Abstract:【Objective】 Paddy soils are extensively polluted by heavy metals (HMs) in China and present significant challenges for safe agricultural use. Thus, this study seeks to address the heavy metal pollution in Chinas paddy soils, focusing on polluted soils from the Dabaoshan mining areas in Shaoguan, Guangdong Province.【Method】 A flooding-drainage incubation experiment was conducted and combined with chemical extraction, diffusive gradients in thin films (DGT) technology, and correlation analyses. Also, the impact of phosphorus (P) and humic acid (HA) addition on soil Fe speciation and the bioavailability of related elements was evaluated. The selection of P and HA concentrations was based on common agricultural practices to ensure the environmental relevance of this study.【Results】 The results revealed that soil pH gradually became neutralized while redox potential (Eh) decreased during the flooding period. The concentrations of acid-soluble Fe2+, amorphous Fe, and Fe activation degrees increased from 1.5 g·kg-1 to 4.8 g·kg-1, 6.6 g·kg-1 to 10.1 g·kg-1, and 21% to 29%, respectively, with a decrease in amorphous Fe content observed in treatments with added P and HA from 2 to 42 days of flooding. After soil drainage, both amorphous Fe(Feox) and Fe activation degree rapidly decreased to 7.4 g·kg-1 and 21.6%. Regarding CaCl2-extractable heavy metals, the lowest values were observed after 14 days of flooding, whereas levels of As, Cd, Cu, and Zn rapidly increased to 0.1, 0.4, 0.3, and 7.0 mg·kg-1, respectively, after drainage. The addition of P and HA in the early flooding stage increased the As content by 80% and 35% compared to the control, respectively, but decreased the contents of Cu, Zn, and Cd, with HA addition, the reduction rates of CaCl2-extractable Cu, Zn and Cd were over 67%. During flooding, DGT-extracted P and Fe content initially increased then decreased, while Zn content gradually reduced. The addition of P and HA significantly reduced the bioavailable Zn content in the early stages of flooding while the bioavailable content of P and As was primarily controlled by bioavailable Fe.【Conclusion】 During the soil flooding-drainage incubation, significant changes occurred in soil physicochemical parameters including pH, Eh, and speciation of Fe. The addition of P and HA could regulate the bioavailability of elements such as Fe, P, and HMs. These findings offer valuable insights for the remediation of HMs-polluted soils, highlighting the potential for using P and HA in improving soil quality and ensuring safe agricultural production.

Abstract:Soil organic phosphorus(P) is an important component of the soil P pool and its mineralization plays an important role in global P cycling. Understanding the mineralization of soil organic P is beneficial for the efficient utilization and management of P in terrestrial ecosystems. In recent years, the application of advanced techniques such as modern spectroscopy, chromatography, and mass spectrometry has provided crucial avenues for a more comprehensive characterization of the composition and structure of organic P. This review summarizes the applications of these technologies in quantifying changes in soil organic P content. Organic P, following mineralization, is converted into inorganic P(Pi), making it available for direct uptake and utilization by plants and microorganisms. Soil organic P mineralization is orchestrated by two primary pathways: enzymatic and mineral-mediated processes. Delving into the mechanisms of biological catalysis and abiological mineral-mediated catalysis is crucial for elucidating the control pathways of organic P. The mechanisms of soil organic P mineralization can be divided into biological mineralization driven by the oxidation of organic matter by microorganisms (phoA, phoD, and phoX) in response to energy demand, and biochemical mineralization driven by the release of Pi nutrients from plants in response to the demand for P nutrients mediated by phosphatases. Recent investigations have underscored the significance of minerals as an abiological mineralization pathway, shedding light on the mechanisms and actions of mineral-mediated catalysis. The surfaces of minerals (such as iron (hydro) oxides, manganese (hydro) oxides, and aluminum(hydro)oxides)provide an enzyme-like environment, facilitating the cleavage of phosphate ester (P-O-C) and terminal phosphoanhydride(P-O-P)bonds, resulting in the hydrolysis of organic P to Pi. In soil ecosystems, the biogenic elements carbon(C) and