12 cm to 54 cm. CaCO3 illuviation modes could be divided into three types, i.e., no illuviation mode in which the profile has only A horizon or CaCO3 content -1, middle illuviation mode in which CaCO3 accumulated in a middle horizon, and down illuviation mode in which CaCO3 content increases with the depth. CaCO3 weighted content, density and illuviation depth had significant correlation with certain terrain attributes. In general, the altitude sequence is an effective way to study CaCO3 characteristics in the alpine region, and the data of terrain attributes which can influence the precipitation and its redistribution in soil are potential in predicting soil CaCO3 characteristics in the alpine region.

Plant growth contributes to mineral weathering, but this contribution remains poorly understood. Weathering rates in an aggrading forested watershed in subtropical China were studied by means of geochemical mass balance. Rainfall, dry deposition, and streamwater were monitored from March 2007 to February 2012. Samples of vegetative components, rainfall, dry deposition, streamwater, representative soils, and parent rock were collected and determined for mass balance calculation and clarifying plant-driven weathering mechanisms stoichiometrically. Ignoring biomass, weathering rates of Ca 2+ , Mg 2+ , Na + , and Si were 25.6, 10.7, 2.8, and 51.0 kg ha −1 year −1 , respectively. Taking biomass into consideration, weathering rates of Ca 2+ , Mg 2+ , and Si and the sum of weathering rates of Ca 2+ , Mg 2+ , Na + , K + , and Si were 2.6, 1.8, 1.2, and 1.5-fold higher than those ignoring biomass, respectively. This is attributed to plant-driven weathering due to the nutrient ( e.g ., Ca 2+ , Mg 2+ , and K + ) absorption by vegetation and substantial proton production during assimilation of these nutrients, with the former acting as a pump for removing weathering products and the latter being a source of weathering agents solubilizing mineral components. The same pattern of weathering, i.e ., higher rates of weathering with than without including biomass in mass balance calculation, was reported in previous studies; however, the extent to which plants drive weathering rates varied with vegetation types and climatic zones. The documented biological weathering driven by plants is expected to play a critical role in regulating nutrient cycling and material flows within the Earth's Critical Zone.

The stability of soil colloids influences soil physicochemical properties, soil development, and transfer of nutrients and contaminants to surface and ground waters. A better understanding of soil colloids stability dynamics during soil evolution is important for the evaluation of soil's capacity to retain nutrients and/or accommodate toxic contaminants. This study was aimed to determine changes in the stability of water-dispersible soil colloids that accompany mineral transformation and surface charge evolution during pedogenesis using a well characterized chronosequence derived from basalt in the humid tropical region of Hainan Island, South China. The results demonstrated that the pH-dependent colloid stability decreased significantly with tropical soil development, which we attribute to the substantial changes in clay mineral compositions and colloid surface charge properties. Clay minerals in the studied chronosequence were characterized by an increase of kaolinite, gibbsite and Fe oxides and a decrease of quartz and halloysite towards more advanced stages of weathering, which resulted in the decline of permanent negative charges in the older soils. The point of zero charge (pHPZC) increased while ∆ pH decreased across the tropical soil chronosequence, being in good agreement with the observed lower colloid stability in aged soils dominated by kaolinitic minerals. Our study of colloid stability at long-term pedogenic time scale suggests young tropical soils (

The accuracy and precision of predictions of soil texture based on visible–near-infrared (Vis-NIR) spectra are affected by the soil moisture content at the time of measurement. This study aimed to quantify these effects. We also developed a method to improve the accuracy of soil texture prediction when the difference in moisture content is large and unknown, as is usually the case in field measurements. Reflection spectra (380–2400 nm) of 89 soil samples were obtained in the laboratory under nine moisture conditions. Prediction models for each moisture condition were built separately using partial least-squares (PLS) regression. Each model was applied to independent validation sets under the same moisture condition as the calibration sets, and then the model based on air-dried soils (dry model) was applied to the other eight moisture conditions to quantify the effect of soil moisture on the prediction accuracy. Finally, the perpendicular drought index (PDI) was used as an indicator of soil moisture, and the nine moisture conditions were regrouped to four groups using the PDI. Prediction models for each group with the same PDI were built and evaluated. The results show that Vis-NIR spectra can be directly applied to predict the soil texture of soils in different known moisture states. When the moisture state is unknown, the models based on PDI grouping markedly improve prediction accuracy. The RMSEₚ of prediction from PDI-grouped models of prediction of the content of clay and sand separates were between 1 and 2% and between 8 and11%, respectively, for the different moisture classes. This method has potential for direct application in the field with on-the-go sensors.

