Abstract:

Phosphorus underpins energy metabolism and nucleic-acid synthesis, yet >40% of cropland lacks plant-available P, limiting yields. Water-soluble fertilizers (MAP, DAP, and TSP) achieve <20% recovery; residual P is immobilized by Fe/Al oxides or Ca/Mg carbonates and accumulates as legacy P, which drives eutrophication. Escaping mineral-P dependence requires nature-based, closed-loop strategies that unlock soil P through ecological synergy. Over the past two decades, biochar and arbuscular mycorrhizal fungi (AMF) have emerged as the two most intensively investigated strategies for enhancing plant phosphorus (P) acquisition. Biochar primarily operates by modulating soil pH, redox status, and surface chemistry to liberate P, whereas AMF enlarges the exploitable soil volume through extensive extraradical hyphal networks. Although the individual P-mobilizing efficacy of each amendment has been repeatedly validated, the mechanistic synergy and optimized co-application frameworks are poorly understood. Here, we synthesize the latest advances on biochar-AMF combinations, dissecting their cooperative mechanisms across physical, chemical, and biological dimensions, and propose an integrated regulatory framework to foster dynamic P transformation and efficient utilization within the soil-plant system, thereby augmenting the soil P-supplying capacity. At the physical scale, biochar presents a hierarchically organized pore system comprising macropores (>50 µm), mesopores (2-50 µm), and micropores (<2 µm), which are generated by the thermal volatilization of lignocellulosic precursors. These pores markedly increased the total porosity and reduced the bulk density. Upon integration with soil aggregates, they establish a continuous pore network that elevates the water-holding capacity and gaseous diffusivity. Under water-logged or compacted conditions, this mitigates hypoxia at the root apices and creates a favorable niche for arbuscular mycorrhizal fungi (AMF). The rigid carbonaceous framework of biochar further serves as a physical conduit, enabling hyphae to traverse nutrient-depleted rhizosheaths and directly access P-enriched bulk soils. In turn, AMF hyphae exert micro-mechanical pressure on adjacent particles and secrete extracellular polymeric substances (EPS) enriched in hydrophobins; this, in concert with biochar’s mechanical reinforcement, stabilizes the pore architecture, enhances pore-network connectivity and moisture retention, and generates microsites characterized by elevated water and nutrient storage. The resulting “biochar-AMF-root” triadic interaction space extends the effective interception zone for orthophosphate ions from the millimeter-scale rhizosphere to micron-scale pores, thereby substantially improving the utilization efficiency of exogenous phosphorus. At the chemical scale, biochar exhibits a surface chemistry dominated by alkaline functional groups, phenolic (−OH), carboxylic (−COOH), and quinonic (C=O) moieties, whose abundance peaks at pyrolysis temperatures of 300−500 ℃. Temperatures exceeding 600 ℃ intensified decarboxylation, leading to a pronounced decline in the number of these surface groups. When applied at agronomically relevant rates, biochar elevates soil pH, shifting the dissolution equilibria of Fe/Al-bound P (Fe/Al-P) and Ca-bound P (Ca-P) toward the solution phase. Concomitantly, the increased net negative surface charge attenuates the adsorption affinity of variably charged minerals for orthophosphate, thereby enhancing P availability. AMF complement these abiotic mechanisms by exuding low-molecular-weight organic acids (malate, citrate, and oxalate) and acid phosphatases, which further mobilize P. These organic acids form stable complexes with Fe³⁺, Al³⁺, and Ca²⁺, promoting the desorption of P from mineral surfaces and exposing fresh reactive sites for sustained P release. Notably, citrate can also oxidize the condensed aromatic domains of biochar, generating additional carboxyl and hydroxyl functionalities that increase hydrophilicity and cation-exchange capacity (CEC), thereby accelerating the adsorption-desorption turnover of orthophosphate. Furthermore, the localized rhizosphere acidification induced by AMF exudates counteracts the over-liming effect occasionally imposed by high-pH biochars, maintaining P solubility within the optimal range for plant acquisition. These chemically complementary processes underpin the synergistic modulation of P availability by the “biochar-AMF” consortium. At the biological scale, biochar-derived low-molecular-weight carboxylates and phenolics act as signaling analogs or amplifiers of plant-released strigolactones (SLs), thereby accelerating hyphal branching and appressorium formation in AMF and increasing root colonization efficiency. Once symbiosis is established, AMF upregulate the host high-affinity phosphate transporter genes (e.g., PT4 and PT11) and modulate the SPX-PHR signaling module that governs P homeostasis, effectively attenuating plant P-starvation responses. Simultaneously, AMF and their associated microbiota secrete acid phosphatases (ACP) and phytases that mineralize organic P pools, such as phytates and nucleic acids. Co-application of biochar and AMF markedly restructured the rhizospheric microbiome, enriching functional guilds, including inorganic P-solubilizing Pseudomonas fluorescens and Bacillus megaterium, as well as organic P-mineralizing fungi, such as Mortierella alpina and Trichoderma harzianum. The resulting “biochar-AMF-plant-microbe” quaternary consortium establishes a dynamic, closed-loop P cycle: biochar immobilizes P to prevent leaching, AMF and associated microbes continuously release P, plants acquire P via both mycorrhizal and direct uptake pathways, and P contained in plant residues is subsequently re-sequestered by biochar, entering a renewed microbial activation phase. Despite the growing mechanistic elucidation of “biochar-AMF” synergy in enhancing P availability, four critical knowledge gaps continue to constrain its translation into agronomic best-practices: 1) Soil texture, pH, organic matter content, indigenous microbial community structure, and climate variables generate substantial, user-specific interactions that compromise the reproducibility and generalizability of the observed synergy. Machine learning algorithms integrated with meta-analytical frameworks are urgently needed to generate predictive and context-specific recommendation models. 2) The optimal combination of biochar physicochemical traits (feedstock type, pyrolysis temperature, particle size distribution, mineral ash content, surface charge density, and co-loaded nutrients) with AMF ecotypes of varying mycorrhizal dependency and their associated microbial consortia remains to be determined. High-throughput phenomics and metabolomics should be used to identify trait configurations that exhibit maximal complementarity. 3) Repeated fertilization, tillage regimes (no-till vs. inversion tillage), and crop rotation schemes may erode the long-term persistence of biochar-AMF synergistic effects. Multi-year field trials incorporating dual-isotope (³²P/³³P) continuous tracing and tillage disturbance as factorial treatments are required to quantify the durability of P-use efficiency. 4) A life cycle assessment framework coupled with ecotoxicological assays should be adopted to evaluate the risk of biogeographic-barrier transgression and indigenous-microbiome displacement associated with the introduction of non-native AMF strains. Concurrently, dose-response relationships for polycyclic aromatic hydrocarbons (PAHs) and heavy metals within biochars must be established to safeguard environmental and food security in the future. Biochar-AMF synergy reconciles high yields with sustainability in P-limited systems across nano- to field-scale processes, converting soil into a self-regulating “living P reservoir” that boosts the short-term availability and long-term storage of P. Future work should integrate spectro-microscopy, multi-omics, and modelling to decode feedback within the biochar-AMF-plant-microbe network, enabling precision P management, reducing reliance on finite phosphate rock, and accelerating circular nutrient economies.