nitrogen(N) are intimately linked with soil organic P mineralization. From a nutrient factor perspective, elucidating the driving patterns of organic P mineralization can inform strategies to regulate soil P pools. Specifically, C effectively drives microbial mineralization of organic P, whereas N influences enzymatic metabolism, with the interplay between the two elements profoundly influencing the soil organic P mineralization process. The multiple forms of organic P present in soils are susceptible to influences from various external factors, which modulate phosphatase activity and alter organic P content, thereby further affecting the mineralization process. Various factors, including agricultural practices (such as fertilizer application, tillage practices, and biochar application), soil physical and chemical properties (such as pH, temperature, soil water content, and soil aeration status), microbial biomass, soil CO2 concentration, vegetation, and pollutants all impact soil organic P mineralization, resulting in corresponding environmental ecological effects. Therefore, regulating organic P mineralization is crucial for enhancing soil fertility and protecting the environment. Future strategies can focus on enhancing phosphatase activity, altering organic P composition, and increasing the abundance of phosphorus-solubilizing microorganisms to improve soil organic P mineralization. This review summarizes the advances in soil organic P mineralization research, synthesizing the soil processes, influencing factors, and control pathways, and highlighting the existing challenges and prospects.

Abstract:【Objective】 Phosphorus is an essential nutrient element that affects the growth, yield and quality of crops. Due to the application of a large amount of phosphate fertilizer and the lack of scientific management, the utilization rate of phosphate fertilizer is low and the environmental risk increases. Thus, it is important to know the effects of different amounts of exogenous phosphorus(P) addition on P bioavailability components in paddy soil under flooding conditions.【Method】 Three soil samples were collected from the new paddy field (NP, 2-3 years), medium-term paddy field (MP, 20-30 years) and old paddy field (OP, 400-500 years) in Sunjia small watershed of Yingtan, Jiangxi Province. Based on the flooding culture experiment (0-80 days) with different concentrations of exogenous P addition [0(CK), 125(P1), 250(P2), 500(P3), 625(P4), 750(P5) mg·kg-1], the test for simulating the bioactivation process of biologically based P(BBP method) was adopted. The dynamic changes in the increment(Δ) of soluble P(Ca-P), easily activated and released P(Ci-P), easily mineralizable acid phosphatase (En-P), and the potential inorganic P(HC-P) were analyzed, and the correlation between all P components and influencing factors were evaluated.【Result】 The results showed that under flooding conditions, exogenous P addition could significantly increase the available P(Bray-P) and BBP components such as Ca-P, En-P, Ci-P and HC-P in paddy soil, and the increment of P contents of each component increased significantly with an extension of P addition. The increment of BBP components was changed by the order:ΔCa-P < ΔEn-P < ΔCi-P < ΔHC-P. On day 15 of incubation, the ΔCa-P and ΔCi-P in the new paddy field reached a maximum value while on day 60, the ΔCa-P, ΔEn-P, ΔHC-P and ΔBray-P in the medium-term paddy field reached their maximum value. However, the P content of each component did not change significantly in the old paddy field. The ratio of ΔBray-P to ΔTP(ΔBray-P/ΔTP) in paddy soil after exogenous P addition showed the same trend as that of ΔBray-P, but there was no significant difference among different P additions. Path analysis showed that exogenous P addition had a significant direct positive effect on ΔBray-P in new and old paddy soil; ΔCi-P had a significant direct positive effect on ΔBray-P in new paddy field; ΔHC-P and ΔCa-P had a significant direct positive effect on ΔBray-P in medium-term paddy field, and ΔEn-P had a significant direct positive effect on ΔBray-P in old paddy field.【Conclusion】 Although exogenous P addition can significantly increase the components of the bioavailability of P in paddy soil, the emergence stage of the maximum increment bioavailability of P in paddy soil is different. Therefore, it is of great significance to timely and appropriately apply P fertilizers for the improvement of P fertility and the risk control of P loss in paddy fields.