Nitrate accumulation at depth in variable charge soils in humid tropics and subtropics affects nitrogen (N) leaching to groundwater. However, the mechanisms on the nitrate accumulation in relation to regolith (soil and saprolite) properties are poorly understood. In this study, four upland regoliths were selected in a typical red soil Critical Zone of humid subtropical China. These regoliths with a thickness of 6–8 m, which developed from Quaternary red clay underlain by sandstone bedrock, were acidic, highly weathered, and clay loam to clay in texture. We analyzed the relationships between nitrate concentrations and the regolith physiochemical properties. The results showed that regolith pH was negatively correlated with nitrate concentrations, whereas crystalline iron (Fe) and aluminum (Al) and anion exchange capacity (AEC) were positively correlated with nitrate concentrations at depths below both soil surface and 1 m. Bulk density (BD) was significantly related to nitrate concentrations only at depths below soil surface, while depth, clay, amorphous Fe and Al, total organic carbon (C), and total N were significantly related to nitrate concentrations only at depths below 1 m. There was no significant correlation between nitrate concentrations and sand, silt, silicate bound Fe or Al, C/N, or moisture at depths below both soil surface and 1 m. Based on the random forest analysis, the relative contributions of pH, crystalline Fe, depth, AEC, BD, crystalline Al and amorphous Fe to explaining nitrate concentration variations were 28.3%, 9.5%, 4.3%, 3.4%, 2.9%, 2.8% and 2.6%, respectively. The concentration of crystalline Fe was ten times that of crystalline Al, which would account for its larger contribution to the nitrate variations. Nitrate accumulation at depth was most likely controlled both by electrostatic adsorption on crystalline Fe and Al oxides and low hydraulic conductivity of the regolith at depth, which appeared to be largely influenced by regolith pH and competitive anions. Our findings shed light on regolith internal property controls on nitrate legacy in the subtropical vadose zone and may help control nitrate contamination of groundwater in intensified agricultural regions.

Subsurface soil bacterial community composition and the controlling factors remain largely unknown, especially the micro-zone differentiation of community composition within a horizon. We studied a plinthic horizon to determine how different micro-zones in a horizon affect the bacterial community. The plinthic horizon is a net-like horizon characterized by the segregation of iron forms as shown by contrasting red matrix and white veins, which share common macro-environmental conditions such as climate and land use but differ only in physical and chemical compositions. The studied horizon is typical of the red soils of southeastern China and is an important layer in the red soil Critical Zone. The plinthite is considered to have been formed in the Quaternary and thus is a record of the paleo-environment. We evaluated the difference in the bacterial community composition between the red matrix and white veins and explored the possible assembly mechanisms of their co-occurrence patterns. Compared to the eutrophic environments of a red matrix, higher relative abundances of Acidobacteria and Nitrospirae were observed in the white veins. Similarly, more niches led to a higher density of bacterial co-occurrence patterns in the red matrix. The differences in the bacterial community composition and association networks are due to environmental selection, including the legacy of the paleoclimate that is represented by major element contents and contemporary hydrological properties that are mainly controlled by the soil texture. Our study shows that micro-zones even within a same plinthic horizon can provide different habitats and thus select for specific bacterial communities. Furthermore, this study could improve our understanding of the differentiation of bacterial communities among microenvironments caused by both historical and contemporary processes and help to predict how these communities may respond to future environmental changes.

Ground-penetrating radar (GPR) is a convenient tool for volumetric soil water content (VSWC) estimation in hydrological and agricultural studies. Although case studies have been widely carried out, little attention has been paid to subsoil moisture estimates. In this research, we investigated three-dimensional soil moisture variation down to a depth of 1 m and the effect of rainfall events on spatial soil moisture dynamics. GPR surveying lines were conducted both before and after a heavy rainfall event to map the VSWC. Soil sampling and time domain reflectometry (TDR) probe data at different depths (20, 40, 60, 80, and 100 cm) were acquired. Our results demonstrated that there was a significant correlation between the dielectric constants and VSWCs at all depths. The established relationships for the different depth ranges had a low VSWC discrepancy when the dielectric constants ranged from 10 to 15. The effective range of each variogram was larger than 20 m, except for that of the 0–100 cm VSWC map after rainfall. In addition, the validation diagrams using corrected TDR values demonstrated relatively reliable VSWC maps. Approximately 89% of the variation in VSWC could be explained by the dielectric constants in the depth range of 0–40 cm, and VSWC predictions at this soil depth outperformed those at other depth ranges, with an overall RMSE of 0.027 m3 m−3 and R2 of 0.725. Furthermore, we also monitored the effect of precipitation on the accuracy of the VSWC prediction on shallow surfaces. Our study shows that three-dimensional soil moisture dynamics can be accurately estimated at the field scale by integrating GPR interpretation and spatial extrapolation methods.

Although Hainan Island is located in one of the relative least dusty regions in China, Asian dust has been identified in Hainan soils. We used Sr and Nd isotopes of soils, saprolite, bedrocks and Asian dust to not only identify long-range transported dust but also quantify the dust contribution to Hainan soils. Sr and Nd isotopic compositions of the soils are clearly affected by dust deposition. The soils in near-surface horizons have 87Sr/86Sr values as high as 0.7193 that are more radiogenic than the basalts, indicating involvement of Asian dust; whereas deep horizons show a dominantly basaltic signature (87Sr/86Sr = 0.7038). The eNd(0) values for Hainan soils range from − 6.46 to 1.85. The higher value closely approximates the values measured for the Hainan lava, and the lower value indicates the fingerprint of the Asian dust. The 87Sr/86Sr and eNd(0) values for soils also show a progressive trend with time, which implies that dust accumulates with soil age. We calculated the dust accretion rates for Sr-based and Nd-based to quantify the dust contributions to soils. The estimate of dust accreted shows an approximate increasing trend with the ages of soils for Sr-based and Nd-based methods. The soils in the old sites are likely to have undergone long-term dust input and accumulated high dust levels. We also find excellent agreement in the amount of dust present and dust accretion rates based on the two tracers at the 300 ka site and the 1120 ka site, respectively. Besides lower dust deposition in the Pliocene epoch than the late Quaternary over eastern Eurasian, the loss of mineralogical and geochemical tracers over million year time scales also may lead to underestimates of time-averaged dust deposition rates in the old sites. Therefore, the young sites (180 ka and 300 ka) should provide more realistic estimates of average dust accretion rates. The importance of dust in the tropical Hainan soil chronosequence highlights the significance of dust accretion to soils globally.

