Effect of granulated biochar sphere on mineral nutrients in removing potentially toxic elements from contaminated agricultural soils.

Biochar (BC) granulation, yielding BC-based spheres, serves as an eco-friendly, cost-effective and efficient adsorbent for the removal of potential toxic elements (PTEs) from contaminated agricultural soils. The effect of BC-based spheres on mineral nutrients while effectively removing PTEs from contaminated soils is worth investigating. In this study, we utilized natural clay minerals, magnetic minerals and BC to produce water-hardened magnetic composite biochar sphere (WMBCS) that was capable of removing PTEs from composite contaminated agricultural soils. We explored the effect of WMBCS on minerals (Al, Ca, Fe, Mn, Na, Mg, Si, K, P, NH4+, and NO3-) in the removal of soil PTEs. WMBCS was a mineral nutrient-rich, recyclable, alkaline BC-based sphere that removes Cd (23.07-29.20 %), Pb (27.68-31.10 %), and As (26.17-37.48 %) from soils after three regeneration cycles. The effect of WMBCS on mineral nutrients varies depending on element type, BC and soil type. Compared to water-hardened magnetic composite phosphate modified biochar spheres (WMPBCS), water-hardened magnetic composite unmodified biochar spheres (WMUBCS) had more significant effect on Ca, Mg, Mn, Al and NH4+ in alkaline soils, but a greater effect on Ca, Mg, Mn, Fe and NO3- in acidic soils. Additionally, WMBCS displayed a more pronounced impact on mineral nutrients in alkaline soils than in acidic soils. The application of WMBCS reduced the accumulation of PTEs in wheat (18.40-84.70 %) and rice (27.96-88.66 %), but significantly inhibited seed germination and altered the uptake of mineral nutrients by seedlings due to its effects on soil physicochemical properties and mineral nutrient dynamics. Overall, WMBCS is suitable as a potential amendment for the remediation of soils co-contaminated with Cd, As, and Pb, but its effects on mineral nutrients cannot be overlooked, particularly in agricultural soils.

Biochar application for the remediation of soil contaminated with potentially toxic elements: Current situation and challenges.

Recently, biochar has garnered extensive attention in the remediation of soils contaminated with potentially toxic elements (PTEs) owing to its exceptional adsorption properties and straightforward operation. Most researchers have primarily concentrated on the effects, mechanisms, impact factors, and risks of biochar in remediation of PTEs. However, concerns about the long-term safety and impact of biochar have restricted its application. This review aims to establish a basis for the large-scale popularization of biochar for remediating PTEs-contaminated soil based on a review of interactive mechanisms between soil, PTEs and biochar, as well as the current situation of biochar for remediation in PTEs scenarios. Biochar can directly interact with PTEs or indirectly with soil components, influencing the bioavailability, mobility, and toxicity of PTEs. The efficacy of biochar in remediation varies depending on biomass feedstock, pyrolysis temperature, type of PTEs, and application rate. Compared to pristine biochar, modified biochar offers feasible solutions for tailoring specialized biochar suited to specific PTEs-contaminated soil. Main challenges limiting the applications of biochar are overdose and potential risks. The used biochar is separated from the soil that not only actually removes PTEs, but also mitigates the negative long-term effects of biochar. A sustainable remediation technology is advocated that enables the recovery and regeneration (95.0-95.6%) of biochar from the soil and the removal of PTEs (the removal rate of Cd is more than 20%) from the soil. Finally, future research directions are suggested to augment the environmental safety of biochar and promote its wider application.

New Straw Coating Material for Improving the Slow-Release Performance of Fertilizers.

In this work, we extracted cellulose from agricultural waste and produced a new straw coating material (ethyl cellulose, EC) through a series of modification operations. The slow-release properties of ethyl cellulose-coated urea (EU) and its absorption and utilization by plants were evaluated. The surface of EU can form a smooth and fine film, and the initial nutrient release rate is only 37.91% that of the uncoated fertilizer. Compared with common urea, the nitrogen of plants cultivated with EU increased by 17.69%, and the leached nitrogen decreased by 61.29%, indicating that EU can reduce nitrogen waste to the greatest extent and continuously supply nutrients to crops. Therefore, the application of EU could be a more practical, environmentally friendly, and sustainable alternative to nitrogen fertilizers.

Adsorption effect and the removal mechanism of silicate composite biochar particles on cadmium in soil.

