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Article

Effect of Drying–Rewetting Alternation on Phosphorus Fractions in Restored Wetland

by
Mingyue Ren
1,2,
Yingxin Li
1,2,
Lixia Wang
1,*,
Naseer Hussain
3,
Bing Bai
1,2,
Jie Zhou
1,2 and
Yongxing Ren
1
1
State Key Laboratory of Black Soils Conservation and Utilization, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130102, China
2
University of Chinese Academy of Sciences, Beijing 101408, China
3
B.S. Abdur Rahman Crescent Institute of Science and Technology, Chennai 600048, India
*
Author to whom correspondence should be addressed.
Agriculture 2025, 15(16), 1720; https://doi.org/10.3390/agriculture15161720 (registering DOI)
Submission received: 1 July 2025 / Revised: 31 July 2025 / Accepted: 6 August 2025 / Published: 9 August 2025
(This article belongs to the Section Agricultural Soils)
Browse Figures
Versions Notes

Abstract

Wetlands frequently experience drying and rewetting (DRW) alternation, which intricately influences the physical, chemical, and biological processes within the soil matrix. The conversion of agricultural land into wetland ecosystems has raised significant concerns regarding the release of phosphorus. However, a significant knowledge gap persists in understanding the implications of this phenomenon for phosphorus transformation and release dynamics within soils situated in the freeze–thaw zone of Northeast China. To address this gap, we conducted a series of experiments involving the simulation of varying intensities and frequencies of DRW alternation using soil column samples collected from restored wetlands. This study evaluated phosphorus fractions with different levels of mobility and availability using methods based on standardized chemical extraction. We subsequently analyzed the impacts of these alterations on phosphorus fractions from the perspectives of soil chemical properties and microbial community changes. DRW events were found to facilitate the conversion of labile inorganic phosphorus into organic phosphorus fractions, driving the transformation of mobile phosphorus into potentially mobile fractions. Moderate drought events showed a significant increase in soil bacterial diversity and abundance, while both normal and extreme drought events caused a decrease in bacterial diversity. Moreover, DRW treatment increased the relative abundance of Proteobacteria and decreased the relative abundance of Chloroflexi. Redundancy analysis identified organic carbon and bacterial diversity as key drivers influencing phosphorus fractions. Overall, this study contributes to our understanding of the intricate relationships among soil characteristics, microbial ecosystems, and the complex behavior of phosphorus under various DRW regimes in restored wetlands.