Summary The objective of this study was to understand the rates and controlling factors of magnetic depletion and enhancement during anthropogenic soil evolution. To this end, the study compared the dynamic changes in magnetic properties as well as iron oxide species of paddy and non-paddy soil chronosequences with the same parent materials. A two-way analysis of variance (anova) showed that paddy management resulted in significant (P

The share of urban population of the world is expected to reach 75% by around 2050. The need to offer within the cities soil services rose awareness of keeping soil functions in cities. Parallel to strong growth of cities some cities are shrinking. In cope with this, working groups (WG) of urban soils have been established (AK Stadtboeden1987, Germany, ICOMANTH 1995, USA, and internationally the IUSS WG US/SUITMA 1998 (Urban Soils/Soils of Urban, Industrial, Traffic, Mining and Military Areas)).The first attempts at urban soil surveys involved municipal waters and sewers in the USA in the 1970s. In the 1990s, the soils of about 15 cities were mapped, totally or in part, in Germany, and in the USA in Baltimore, and recently in New York. But at all times the soil pollution was a main interest of urban soil investigation. Urban soils are characterized by their particular use on which first attempts of soil classification are based. As knowledge about soil morphology and soil composition increased, soil survey concepts were developed. These concepts take into account relicts of the original, natural soils, as well as new, man-modified and man-made materials. At least in humid climate Regosols, as well as other urban soils such as Carbonatosols, Dialeimmasols, Intrusols, Reductosols and Acid Sulfuric Soils, can develop within 25 years or less. Technosols as soils from man-made materials are already included in the WRB. Besides pollution, a major feature of urban soils is soil sealing by construction. This restricts the availability of soils for urban greenery and other functions. Some particular problems of urban soils are high stone content, high differences of bulk density, shape of particles and soil structure performance. This lowers the quality of soils to fulfill functions. Specific methods are necessary to determine many urban soil characteristics. Specific instructions have been developed for soil and soil sealing surveys, and stormwater infiltration. However, for other cases, feasible methods are still missing. The European Commission’s “Thematic Strategy of Soil Protection” did raise political awareness of urban soils, with a main focus on sealing.

Soil nutrient stoichiometry and its environmental controllers play vital roles in understanding soil-plant interaction and nutrient cycling under a changing environment, while they remain poorly understood in alpine grassland due to lack of systematic field investigations. We examined the patterns and controls of soil nutrients stoichiometry for the top 10 cm soils across the Tibetan ecosystems. Soil nutrient stoichiometry varied substantially among vegetation types. Alpine swamp meadow had larger topsoil C:N, C:P, N:P, and C:K ratios compared to the alpine meadow, alpine steppe, and alpine desert. In addition, the presence or absence of permafrost did not significantly impact soil nutrient stoichiometry in Tibetan grassland. Moreover, clay and silt contents explained approximately 32.5% of the total variation in soil C:N ratio. Climate, topography, soil properties, and vegetation combined to explain 10.3–13.2% for the stoichiometry of soil C:P, N:P, and C:K. Furthermore, soil C and N were weakly related to P and K in alpine grassland. These results indicated that the nutrient limitation in alpine ecosystem might shifts from N-limited to P-limited or K-limited due to the increase of N deposition and decrease of soil P and K contents under the changing climate conditions and weathering stages. Finally, we suggested that soil moisture and mud content could be good predictors of topsoil nutrient stoichiometry in Tibetan grassland.

This study aims to understand how clay minerals change sequentially with paddy cultivation age and how parent materials (or original soils) affect the clay mineral behavior of paddy soils. Three paddy soil chronosequences in the hilly regions of South China, derived from purple sandy shale (PS), Quaternary red clays (RC), and red sandstone (RS), were selected to explore the dynamic changes in clay mineralogy, by comparing physical, chemical, and mineralogical properties of soil sequences. For RC and RS soils, both of which have a low K content, there was little change in the clay minerals. Long-term paddy cultivation can promote formation of illite-like minerals; however, this form of K storage was limited under present farming conditions. In PS soils, which are abundant in K-bearing minerals, the depotassication was strong, accompanied by marked transformation of clay minerals. Kaolinite-like minerals gradually decreased with paddy cultivation age; by contrast, derivative clay minerals such as secondary chlorite and halloysite gradually increased. Strong depotassication mainly occurred in the nonclay fractions. The rate of depotassication and the generation of clay fractions were much faster than in natural soils. The clay minerals of paddy soils mainly followed the feature of their original soils. Their evolutions could be distinguished based on their constituents, which are greatly affected by their parent materials. Moreover, paddy cultivation is able to modify clay mineralogy, according to the original mineralogy and paddy soil management.

Anaerobic ammonium oxidation coupled to iron(III) reduction (termed Feammox) with dinitrogen, nitrite, or nitrate as the end-product is a recently discovered process of nitrogen cycling. However, Feammox has not been described in paddy soils, which are rich in iron(III) oxides and subjected to intensive nitrogen fertilization. Here, evidence for Feammox in a paddy soil chronosequence with a gradient of microbially reducible iron(III) levels was obtained in Southern China using (15)N-labeled ammonium-based isotopic tracing and acetylene inhibition techniques. Our study demonstrated the occurrence of Feammox in the chronosequence, and direct dinitrogen production was shown to be the dominant Feammox pathway. Within the chronosequence, three paddy soils with higher microbially reducible iron(III) levels had higher Feammox rates (ranged from 0.17 to 0.59 mg N kg(-1) d(-1)) compared to an uncultivated soil (0.04 mg N kg(-1) d(-1)). It is estimated that a loss of 7.8-61 kg N ha(-1) year(-1) is associated with Feammox in the examined paddy soils. Overall, we discover that rice cultivation could enrich microbially reducible iron(III), accelerate Feammox reaction and thus fuel nitrogen loss from soils, and suggest that Feammox could be a potentially important pathway for nitrogen loss in paddy soils.