Abstract:Polycyclic aromatic hydrocarbons (PAHs) are a class of persistent organic pollutants, which readily accumulate in the soil with significant toxicity. However, PAHs could be strongly adsorbed by the solid phase of soil, the exposure based on the total pollution mass will lead to the overestimation of human health risks. Based on 123 papers published from 2000 to 2020, this study summarized the distribution and composition characteristics of soil concentrations of 16 priority-controlled PAHs (Σ16PAHs) in China, introduced 11 commonly used testing methods and main influencing factors for modelling bioaccessibilities, and summarized the ranges of bioaccessibility coefficients of PAHs. The results showed that the maximum and average concentrations of Σ16PAHs in soil were 23 250 and 1 314.7 μ·kg-1 respectively. In recent years, mainly based on the physiologically-based extraction tests (PBET), the bioaccessibility test methods of PAHs are continuously improved and modified in enriched simulation of digestion processes and adsorbents, while the digestion conditions and soil properties etc. have a great impact on the bioaccessibility results. The average bioaccessibilities of 16 PAHs ranged from 13.2% to 72.4%, among which higher values were identified for chrysene and benzo (b) fluoranthene, contributing significantly to the total Σ16PAHs exposure. This study therefore provided an important theoretical ground for undertaking detailed risk assessment of PAHs.

Abstract:【Objective】Ingestion of soil is a pathway of human exposure to several environmental contaminants, including several heavy metals. Risk assessment of soils has typically been performed on total concentrations of target heavy metals. However, it may overestimate the potential adverse effects. To refine exposure risk, bioaccessibility and bioavailability measurements have been employed in many studies. Bioaccessibility tests are used to measure the bioaccessible fractions of contaminants in soils while bioavailability evaluates the fraction of heavy metals that reach the systemic circulation. These tests are both considered accurate approaches to evaluate the potential health risk of contaminants. However, there are few studies on the health risk assessment of heavy metals from mining soils via bioavailability. 【Method】In this study, five soil samples from the mining area of Wenshan, Yunnan Province were collected and the levels of Cd, Pb, Zn and Cu were analyzed using inductively coupled plasma mass spectrometry (ICP-MS). The bioaccessibility of Cd, Pb, Zn and Cu was detected using in vitro digestion method (SBRC), and the relative bioavailability (RBA) of Cd was determined by BALB/c mice models. The health risks of the four heavy metals in mining soils were assessed based on total, bioaccessible, and bioavailable data, respectively.【Result】The results showed that Cd pollution in this study area was serious, with the content being 2.06 mg·kg-1, which was 9.36 folds higher than the limit of soil background value in Yunnan. The bioaccessibility of Cd, Pb, Zn and Cu in the gastric phase was 24.29%-50.55%, 7.68%-17.87%, 24.61%-32.18%, 7.75%-37.87%, respectively, while in intestinal phase, they were 22.78%-44.32%, 1.64%-5.22%, 14.10%-28.11%, 8.51%-31.49%, respectively. As evident, the bioaccessibility of Cd was the highest among the four heavy metals. The RBA of Cd measured in vivo was 1.31%-48.39% in the liver, 2.83%-8.58% in the kidney, and 4.60%-50.95% in the liver and kidney. Compared with a single endpoint, Cd-RBA in the liver and kidney provided better repeatability and were ideal target organs for the determination of Cd-RBA. In vivo-in vitro correlation showed that the bioavailability of Cd determined by SBRC had a poor potential to predict Cd-RBA in contaminated soils from the mining area in China. Health risk assessment of the mining soil based on the target heavy metals, bioaccessibility and bioavailability revealed that the assessment using the total heavy metals had greater human health risk, while the data based on bioaccessibility and bioavailability showed a significantly reduced risk. 【Conclusion】The health risks assessment based on the total heavy metals in soil may be overestimated, and the establishment of a new method based on the bioavailability data will be more accurate. Our results provide a scientific basis for the health risk assessment of contaminated soils in China.