1. Introduction

The accumulation of surplus phosphorus (P) in agricultural soil significantly contributes to the non-point source pollution and degradation of water [1]. Of the phosphorus applied to farmland in a growing season, only 10–20% is absorbed and utilized by crops, while the remaining P forms insoluble phosphates that are rapidly adsorbed by soil minerals or transformed into inorganic fractions and immobilized by microbial communities [2]. The hydrological dynamics experience alteration following the conversion of farmland to wetland, facilitating the retention of exogenous P through matrix adsorption, plant absorption, and microbial accumulation. Nevertheless, the accumulation of soil organic matter within wetland soil diminishes or alters its ability to purify water, consequently transitioning into a potential source of P release [3]. Between 1990 and 2010, China converted 1369 km2 of land area from farmland to wetland to strengthen the establishment of wetland-based ecological systems [4]. By the end of 2020, wetland expansion in China had reached approximately 41.2 × 104 km2, positioning it at the forefront among Asian nations. China is committed to furthering policies that encompass the reconversion of pasture and farmland into wetland habitats, with the objective of safeguarding 55% of wetland areas by the year 2025 [5]. Evidently, the potential for P release during the restoration and reconstruction of wetlands demands considerable attention.
Restored wetlands exhibit distinctive characteristics inherent to both farmland and wetland ecosystems, with their elemental composition, organic matter, and hydrological dynamics occupying an intermediate position between natural wetlands and paddy fields [6]. The drying–rewetting process plays a significant role in the biogeochemical P cycle [7]. The fluctuations between aerobic and anaerobic conditions during DRW exert an intricate influence on soil’s physical, chemical, and biological processes. This includes aspects such as structural composition, organic matter decomposition and mineralization, ionic transport and transformation, and the dynamics of microbial communities [8]. As the nutrient levels in the soil manifest higher concentrations in newly restored wetlands [9], the process of DRW may trigger P release, thereby elevating the risk of P discharge into surface- and groundwater [10].
Phosphorus mobility within soil is intricately linked to its chemical fractions. In the soil matrix, P can be broadly classified into inorganic and organic fractions [11]. Within the realm of inorganic P, distinct subtypes emerge, including mobile P (L-P), iron–aluminum-bound P (Fe/Al-P), calcium-bound P (Ca-P), and occluded-accumulation P (O-P) [12]. Based on the degree to which plants can absorb and utilize them, L-P mainly refers to P compounds that are readily soluble in water, can move in soil with water flow through diffusion, and can be quickly absorbed by plants. Fe/Al-P refers to P that is encapsulated by iron, manganese, aluminum oxides, and their hydroxides. It has a relatively low stability and can be directly absorbed and utilized by plants. It constitutes a relatively large proportion of inorganic P. Ca-P and O-P tend to achieve stabilization within the soil matrix, rendering them unavailable to plant absorption and utilization [13]. Organic P, characterized by its predominant presence within plant and microbial residues, displays a greater mobility than inorganic fractions due to reduced immobilization within the soil matrix. Notably, Fe/Al-P can be converted to dissolved P under specific redox potential and pH changes, particularly under anoxic conditions during the reduction and dissolution of iron hydroxide [14]. Overall, the distribution of P in the soil matrix is intricate and dictated by multifarious factors, including organic matter content, soil pH, Eh, microbial activity, and iron and aluminum oxides [15].
Alterations in soil moisture play a pivotal role in P release dynamics. Longterm simulations on wheat–corn rotation farmland soils underscored that extreme drought conditions (holding capacity of 5%) lead to increased bicarbonate-extractable inorganic P and phytate-like P content [16]. Likewise, Turner and Haygarth et al. [17] demonstrated elevated water-soluble P under drying conditions in 29 types of lowland permafrost in England and Wales. Additionally, microbial P release and soil organic matter degradation contribute to an increase in labile P in forest environments [18]. Therefore, we hypothesize that changes in soil physical and chemical properties and microbial community structure under DRW conditions will affect the fractions of P and lead to variations in its mobility. Despite extensive investigations into P fraction changes in farmland, forest, and grassland, etc., there remains a notable research gap concerning P fraction alterations and release risk in restored wetlands during DRW events.
To address this gap, a series of drying–rewetting experiments were conducted, encompassing varying frequencies and simulated degrees of drought severity. The primary objective was to investigate the implications of DRW for P fractions within restored wetland ecosystems, focusing on consequential shifts in soil chemical attributes and microbial communities. Furthermore, our findings provide theoretical insights into the formulation of strategies for wetland conservation, mitigating the potential risk of P loss and wetland water eutrophication.

2. Materials and Methods

2.1. Sampling Site

The soil samples for this experiment were meticulously collected in October 2020 in the Sanjiang Wetland Nature Reserve in the northeastern part of Heilongjiang Province, China (133°43′20″–134°46′40″ E, 47°26′–48°22′50″ N). Notably, this site was restored as wetland two years before the sampling phase. Subsequently, the samples were brought back to the laboratory for a 49-day drying–rewetting alternating simulation experiment, and then experimental analysis was conducted to obtain data. The basic soil chemical properties were obtained (Table 1). This geographical location is classified as having a temperate humid and semi-humid continental monsoon climate, which is distinctively characterized by seasonal fluctuations in temperature. The duration of freezing temperatures extends for approximately 7–8 months per annum, with an average annual temperature range spanning from 1.4 °C to 4.3 °C. The region is subject to an average annual precipitation of 595.7 mm.