Information on the dynamics of phosphorus (P) fraction and sorption–desorption characteristics of paddy soils with long cultivation history is essential to improve our understanding of P transformation and transport in agro-ecosystems and critical for sustainable and environment-friendly soil management. We investigated changes in P fraction and sorption–desorption characteristics in topsoils from two contrasting paddy chronosequences developed on calcareous marine sediments at Cixi and acidic quaternary red clays at Jinxian, respectively, in subtropical China. Both chronosequences showed similar patterns and pedogenetic trends of their topsoil P status. Total P (PT) and various P fractions (calcium-associated P (PCa), organic P (Po), non-occluded and occluded P) accumulated to a maximum after 50-yrs and 150-yrs of cultivation, respectively, at Cixi and Jinxian due to P addition, consistent with the prior agronomic field studies on decadal time scales. However, rapid decrease of PT occurred in the older paddy soils at both sites, despite the continuous P additions, which we attributed to the decline of P sorption capacity since P sorbents (CaCO3, Fe- and Al-oxides, and clay) rapidly decreased. Compared with Walker and Syers’ model of gradual P depletion and decreasing bioavailabity in natural ecosystems, our results show that long-term paddy cultivation alters both the rate and trajectory of topsoil P transformations during agro-ecosystem development. The ability of the topsoil to sorb added P decreased with paddy cultivation history in both chronosequences, due to the decline in the maximum P sorption capacity (Smax); whereas the P release potential increased as a result of paddy cultivation at both sites, as indicated by the higher degree of P saturation (DPS) and greater P desorbability in the paddy soils than the uncultivated soils. Our study suggests that after long-term cultivation paddy soils are getting degraded by lowering their P holding capacity so may enhance the P release potential and lead to increasing environmental risk through P loss.

Alfalfa cropping has been considered an efficient method of increasing soil fertility. Usually nitrogen increase in root nodules is considered to be the major beneficial effect. A 21-year time series (five sampling periods) of alfalfa cultivation plots on a loess soil, initially containing illite and chlorite, in Lanzhou of northwestern China was selected to investigate the relationships among alfalfa cropping, soil potassium (K) content and soil clay minerals. The results indicated that soil K significantly accumulated after cropping, with a peak value at about 15 years, and decreased afterwards. The accumulated K was associated with the K increase in the well-crystallized illite, which was not extracted by the traditional laboratory K extraction methods in assessing bioavailability. The steep decline in soil K content after 15-year cropping was in accord with the observed fertility loss in the alfalfa soil. Plant biomass productivity peaked at near 9 years of culture, whereas soil K and clay minerals continued to increase until cropping for 15 years. This suggested that K increased in the topsoil came from the deep root zone. Thus alfalfa continued to store K in clays even after peak production occurred. Nitrogen did not follow these trends, showing a general decline compared with the native prairie soils that had not been cropped. Therefore, the traditional alfalfa cropping can increase K content in the topsoil.

Urban soil characteristics, especially soil physical properties, are subject to dramatic changes due to compaction by intensive human activities, which may cause frequent flood events during the rainy seasons. The aim of this study was to explore the water infiltration characteristics of urban soils with different degrees of compaction and to determine the effects of infiltration on environmental problems in urban areas. Ten typical land use patterns with various vegetation and age were selected in Nanjing City, China to determine the infiltration rates by the dual-ring method. Three replicated sites were done in the same combined mode. At the same time, other soil physical and chemical properties that may affect infiltration rate were also determined. In addition, urban surface runoff coefficients were estimated according to final infiltrations and rain intensities in the different degrees of soil compaction. The quality of flood water from a strong rainstorm in a main road in Nanjing was analyzed. The final infiltration rates of urban soils were highly variable (from very slow to very fast). Low soil infiltration rates are related to many factors; however, urban soil compaction is the leading one. Infiltration rates decreased with an increase in the bulk density and with a reduction in the air-filled porosity. Thus, large amounts of macropores increase the infiltration rate and reduce surface runoff. Runoff coefficients of compacted soils with low infiltration rates were high, especially for extremely compacted soil. As a result, the prevalence of flooding is high in compacted soils, and the quality of surface runoff water is reduced during flooding events. The concentrations of NO 3 − -N, total nitrogen, molybdate-reactive phosphorus, total phosphors, and suspended material in urban surface runoff were significantly higher than those observed in forested or agricultural watersheds. In urban areas, low infiltration rates caused by soil compaction have negative effects on the eco-environment of the city and result in increased instantaneous flooding and poor surface water quality. Thus, to improve the eco-environment of urban areas, the amount of greenbelt soil must be increased and soil compaction must be reduced.