Abstract:[Objective] This study aimed at accurate assessment of health risks of arsenic (As) in typical soils of China and comprehensive investigation of their influencing factors, based on oral bioaccessibility/bioavailability of soil As.[Method] Samples of five typical soils of the country, i.e. red soil, black soil, cinnamon soil, brown soil and yellow soil, were collected and prepared into As-contaminated samples by spiking As at 600 mg·kg-1. After one-month aging, bioaccessibility/bioavailability of soil As in the gastrointestinal tract of a human being were studied, and its health risk was evaluated by means of in vitro test (PBET-UF model). Besides, from the perspective of soil properties (soil basic physicochemical properties and As fractions), influencing factors of As bioaccessibility/bioavailability were explored and variation of As bioaccessibility/bioavailability with the soil analyzed.[Result] Results show that As bioaccessibility/bioavailability significantly varied between soils, as As bioaccessibility ranged from 37.2% to 71.8% in the gastric phase and from 49.0% to 73.3% in the small intestinal phase, while As bioavailability in the small intestinal phase ranged from 48.6% to 72.1%. It was indicated that from the stomach to the small intestine soil As was gradually dissolved by digestive fluids, and almost all the dissolved As in the small intestine could pass through the specific ultrafiltration membrane used to simulate small intestinal epithelium. Also, health risks of soil As through oral ingestion significantly varied with the soil. Based on the estimated As bioaccessibility in the gastric phase, carcinogenic and non-carcinogenic risks of As in the five soils varied in the range of 4.28×10-4~8.26×10-4and in the range of 14.86~28.69, respectively; and based on the estimated As bioavailability in the small intestinal phase, they did in the range of 5.59×10-4~8.30×10-4and in the range of 19.40~28.81, respectively. The carcinogenic risk and non-carcinogenic risk was two orders and one order of magnitude respectively higher than their respective acceptable limit. In addition, correlation analysis shows that between bioaccessibility/bioavailability of soil As was significantly related to certain soil properties.[Conclusion] Oral bioaccessibility/bioavailability of soil As is significantly and positively related to soil pH, migration coefficientS and migration coefficientW, but negatively to content of free Fe and Al oxides. Migration coefficientS is the main influencing factor determining bioaccessibility of soil As in the gastric phase, whereas soil pH is in the small intestinal phase. Although free Fe and Al oxides in the soils are not the determining factors directly affecting bioaccessibility/bioavailability of soil As in the gastric phase, they form new minerals with As, thus reducing the activity and consequent bioaccessibility/bioavailability of soil As. It is expected that all the findings in this study may contribute positively to making future field investigations of soil environmental quality of China more in line with the reality, and consummation of the methodologies for health risk assessment of heavy metal (loid) -contaminated soil.