Key words: phosphorus limitation, biochar, infection, phosphorus forms, arbuscular mycorrhizal fungi (AMF)

摘要：

磷是植物必需的营养元素，但土壤有效磷匮乏已限制农业绿色高效发展。传统补磷措施利用率低且易引发环境风险。近年研究表明，生物炭和丛枝菌根真菌（AMF）均可提升土壤磷有效性，但协同机理仍缺乏系统阐释。该文从3个角度探讨其协同解锁土壤磷库的机理。生物炭多级孔结构改善土壤孔隙度和通气性，为AMF菌丝延伸提供通道；AMF以胞外聚合物加固孔壁，扩大磷捕获范围并提高外源磷利用率。生物炭碱性组分提高土壤pH，削弱铁/铝/钙对磷的固定，促进磷向可溶态转化；AMF分泌有机酸与磷酸酶络合金属离子，释放磷位点，并氧化生物炭表面，增强其亲水性及阳离子交换能力，促进磷的解吸循环，实现磷活化。生物炭释放的信号分子诱导AMF的分枝与侵染；共生后AMF调控高亲和力磷转运蛋白及SPX-PHR模块、分泌酶矿化有机磷，并重塑根际微生物群落，富集溶磷菌和有机磷矿化真菌，构建“生物炭-AMF-植物-微生物”闭环磷循环。尽管机制日趋明晰，但其环境适应性、材料-微生物匹配度及长期效应仍有待探索。未来需基于系统框架，结合光谱显微与多组学研究，解析网络互作与动态反馈，为精准磷管理提供理论支撑，推动农业向循环养分经济转型。

关键词: 磷限制, 生物炭, 侵染, 磷形态, 丛枝菌根真菌