Soil samples were collected from eight basaltderived chronosequence soils with the ages of 0.01, 0.58, 0.92, 1.33, 2.04, 3.04, 3.76 and 6.12 Ma respectively from Leizhou Peninsula and northern Hainan Island of tropical southern China. Magnetic parameters of magnetic susceptibility (MS), percentage of frequency-dependent magnetic susceptibility (FDS%), anhysteretic remanent magnetization (ARM), saturation isothermal remanent magnetization (SIRM), soft and hard isothermal remanent magnetization (IRMs and IRMh) of the collected samples were measured to study the evolution and the significance of the magnetism with soil age. The results show that the magnetic parameters changed fast from Primosols to Ferrosols (0.01 - 0.92 Ma) but slowly at Ferralosols stage (1.33 Ma~), it suggests a stable phase occurred for soil magnetism at Ferralosols, the existence of this phase could be supported by the little changes in the contents of clay, Fet and Fed. Obvious differences existed in the values of magnetic parameters between Ferralosols and other soil types (Primosols and Ferrosols), FDS%: Ferralosols > 10%, Primosols and Ferrosols 8000 × 10 –8 SIm 3 ·kg –1

Soil ingestion is an important human exposure pathway of heavy metals in urban environments with heavy metal contaminated soils. This study aims to assess potential health risks of heavy metals in soils sampled from an urban environment where high frequency of human exposure may be present. A bioaccessibility test is used, which is an in vitro gastrointestinal (IVG) test of soluble metals under simulated physiological conditions of the human digestion system. Soil samples for assessing the oral bioaccessibility of arsenic (As) and lead (Pb) were collected from a diverse range of different land uses, including urban parks, roadsides, industrial sites and residential areas in Guangzhou City, China. The soil samples contained a wide range of total As (10.2 to 61.0 mg kg(-1)) and Pb (38.4 to 348 mg kg(-1)) concentrations. The bioaccessibility of As and Pb in the soil samples were 11.3 and 39.1% in the stomach phase, and 1.9 and 6.9% in the intestinal phase, respectively. The As and Pb bioaccessibility in the small intestinal phase was significantly lower than those in the gastric phase. Arsenic bioaccessibility was closely influenced by soil pH and organic matter content (r (2) = 0.451, p 0.01) in the stomach phase, and by organic matter, silt and total As contents (r (2) = 0.604, p 0.001) in the intestinal phase. The general risk of As and Pb intake for children from incidental ingestion of soils is low, compared to their maximum doses, without causing negative human health effects. The exposure risk of Pb in the soils ranked in the order of: industrial area/urban parks residential area/road side. Although the risk of heavy metal exposure from direct ingestion of urban soils is relatively low, the risk of inhalation of fine soil particulates in the air remains to be evaluated.

Energy use in urbanization has fundamentally changed the pattern and fluxes of carbon cycling, which has global and local environmental impacts. Here we have investigated organic carbon (OC) and black carbon (BC) in six soil profiles from two contrast zones in an ancient city (Nanjing) in China. BC in soils was widely variable, from 0.22 to 32.19 g kg(-1). Its average concentration in an ancient residential area (Zone 1) was, 0.91 g kg(-1), whereas in Zone 2, an industrial and commercial area, the figure was 8.62 g kg(-1). The ratio of BC/OC ranged from 0.06 to 1.29 in soil profiles, with an average of 0.29. The vertical distribution of BC in soil is suggested to reflect the history of BC formation from burning of biomass and/or fossil fuel. BC in the surface layer of soils was mainly from traffic emission (especially from diesel vehicles). In contrast, in cultural layers BC was formed from historical coal use. The contents of BC and the ratio of BC/OC may reflect different human activities and pollution sources in the contrasting urban zones. In addition, the significant correlation of heavy metals (Cu, Pb, and Zn) with BC contents in some culture layers suggests the sorption of the metals by BC or their coexistence resulted from the coal-involved smelting.

Pedogenetic soil horizons are one of the fundamental building blocks of modern soil classiflcation; however, in soils of urban areas which are often strongly disturbed by human activities, horizons are difficult to distinguish but substitutive morphological layers may be identifled. To identify the characteristic soil layers in an urban environment, 224 soil layers of 36 in-situ pedons were examined and described in urban and suburban Nanjing, and 27 variables were extracted for multivariate analysis. Three groups and six subdivisions were identifled by TwoStep cluster analysis combined with hierarchical cluster analysis based on factor scores. Soil forming factors and soil forming processes could be interpreted from the principal component analysis (PCA) of variables, cluster analysis of soil layers, and discriminant analysis of soil layer groups and their subdivisions. Parent materials, moisture regimes, organic matter accumulation, and especially nutrient accumulation were the main causes of characteristic soil layer formations. The numerical approaches used in this study were useful tools for characteristic soil layer identiflcation of urban soils.

The vertical distribution and migration of Cu, Zn, Pb, and Cd in two forest soil proflles near an industrial emission source were investigated using a high resolution sampling method together with reference element Ti. One-meter soil proflle was sectioned horizontally at 2 cm intervals in the flrst 40 cm, 5 cm intervals in the next 40 cm, and 10 cm intervals in the last 20 cm. The migration distance and rate of heavy metals in the soil proflles were calculated according to their relative concentrations in the proflles, as calibrated by the reference element Ti. The enrichment of heavy metals appeared in the uppermost layer of the forest soil, and the soil heavy metal concentrations decreased down the proflle until reaching their background values. The calculated average migration rates of Cd, Cu, Pb, and Zn were 0.70, 0.33, 0.37, and 0.76 cm year −1 , respectively, which were comparable to other methods. A simulation model was proposed, which could well describe the distribution of Cu, Zn, Pb, and Cd in natural forest soils.