Ordinary biochar has the disadvantages of low strength and fragility, and it is difficult to be separated in heavy metal contaminated soil after the remediation process. In order to realize the recovery and reuse of biochar, we prepared silicate composite biochar (SCB) and the magnetic silicate composite biochar (MSCB) with consistent particle size and high hardness. As well as the passivation effect and mechanism of the material on cadmium in soil was also investigated. The results showed that: (1) The MSCB had good hydraulic properties and strong magnetism, which can be quickly separated from the soil under the condition of external magnetic field. (2) The MSCB can remove 30.32%-38.80% of cadmium in the soil after three times of "application-separation-desorption-reuse", as well as the SCB can remove 28.30%-35.78% of cadmium from the soil. (3) The recovered SCB and MSCB had a certain mass loss, the mass loss rate of the biochar particles was in the range of 2.65%-4.90% after each time of recycling. (4) MSCB mainly immobilized cadmium ions through pore interception, complexation of oxygen-containin/iron-containin functional group and precipitation reaction.

Characteristics and preparation of oil-coated fertilizers: A review.

As the slow-release fertilizer, oil-coated fertilizer can not only slow down the nutrients loss, but also have outstanding advantages in controlling the nutrients release. Based on a large number of literature, this paper systematically investigated the composition, classification, properties and preparation of oil-coated fertilizers, summarizes the challenges faced by the oil-coated fertilizers and offers a few suggestions for the future research. Through literature research, some important conclusions were found: (1) Oil-coated fertilizers are generally composed of core fertilizers and coated oil layers, and some have active interlayers. (2) Vegetable oils has the characteristics of easy degradation, water resistance and impact resistance, and the nutrient release curves of vegetable oil coated fertilizer in soil and still water are "S" type. (3) The modified polyurethane exhibits good compatibility with urea, and can control the release of N in a long period of time, which is 30 days longer than the N release life of ordinary polyurethane-coated fertilizers. (4) Oil-coated fertilizers can reduce the loss of N by slowing down the hydrolysis rate of urea and the nitrification from NH4+ to NO3-, which reduces the N2O release by 70-80% compared to the uncoated fertilizers. Moreover, the paper also proposes a new preparation method of oil-coated material.

Effects of ecological protection and restoration on phytoplankton diversity in impounded lakes along the eastern route of China's South-to-North Water Diversion Project.

Traditional lake phytoplankton diversity studies do not take into account the impact of ecological protection and restoration project policies. Here, a difference-in-differences (DID) model, which is commonly used to analyze the relative importance of economic factors, was used to evaluate the impact of such policies on phytoplankton diversity in lakes. Dongping Lake was used as the experimental group, and the upstream Nansi Lake was used as the control group. The phytoplankton diversity index of the experimental group and the control group was used as the explanatory variable of the DID model. Six environmental and socioeconomic factors, temperature and precipitation, were used as control variables in the DID model. The effects of ecological protection and restoration project policy on phytoplankton diversity in lakes were analyzed. Under the influence of policy implementation, the phytoplankton diversity in the experimental lake was improved by 2.79% compared with that in the control lake. Temperature and precipitation were the main factors affecting phytoplankton diversity in the two connected shallow lakes in the Shandong Peninsula. This study verified that DID models can be used to quantitatively analyze the impact of ecological protection and restoration project policies on phytoplankton diversity in lakes.

Modeling water quantity and quality for a typical agricultural plain basin of northern China by a coupled model.

Water crisis across the globe has placed high pressure on social development due to the need to balance the water consumption between sustainable economy and functioning ecosystem. Integrated process-based modeling has been reported as an effective tool to better understand the complex mechanisms of water issues on a basin scale. Considering that it is still relatively difficult to simulate the water quantity-quality processes simultaneously, this study proposed an integrated modeling framework by coupling a hydrological model with a water quality model. Taking the Xiaoqing River Basin in the Shandong Province of northern China as an example, this study coupled a distributed hydrological model, SWAT, with a one-dimensional hydrodynamic-water quality model, HEC-RAS, to investigate its ability to simulate water quality and quality at the basin scale. The coupling of the two models adopted the "output-input" scheme, where the runoff modeling results from SWAT are input into HEC-RAS for hydrodynamic and water quality simulations of the river channel. The results show that the SWAT model can adequately reproduce runoff with accepted accuracy for the calibration and validation periods with acceptable R2 and Nash-Sutcliffe coefficients for the two hydrological stations. Further analysis also shows that the coupled model can simulate the concentration of ammonia nitrogen (NH4-N) and the chemical oxygen demand (COD) in the middle and upper stream of the river for both low and high flow periods. The coupling of the hydrological and hydraulic models in this study provides a good tool for identifying the spatial patterns of the water pollutants over the basin and, thus, helps simplify precision water management.

