Ylva Lekberg, Jan Jansa, David Johnson, Paul Milham, Chad Penn, Benjamin P. Colman
{"title":"通过菌根真菌追踪磷从土壤到植物的过程。","authors":"Ylva Lekberg, Jan Jansa, David Johnson, Paul Milham, Chad Penn, Benjamin P. Colman","doi":"10.1111/nph.20217","DOIUrl":null,"url":null,"abstract":"<p>A recent paper about carbon and phosphorus (P) transfer between arbuscular mycorrhizal (AM) fungi and plants (Lekberg <i>et al</i>., <span>2024</span>) was cited by Spohn & Wanek (<span>2025</span>; pp. 443–445, in this issue) to highlight potential pitfalls of using P radioisotopes (in this case <sup>32</sup>P, although equally relevant for <sup>33</sup>P as they behave similarly; Frossard <i>et al</i>., <span>2011</span>). Specifically, the paper by Spohn & Wanek (<span>2025</span>) states that without knowing the specific activity in soil solution (i.e. the ratio of <sup>32</sup>P : <sup>31</sup>P), the conclusion in Lekberg <i>et al</i>. (<span>2024</span>) that more P was delivered by AM fungi in high-P than low-P soils may not be valid if the three low-P soils sorbed more <sup>32</sup>P. We agree with Spohn & Wanek (<span>2025</span>) that tracer experiments should be interpreted with caution, and we appreciate the opportunity to explore some of the pitfalls highlighted in that paper in more detail.</p><p>Sorption of inorganic orthophosphate (H<sub>2</sub>PO<sub>4</sub><sup>−</sup> and HPO<sub>4</sub><sup>2−</sup>) – the main P form taken up by plants, fungi, and prokaryotes from soil solution (Bucher, <span>2007</span>) – is determined by pH, organic matter, hydrous oxides of aluminum and iron, and calcium carbonate (Frossard <i>et al</i>., <span>1995</span>; Daly <i>et al</i>., <span>2001</span>; Barrow, <span>2017</span>). In Lekberg <i>et al</i>. (<span>2024</span>), soils were collected from two regions, one with high-P availability and one with low-P availability. In both regions, soils were classified as fine to gravely, loamy Mollisols, and neither soil pH (6.80 ± 0.38 vs 6.53 ± 0.38, means ± SE) nor organic matter concentrations (4.50 ± 0.69% and 5.03 ± 2.00%) differed (<i>P</i> > 0.5) between the high-P and low-P soils, respectively. Total organic P stocks also did not differ significantly between the high-P and low-P soils (1178 ± 302 and 684 ± 79 mg kg<sup>−1</sup>) and the strong regional difference in P availability based on Bray 1 extractions were likely due to soil mineralogy.</p><p>There is another reason why differences in sorption likely did not influence results in Lekberg <i>et al</i>. (<span>2024</span>). One of the main assumptions of plant–soil–isotope experiments is that the concentration of the isotope-tracer must be negligible compared to that of the nonlabelled nutrient in solution. If the amount of <sup>32</sup>P added is similar to <sup>31</sup>P, the solution and solid-phase P equilibrium is disrupted and net sorption of total and <sup>32</sup>P will occur. The solution concentration where no net sorption or desorption occurs in soil is known as the ‘equilibrium P concentration at net zero sorption’ (EPC<sub>0</sub>). Specific to each soil, EPC<sub>0</sub> depends on the same variables that determine sorption, as well as P desorption kinetics, and microbial P immobilization and release. A recent meta-analysis by Simpson <i>et al</i>. (<span>2021</span>) illustrates that many soils have an EPC<sub>0</sub> of 0.1 mg l<sup>−1</sup>, and the lowest value reported was 0.5 μg l<sup>−1</sup>. The <sup>32</sup>P concentration used in Lekberg <i>et al</i>. (<span>2024</span>) was 3.2 pmol l<sup>−1</sup> or 0.1 ng l<sup>−1</sup>; at least three orders of magnitude lower than the lowest EPC<sub>0</sub> ever reported in the literature. Net sorption of radioisotopes due to disrupted soil solution and solid-phase equilibrium is, therefore, unlikely to occur when using ‘carrier-free’ tracers as was done in Lekberg <i>et al</i>. (<span>2024</span>).