Abstract:Geological and human activities in quite a number of regions of the world are found to have brought about serious arsenic (As) pollution in soil and groundwater, gravely threatening the ecosystems and human health in those regions. In order to effectively control As pollution risk at large scales, it is necessary to accurately evaluate interfacial behaviors of As in different media. Being regulated by chemical and microbiological factors migration and transformation of the element in certain typical environmental interfaces, like that of soil-water and rhizosphere, exhibit the characteristics of drastic changes in species at μm-to-mm-scales. Conventional active sampling techniques, which mostly consist of destructive field sampling and afterwards sample analysis in lab, have proved to be not good enough to meet the demands of the study on interfacial process of the element, such as handling an element varying drastically in species, quantifying the element at trace levels, and time- and labor-saving. In recent years, passive sampling technology, represented by diffusive gradients in thin-films (DGT), diffusive equilibrium in thin-films (DET), in-situ porewater iterative sampler (IPI) and dialysis sampler (Peeper), has emerged, displaying great advantages over the conventional ones in the research. The DGT device is composed of filter membranes, diffusion gel, binding gel and plastic bases/caps used to fix the three layers of membrane/gel. The filter membrane is mainly used to prevent particles in the environment to be tested from entering the device; the diffusion gel to facilitate free diffusion of ions and formation of a diffusion gradient; and the binding gel, chosen according to the purpose of the experiment, to absorb the pollutants to be tested. DET is a sister technique of DGT, omitting the binding gel phase. The IPI sampler consists of hollow fiber membrane sampling tubes and catheters. For sampling, the sampling tube is filled with deionized water in advance, and ions and small molecules in the environment diffuse into the tube. After the diffusion reaches equilibrium, the solution in the sampling tube is directly pumped out for measurement of concentrations of the ions tested. In principle, Peeper is similar to DET and IPI, but lower in spatial resolution for measurement of porewater concentration. These passive sampling techniques have been used to determine in situ of total As and As speciations in water and soil porewater, and their one-dimensional distribution profiles. DGT-measured As concentration in soil has a good correlation with its content in plants, showing that DGT is suitable for the evaluation of As phytoavailability. It turns out in recent years to be an important trend to use these passive samplers to study two-dimensional spatio-temporal distribution of As at the soil/sediment-water interface. DGT has been used to characterize the two-dimensional distribution of As at soil/sediment-water interface and plant rhizosphere in submillimeter high-resolution, so it cherishes great advantages in the study on spatial distribution of As, whereas IPI can sample iteratively with low disturbance, thus being one of the few tools that can be used to study dynamic distribution of As relative to species. These studies elucidate biogeochemical behaviors of As from a microscale perspective. In the end, the paper describes a prospect of the research in future, including:1) taking advantage of the merits of the passive sampling techniques in future studies on dynamic-controlled processes of As uptake by plants; 2) developing novel passive sampling techniques with both the spatial resolution and the temporal resolution of As concentration taken into account; 3) combining the passive sampling techniques with other 2D sampling techniques, such as planar optodes and soil zymography, in comprehensive studies on biogeochemical process of As in soils and sediments; 4) extending the use of passive sampling techniques to the study on processes of As uptake by fauna living in soils and sediments; and 5) building models of As transporting across interfaces based on data of changes in spatiotemporal concentration of As at the interfaces in complex environmental matrix.

Abstract:【Objective】How to monitor and assess soil Cd pollution and bioavailability is currently an issue of great concern. However, measurements of soil available Cd contents using the conventional chemical methods may only serve as certain reference, because they are not good enough to intuitively and truly reflect the damages soil Cd2+ does to plants. Bryophyta are simple in structure, free of any cuticle on their surface and quite sensitive to pollutants, so they are usually used in monitoring environmental pollution. In this paper, an effective method was preliminarily studied to monitor and evaluate soil Cd pollution and Cd bioavailability using Pogonatum inflexum, a species of bryophyte highly sensitive to soil Cd. A mature plant of the sporophytic generation of Pogonatum inflexum could be as high as 10 cm. Besides, it is simple in surface structure with no vascular bundle differentiation, but with sporophytes parasitizing on gametophytes. Furthermore, the various organs of Pogonatum inflexum are homogeneous in cell structure and mostly monolayer cells.【Method】In the experiment to validate effectiveness of the method, the tested soil was prepared into media, different in Cd contamination degree (1~5 mg kg-1), for culture of Pogonatum inflexum. Growth, Cd enrichment, chlorophyll content, soluble protein content and MDA concentration of the bryophyte was observed and/or determined. Soil available Cd contents extracted with acetic acid were cited as indicator parameter for soil Cd bioavailability, and then analysis was done of correlations of the above described indices with content of soil total Cd and content of soil available Cd, separately.