Dynamic landscapes and the driving forces in the Yellow River Delta wetland region in the past four decades.

Wetlands provide a broad range of ecosystem services, such as flood control, groundwater replenishment, and water purification. These services are particularly important in the Yellow River Delta, one of four estuarine deltas in China. The aim of this study was to examine the patterns and drivers forces of wetland landscape in the Yellow River Delta. We analyzed the spatio-temporal characteristics of land use change and dynamic landscape patterns changes between 1980 and 2018, divided into eight periods, from land use and remote sensing data. We also analyzed data for annual precipitation, annual temperature, annual evapotranspiration, digital elevation, slope, distance from the main river, built-up land, GDP, and population with principal component analysis, to identify the main drivers behind the changes in the wetland landscape. The results showed that, from 1980 to 2018, the total area of wetland decreased first and then increased the total area of wetland increased and was 1172.73 km2 greater in 2018 than it was in 1980, and the types of wetland in the Yellow River Delta changed frequently. From 1980 to 2005, the area of wetland decreased and the landscape dominance and degree of fragmentation were relatively high. From 2006 to 2018, as environmental policies were implemented and wetland protection became more important, the rate of development of wetland areas increased in a north-south direction, the proportion of landscape types became more balanced, and the spatial distribution homogenized. The main drivers of change in the wetland landscapes were GDP, population, precipitation, and temperature which were included 81.852% of the original information. The findings from this study provide us with an improved understanding of how land use and wetland landscapes changed from 1980 and 2018 and may have implications for the protection of wetland ecosystems, species diversity, and sustainable development.

Mechanism of cadmium removal from soil by silicate composite biochar and its recycling.

Biochar added to the soil is generally difficult to separate. In order to solve the problem of separating biochar from soil, this paper applies a hydraulic silicate gel material to the preparation of biochar. Non-magnetic silicate bonded biochar (SBC) and magnetic silicate bonded biochar (MSBC) with hydraulic properties were prepared. The new silicate bonded biochar has good adsorption performance, separation and recovery characteristics. The findings are as follows: (1) after three times of soil remediation, the silicate bonded biochar still had good mechanical properties, and the compressive strength was not attenuated, remaining between 210 and 270 N. (2) After three times of SBC and MSBC remediation, total Cd in soil decreased by 29.33% and 31.82% respectively, and available Cd decreased by 60.82% and 62.74% respectively. (3) After three cycles, the recovery rates of SBC and MSBC both exceeded 94.88%, and the highest adsorption regeneration rates of SBC and MSBC reached 83.09% and 92.06%, respectively. (4) The Cd content of wheat after SBC and MSBC repair was reduced by 29.67-37.36% and 47.25-63.74%, respectively.

Effect of returning biochar from different pyrolysis temperatures and atmospheres on the growth of leaf-used lettuce.

The aim of this study is to evaluate the effects of biochar on the plant's growth. A pot experiment was carried out in our study. Rice straw-derived biochar were charred at two heating temperatures (400 °C/800 °C) and two oxygen-limited atmospheres (CO2/N2), respectively. The FESEM/EDS technique (field emission scanning electron microscopy with X-ray energy-dispersive spectroscopy) was used to study soils, biochar and plant samples. FESEM images indicated that the structure of the biochar was highly heterogeneous with larger macropores, which can enhance soil porosity. Fine soil mineral particles blocked the biochar inner pores and channels after returning biochar to soil. EDS analysis indicated that the Al and Fe contents increased on the surface of biochar after their returning, which reduced the toxicity of Al and Fe in the soil. The returning straw directly inhibited the growth of leaf-used lettuce. Four returning biochar all significantly improved leaf-used lettuce growth, and the effects of biochar prepared under 400 °C and a CO2 atmosphere were better than those prepared under 800 °C and a N2 atmosphere. Changes of nitrogen content in the biochar before and after their returning were consistent with the improvement of soil available nitrogen, and plant growth was positively correlated with the nitrogen content of biochar. This study explored the impact of biochar on soil nutrients and revealed the mechanism of biochar returning to the field to promote plant growth. It is of great significance in studying and improving the characteristics of soil nutrients.