2.2. Experimental Design

Soil column samples were systematically collected from the designated experimental site at a depth interval of 0–20 cm following the random sampling method. These columns were tightly sealed at the base using a polyvinyl chloride (PVC) cap. The experimental unit was a PVC cylindrical column 25 cm in height and 15 cm in diameter. The samples were recorded on site. Prior to the drying and rewetting experiments, the soil samples in all PVCs were saturated with distilled water until a 2 cm water layer remained above the soil layer. This preliminary phase ensured a consistent and uniform moisture level across all experimental units.
Considering that in the summer months of June, July, and August in Heilongjiang Province, the average monthly number of precipitation days is 13.6 days, 14.9 days, and 13.8 days, respectively, the summer precipitation is relatively abundant [19]. In the DRW experiment, the columns of the first group were initially flooded with distilled water for 1 day. This was followed by a 2-day drying period, after which the columns were subjected to another cycle of flooding and drying, corresponding to the pattern depicted in Figure 1. In the second group of columns, the soil samples were allowed to dry for 6 days after 1 day of flooding, simulating a moderate drought scenario. Meanwhile the third group of columns were exposed to extreme drought simulations, with 12 days of drying after 1 day of flooding [20].
The experiment was continued for a period of seven weeks. Soil samples from each of the experimental groups were collected on days 0, 17, 33, and 49 for further analysis (sampling was conducted after reflooding for one day before each drying period). Each soil column was set up with three replicates. To ensure the uniformity of the samples, after the experiment, each layer (upper, middle, and lower) of the soil column were evenly mixed to form a single sample. The collected soil samples were divided into the following two parts: one part of the samples was stored at 4 °C for measuring the soil physicochemical properties, and the other was stored at −80 °C to extract DNA for high throughput sequencing analysis.

2.3. Physicochemical Analysis of Soil

The pH of the soil samples was determined in a soil–water suspension (1:2.5) using a multifunctional water quality analyzer (Elemental Analyzer System Vario MACRO cube, Langenselbold, Germany). Total nitrogen (TN) and carbon (TC) were determined using an elemental analyzer. Dissolved organic carbon (DOC) was determined following the extraction of samples in soil columns and the analysis of the resultant extract in a total organic carbon analyzer (TOC-L; 5000TOC, Kyoto, Japan). Ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3−-N) were determined with a SmartChem200 (Skalar 5000, Breda, The Netherlands), using the indophenol blue colorimetric method and the azo staining colorimetric method, respectively.
We referred to the improved P classification scheme by Hedley et al. [21], using NaHCO3 (0.5 mol·L−1), NaOH (0.5 mol·L−1), and HCl (0.5 mol·L−1) solutions as extractants. The P in the soil can be classified as follows: L-Pi, L-Po, Fe/Al-P, Hu-P, Ca/Mg-P, Ml-Po, and Re-P. Based on plant mobility in soil sediments and solutions, the P fractions in soil can be classified into available P (Pavail), moderately available P (PMA), and unavailable P (PUA) [22]. Available P was extracted by NaHCO3 and included orthophosphates, phosphate, monophosphatase, and DNA, which are mobile in soil and can be directly absorbed and utilized by plants. Moderately available P was extracted by NaOH and included Fe/Al-P and Hu-P, which have a high mobility in the soil and convert to Pavail, increasing the ecological risk of soil P. Unavailable P included HCl-P and Residual P (Re-P). The specific operation process is shown in Figure 2.
After the extract reached equilibrium, we placed the centrifuge tubes in a centrifuge (SIGMA318K) and centrifuged at a constant temperature (25 °C) for 10 min (6000 r·min−1). We passed the supernatant through 0.45 μm filter membrane. We took an appropriate amount of the filtrate to determine the orthophosphate concentration (Pi) (adjusted to about pH = 3). Then, the contents of L-Pi (NaHCO3-Pi), Fe/Al-P (NaOH-Pi), and Ca/Mg-P (HCl-Pi) in the soil could be obtained. Meanwhile, the total phosphorus (TP) content in the filtrate was determined by the high-temperature digestion method of H2SO4-K2S2O8. The differences between the two were the L-Po (NaHCO3-Po), Hu-P (NaOH-Po), and Ml-Po (HCl-Po) contents of the soil. Re-P was determined by the H2SO4-HClO4 oxidation-molybdenum antimony colorimetric method.