【Result】Results show that in the test soil, bioavailable Cd accounted for about 20%~40% of total Cd. All the indexes of Pogonatum inflexummentioned above were closely related to soil Cd stress, especially when soil Cd concentration was higher than 3 mg kg-1. Pogonatum inflexum was low in Cd enrichment and in tolerance to Cd as well. It could enrich as high as Cd 1.627 mg kg-1, with enrichment coefficient being 63.9% and Cd concentration in Pogonatum inflexum was more closely related to soil bioavailable Cd than to soil total Cd. Pogonatum inflexum responded quite apparently to soil Cd pollution, with visible symptoms such as damaged sporophytes and gametophytes. When soil Cd concentration was higher than 1~2 mg kg-1 (available Cd concentration was higher than 0.559 mg kg-1), leaves ofPogonatum inflexum turned yellow and brown; when soil Cd concentration got up to 5 mg kg-1, seta softened and kinked, till the plants withered dead. Changes in physiological and biochemical indexes of the tested plants, such as contents of chlorophyll, soluble protein and MDA, were apparently related to soil Cd stress, especially to soil bioavailable Cd contents, and corresponded well to changes in soil Cd pollution level.【Conclusion】Therefore, the contents of chlorophyll, soluble protein and MDA in Pogonatum inflexum can be used as indicators to monitor and evaluate soil Cd pollution and bioavailabilty. Pogonatum inflexum is an ideal material to be used to effectively monitor soil Cd pollution thanks to its high sensitivity to soil Cd2+ and its readiness and intuitiveness in displaying damage symptoms.

Abstract:Biochar is a kind of highly aromatic carbonized material produced through thermal decomposition of biomass under reductive conditions (i.e. in the absence of or with a limited supply of oxygen). Biochar is found to be able to play an important role in mitigating global climate change, removing pollutants from water and soil, as well as maintaining functions of ecosystems. During the pyrolytic processes of biological materials, a certain amount of organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), would form and remain on the surface of the biochar. Consequently, increasing application of biochar may bring about a certain risk to the environment. Current researches pay much attention to the positive effects biochar may have, while ignoring its potential hazards to the ecosystem. To assess environmental risk of the PAHs in biochar, it is necessary to determine the contents of total and bioavailable PAHs in biochar. At present, the following four methods, i.e. Soxhlet extraction, accelerated solvent extraction (ASE), ultrasonication extraction and thermal extraction, are available for determining total PAHs in biochar. However, the four methods were often used to determine semivolatile organic compounds in solid matrix (soil or sediment). Among the four methods, the Soxhlet extraction and ASE methods are the most commonly used ones, because of their higher recoveries of target compounds. However, when they are used to extract PAHs in biochar, PAHs recoveries depend highly on solvents and the biochar per se. In the case of determining bioavailable PAHs, limited information is available besides the polyoxymethylene (POM) passive sampling method. Altough PAHs in biochar are formed mainly through two pathways, i.e. low temperature pyrolysis (< 500℃) and high temperature pyrolysis (> 500℃), the formation process is still very complicated, because there are a lot of factors that affect yield and composition of PAHs in biochar, including feedstock resource, pyrolysis temperature, heating rate, holding time, etc. With the respect of feedstock, little information is available concerning relationship between content of lignin and/or cellulose and PAHs in biochar. As regards pyrolysis temperature, biochar out of low-temperature pyrolysis generally contains more low-molecule-weight/high-vapor-pressure PAHs, whereas biochar out of high-temperature pyrolysis contains more high-molecule-weight /lower-vapor-pressure PAHs. However, the relationship between temperature and PAHs yield is still controversial. Heating rate and holding time of the pyrolysis are two important factors influencing PAHs yield in biochar. Generally speaking, during the process of slow pyrolysis with long holding time, PAHs are more likely to escape into the atmosphere as gas whereas during the process of fast pyrolysis, they are more likely to get condensed and adsorbed onto the surface of biochar. The other factors that influence PAHs content in biochar include ash content and moisture content of the feedstock, and presence of oxygen during the process of pyrolysis or the post-pyrolysis cooling process. Researches demonstrate that feedstock is high in ash and moisture content plus presence of a little oxygen facilitates formation of more PAHs in biochar.To minimize environmental risk of the PAHs in biochar, it is recommended firstly that feedstock free of PAHs contamination should be used for biochar preparation, and secondly that the technology of slow pyrolysis (heating rate <100 K min-1 and pyrolysis temperature <400℃) could reduce apparent total PAHs and bioavailable PAHs concentration in biochar. However, it should be noticed that high-temperature biochar is much higher than low-temperature biochar in specific surface area and adsorption capacity, and hence in applicability to pollutant removal, moreover, the PAHs in high-temperature biochar is lower in bioavailability and therefore in environmental risk, too. Obviously, high-temperature biochar (>600℃) is a better option. In order to find a professional and standardized protocol for quantitative analysis of PAHs or other toxic organic compounds in biochar, it is essential to do more researches that should lay more emphasis on pollutant yield relative to property variability of the biochar per se. In addtion, in-depth studies should also be done on long-term impacts of biochar on ecological environment. Both laboratory researches and field experiments should work jointly to deepen our understanding of how various biological and non-biological factors influence environmental behavior of PAHs in biochar.

