Could biochar amendment be a tool to improve soil availability and plant uptake of phosphorus? A meta-analysis of published experiments

Combined use of biochar and phosphate rocks on phosphorus and heavy metal availability: A meta-analysis

Biochar (BC) and phosphate rocks (PR) are alternative nutrient sources with multiple benefits for sustainable agriculture. The combination of these soil amendments serves two main purposes: to increase soil phosphorus (P) availability and to remediate heavy metal (HM) contamination. However, a further demonstration of the benefits and risks associated with the combined use of BC and PR (BC + PR) is needed, considering the specific characteristics of raw materials, soil types, experimental conditions, and climatic contexts. This meta-analysis is based on data from 28 selected studies, including 581 paired combinations evaluating effects on extraction and fractionation of cadmium (Cd) and lead (Pb), and 290 paired combinations for soil labile and non-labile P. The results reveal that BC, PR, and BC + PR significantly increase soil labile and non-labile P, with BC + PR showing a 150% greater increase compared to BC alone. In tropical regions, substantial increases in P levels were observed with BC, PR, and BC + PR exhibiting increments of 317, 798, and 288%, respectively. In contrast, temperate climate conditions showed lower increases, with BC, PR, and BC + PR indicating 54, 123, and 88% rises in soil P levels. Moreover, BC, PR, and BC + PR effectively reduce the bioavailability of Cd and Pb in soil, with BC + PR demonstrating the highest efficacy in immobilizing Cd. The synergistic effect of BC + PR highlights their potential for Cd remediation. BC + PR effectively reduces the exchangeable fraction of Cd and Pb in soil, leading to their immobilization in more stable forms, such as the residual fraction. This study provides valuable insights into the remediation potential and P management benefits of BC and PR, highlighting their importance for sustainable agriculture and soil remediation practices.

Unlocking the potential of biochar in the remediation of soils contaminated with heavy metals for sustainable agriculture

Agricultural soils contaminated with heavy metals (HMs) impose a threat to the environmental and to human health. Amendment with biochar could be an eco-friendly and cost-effective option to decrease HMs in contaminated soil. This paper reviews the application of biochar as a soil amendment to immobilise HMs in contaminated soil. We discuss the technologies of its preparation, their specific properties, and effect on the bioavailability of HMs. Biochar stabilises HMs in contaminated soil, enhance the overall quality of the contaminated soil, and significantly reduce HM uptake by plants, making it an option in soil remediation for HM contamination. Biochar enhances the physical (e.g. bulk density, soil structure, water holding capacity), chemical (e.g. cation exchange capacity, pH, nutrient availability, ion exchange, complexes), and biological properties (e.g. microbial abundance, enzymatic activities) of contaminated soil. Biochar also enhances soil fertility, improves plant growth, and reduces the plant availability of HMs. Various field studies have shown that biochar application reduces the bioavailability of HMs from contaminated soil while increasing crop yield. The review highlights the positive effects of biochar by reducing HM bioavailability in contaminated soils. Future work is recommended to ensure that biochars offer a safe and sustainable solution to remediate soils contaminated with HMs.

An overview of characteristic factors of biochar as a soil improvement tool in rice growth- A review

Biochar is a growing tool for bioremediation and soil improvement applications. Researchers are focusing on biochar due to its efficacy, eco-friendly composition, and cost-effective solutions to a variety of environmental issues. In recent times biochar has been used in enhancing the soil, increasing nutrient content, and sequestering carbon in paddy cultivation soils. India and Southeast Asian countries consume paddy as a major source of food in large quantities. Therefore, improving the growth condition of paddy fields using an easily available and safe technique will help increase the production rate. This will fulfill the needs of the growing population. Biochar is developed by the thermal decomposition of organic materials in low or no oxygen through pyrolysis, gasification, and co-pyrolysis methods. It improves paddy soil fertility due to its special physicochemical properties such as porosity, high surface area, efficient slow release, nutrient holding capacity, and maintenance of soil microbiota. Considering the importance of biochar in paddy soil fertility, the present work reviews the sources of biochar, functionalization of biochar, mechanism, and beneficial role of biochar.