</p><p>Even without net sorption of P, isotopic dilution will occur through time after tracer addition due to isotopic exchange of <sup>32</sup>P between the liquid and solid-phase with more <sup>32</sup>P being sorbed over time, accompanied by desorption of <sup>31</sup>P, radioactive decay, and microbial immobilization and release (Helfenstein <i>et al</i>., <span>2018</span>). Desorption of <sup>31</sup>P was not specifically highlighted in Spohn & Wanek (<span>2025</span>), although ‘P release’ was mentioned, which also includes processes like P mineralization and solubilization. However, the implication of variation in P desorption is apparent in their fig. 1, which shows higher <sup>31</sup>P concentration in the P-rich soil. Given the 10-fold greater background concentration of bioavailable <sup>31</sup>P in the high-P soils in Lekberg <i>et al</i>. (<span>2024</span>), and because the same amount of <sup>32</sup>P was added to all pots, for every <sup>32</sup>P disintegration measured in plants growing in the low- and high-P soils, 10 times more <sup>31</sup>P would have been taken up and delivered by AM fungi in the high-P than in the low-P soils. Thus, as discussed in Lekberg <i>et al</i>. (<span>2024</span>), the estimated higher AM fungal P delivery in the high-P soils based solely on the measured <sup>32</sup>P is, if anything, likely a conservative estimate.</p><p>While patterns observed in Lekberg <i>et al</i>. (<span>2024</span>) are substantiated due to the reasons outlined above, we agree with Spohn & Wanek (<span>2025</span>) that estimates of resource uptake would be strengthened by measurements of both <sup>31</sup>P and <sup>32</sup>P in the bioavailable orthophosphate fraction through time, especially if different soils are used. However, proper execution of such measurements through time is challenging for several reasons. First, estimating specific activity of <sup>32</sup>P assumes even spatial distribution of <sup>32</sup>P, which is almost always violated given the low mobility of P and because radioisotopes are often added as a point source (including in Lekberg <i>et al</i>., <span>2024</span>). Second, repeated sampling of soil after tracer addition involves some degree of disturbance that could affect soil structure, possibly promote mineralization, and disrupt fungal hyphae and roots. This would inevitably change conditions following the first sampling as a function of the amount of soil sampled and degree of disturbance. Also, there is some debate about how to best assess and quantify the plant- and/or mycorrhiza-available P pool (Bühler <i>et al</i>., <span>2003</span>; Mason <i>et al</i>., <span>2013</span>). Using extractants (e.g. Bray 1) as suggested by Spohn & Wanek (<span>2025</span>) and used in Lekberg <i>et al</i>. (<span>2024</span>), can generate artifacts. This last issue could be avoided by directly quantifying P in soil solution, but collecting and reliably analyzing inorganic orthophosphate in microliters of soil solution is extremely difficult given its low concentration and possible interferences due to soil colloids and dissolved organic P. To acquire specific <sup>32</sup>P activity in soil solutions of the different soils over time, an alternative option is to conduct parallel laboratory batch experiments and use isotope exchange kinetics (Frossard & Sinaj, <span>1997</span>) although such an approach may not be fully representative of patterns in soils with plants and mycorrhizal fungi. Related to this, whether <sup>32</sup>P specific activity that changes over time in the soil solution reflects P acquisition by AM fungi at all time points (e.g. 1 min, 1 h, and 1 d after tracer addition, or during daytime vs nighttime) is uncertain given that so little is known about temporal and spatial acquisition patterns and mechanisms (but see attempts by Dessureault-Rompré <i>et al</i>., <span>2007</span>), interactions with other soil biota that immobilize, mineralize, and solubilize P, and effects of water and other nutrient availabilities. Add to this the difficulty of measuring declining levels of <sup>32</sup>P radioactivity across longer timescales with confidence given radioactivity from naturally occurring isotopes such as <sup>40</sup>K, <sup>14</sup>C and <sup>3</sup>H. Given these challenges, perhaps it is not surprising that most models fail to accurately predict P acquisition in plants (Hinsinger <i>et al</i>., <span>2011</span>), although different estimates of soil available P often correlate reasonably well with P uptake and each other (Steinfurth <i>et al</i>., <span>2021</span>). Ultimately, we need to refine the assumptions and better understand the physical, chemical, and biological properties of soils that govern P availability to make radioisotope tracing of P acquisition from soil to AM fungi and plants truly quantitative. This is a difficult, albeit not impossible, task.