Abstract:Release of non-exchangeable potassium (NEK) in soil is closely related to bioavailability of the potassium. The aim of this study was to characterize release and bioavailability of NEK in 8 soils, using pot K depletion experiments, 0.2 mol L-1sodium tetraphenylboron(NaTPB) extraction method and kinetic models. From each soil, 3 portions, 0.5 g each as replicate, were taken out for incubation in 3 ml of extracting solution (0.01 mol L-1 EDTA and 0.2 mol L-1 NaTPB) separately for a period varying from 0.5 to 96h. Four kinetic models (First-order, Elovich, power function and parabolic diffusion) were used to describe NEK release characteristics. Results show that NEK release rate in soil varied sharply with the agro-ecological region, being the highest in Northwest and Northeast China, and the lowest in Central and East China. NEK release in soil could be divided into two phases, fast and slow ones. The fast phase occurred at the beginning of the release, when NEK at the edges and wedge zones of clay mineral interlattices broke up bondage of the minerals by diffusion, while the slow phase came late when NEK inside the interlattices became the major source of K release. All the four models, the first-order (R2 = 0.817~0.926, SE = 11.9~215.1), Elovich equations (R2 = 0.952~0.997, SE = 4.6~66.9), power function (R2 = 0.869~0.990, SE = 3.9~127.8) and parabolic diffusion (R2 = 0.790~0.963, SE = 7.5~211.4), were good enough to describe NEK release kinetics. However, in choosing a proper one, it is essential not only to compare them in fitting degree, but also take into account their practicality and physical significance of the information they may provide. The Elovich equation failed to describe the early period of NEK release adequately, while the parabolic diffusion model yielded more than one NEK release rates. Thus, the NEK release rate described by the power function model might be a good indicator of NEK potential of soil. By such an indicator, the 8 soils were found to be declining in NPK release potential from west to east and from north to south of the country. Ryegrass were grown for 8 harvests in a pot experiment to evaluate K supplying capacity of the 8 soils. It was found that soil K supplyingpotential consisted with soil K release capacity, and that in soils high in K release amount and rate, the ryegrass without any K fertilizer applied was still quite high in relative and cumulative biomasses, cumulative K uptake and relative K content; and vice versa. The soil NEK release rates described with the four kinetic models was significantly related to K content and K uptake of the ryegrass without K fertilizer applied. In order to judge whether K supply of a soil was adequate or not, relationships between relative biomass and K content of the ryegrass without K fertilizer applied and between K content of the ryegrass and soil NEK release rate were determined. Based on the relationships, 90% of the relative biomass of the ryegrass without K fertilizer appliedwas set as threshold of the index of soil K nutrition, equaling to 34.0 g kg-1in soil K content and 396 mg kg-1 h-1 in soil NEK release rate. It is, therefore, concluded that out of the 8 soils, only Aridosol in Xinjiang and Isohumosol in Shaanxi and Heilongjiang can ensure ryegrass a good yield without showing any potassium deficiency symptomin a short termwithout K fertilizer application.