Pyrolytic and hydrothermal carbonization affect the transformation of phosphorus fractions in the biochar and hydrochar derived from organic materials: A meta-analysis study

Carbonized organic materials are widely used to achieve soil improvement and alleviate soil pollution. The carbonization process significantly changes the total phosphorus (P) content and the P form in the solid phase derived from organic materials, which in turn has a significant impact on the P fertilizer effect in soils. In the present study, a meta-analysis with 278 observational data was conducted to detect the impact of the carbonization process (including pyrolytic carbonization and hydrothermal carbonization) on the transformation of P fractions in biochar or hydrochar derived from different organic materials. The results showed that the carbonization process significantly increased the total P content of the solid phase by 67.9%, and that the rate of P recovery from raw materials stayed high with a mean value of 86.8%. Among them, the impact of sludge-derived char was smaller when compared to the manure-derived char and biomass-derived char. The increase of total P in the biochar (or hydrochar) produced at >500 °C (or >200 °C) was more notable than that at <500 °C (or <200 °C). Simultaneously, the carbonization process significantly decreased the proportion of available P pool in the solid phase by 51.7% on average and increased the proportion of stable P pool in the solid phase by 204%. Appropriate production temperature helps to adjust the proportion of stable P pool in the solid phase. This meta-analysis pointed out that the carbonized solid phase recovers most of the P in the feedstock and that it promotes a significant transformation of available P pool in the feedstock to stable P in the carbonized solid phase. These findings provide useful information for the rational use of carbonization technology, the development of corresponding field management strategies, and the potential value of carbonized solid phase utilization.

Effects of different remediation methods on phosphorus transformation and availability

The effects of different heavy metal pollution remediation methods on soil nutrient transformation and soil health remain unclear. In this study, the effects of phytoextraction (PE) and passivation remediation (PR) on Cd-polluted soil phosphorus transformation and availability were compared by pot experiment. The results showed that PE significantly reduced the concentrations of total and available Cd (both H2O-Cd and DTPA-Cd) in soil, PR also decreased available Cd content but had no significant effect on total Cd content. PE slightly increased soil pH and NH4+-N content, while PR significantly increased soil pH, NO3--N and AK content. PE promoted the conversion of stable P (including HCl-Pi and residual-Pt), and increased the content of labile P (including H2O-Pi, NaHCO3-Pi and NaHCO3-Po) and the proportion of moderately labile P (including NaOH-Pi and NaOH-Po), while PR showed the opposite trend. PE showed a higher soil phoC gene abundance and acid phosphatase (ACP) activity, while PR showed a higher phoD gene copies and alkaline phosphatase (ALP) activity. Soil bacteria and phoD-harboring bacteria community was significantly affected by remediation methods and soil types. Compared with PR, PE reduced phoD-harboring bacterial diversity but significantly increased the abundance of genera associated with P dissolution (Streptomyces) and P conversion (Bradyrhizobium and Frankia), both of which were significantly positively correlated with labile P or moderately labile P. In general, compared with PR, PE can effectively remove soil Cd pollution, while maintaining a higher content of labile P and a higher proportion of moderately labile P, which can be considered as a green and sustainable remediation strategy conducive to soil quality.

Biochar Enhances Soil Resource Availability and Suppresses Microbial Metabolism Genes in the Rhizosphere of Wheat

Despite the well-documented role of biochar in promoting soil quality and crop productivity, the underlying biological mechanisms remain poorly understood. Here, we explored the effects of straw biochar on soil microbiome in the rhizosphere from wheat using metagenomic sequencing. Our results showed that straw return decreased the yields of wheat, while the straw biochar return increased the wheat yields. Further, both the richness and community composition confirmed different effects of the straw return and straw biochar return. The straw biochar return also resulted in greater rhizosphere effects from wheat, represented by resource availability, including soil organic carbon, soil total nitrogen, available phosphorus, and available potassium. The rhizosphere effects from wheat, represented by microbial metabolism genes involved in carbon, nitrogen, phosphorus, and potassium cycling, however, were decreased by straw biochar returning. In addition, the rhizosphere effects from nitrogen content and the nitrogen cycling genes showed negative relationships with wheat yields. Together, these results revealed that straw biochar enhanced soil resource availability but suppressed microbial metabolism genes in the rhizosphere from wheat, supporting the idea that straw biochar serves as a nutrient pool for crops.