Contributions and mechanisms of components in modified biochar to adsorb cadmium in aqueous solution.

Recently, researchers have carried out a large number of studies on the adsorption of heavy metals by modified biochar, but there have been fewer explorations of the contributions and mechanisms of components in biochar composites on heavy metals adsorption. In this paper, the biochar was modified by Fe2+/Fe3+ and NaOH, and a further analysis of the adsorption of cadmium on the new biochar was conducted. It was found that (1) the adsorption capacity for cadmium of the modified biochar (M85) was 406.46 mg/g, which was 16 times that of the original biochar (C800); (2) the increased adsorption of cadmium onto the modified biochar had little correlation with the specific surface area, and the pure iron component was not the decisive factor for the huge adsorption capacity; and (3) the modified biochar was a kind of composite material with special construction, where the C-O-Fe structure that formed on its surface was the main reason for the sharp increase in adsorption. Among the iron components, iron oxides (Fe3O4, γ-Fe2O3 and Fe-O-Fe), iron-containing functional groups (-Fe-R-COOH and Fe-R-OH, etc.) and the mineral crystal XiFeYjOk reacted with the cadmium ion in aqueous solution to exchange, form complexes and precipitate, achieving the purpose of fixing the heavy metal. In addition, the aromatic structure C=Cπ can also adsorb Cd2+ to generate C=Cπ-Cd.

Preparation of high porosity biochar materials by template method: a review.

Biochar plays an important role in soil improvement, pollutant removal, and nitrogen reduction. The excellent adsorption performance of biochar is closely related to its pore structure. Therefore, this paper combines a large amount of literatures to investigate the principle and method of preparing carbon materials by using the template method, and the idea of preparing high porosity biochar by template method was proposed. The results show that: (1) The specific surface area of the carbon materials prepared by the template method is more than 400 m2 g-1, and the total pore volume is more than 0.3 cm3 g-1, which is much higher than the biochar materials prepared under the traditional high temperature anoxic pyrolysis. (2) Compared with the hard template method, a soft template method with simple operation, low toxicity of the compound, and low cost is selected. (3) The lignin, which is also a hydrophilic carbon source similar to phenolic resin, can be used as an ideal carbon precursor. (4) In the selection of templating agents, the specific surface area and total pore volume of carbon materials prepared by using F127 as a template are relatively large, showing more excellent pore size performance. (5) Finally, the idea of using template method to prepare high porosity biochar is proposed: lignin extracted from straw material is used as precursor, block polymer F127 is used as template, an appropriate amount of a cross-linking agent and a solvent is added, and finally the target biochar material is prepared by pyrolysis carbonization.

Effects of pyrolysis conditions on migration and distribution of biochar nitrogen in the soil-plant-atmosphere system.

The use of biochar to amend soil has been gaining increasing attention in recent years. In this study, the 15N tracer technique was used together with elemental analysis-stable isotope ratio analysis and gas isotope mass spectrometry to characterise biochar, soil, plant, and gas samples in order to explore the nitrogen transport mechanisms in the biochar-soil-plant-atmosphere system during the process of returning biochar to the soil (RBS). The results showed that the nitrogen retention rate of biochar was negatively correlated with the pyrolysis temperature during the preparation process, but was less affected by the pyrolysis atmosphere. In the RBS process, the migration of biochar nitrogen to plants was significantly greater than that of straw nitrogen, and it showed an overall decreasing trend with the increase in pyrolysis temperature, but was less influenced by the pyrolysis atmosphere. At temperatures of 300-500 °C, the pyrolysis atmosphere had a slightly smaller effect on the migration of biochar nitrogen to the air, plant, and soil system, and the pyrolysis temperature was much more important. However, the activation with CO2 gas at a higher temperature (600 °C) significantly enhanced the pore structure of biochar, particularly the structure of small pores; therefore, biochar prepared under a CO2 atmosphere at 600 °C reduces gaseous nitrogen emissions better than that under a N2 atmosphere. In the future, more pyrolysis conditions should be examined and their optimal combination should be further explored to reduce gaseous nitrogen emissions.

Mechanism of negative surface charge formation on biochar and its effect on the fixation of soil Cd.