</p><p>Alternative methodologies to radio-phosphorus tracers exist, and hydroxyapatite conjugated to quantum dots is increasingly being used to assess P transfer in AM (e.g. Whiteside <i>et al</i>., <span>2019</span>; van't Padje <i>et al</i>., <span>2021</span>). Quantum dots are appealing in many ways due to their greater longevity than radioactive P isotopes and the possibility of <i>in vivo</i> tracking on cellular levels (Färkkilä <i>et al</i>., <span>2021</span>). Previous research has shown acquisition and delivery of quantum dots to plants by AM fungi and have elucidated fascinating patterns involved in plant–fungal interactions. However, relying on these approaches is not without challenges and uncertainties because the uptake and delivery mechanisms may differ from those that govern the uptake and delivery of orthophosphate. It is assumed that most P is acquired by AM fungi and delivered to plants via substrate-specific membrane transporters (Ezawa & Saito, <span>2018</span>). Transported molecules are ionic and have a hydrated diameter of < 1 nm, whereas quantum dots coated with apatite are 8 nm diameter, which sometimes are combined into complexes reaching 200 nm in diameter (van't Padje <i>et al</i>., <span>2021</span>). In this size range, quantum dots are likely taken up by endocytosis (Raven, <span>2022</span>), which is a completely different mechanism compared to inorganic orthophosphate transporters. We consequently echo the suggestion in Raven (<span>2022</span>) that isotopes could be an excellent way to assess if and how quantum dots accurately reflect phosphate uptake and delivery by AM fungi.</p><p>With increasing realism in experiments comes increasing difficulty. As we continue to try to understand the functioning of mycorrhizal symbioses in nature, it is essential that we use appropriate methods and are aware of potential limitations. As such, the discussion generated by Spohn & Wanek (<span>2025</span>) is very timely.</p><p>None declared.</p><p>YL conceived of the idea and wrote the first draft, YL and BPC conducted the sorption tests, and BPC calculated sorption. YL, BPC, JJ, DJ, PM and CP edited the manuscript draft and provided critical input.</p>","PeriodicalId":214,"journal":{"name":"New Phytologist","volume":"245 2","pages":"446-449"},"PeriodicalIF":8.3000,"publicationDate":"2024-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/nph.20217","citationCount":"0","resultStr":"{\"title\":\"Tracing phosphorus from soil through mycorrhizal fungi to plants\",\"authors\":\"Ylva Lekberg, Jan Jansa, David Johnson, Paul Milham, Chad Penn, Benjamin P. Colman\",\"doi\":\"10.1111/nph.20217\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<p>A recent paper about carbon and phosphorus (P) transfer between arbuscular mycorrhizal (AM) fungi and plants (Lekberg <i>et al</i>., <span>2024</span>) was cited by Spohn & Wanek (<span>2025</span>; pp. 443–445, in this issue) to highlight potential pitfalls of using P radioisotopes (in this case <sup>32</sup>P, although equally relevant for <sup>33</sup>P as they behave similarly; Frossard <i>et al</i>., <span>2011</span>). Specifically, the paper by Spohn & Wanek (<span>2025</span>) states that without knowing the specific activity in soil solution (i.e. the ratio of <sup>32</sup>P : <sup>31</sup>P), the conclusion in Lekberg <i>et al</i>. (<span>2024</span>) that more P was delivered by AM fungi in high-P than low-P soils may not be valid if the three low-P soils sorbed more <sup>32</sup>P. We agree with Spohn & Wanek (<span>2025</span>) that tracer experiments should be interpreted with caution, and we appreciate the opportunity to explore some of the pitfalls highlighted in that paper in more detail.</p><p>Sorption of inorganic orthophosphate (H<sub>2</sub>PO<sub>4</sub><sup>−</sup> and HPO<sub>4</sub><sup>2−</sup>) – the main P form taken up by plants, fungi, and prokaryotes from soil solution (Bucher, <span>2007</span>) – is determined by pH, organic matter, hydrous oxides of aluminum and iron, and calcium carbonate (Frossard <i>et al</i>., <span>1995</span>; Daly <i>et al</i>., <span>2001</span>; Barrow, <span>2017</span>). In Lekberg <i>et al</i>. (<span>2024</span>), soils were collected from two regions, one with high-P availability and one with low-P availability. In both regions, soils were classified as fine to gravely, loamy Mollisols, and neither soil pH (6.80 ± 0.38 vs 6.53 ± 0.38, means ± SE) nor organic matter concentrations (4.50 ± 0.69% and 5.03 ± 2.00%) differed (<i>P</i> > 0.5) between the high-P and low-P soils, respectively. Total organic P stocks also did not differ significantly between the high-P and low-P soils (1178 ± 302 and 684 ± 79 mg kg<sup>−1</sup>) and the strong regional difference in P availability based on Bray 1 extractions were likely due to soil mineralogy.