Exploring biochar and fishpond sediments potential to change soil phosphorus fractions and availability

Phosphorus (P) availability in soil is paradoxical, with a significant portion of applied P accumulating in the soil, potentially affecting plant production. The impact of biochar (BR) and fishpond sediments (FPS) as fertilizers on P fixation remains unclear. This study aimed to determine the optimal ratio of BR, modified biochar (MBR), and FPS as fertilizer replacements. A pot experiment with maize evaluated the transformation of P into inorganic (Pi) and organic (Po) fractions and their contribution to P uptake. Different percentages of FPS, BR, and MBR were applied as treatments (T1-T7), T1 [(0.0)], T2 [FPS (25.0%)], T3 [FPS (25.0%) + BR (1%)], T [FPS (25%) +MBR (3%)], T5 [FPS (35%)], T6 [FPS (35%) +BR (1%)], and T7 [FPS (35%) + MBR (1%)]. Using the modified Hedley method and the Tiessen and Moir fractionation scheme, P fractions were determined. Results showed that various rates of MBR, BR, and FPS significantly increased labile and moderately labile P fractions (NaHCO3-Pi, NaHCO3-Po, HClD-Pi, and HClC-Pi) and residual P fractions compared with the control (T1). Positive correlations were observed between P uptake, phosphatase enzyme activity, and NaHCO3-Pi. Maximum P uptake and phosphatase activity were observed in T6 and T7 treatments. The addition of BR, MBR, and FPS increased Po fractions. Unlike the decline in NaOH-Po fraction, NaHCO3-Po and HClc-Po fractions increased. All Pi fractions, particularly apatite (HClD-Pi), increased across the T1-T7 treatments. HClD-Pi was the largest contributor to total P (40.7%) and can convert into accessible P over time. The T5 treatment showed a 0.88% rise in residual P. HClD-Pi and residual P fractions positively correlated with P uptake, phosphatase activity, NaOH-Pi, and NaOH-Po moderately available fractions. Regression analysis revealed that higher concentrations of metals such as Ca, Zn, and Cr significantly decreased labile organic and inorganic P fractions (NaHCO3-Pi, R 2 = 0.13, 0.36, 0.09) and their availability (NaHCO3-Po, R 2 = 0.01, 0.03, 0.25). Excessive solo BR amendments did not consistently increase P availability, but optimal simple and MBR increased residual P contents in moderately labile and labile forms (including NaOH-Pi, NaHCO3-Pi, and HClD-Pi). Overall, our findings suggest that the co-addition of BR and FPS can enhance soil P availability via increasing the activity of phosphatase enzyme, thereby enhancing plant P uptake and use efficiency, which eventually maintains the provision of ecosystem functions and services.

Activated ZnCl2 biochar and humic acid as additives in monoammonium phosphate fertilizer: Physicochemical characterization and agronomic effectiveness

Activated zinc biochar (ZnBC) and humic acid (HA) were used as coating agents in a soluble monoammonium phosphate (MAP) to modify phosphorus (P) use efficiency by altering adsorption/desorption kinetics between the granule region and the soil. The coated treatments MAPZnBC and MAPHA were compared with MAP through P diffusivity, kinetics, and agronomic evaluation. Eucalyptus sawdust was used as biomass for biochar synthesis, and a pre-pyrolysis treatment with zinc chloride (ZnCl₂) was applied. The P diffusivity was evaluated in the fertosphere zone. Adsorption and desorption potential of the ZnBC compared with control biochar (BC) was evaluated separately. Desorption kinetics of P from soil was assessed after incubation with MAPZnBC and MAPHA. The shoot dry matter yield (SDM), P uptake, and P use efficiency (PUE) were evaluated with a pot experiment in a clay Oxisol sown with maize and soybeans as successive plant trials, under glasshouse conditions. Surface area values of 940 and 305 m² g^-1 combined with adsorption capacities of 106 and 53 mg P g^-1 for ZnBC and BC, respectively, confirm the increased capacity of activated biochar to adsorb P. Both MAPZnBC and MAPHA decreased P diffusivity compared to MAP after 20 days of incubation. Moreover, MAPZnBC and MAPHA presented 20% and 34% more water-soluble phosphorus recovery. MAPZnBC expressed an increase in SDM while MAPHA highlighted P uptake and PUE compared with MAP. Both kinetic studies and agronomic evaluations showed that ZnBC and HA are suitable as coatings for phosphate fertilizers in terms of increasing P efficiency in the fertosphere on high P-fixing soils.