There is a need for accurate estimate of soil organic carbon (SOC) stocks for understanding the role of alpine soils in the global carbon cycle. We tested a method for mapping digitally the continuous distribution of the SOC stock in three dimensions in the northeast of the Tibetan Plateau. The approach integrated the spatial distribution of the mattic epipedon which is a special surface horizon widespread and rich in organic matter in Tibetan grasslands. Prediction models resulted in high prediction accuracy. An average SOC stock in the mattic epipedon was estimated to be 4.99 kg m−2 in a mean depth of 14 cm. The amounts of SOC in the mattic epipedon, the upper 30 cm and 50 cm accounted for about 21%, 80% and 89%, respectively, of the total SOC stock in the upper 1 m depth. Compared with previous estimates, our approach resulted in more reliable predictions. The mattic epipedon was proven to be an important factor for modelling the realistic distribution of the SOC stock in Tibetan grasslands. Vegetation-related covariates have the most important influence on the distribution of the mattic epipedon and the SOC stock in the alpine grassland soils of northeast Tibetan Plateau.

Potential toxic elements (PTEs) in urban soils are of great environmental concern because of their potential long-term effects on human health. A systematic soil geochemical survey was performed across central urban districts of Guangzhou, the largest city in southern China. Surface soil samples (0–10 cm) were collected from 426 sites in order to evaluate the environmental quality of this urban area. Geostatistical and multivariate analysis (principal component, clustering, and correlation) were applied to generate spatial distribution maps of PTEs concentrations as well as identify possible sources of metals. The results showed that PTEs ranged widely in value; Arsenic, Hg, Pb, Cu Zn, Cd, Ni, and Mn ranged from 1.4 to 144, 0.01 to 12.2, 18.5 to 4903, 5.0 to 417, 10.1 to 1795, 0.03 to 2.41, 2.5 to 77.6, and 21.2 to 1286 mg/kg, respectively. Iron ranged from 6.1 to 61.8 g/kg. Soils were enriched with Cd, Cu, Hg, Mn, Pb and Zn as compared with soil background values of Guangzhou. Spatial distribution maps indicated similar distribution patterns of Cd, Cu, Pb and Zn. Urbanization history and industrial activities affected the accumulation of PTEs. Relatively higher concentrations of As, Cd, Cu, Hg, Pb and Zn were found in districts with a longer history of urbanization and industrial activities, such as the Liwan district along the Pearl River in western Guangzhou. This area was the most highly contaminated, whereas the Tianhe, a district with a limited time of development, was the least contaminated. Multivariate statistical analysis showed distinctly different associations among the studied metals; suggesting that Fe, Ni and Mn were predominantly derived from a natural (geological) source; As, Cu, Hg, Pb and Zn from anthropogenic sources; and Cd from both sources. Present PTEs levels were also compared with those reported from other urban areas around world, and results indicate that further studies on assessing both the human and ecosystem risks associated with urban contaminated soils will be necessary.

Crop-residue burning and frequent paddy-soil flooding can lead to a substantial accumulation of black carbon (BC), thus contributing to long-term C sequestration. There is evidence that the turnover of BC in soils also depends on the soil mineral assembly. We studied the effects of paddy and non-paddy soil management and different major reference soil groups on BC storage. We hypothesized that overall BC storage in soil relates to paddy management and the abundance of reactive mineral phases such as Fe and Al oxides, and clay-sized minerals. Parallel to BC, black nitrogen (BN) should accumulate in soil. Paddy and non-paddy soils were sampled in three different climate zones (tropical, subtropical, and temperate). The soil profiles comprised six replicates of Andosols, Alisols and Vertisols from Java (Indonesia), and Alisols and Cambisols from China, as well as one Fluvisol and Gleysol from Northern Italy. Samples were taken by horizon down to > 1 m depth and analyzed for soil organic carbon (SOC) and BC. The latter was analyzed by oxidation to benzene polycarboxylic acids. Abundance of BN (as aromatic N) was estimated by X-ray photoelectron spectroscopic analyses of selected topsoil horizons. In topsoils BC vs. SOC accumulation was affected by management for Andosol, Alisols in China, and Vertisols. However, both flooding and crop-residue management seemed to control this. BC contents relative to SOC also differed between the reference soil groups, independent of management (p 50% of total N after combustion of rice straw. However, aromatic N was barely, or not detectable in soil, and there was no correlation to BC. We conclude that burned crop residues were not a major source for aromatic N in soil. BC and aromatic N showed no distinct relations to soil properties, such as the abundance of clay-sized minerals, and Al and Fe oxides. Differences in BC stocks between the soils were most pronounced in the subsoils, likely caused by physical processes, such as swelling and shrinking of clays and/or translocation by leaching. Climate and regional soil-adjusted management also affected BC accumulation, but this first snapshot indicates that global BC maps may be linked to global soil maps.

To study the feasibility of using magnetic techniques for monitoring soil pollution in Shanghai, magnetic properties and heavy metals in the topsoils in an urban site (Songnan Town) and a less-urbanized agricultural site (Luojing Town) in Baoshan District, Shanghai, were studied. Compared with the background, magnetic signals of the urban topsoils are extremely enhanced with magnetic susceptibility ( χ lf ) from 127.3–1959 × 10 − 8 m 3 kg − 1 ; while those of the agricultural topsoils are only slightly increased. However, both the urban and agricultural topsoils contain few pedogenic SP grains, as indicated by their low χ fd % ( χ arm /SIRM, χ arm / χ lf and SIRM/ χ lf indicate that the grain size of magnetic minerals in the urban topsoils is significantly coarser than that in the background and the agricultural topsoils. Furthermore, the urban topsoils show low coercivity and magnetic soft behaviors, as indicated by higher SOFT%, lower HARD%, higher IRM 300 mT /SIRM (close to 1) and lower IRM − 200 mT /SIRM (close to − 1). It suggests that the urban topsoils have received some coarse ferrimagnetic particles. Heavy metals are highly enriched in the magnetic fractions of the topsoils. Geochemical properties of the magnetic fraction of the urban topsoils are significantly different from those of the agricultural topsoils, further indicating that the extra magnetic minerals accumulated in the urban topsoils are neither inherited from soil parent materials nor from pedogenic processes, but originate from anthropogenic activities. The significant correlations between heavy metals and χ lf , χ arm , SIRM, SOFT and HIRM of the topsoils in the district indicate that the magnetic techniques can be used for monitoring soil pollution in Shanghai. The soils with χ lf from 39–50 × 10 − 8 m 3 kg − 1 in the district are tentatively defined as “slightly polluted soils”; those with χ lf > 50 × 10 − 8 m 3 kg − 1 are defined as “polluted soils”.