The studies of the mechanism of Cd fixation by biochar have mainly focused on the pore size, pH, and oxygen-containing functional groups, and few researches have paid close attention to the effect of the negative charge in biochar surface. In this paper, biochar was produced in the CO2 atmosphere at different pyrolysis temperatures, and the influence of the pyrolysis temperature on biochar surface charge were explored. The cause of the negative charge on the biochar surface has been analysed, and the optimal preparing temperature for the biochar with the best effect of cadmium immobilization from soil has been found. The results show that with the increasing temperature from 300 to 700 °C, the negative surface charge on biochar surface gradually decreases, while the fixed amount of Cd increases. The factors affecting the surface charge of biochar are ash content, pH, oxygen-containing functional groups, polar groups, and hydrogen bonds. Among them, the pH, oxygen-containing functional groups and polar groups have positive effects on the surface negative charge, whereas the hydrogen bond has a negative effect. The determinant of surface charge is the hydroxyl group, the content of hydroxyl group decreases as increasing temperature, resulting in a decrease in surface negative charge.

Preparation of biochar with high absorbability and its nutrient adsorption-desorption behaviour.

To study the formation of biochar with high absorbability, experiments were carried out at different carbonization temperature (300, 400, 600, and 800 °C) and under different carbonization atmosphere (activating gases (steam and CO2) and inert gas (N2)) to prepare biochar. In this paper, the effects of the carbonization atmosphere on the biochar pore structure were studied, and the influence of the biochar pore structure on the adsorption-desorption behaviour of nutrients (NH4+-N, NO3--N, P, and K) was investigated. Experimental results: (1) The activating gases (steam and CO2) can catalytically crack activated carbon atoms and tar blocking the biochar pores at high temperatures (T > 600 °C), and the activating gas promotes the formation of microporous biochar (d < 2 nm). (2) Micropores with a pore diameter distribution of 0.6-2 nm in biochar have the strongest nutrients adsorption, and pores with a diameter below 0.6 nm cannot adsorb hydrated ions of nutrients. (3) Biochar prepared at 600 °C and CO2 atmosphere has the best adsorption effect on nutrients. The adsorption kinetic was well described by Pseudo-second-order model. (4) After 5 cycles of biochar, the adsorption of the nutrients is still >40% of the first adsorption. Biochar has relatively high reusability.

Vertical variations of soil water and its controlling factors based on the structural equation model in a semi-arid grassland.

Soil water content (SWC) of a vertical profile plays an important role in the soil-plant-atmosphere continuum system through eco-hydrological process, which was controlled by multiple factors. Previous studies ignored soil water from a systematic perspective because of the lack of suitable methods to deal with interrelated factors. We developed a meta-model based on structural equation model (SEM) to identify the factors contributing to soil water, and the interactions among these factors, in a semi-arid grassland system. The model was based on the hypothesis that soil water is affected by hydrological variables (precipitation: P, evapotranspiration: E and underground water: GW), vegetation (vegetation coverage: VC and above ground biomass: AGB), and soil parameters (soil organic matter: SOM and bulk density: BD). E and AGB decrease soil water content, while VC and SOM help to retain soil water content. The proportion of explained variation in soil water increased with depth due to increasing stability. The most important contributors were AGB (r∂ = -0.15) and VC (r∂ = 0.39), and their contributions were opposite because their mechanisms differed. The accumulation of AGB in the growth season consumed soil water through root uptake. The contribution of AGB increased with depth, inferring that grassland species are xerophytes with deep roots to access soil water during drought. Coverage positively contributed to soil water, but its influence decreased with depth because its main effects (intercepting rainfall and providing shade) were at the surface. This systematic perspective of how hydrological, vegetation, and soil properties affect soil water will be useful to guide the management of semi-arid grasslands.

Effect mechanism of biochar's zeta potential on farmland soil's cadmium immobilization.

In situ passivation of heavy metals by biochar mainly focuses on the effect of biochar's pH, surface oxygen-containing functional groups (OCFGs), and ash content. In this paper, starting with the measurement of biochar's electrical properties under different pyrolysis atmospheres and temperatures, the changes in the zeta potential of biochar and the consequent effects on cadmium immobilization in soil are studied. The results show that the zeta potential of biochar from the pyrolysis of high temperature (800 °C) is higher than that of biochar at low temperatures, so its electronegativity is weaker than that of biochar at low temperatures, but the protective effect on wheat is stronger than that of biochar obtained at low temperatures. The zeta potential of biochar obtained under a CO2 atmosphere was higher than that of biochar prepared under a N2 atmosphere, so its protective effect on wheat was stronger than that of biochar under N2. The reason is that biochar particles with a high zeta potential and weak electronegativity have higher cohesion and are better at in situ passivation of Cd in soils. Namely, biochar obtained at high pyrolysis temperatures (800 °C) and prepared under a CO2 atmosphere has better effect on Cd immobilization.