</p><p>There is another reason why differences in sorption likely did not influence results in Lekberg <i>et al</i>. (<span>2024</span>). One of the main assumptions of plant–soil–isotope experiments is that the concentration of the isotope-tracer must be negligible compared to that of the nonlabelled nutrient in solution. If the amount of <sup>32</sup>P added is similar to <sup>31</sup>P, the solution and solid-phase P equilibrium is disrupted and net sorption of total and <sup>32</sup>P will occur. The solution concentration where no net sorption or desorption occurs in soil is known as the ‘equilibrium P concentration at net zero sorption’ (EPC<sub>0</sub>). Specific to each soil, EPC<sub>0</sub> depends on the same variables that determine sorption, as well as P desorption kinetics, and microbial P immobilization and release. A recent meta-analysis by Simpson <i>et al</i>. (<span>2021</span>) illustrates that many soils have an EPC<sub>0</sub> of 0.1 mg l<sup>−1</sup>, and the lowest value reported was 0.5 μg l<sup>−1</sup>. The <sup>32</sup>P concentration used in Lekberg <i>et al</i>. (<span>2024</span>) was 3.2 pmol l<sup>−1</sup> or 0.1 ng l<sup>−1</sup>; at least three orders of magnitude lower than the lowest EPC<sub>0</sub> ever reported in the literature. Net sorption of radioisotopes due to disrupted soil solution and solid-phase equilibrium is, therefore, unlikely to occur when using ‘carrier-free’ tracers as was done in Lekberg <i>et al</i>. (<span>2024</span>).</p><p>Even without net sorption of P, isotopic dilution will occur through time after tracer addition due to isotopic exchange of <sup>32</sup>P between the liquid and solid-phase with more <sup>32</sup>P being sorbed over time, accompanied by desorption of <sup>31</sup>P, radioactive decay, and microbial immobilization and release (Helfenstein <i>et al</i>., <span>2018</span>). Desorption of <sup>31</sup>P was not specifically highlighted in Spohn & Wanek (<span>2025</span>), although ‘P release’ was mentioned, which also includes processes like P mineralization and solubilization. However, the implication of variation in P desorption is apparent in their fig. 1, which shows higher <sup>31</sup>P concentration in the P-rich soil. Given the 10-fold greater background concentration of bioavailable <sup>31</sup>P in the high-P soils in Lekberg <i>et al</i>. (<span>2024</span>), and because the same amount of <sup>32</sup>P was added to all pots, for every <sup>32</sup>P disintegration measured in plants growing in the low- and high-P soils, 10 times more <sup>31</sup>P would have been taken up and delivered by AM fungi in the high-P than in the low-P soils. Thus, as discussed in Lekberg <i>et al</i>. (<span>2024</span>), the estimated higher AM fungal P delivery in the high-P soils based solely on the measured <sup>32</sup>P is, if anything, likely a conservative estimate.</p><p>While patterns observed in Lekberg <i>et al</i>. (<span>2024</span>) are substantiated due to the reasons outlined above, we agree with Spohn & Wanek (<span>2025</span>) that estimates of resource uptake would be strengthened by measurements of both <sup>31</sup>P and <sup>32</sup>P in the bioavailable orthophosphate fraction through time, especially if different soils are used. However, proper execution of such measurements through time is challenging for several reasons. First, estimating specific activity of <sup>32</sup>P assumes even spatial distribution of <sup>32</sup>P, which is almost always violated given the low mobility of P and because radioisotopes are often added as a point source (including in Lekberg <i>et al</i>., <span>2024</span>). Second, repeated sampling of soil after tracer addition involves some degree of disturbance that could affect soil structure, possibly promote mineralization, and disrupt fungal hyphae and roots. This would inevitably change conditions following the first sampling as a function of the amount