Information on carbon (C) storages and the changes under the shifting of land use is of particular interest for estimating the gains and losses of soil C at a regional scale. The present study attempted to quantify the soil organic carbon (SOC) storage in the Hong Kong soils for the first time as well as to demonstrate the changes of SOC density after the transformations of paddy fields to other land usages due to the decline of agricultural activities. SOC storages were estimated by combining the land-use areas and profile data derived from our recent soil survey (248 samples were collected) and other reports. The results indicated that SOC densities to the upper 100 cm ranged from 25.06 Mg ha − 1 of urban park land to 288.65 Mg ha − 1 of swamp and mangrove, and total SOC storage in the upper 100 cm was 8.8548 × 10 6 t, approximately 0.51% of that in the soils of Guangdong Province China. Approximately 46.5% to 70.5% of the SOC pool was stored in the upper 40-cm depth, and SOC densities were much consistent in depth distributions under different land uses. Different changes of SOC density were evidenced after the conversion from paddy fields to orchards, vegetable land and grassland. The uncertainties were also discussed for lack of the data from a long term survey at a particular location.

Human activities can affect the biogeochemical cycling of phosphorus substantially. However, the relationship between P accumulation and urban development process is largely unknown. This study investigated the influence of urban development on the P accumulation in urban and suburban soils, using Nanjing (China) as a case. Based on its urban development history, Nanjing was divided into suburban and urban area, and the urban area was subdivided into urban north and urban south. Soil total P (TP) and available P (AP) of 578 samples from 68 pedons at different locations were measured. Thickness-weighted mean P content of each pedon (P w ), P content of the surface soil layer (P s ), the highest P content of each pedon (P h ), and the lowest P content (P l ) of each pedon were selected as statistical indices. Compared with the background value, urban and suburban soils were enriched in P. The highest TP content was up to 11.14 g P kg -1 , and the highest AP content was up to 360 mg P kg -1 . However, analysis of variance (ANOVA) and multiple comparisons of P w, P s, P h, P l showed that urban south with longest residential history had the highest TP w, TP s TP h, and AP w, AP l contents, and urbanizing suburban had the lowest TP w and AP w, AP l contents. For both APs and AP h, there was no significant difference between suburban and urban area. However, significant differences between urban south and urban north were observed. The results demonstrated that urban development process, including population quantity and level of urban infrastructure, could influence soil P accumulation and distribution in urban environment. A more detailed assessment is required to avoid the potential secondary eutrophication caused by excess P release from those anthropogenic high-P soils.

A total of 66 surface (0–10 cm) soil samples covering the whole territory of Hong Kong were collected for analysis of the seven PCB congeners. The detectable concentrations of PCBs in the soils of Hong Kong ranged from 0.07 to 9.87 μg kg− 1, with a higher concentration in the urban area than in the countryside. PCB patterns were dominated by the low chlorinated congeners. PCB118, a dioxin-like congener, was found in samples from Tsuen Wan and Sha Tin and was assumed to be deposited from local sources. Principal component analysis (PCA) revealed that the sources of PCBs were mainly associated with Aroclor 1242 and Aroclor 1248. A significant linear correlation between the concentrations of soil organic matter (SOM) and the sum of 7 PCBs and of SOM and PCB153 was observed. In total, PCBs were not a severe contamination in the soils of Hong Kong from the current results. However, long-term monitoring is still recommended to keeping, especially in the landfill deposits and some urban parks.

Conventional classification systems based on vegetation and land use are frequently used to characterize or describe urban soils to determine the influence of urbanization on soils. In this study, the sensitivity of different grouping methods in reflecting soil variations along an urban–rural gradient was compared. The objective of this study was to determine the most sensitive grouping system in depicting and explaining variations of soil attributes around an urban area. Grouping methods, including urban–rural division, in situ vegetation type, land use types in different scales and numerical clustering, were compared for both single soil attributes and “soil set” defined by multiple variables. The result shows urbanization has a strong impact on many soil properties, especially that of gravel content, sand content, pH, phosphorus and soil compaction. In terms of the variations of soil attributes, in situ vegetation type is the most sensitive in comparison with local land use types and district-viewed land use types. In other words, soil properties in this study are not sensitive to coarser spatial resolution. Therefore, it's hard to interpret the spatial variation of urban soil by regular methods using natural soil-landscape paradigm. Furthermore, vegetation would best proxy the delineation of single attribute of urban soils. Numerical clusters effectively reflect the land use types and their change during urbanization. All clusters were interpreted as different sets with practical meanings: soil in abandoned greenbelt, soil in ill-managed greenbelt, soil in new vegetable land, extreme urban conditioned soil, soil in well-managed greenbelt, soil in highly mellowed vegetable land, soil in common urban–peri-urban greenbelt and weak-urban-impacted soil. They can be used as bases for soil regionalization in urban and peri-urban environment.