More microbial manipulation and plant defense than soil fertility for biochar in food production: A field experiment of replanted ginseng with different biochars

The role of biochar-microbe interaction in plant rhizosphere mediating soil-borne disease suppression has been poorly understood for plant health in field conditions. Chinese ginseng (Panax ginseng C. A. Meyer) is widely cultivated in Alfisols across Northeast China, being often stressed severely by pathogenic diseases. In this study, the topsoil of a continuously cropped ginseng farm was amended at 20 t ha^-1, respectively, with manure biochar (PB), wood biochar (WB), and maize residue biochar (MB) in comparison to conventional manure compost (MC). Post-amendment changes in edaphic properties of bulk topsoil and the rhizosphere, in root growth and quality, and disease incidence were examined with field observations and physicochemical, molecular, and biochemical assays. In the 3 years following the amendment, the increases over MC in root biomass were parallel to the overall fertility improvement, being greater with MB and WB than with PB. Differently, the survival rate of ginseng plants increased insignificantly with PB but significantly with WB (14%) and MB (21%), while ginseng root quality was unchanged with WB but improved with PB (32%) and MB (56%). For the rhizosphere at harvest following 3 years of growing, the total content of phenolic acids from root exudate decreased by 56, 35, and 45% with PB, WB, and MB, respectively, over MC. For the rhizosphere microbiome, total fungal and bacterial abundance both was unchanged under WB but significantly increased under MB (by 200 and 38%), respectively, over MC. At the phyla level, abundances of arbuscular mycorrhizal and Bryobacter as potentially beneficial microbes were elevated while those of Fusarium and Ilyonectria as potentially pathogenic microbes were reduced, with WB and MB over MC. Moreover, rhizosphere fungal network complexity was enhanced insignificantly under PB but significantly under WB moderately and MB greatly, over MC. Overall, maize biochar exerted a great impact rather on rhizosphere microbial community composition and networking of functional groups, particularly fungi, and thus plant defense than on soil fertility and root growth.

Improved ginseng production under continuous cropping through soil health reinforcement and rhizosphere microbial manipulation with biochar: a field study of Panax ginseng from Northeast China

The production of ginseng, an important Chinese medicine crop, has been increasingly challenged by soil degradation and pathogenic disease under continuous cropping in Northeast China. In a field experiment, an Alfisol garden continuously cropped with Chinese ginseng (Panax ginseng C. A. Meyer) was treated with soil amendment at 20 t ha^-1 with maize (MB) and wood (WB) biochar, respectively, compared to conventional manure compost (MC). Two years after the amendment, the rooted topsoil and ginseng plants were sampled. The changes in soil fertility and health, particularly in the soil microbial community and root disease incidence, and in ginseng growth and quality were portrayed using soil physico-chemical assays, biochemical assays of extracellular enzyme activities and gene sequencing assays as well as ginsenoside assays. Topsoil fertility was improved by 23% and 39%, ginseng root biomass increased by 25% and 27%, and root quality improved by 6% and 18% with WB and MB, respectively, compared to MC. In the ginseng rhizosphere, fungal abundance increased by 96% and 384%, with a significant and insignificant increase in bacterial abundance, respectively, under WB and MB. Specifically, the abundance of Fusarium spp. was significantly reduced by 19-35%, while that of Burkholderia spp. increased by folds under biochar amendments over MC. Relevantly, there was a significant decrease in the abundance proportion of pathotrophic fungi but a great increase in that of arbuscular mycorrhizal fungi, along with an enhanced microbial community network complexity, especially fungal community complexity, under biochar amendments. Thus, biochar, particularly from maize residue, could promote ginseng quality production while enhancing soil health and ecological services, including carbon sequestration, in continuously cropped fields.