of soil sampled and degree of disturbance. Also, there is some debate about how to best assess and quantify the plant- and/or mycorrhiza-available P pool (Bühler <i>et al</i>., <span>2003</span>; Mason <i>et al</i>., <span>2013</span>). Using extractants (e.g. Bray 1) as suggested by Spohn & Wanek (<span>2025</span>) and used in Lekberg <i>et al</i>. (<span>2024</span>), can generate artifacts. This last issue could be avoided by directly quantifying P in soil solution, but collecting and reliably analyzing inorganic orthophosphate in microliters of soil solution is extremely difficult given its low concentration and possible interferences due to soil colloids and dissolved organic P. To acquire specific <sup>32</sup>P activity in soil solutions of the different soils over time, an alternative option is to conduct parallel laboratory batch experiments and use isotope exchange kinetics (Frossard & Sinaj, <span>1997</span>) although such an approach may not be fully representative of patterns in soils with plants and mycorrhizal fungi. Related to this, whether <sup>32</sup>P specific activity that changes over time in the soil solution reflects P acquisition by AM fungi at all time points (e.g. 1 min, 1 h, and 1 d after tracer addition, or during daytime vs nighttime) is uncertain given that so little is known about temporal and spatial acquisition patterns and mechanisms (but see attempts by Dessureault-Rompré <i>et al</i>., <span>2007</span>), interactions with other soil biota that immobilize, mineralize, and solubilize P, and effects of water and other nutrient availabilities. Add to this the difficulty of measuring declining levels of <sup>32</sup>P radioactivity across longer timescales with confidence given radioactivity from naturally occurring isotopes such as <sup>40</sup>K, <sup>14</sup>C and <sup>3</sup>H. Given these challenges, perhaps it is not surprising that most models fail to accurately predict P acquisition in plants (Hinsinger <i>et al</i>., <span>2011</span>), although different estimates of soil available P often correlate reasonably well with P uptake and each other (Steinfurth <i>et al</i>., <span>2021</span>). Ultimately, we need to refine the assumptions and better understand the physical, chemical, and biological properties of soils that govern P availability to make radioisotope tracing of P acquisition from soil to AM fungi and plants truly quantitative. This is a difficult, albeit not impossible, task.</p><p>Alternative methodologies to radio-phosphorus tracers exist, and hydroxyapatite conjugated to quantum dots is increasingly being used to assess P transfer in AM (e.g. Whiteside <i>et al</i>., <span>2019</span>; van't Padje <i>et al</i>., <span>2021</span>). Quantum dots are appealing in many ways due to their greater longevity than radioactive P isotopes and the possibility of <i>in vivo</i> tracking on cellular levels (Färkkilä <i>et al</i>., <span>2021</span>). Previous research has shown acquisition and delivery of quantum dots to plants by AM fungi and have elucidated fascinating patterns involved in plant–fungal interactions. However, relying on these approaches is not without challenges and uncertainties because the uptake and delivery mechanisms may differ from those that govern the uptake and delivery of orthophosphate. It is assumed that most P is acquired by AM fungi and delivered to plants via substrate-specific membrane transporters (Ezawa & Saito, <span>2018</span>). Transported molecules are ionic and have a hydrated diameter of < 1 nm, whereas quantum dots coated with apatite are 8 nm diameter, which sometimes are combined into complexes reaching 200 nm in diameter (van't Padje <i>et al</i>., <span>2021</span>). In this size range, quantum dots are likely taken up by endocytosis (Raven, <span>2022</span>), which is a completely different mechanism compared to inorganic orthophosphate transporters. We consequently echo the suggestion in Raven (<span>2022</span>) that isotopes could be an excellent way to assess if and how quantum dots accurately reflect phosphate uptake and delivery by AM fungi.</p><p>With increasing realism in experiments comes increasing difficulty. As we continue to try to understand the functioning of mycorrhizal symbioses in nature, it is essential that we use appropriate methods and are aware of potential limitations. As such, the discussion generated by Spohn & Wanek (<span>2025</span>) is very timely.