Application of the 15N tracer method to study the effect of pyrolysis temperature and atmosphere on the distribution of biochar nitrogen in the biomass-biochar-plant system.

Biochar nitrogen is key to improving soil fertility, but the distribution of biochar nitrogen in the biomass-biochar-plant system is still unclear. To provide clarity, the 15N tracer method was utilised to study the distribution of biochar nitrogen in the biochar both before and after its addition to the soil. The results can be summarised as follows. 1) The retention rate of 15N in biochar decreases from 45.23% to 20.09% with increasing pyrolysis temperature from 400 to 800°C in a CO2 atmosphere. 2) The retention rate of 15N in biochar prepared in a CO2 atmosphere is higher than that prepared in a N2 atmosphere when the pyrolysis temperature is below 600°C. 3) Not only can biochar N slowly facilitate the adsorption of N by plants but the addition of biochar to the soil can also promote the supply of soil nitrogen to the plant; in contrast, the direct return of wheat straw biomass to the soil inhibits the absorption of soil N by plants. 4) In addition, the distribution of nitrogen was clarified; that is, when biochar was prepared by the pyrolysis of wheat straw at 400°C in a CO2 atmosphere, the biochar retained 45.23% N, and after the addition of this biochar to the soil, 39.99% of N was conserved in the biochar residue, 4.55% was released into the soil, and 0.69% was contained in the wheat after growth for 31days. Therefore, this study very clearly shows the distribution of nitrogen in the biomass-biochar-plant system.

Study of the mechanism of remediation of Cd-contaminated soil by novel biochars.

This article used novel non-magnetized and magnetized biochars prepared under a CO2 atmosphere returned to Cd-contaminated soil and compared these to the effects of conventional biochars prepared under a N2 atmosphere with regard to Cd-contaminated soil remediation. A pot experiment with lettuce (Lactuca sativa) was conducted to investigate the relative soil remediation effects of these biochars. The soil used for the pot experiment was spiked with 20 mg kg-1 Cd and amended with 5% of a biochar before sowing. Through these research works, some important results were obtained as follows: (1) applying biochar treated by pyrolysis under a CO2 atmosphere can obtain the best remediation effect of Cd-contaminated soil that the content of cadmium in the lettuce roots, stems, and leaves was reduced 67, 62, and 63%, respectively; (2) the magnetic biochar aggregation for the soil is weak, so the heavy metal cadmium in the soil could not be immobilized well by the magnetic biochar; (3) The remediation mechanism of novel biochars is that biochar includes a large number of organic functional groups (-C-OH, -C=O, COO-) that can act in a complexing reaction with heavy metal Cd(II) and the inorganic salt ions (Si, S, Cl, etc.) that can combine with cadmium and generate a stable combination.

Mechanistic study of the influence of pyrolysis conditions on potassium speciation in biochar "preparation-application" process.

Biochar samples produced from rice straw by pyrolysis at different temperatures (400°C and 800°C) and under different atmospheres (N2 and CO2) were applied to lettuce growth in a 'preparation-application' system. The conversion of potassium in the prepared biochar and the effect of the temperature used for pyrolysis on the bioavailability of potassium in the biochar were investigated. Root samples from lettuce plants grown with and without application of biochar were assayed by X-ray photoelectron spectroscopy (XPS). The optimal conditions for preparation of biochar to achieve the maximum bioavailability of potassium (i.e. for returning biochar to soil) were thus determined. Complex-K, a stable speciation of potassium in rice straw, was transformed into potassium sulfate, potassium nitrate, potassium nitrite, and potassium chloride after oxygen-limited pyrolysis. The aforementioned ionic-state potassium species can be directly absorbed and used by plants. Decomposition of the stable speciation of potassium during the pyrolysis process was more effective at higher temperature, whereas the pyrolysis atmosphere (CO2 and N2) had little effect on the quality of the biochar. Based on the potassium speciation in the biochar, the preparation cost, and the plant growth and rigor after the application of returning biochar to soil, 400°C and CO2 atmosphere were the most appropriate conditions for preparation of biochar.