Soils in urban areas are often artificially formed and preserved as cultural layers in which soil materials are mixed with artifacts. The vertical change of soil properties denotes the historical change of human activities, and therefore, urban soil can be considered as a record of history of urban development. We studied a deep profile of urban soil (600 cm) in Nanjing, China, by intensively sampling at every 5 cm. Soil samples were analyzed for lead content. Charcoal from several layers was found and was dated using 14C to recognize archaeological cultural layers that might be formed in different Chinese dynasties. Pb isotope ratios were determined by thermal ionization mass spectrometry (TIMS) technology. The study found that artificial deposition, i.e., the formation of cultural layer started about 1700 years ago and lasted until recently. All cultural layers above the original loess were heavily contaminated by Pb with total content varying from 100 mg kg− 1 to more than 2000 mg kg− 1. There were several historical periods in which Pb content peaked, indicating significant contamination by human activities, such as ancient ore smelting for various purposes and use of Pb-containing materials for handicraft manufacture. Various Pb isotope ratios of the cultural layers differed substantially from that of the original undisturbed loess in the deepest position. 206Pb/207Pb value decreased gradually from the bottom layer to top layer, although with occasional exceptions, indicating a long-lasting mixing of extraneous sources of Pb during the entire history since ca. 300 A.D. In comparison with the isotope ratios of lead ores of different sources and that of aerosols, it was illustrated that the source of Pb in cultural layers might come from lead ores of southern China in the earlier dynasties, while Pb from north China might have contributed to the Pb source in the more recent years, suggested by the lowering of 206Pb/207Pb ratio in the near-surface cultural layers. However, the impact of petrol burning on cultural layers was basically excluded, considering the relatively short history of petroleum use in this area.

A geochemical baseline provides the means to distinguish between the pedogenic origin and the anthropogenic origin of the trace element in the environmental compartments. We collected 271 soil samples representative of different parent rocks and soil types from the whole territory of Hong Kong and analyzed the composition of clay mineralogy and the contents of 15 chemical elements (Fe, Cd, As, etc.) for these samples. The baseline was predicted with the method of the normalization procedure combined with the relative cumulative frequency curve. The result indicated that Fe was the best reference element for the normalization procedure among the five potential reference elements (Fe, Al, Sc, Ti, and Mn), followed by Sc and Ti. A poor correlation was found between Sc, Ti, and Cu. The predicted baseline was much lower than the A-value of the Dutch List used usually in screening the polluted soil of Hong Kong, implying that the extent of heavy metal pollution might have been underestimated with respect to local lands. We also applied the cluster analysis to distinguish the geochemical associations of the trace elements due to its importance to the baseline. Approximately three major associations including the Fe–Mn-oxides related, Al oxides or Al-bearing-clay-mineralogy related and sulfide- related associations were observed from the dendrogram.

Surface soil (0–10 cm) samples from 53 sampling sites including rural and urban areas of Hong Kong were collected and analyzed for 16 EPA priority polycyclic aromatic hydrocarbons (PAHs). Total PAH concentrations were in the range of 7.0–410 μg kg−1 (dry wt), with higher concentrations in urban soils than that in rural soils. The three predominant PAHs were Fluoranthene, Naphthalene and Pyrene in rural soils, while Fluoranthene, Naphthalene and Benzo(b + k)fluoranthene dominated the PAHs of urban soils. The values of PAHs isomer indicated that biomass burning might be the major origin of PAHs in rural soils, but vehicular emission around the heavy traffic roads might contribute to the soil PAHs in urban areas. A cluster analysis was performed and grouped the detectable PAHs under 4 clusters, which could be indicative of the PAHs with different origins and PAHs affected by soil organic carbon contents respectively.

Paddy soils are Hydragric Anthrosols and are an important soil resource for food production. They are widely distributed in China and Asia. Soils that can be used for paddy cultivation vary considerably but basically are grouped into three kinds of landscape, i.e., well-drained sloping uplands, alluvial plains with groundwater fluctuation, and poorly drained polder areas with a near-surface water table. Typical soil chronosequences were studied to clarify the pedogenic changes after wetland rice cultivation, especially in relation to management practices. All of them are in important rice production regions of China. The study focussed on redox condition and iron oxides dynamics. On alluvial plains, the soil conditions range from continuously reducing controlled by high groundwater to alternating reduction and oxidation because of artificial submergence and groundwater fluctuations. Artificially induced leaching losses, previously prevented by a high groundwater table, are partly compensated during paddy cultivation by the rejuvenation process of alluvial deposition. Soils on an older delta, which no longer receives river-sediments, are more strongly developed and showed a decrease of DCB-extractable iron in surface horizon of young paddy soils and more profoundly throughout the profile in old paddy soils. In the polder area, the apparent lowering of groundwater table by deposition of sediment has changed the soil hydrology and increased redox potential. With the better soil aeration, crystalline iron oxides and their ratio to total iron increase with cultivation age. On well-drained terraces, the change of soil moisture regime is overwhelming. Previously well-aerated upland soils are periodically reduced, which accelerates leaching and loss of iron oxides from surface horizons, and probably clay decrease by lateral movement and ferrolysis processes superimposed on the difference of parent soils. An iron-enriched subsurface horizon has formed rapidly and it is a diagnostic feature of this type of paddy soil.