</p><p>None declared.</p><p>YL conceived of the idea and wrote the first draft, YL and BPC conducted the sorption tests, and BPC calculated sorption. YL, BPC, JJ, DJ, PM and CP edited the manuscript draft and provided critical input.</p>\",\"PeriodicalId\":214,\"journal\":{\"name\":\"New Phytologist\",\"volume\":\"245 2\",\"pages\":\"446-449\"},\"PeriodicalIF\":8.3000,\"publicationDate\":\"2024-11-21\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"https://onlinelibrary.wiley.com/doi/epdf/10.1111/nph.20217\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"New Phytologist\",\"FirstCategoryId\":\"99\",\"ListUrlMain\":\"https://onlinelibrary.wiley.com/doi/10.1111/nph.20217\",\"RegionNum\":1,\"RegionCategory\":\"生物学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q1\",\"JCRName\":\"PLANT SCIENCES\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"New Phytologist","FirstCategoryId":"99","ListUrlMain":"https://onlinelibrary.wiley.com/doi/10.1111/nph.20217","RegionNum":1,"RegionCategory":"生物学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"PLANT SCIENCES","Score":null,"Total":0}
引用次数: 0
摘要
最近一篇关于丛枝菌根(AM)真菌和植物之间碳和磷(P)转移的论文(Lekberg et al., 2024)被Spohn &;Wanek (2025;以强调使用P放射性同位素(在本例中为32P,尽管与33P同样相关,因为它们的行为相似)的潜在缺陷;Frossard et al., 2011)。具体来说,Spohn &;Wanek(2025)指出,在不知道AM真菌在土壤溶液中的具体活性(即32P: 31P的比例)的情况下,Lekberg等人(2024)认为AM真菌在高磷土壤中比低磷土壤中输送更多的P,如果三种低磷土壤吸收更多的32P,则该结论可能不成立。我们同意Spohn &;Wanek(2025)认为,示踪剂实验应谨慎解释,我们感谢有机会更详细地探索该论文中强调的一些陷阱。无机正磷酸盐(H2PO4−和HPO42−)——植物、真菌和原核生物从土壤溶液中吸收的主要磷形式(Bucher, 2007)——的吸附是由pH值、有机物、铝和铁的氢氧氧化物和碳酸钙决定的(Frossard等人,1995;Daly et al., 2001;巴罗,2017)。Lekberg等人(2024)从两个区域收集土壤,一个是高磷有效度区域,另一个是低磷有效度区域。两个地区的土壤均为细土-重土,土壤pH值(6.80±0.38 vs 6.53±0.38,平均值±SE)和有机质浓度(4.50±0.69%和5.03±2.00%)在高磷和低磷土壤之间均无差异(P > 0.5)。高磷土壤和低磷土壤的总有机磷储量(分别为1178±302和684±79 mg kg - 1)差异不显著,Bray - 1提取磷有效性的区域差异可能与土壤矿物学有关。Lekberg等人(2024)的研究表明,吸附差异可能不会影响结果还有另一个原因。植物-土壤-同位素实验的一个主要假设是,与溶液中未标记的营养物相比,同位素示踪剂的浓度必须可以忽略不计。如果32P的加入量与31P相近,则溶液和固相P平衡被打破,总P和32P发生净吸附。土壤中不发生净吸附或解吸的溶液浓度称为“净零吸附平衡磷浓度”(EPC0)。具体到每种土壤,EPC0取决于相同的变量,这些变量决定了吸附、磷解吸动力学以及微生物对磷的固定和释放。Simpson等人(2021)最近的一项荟萃分析表明,许多土壤的EPC0为0.1 mg l - 1,报道的最低值为0.5 μg l - 1。Lekberg等人(2024)使用的32P浓度为3.2 pmol l - 1或0.1 ng l - 1;至少比文献中报道的最低EPC0低三个数量级。因此,当使用Lekberg等人(2024)所做的“无载流子”示踪剂时,由于土壤溶液和固相平衡被破坏而导致的放射性同位素净吸收不太可能发生。即使没有P的净吸附,由于32P在液相和固相之间的同位素交换,示踪剂添加后也会随着时间的推移发生同位素稀释,随着时间的推移,更多的32P被吸附,伴随着31P的解吸、放射性衰变和微生物的固定和释放(Helfenstein et al., 2018)。在Spohn &;中没有特别强调31P的解吸Wanek(2025),虽然提到了“P释放”,但也包括P矿化和增溶等过程。然而,在他们的图1中,P解吸变化的含义很明显,表明富P土壤中31P浓度较高。考虑到Lekberg等人(2024)在高磷土壤中生物可利用31P的背景浓度高出10倍,并且由于所有盆栽中添加的32P量相同,在低磷和高磷土壤中生长的植物中测量到的每32P崩解,高磷土壤中AM真菌吸收和输送的31P是低磷土壤中的10倍。因此,正如Lekberg等人(2024)所讨论的那样,仅根据测量的32P估算出的高磷土壤中AM真菌P排泄量较高,如果有的话,可能是保守估计。虽然由于上述原因,Lekberg等人(2024)观察到的模式得到了证实,但我们同意Spohn &;Wanek(2025)认为,随着时间的推移,特别是在使用不同土壤的情况下,通过测量生物可利用的正磷酸盐部分中的31P和32P,可以加强对资源吸收的估计。然而,由于几个原因,随着时间的推移正确执行这些测量是具有挑战性的。首先,估计32P的比活度假设32P的空间分布均匀,考虑到P的低迁移率,并且由于放射性同位素经常作为点源添加(包括Lekberg等人,2024),这几乎总是违反的。 其次,添加示踪剂后的土壤重复采样存在一定程度的干扰,可能会影响土壤结构,促进矿化,破坏真菌菌丝和根。这将不可避免地改变第一次采样后的条件,作为采样土壤量和干扰程度的函数。此外,关于如何最好地评估和量化植物和/或菌根可利用磷库也存在一些争论(b<s:1> hler等人,2003;Mason et al., 2013)。按照Spohn &;的建议使用萃取剂(例如Bray 1)。Wanek(2025)和Lekberg等人(2024)使用的方法可以产生伪影。最后一个问题可以通过直接定量土壤溶液中的磷来避免,但由于无机正磷酸盐的浓度低,并且可能受到土壤胶体和溶解的有机磷的干扰,因此收集和可靠地分析微升土壤溶液中的无机正磷酸盐是极其困难的。另一种选择是进行平行实验室批量实验,并使用同位素交换动力学(Frossard &;Sinaj, 1997),尽管这种方法可能不能完全代表有植物和菌根真菌的土壤模式。与此相关的是,土壤溶液中32P比活性随时间的变化是否反映了AM真菌在所有时间点(例如添加示踪剂后1分钟、1小时和1天,或白天与夜间)对P的获取尚不确定,因为对时空获取模式和机制知之甚少(但参见dessureault - rompreve等人的尝试,2007),与其他土壤生物的相互作用可以固定、矿化和溶解P,以及水和其他营养物质的影响。考虑到天然存在的同位素(如40K、14C和3H)的放射性,在更长的时间尺度上,很难有把握地测量32P放射性水平的下降。考虑到这些挑战,大多数模型无法准确预测植物对磷的获取(Hinsinger等人,2011)也就不足为奇了,尽管对土壤有效磷的不同估计通常与磷吸收以及彼此之间的关系相当好(Steinfurth等人,2021)。最终,我们需要完善这些假设,更好地了解控制磷有效性的土壤的物理、化学和生物特性,从而使土壤对AM真菌和植物的磷获取的放射性同位素示踪真正定量。这是一项艰巨的任务,尽管并非不可能。存在放射性磷示踪剂的替代方法,羟基磷灰石与量子点共轭越来越多地被用于评估AM中的P转移(例如Whiteside等人,2019;van't Padje et al., 2021)。量子点在许多方面都很有吸引力,因为它们比放射性P同位素寿命更长,并且可以在细胞水平上进行体内跟踪(Färkkilä et al., 2021)。先前的研究表明AM真菌获得和传递量子点给植物,并阐明了植物与真菌相互作用的有趣模式。然而,依赖这些方法并非没有挑战和不确定性,因为摄取和递送机制可能与控制正磷酸盐的摄取和递送的机制不同。据推测,大多数磷是由AM真菌获得的,并通过底物特异性膜转运体传递给植物(Ezawa &;斋藤,2018)。传输的分子是离子型的,水合直径为1nm,而包覆磷灰石的量子点直径为8nm,有时会组合成直径达到200nm的配合物(van't Padje et al., 2021)。在这个尺寸范围内,量子点很可能被内吞作用所占据(Raven, 2022),与无机正磷酸盐转运体相比,这是一个完全不同的机制。因此,我们呼应Raven(2022)的建议,即同位素可能是评估量子点是否以及如何准确反映AM真菌对磷酸盐的吸收和传递的极好方法。随着实验真实感的增加,难度也随之增加。当我们继续试图了解自然界中菌根共生的功能时,我们必须使用适当的方法并意识到潜在的局限性。因此,由Spohn &;Wanek(2025)非常及时。没有宣布。YL构思构思并撰写初稿,YL与BPC进行吸附试验,BPC计算吸附量。YL, BPC, JJ, DJ, PM和CP编辑稿件草稿并提供重要意见。
