Abstract:

Iron (Fe) and Phosphorus (P) are key nutrient elements in paddy soil that have a profound impact on rice growth. The migration, transformation, and bioavailability of Fe and P in paddy soil ecosystems are governed by a series of complex biogeochemical processes. Rice production contributes approximately one-third of China’s total grain output. Its yield is intrinsically linked to both per-unit-area productivity and soil fertility, and is critically contingent upon the bioavailability of soil P. Consequently, improving P utilization efficiency in paddy soils has emerged as a pressing scientific challenge essential for achieving sustainable agricultural development. The alternating flooding and drainage in paddy fields dynamically regulates the reductive transformation of Fe minerals, which in turn governs the mobilization and re-immobilization of the mineral-bound P. This process critically influences phosphorus bioavailability and, consequently, rice growth and yield. As a common P-containing Fe mineral in the natural environment, the dissolution and transformation processes of ferric phosphate (FePO₄) play an important role in improving P availability in paddy soil. Dissimilatory Fe-reducing bacteria, Shewanella oneidensis MR-1 (S. oneidensis MR-1), are capable of using Fe minerals as terminal electron acceptors for metabolic growth, playing an important role in the biogeochemical cycling of Fe and P. Current research predominantly focuses on the mutual reactions of FePO₄ and MR-1 in microcosm systems, whereas systematic studies on the fate and transport of Fe and P under field-scale paddy soil conditions remain limited. Herein, to investigate the underlying influences of the addition of P-containing Fe minerals and Fe-reducing bacteria on the biological availability of P and Fe in rice paddy soils, a series of pot experiments were conducted using a typical paddy soil from Zhishan Town, Jiangmen City, Guangdong Province as the growth medium, and five treatments were set up, including a control treatment (CK), a low-level FePO₄ treatment, a high-level FePO₄ treatment, a low-level FePO₄+MR-1 treatment, and a high-level FePO₄+MR-1 treatment. At specific times (1, 15, 30, 80, 90, 105, and 122 d), soil and pore water samples were collected to determine pH, Eh, and dissolved Fe(II) content, and rhizosphere soil was collected for HCl-extractable Fe(II) and soil P fractionation determination. At the rice maturity stage, rice roots were sampled to determine Fe and P contents in the Fe plaque, and P content in different organs of rice was determined. Lastly, the aboveground biomass and grain yield of rice were measured to evaluate the effects of different treatments on rice growth and production. During the early flooding stage, the dissolved Fe(II) concentration in soil pore water increased significantly. Relative to CK treatment, the low-level FePO₄, high-level FePO₄, and low-level FePO₄+MR-1 treatments showed increases of 20.88%, 11.70%, and 11.72%, respectively. Moreover, on day 15, the HCl-extractable Fe(II) content in the low-level FePO₄+MR-1 treatment was significantly higher than that in other treatments. During the drainage stage, Fe(II) in the porewater was rapidly oxidized and precipitated. In contrast, HCl-extractable Fe(II) exhibited an initial increase followed by a decrease, indicating the reductive dissolution and subsequent precipitation of Fe minerals. The P fractionation revealed that after 15 d of flooding, H₂O-P and NaHCO₃-P contents in the high-level FePO₄+MR-1 treatment were 93.21% and 10.01%, respectively, which were higher than those in the CK treatment. After entering the drainage stage, different P speciation gradually tended to stabilize. Quantitative analysis of root Fe plaque revealed that compared to CK treatment, Fe and P concentrations in the high-level FePO₄+MR-1 treatment increased by 28.58% and 36.44%, respectively. At the maturity stage, the rice root was the primary site of P distribution, comprising 36.87%‒47.41% of total rice plant P, with its proportion being significantly higher in all treatments than that in the CK treatment. Compared to CK treatment, high-level FePO₄+MR-1 treatment increased root P content by 59.00%, with a smaller increase in aboveground P. This may be attributed to the increasing reductive dissolution of P-bearing minerals and FePO₄ mediated by MR-1 during flooding. In the late flooding stage, Fe(II) concentration was lowest in the high-level FePO₄+MR-1 treatment, while it was highest in the low-level FePO₄ treatment. This may be ascribed to the released P from paddy soil in the high-level FePO₄ treatment, which, in conjunction with MR-1, could occupy the reduction sites on Fe minerals, thereby suppressing more Fe(II) generation. Regarding P speciation, NaHCO₃-P concentration on day 15 was lower in the CK treatment than that in other treatments, with the high-level FePO₄ treatment showing a relatively higher value. This may be because a portion of P released through the reductive dissolution of Fe minerals may be readily re-adsorbed by Fe minerals or precipitated with Ca²⁺, thereby increasing the content of Fe-bound or Ca-bound P. Substantial increases in Fe and P within the root Fe plaque were observed in the high-level FePO₄ and high-level FePO₄+MR-1 treatments. These results demonstrated that the addition of elevated FePO₄, particularly when interacting with MR-1, significantly increased Fe and P content in the Fe plaque, which contributed to the P absorption and transport by rice plants. These above changes significantly improved rice growth, P uptake, and yield. Specifically, the rachis biomass increased by 20.74% in the low-level FePO₄+MR-1 treatment and 21.90% in the high-level FePO₄+MR-1 treatment compared to the CK treatment (1.07 g·plant⁻¹). Stem biomass showed a similar trend, reaching 12.28 and 12.32 g·plant⁻¹ in the low-level FePO₄+MR-1 and high-level FePO₄+MR-1 treatments versus 10.79 g·plant⁻¹ in the CK treatment, indicating overall enhanced plant growth after amendment with FePO₄ and S. oneidensis MR-1. Ultimately, high-level FePO₄+MR-1 treatment yielded the highest brown rice biomass (22.38 g·plant⁻¹), which was significantly greater than that of CK (18.61 g·plant⁻¹) and other treatments. This study elucidated the interaction between FePO₄ and dissimilatory Fe-reducing bacteria in the pot experiment and microscopic mechanisms of the migration and transformation of Fe and P at the soil-water and the soil-rice plant interfaces, thereby advancing our understanding of the geochemical cycling of Fe and P in paddy fields. These findings provide a scientific basis for developing microbial strategies to enhance soil P availability and reduce fertilizer dependency, which is beneficial for increasing rice yields in the future.