Synergistic Effects of Acacia Prunings-Derived Biochar and Nitrogen Application on the Mineral Profile of Maize (Zea mays L.) Grains

Despite the proven impact of biochar (BC) on crop yield, little is known about its effect on cereal grain quality. We explored the effect of acacia pruning-derived biochar and nitrogen (N) applications on the mineral profile of maize grains in a field study. Biochar was applied at the rates of 5, 10, 15, and 20 t ha−1 and N at 100, 150, and 200 kg ha−1 along with the control (BC or N not applied) in a split-plot arrangement using a randomized complete block design. At crop maturity, the grains were analyzed for K, P, Ca, Mg, Zn, Fe, and Cu content. The results showed that BC application at the rate of 10 t ha−1 along with N at 200 kg ha−1 resulted in the highest concentrations of K, P, Ca, and Mg in grains. The Fe content was the maximum at the N application rate of 200 kg ha−1 while Zn and Cu had the highest concentration at 150 kg N ha−1 with no BC. It was concluded that the integrated use of BC and N could be a valuable strategy to improve the nutritional quality of maize grains. The enrichment of BC with micronutrients is recommended to achieve the desired concentration of micronutrients in maize grains to help cure malnutrition. However, further investigation is warranted to validate the impact of BC made of different feedstocks on soils of contrasting mineralogy as organo-mineral interactions might mask the true potential of BC.

The effects of biochar soil amendment on rice growth may vary greatly with rice genotypes

While plant growth promotion with increased nutrient uptake had been well addressed for biochar soil amendment in agriculture, there was limited knowledge on the variation of such effects with crop genotypes. In a rice field experiment without and with biochar soil amendment at 20 t ha^-1, 19 mutants of a rice cultivar Wuyunjing 7 (Oryza sativa L.) were tested for plant growth in split plots respectively. At harvest, the biomass of grain, stem and leaves were measured and soil and plant samples were collected for measuring N, P and K nutrients. Across the 19 mutants, relative change with biochar soil amendment varied in a range of -41.6% to +35.6% for biomass production and agronomic traits, and -87.0% to +117% for nutrient accumulation. For the nutrients content, the relative change for N was seen in a narrow range of -29.4% to +16.6%, being similar among grain, leaf and shoot samples while that for P in a wide range of -109% to +105%. With factor analysis, variation of biomass and nutrient uptake was least explained with biochar effect (up to 7.0%) but largely by genotype effect (mostly by 40-70%). However, the genotype × biochar interaction effect could also explain 10-40% of the total variations though the interaction explained 40-70% of leaf P variation. Therefore, mutant and mutant × biochar interactions dominated the agronomic variation of rice production of the Wuyunjing 7 cultivar. Furthermore, across the traits analyzed, genotype effects were shown very significantly but negatively correlated to biochar effects. In other words, biochar soil amendment provided little growth or nutrient enhancement for those mutants bred for high efficiency. Hence, genotype selection should be considered in optimizing prioritizing biochar application in crop production. Of course, variation of biochar effect with crop genotypes deserved further plant physio-ecological studies.

A critical review of the possible adverse effects of biochar in the soil environment

Biochar has received extensive attention because of its multi-functionality for agricultural and environmental applications. Despite its many benefits, there are concerns related to the long-term safety and implications of its application, mainly because the mechanisms affecting soil and organism health are poorly quantified and understood. This work reviews 259 sources and summarises existing knowledge on biochar's adverse effects on soil from a multiangle perspective, including the physicochemical changes in soil, reduced efficiency of agrochemicals, potentially toxic substances in biochar, and effects on soil biota. Suggestions are made for mitigation measures. Mixed findings are often reported; however, the results suggest that high doses of biochar in clay soils are likely to decrease available water content, and surface application of biochar to sandy soils likely increases erosion and particulate matter emissions. Furthermore, biochar may increase the likelihood of excessive soil salinity and decreased soil fertility because of an increase in the pH of alkaline soils causing nutrient precipitation. Regarding the impact of biochar on (agro)chemicals and the role of biochar-borne toxic substances, these factors cannot be neglected because of their apparent undesirable effects on target and non-target organisms, respectively. Concerning non-target biota, adverse effects on reproduction, growth, and DNA integrity of earthworms have been reported along with effects on soil microbiome such as a shift in the fungi-to-bacteria ratio. Given the diversity of effects that biochar may induce in soil, guidelines for future biochar use should adopt a structured and holistic approach that considers all positive and negative effects of biochar.