Tracing phosphorus from soil through mycorrhizal fungi to plants
A recent paper about carbon and phosphorus (P) transfer between arbuscular mycorrhizal (AM) fungi and plants (Lekberg et al., 2024) was cited by Spohn & Wanek (2025; pp. 443–445, in this issue) to highlight potential pitfalls of using P radioisotopes (in this case 32P, although equally relevant for 33P as they behave similarly; Frossard et al., 2011). Specifically, the paper by Spohn & Wanek (2025) states that without knowing the specific activity in soil solution (i.e. the ratio of 32P : 31P), the conclusion in Lekberg et al. (2024) that more P was delivered by AM fungi in high-P than low-P soils may not be valid if the three low-P soils sorbed more 32P. We agree with Spohn & Wanek (2025) that tracer experiments should be interpreted with caution, and we appreciate the opportunity to explore some of the pitfalls highlighted in that paper in more detail.
Sorption of inorganic orthophosphate (H2PO4− and HPO42−) – the main P form taken up by plants, fungi, and prokaryotes from soil solution (Bucher, 2007) – is determined by pH, organic matter, hydrous oxides of aluminum and iron, and calcium carbonate (Frossard et al., 1995; Daly et al., 2001; Barrow, 2017). In Lekberg et al. (2024), soils were collected from two regions, one with high-P availability and one with low-P availability. In both regions, soils were classified as fine to gravely, loamy Mollisols, and neither soil pH (6.80 ± 0.38 vs 6.53 ± 0.38, means ± SE) nor organic matter concentrations (4.50 ± 0.69% and 5.03 ± 2.00%) differed (P > 0.5) between the high-P and low-P soils, respectively. Total organic P stocks also did not differ significantly between the high-P and low-P soils (1178 ± 302 and 684 ± 79 mg kg−1) and the strong regional difference in P availability based on Bray 1 extractions were likely due to soil mineralogy.
There is another reason why differences in sorption likely did not influence results in Lekberg et al. (2024). One of the main assumptions of plant–soil–isotope experiments is that the concentration of the isotope-tracer must be negligible compared to that of the nonlabelled nutrient in solution. If the amount of 32P added is similar to 31P, the solution and solid-phase P equilibrium is disrupted and net sorption of total and 32P will occur. The solution concentration where no net sorption or desorption occurs in soil is known as the ‘equilibrium P concentration at net zero sorption’ (EPC0). Specific to each soil, EPC0 depends on the same variables that determine sorption, as well as P desorption kinetics, and microbial P immobilization and release. A recent meta-analysis by Simpson et al. (2021) illustrates that many soils have an EPC0 of 0.1 mg l−1, and the lowest value reported was 0.5 μg l−1. The 32P concentration used in Lekberg et al. (2024) was 3.2 pmol l−1 or 0.1 ng l−1; at least three orders of magnitude lower than the lowest EPC0 ever reported in the literature. Net sorption of radioisotopes due to disrupted soil solution and solid-phase equilibrium is, therefore, unlikely to occur when using ‘carrier-free’ tracers as was done in Lekberg et al. (2024).
Even without net sorption of P, isotopic dilution will occur through time after tracer addition due to isotopic exchange of 32P between the liquid and solid-phase with more 32P being sorbed over time, accompanied by desorption of 31P, radioactive decay, and microbial immobilization and release (Helfenstein et al., 2018). Desorption of 31P was not specifically highlighted in Spohn & Wanek (2025), although ‘P release’ was mentioned, which also includes processes like P mineralization and solubilization. However, the implication of variation in P desorption is apparent in their fig. 1, which shows higher 31P concentration in the P-rich soil. Given the 10-fold greater background concentration of bioavailable 31P in the high-P soils in Lekberg et al. (2024), and because the same amount of 32P was added to all pots, for every 32P disintegration measured in plants growing in the low- and high-P soils, 10 times more 31P would have been taken up and delivered by AM fungi in the high-P than in the low-P soils. Thus, as discussed in Lekberg et al. (2024), the estimated higher AM fungal P delivery in the high-P soils based solely on the measured 32P is, if anything, likely a conservative estimate.