Key words: rice, ferric phosphate, dissimilatory iron-reducing bacteria, root iron plaques, phosphorus migration and transportation

摘要：

含磷铁矿物与铁还原菌之间的相互作用是影响稻田土壤铁和磷迁移性和生物有效性的关键过程，然而，关于含磷铁矿物与铁还原菌互作对稻田土壤中铁和磷的迁移转运机制尚不清楚。选取典型水稻土开展盆栽试验，设置磷酸铁和异化铁还原菌（Shewanella oneidensis MR-1）的添加实验：对照组（CK）、低水平磷酸铁添加（FeP1）、高水平磷酸铁添加（FeP2）、低水平磷酸铁+MR-1添加（FeP1+MR-1）和高水平磷酸铁+MR-1（FeP2+MR-1）添加。在水稻生育期内，测定土壤孔隙水和植株中铁磷质量分数及植株和籽粒单株生物量变化，以此探究添加磷酸铁与异化铁还原菌对土壤-水稻体系中铁和磷迁移与转运的影响。结果表明，在淹水末期，与CK相比，FeP1、FeP1+MR-1和FeP2+MR-1处理的盐酸提取态Fe(II)质量分数分别高25.0 %、45.7 %、23.6 %；在磷酸铁+MR-1添加组中，孔隙水Fe(II)与盐酸提取态Fe(II)质量分数高于CK以及只添加磷酸铁的处理，说明异化铁还原菌促进了磷酸铁的还原和铁的释放。成熟期时，根表铁膜中的Fe质量分数，FeP1、FeP2、FeP1+MR-1、FeP2+MR-1处理组分别比CK（69.3 g·kg⁻¹）高出24.4%、17.8%、20.1%、28.9%；对于根表铁膜中的P质量分数，FeP1、FeP2、FeP1+MR-1、FeP2+MR-1处理分别比CK（3.95 g·kg⁻¹）高出24.5%、31.2%、24.7%、36.4%。在磷酸铁+MR-1添加组中，根际磷的生物有效性和植株磷的积累显著提高，表现为根表铁膜、茎、叶和稻壳的磷质量分数增加。FeP1、FeP2、FeP1+MR-1和FeP2+MR-1 4个处理组的糙米单株生物量分别为20.2、21.1、21.6和22.4 g·plant⁻¹，均显著高于CK（18.6 g·plant⁻¹）。淹水-排水稻田中磷酸铁与铁还原菌在根际环境中的协同作用，可有效促进铁和磷在土壤-孔隙水-植株系统中的迁移和转运。

关键词: 水稻, 磷酸铁, 异化铁还原菌, 根表铁膜, 磷迁移转运