While patterns observed in Lekberg et al. (2024) are substantiated due to the reasons outlined above, we agree with Spohn & Wanek (2025) that estimates of resource uptake would be strengthened by measurements of both 31P and 32P in the bioavailable orthophosphate fraction through time, especially if different soils are used. However, proper execution of such measurements through time is challenging for several reasons. First, estimating specific activity of 32P assumes even spatial distribution of 32P, which is almost always violated given the low mobility of P and because radioisotopes are often added as a point source (including in Lekberg et al., 2024). Second, repeated sampling of soil after tracer addition involves some degree of disturbance that could affect soil structure, possibly promote mineralization, and disrupt fungal hyphae and roots. This would inevitably change conditions following the first sampling as a function of the amount of soil sampled and degree of disturbance. Also, there is some debate about how to best assess and quantify the plant- and/or mycorrhiza-available P pool (Bühler et al., 2003; Mason et al., 2013). Using extractants (e.g. Bray 1) as suggested by Spohn & Wanek (2025) and used in Lekberg et al. (2024), can generate artifacts. This last issue could be avoided by directly quantifying P in soil solution, but collecting and reliably analyzing inorganic orthophosphate in microliters of soil solution is extremely difficult given its low concentration and possible interferences due to soil colloids and dissolved organic P. To acquire specific 32P activity in soil solutions of the different soils over time, an alternative option is to conduct parallel laboratory batch experiments and use isotope exchange kinetics (Frossard & Sinaj, 1997) although such an approach may not be fully representative of patterns in soils with plants and mycorrhizal fungi. Related to this, whether 32P specific activity that changes over time in the soil solution reflects P acquisition by AM fungi at all time points (e.g. 1 min, 1 h, and 1 d after tracer addition, or during daytime vs nighttime) is uncertain given that so little is known about temporal and spatial acquisition patterns and mechanisms (but see attempts by Dessureault-Rompré et al., 2007), interactions with other soil biota that immobilize, mineralize, and solubilize P, and effects of water and other nutrient availabilities. Add to this the difficulty of measuring declining levels of 32P radioactivity across longer timescales with confidence given radioactivity from naturally occurring isotopes such as 40K, 14C and 3H. Given these challenges, perhaps it is not surprising that most models fail to accurately predict P acquisition in plants (Hinsinger et al., 2011), although different estimates of soil available P often correlate reasonably well with P uptake and each other (Steinfurth et al., 2021). Ultimately, we need to refine the assumptions and better understand the physical, chemical, and biological properties of soils that govern P availability to make radioisotope tracing of P acquisition from soil to AM fungi and plants truly quantitative. This is a difficult, albeit not impossible, task.
Alternative methodologies to radio-phosphorus tracers exist, and hydroxyapatite conjugated to quantum dots is increasingly being used to assess P transfer in AM (e.g. Whiteside et al., 2019; van't Padje et al., 2021). Quantum dots are appealing in many ways due to their greater longevity than radioactive P isotopes and the possibility of in vivo tracking on cellular levels (Färkkilä et al., 2021). Previous research has shown acquisition and delivery of quantum dots to plants by AM fungi and have elucidated fascinating patterns involved in plant–fungal interactions. However, relying on these approaches is not without challenges and uncertainties because the uptake and delivery mechanisms may differ from those that govern the uptake and delivery of orthophosphate. It is assumed that most P is acquired by AM fungi and delivered to plants via substrate-specific membrane transporters (Ezawa & Saito, 2018). Transported molecules are ionic and have a hydrated diameter of < 1 nm, whereas quantum dots coated with apatite are 8 nm diameter, which sometimes are combined into complexes reaching 200 nm in diameter (van't Padje et al., 2021). In this size range, quantum dots are likely taken up by endocytosis (Raven, 2022), which is a completely different mechanism compared to inorganic orthophosphate transporters. We consequently echo the suggestion in Raven (2022) that isotopes could be an excellent way to assess if and how quantum dots accurately reflect phosphate uptake and delivery by AM fungi.
With increasing realism in experiments comes increasing difficulty. As we continue to try to understand the functioning of mycorrhizal symbioses in nature, it is essential that we use appropriate methods and are aware of potential limitations. As such, the discussion generated by Spohn & Wanek (2025) is very timely.
None declared.
YL conceived of the idea and wrote the first draft, YL and BPC conducted the sorption tests, and BPC calculated sorption. YL, BPC, JJ, DJ, PM and CP edited the manuscript draft and provided critical input.
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