2.4. Analyses of Microbial

To determine changes in the soil bacterial composition during the DRW process, DNA was extracted from freeze-dried soil samples (stored at −80 °C) using the SDS method for DNA extraction and the extracted bacterial DNA was detected by 1% agarose gel electrophoresis. The forward and reverse primers 338F (50-ACTCCTACGGGAGGCAGCA-30) and 806R (50-GGACTACHVGGGTWTCTAAT-30) were used to perform PCR amplification of the V3−V4 region of all 16S rRNA of bacteria. The amplification cycle was performed 30 times.
The amplification products were detected by 2% concentration agarose gel electrophoresis, and the PCR products were purified using the AxyPrep DNA gel recovery kit (Axygen Company, Corning, New York, NY, USA). The sequencing of purified amplicons was conducted in a paired-end format with the Illumina MiSeq platform. The sequences were clustered under operational taxonomic units (OTUs) based on 97% similarity using UPARSE software and underwent chimera and single-sequence filtering. The taxonomy of each sequence was analyzed by RDP Classifier, then compared with the Silva 16S rRNA database, and the confidence threshold was set to 70% (Novozymes Technology Co., Ltd. Tianjin, China).

2.5. Statistical Analysis

In this study, soil physicochemical properties and P fraction data were calculated and processed using Microsoft Office Excel 2019. To determine statistical significance, one-way analysis of variance (ANOVA) tests were carried out with Tukey’s Honestly Significant Difference post hoc testing in SPSS 27.0. The significance level was set at p < 0.05. The drawing was conducted using Origin 2023b. Mothur and SILVA138’s SSUrRNA databases were used for species annotation analysis (set threshold of 0.8~1), and Qiime (Version 1.9.1) was used to calculate the Alpha diversity indices of samples such as observed species and Shannon index. Based on the Pearson correlation index, heat maps between bacterial species at the phylum level and different forms of P were fabricated by applying Origin 2023b, and redundancy analysis was performed using Canoco 5.0.

3. Results

3.1. Effect of DRW on Chemical Properties of Soil

The impacts of varying intensities and frequencies of DRW significantly affected the chemical attributes of the soil (Figure 3). The pH in all the three groups marginally dropped at all DRW levels. With an increased duration of DRW treatment, the pH value showed an overall downward trend under normal DRW and moderate DRW conditions, while the pH value dropped sharply on day 17 in the extreme DRW treatment, indicating that the stress effect of extreme drought led to more severe soil acidification. Soil DOC significantly decreased after 17 days of DRW conditions, and the decrease in soil DOC content was the greatest under the extreme drought treatment.
The soil NH4+-N content recorded an increase 17 days before being exposed to DRW conditions, then a slight decrease from 17 days to 33 days after the alternation, and a significant and substantial increase after 33 days of the alternation. There was no significant change in soil NO3−-N content before or after the DRW treatment. Overall, extreme drought significantly affected pH and DOC. NH4+-N showed significant variations under the moderate drought treatment.

3.2. Effect of DRW on Soil P Fractions

DRW treatment led to changes in the distribution of P accumulation fractions across all levels in all soil samples (Figure 4a–c), however, the observed trends were inconsistent. The effects of normal and extreme DRW conditions on P mobility exhibited similar trends in soil samples on day 49. Soil Pavail content decreased by 10% and 6%, while the PMA content increased by 7% and 4% and the PMA content increased by 3% and 2%, respectively, in both the DRW conditions. Contrarily, under moderate DRW conditions, no significant change was recorded in all P fractions after day 49. Consequently, the associated risk of P movement remained negligible in moderate DRW conditions.
DRW events had a certain degree of influence on soil P mineralization (Figure 4d–f), but the change in soil P mineralization was inconsistent with the increase in the frequency of DRW. On day 33, the concentration of Piorg was reduced by 7% and 6% in normal and moderate DRW conditions, respectively, with a proportional increase in Porg. Conversely, under extreme DRW conditions, no significant difference was observed in P mineralization. However, at the end of the experiment (day 33–day 49), Piorg and Porg levels were restored to their initial concentrations, irrespective of the DRW intensity.

3.3. Effect of DRW on Soil Microbial Community

3.3.1. Changes in Soil Bacterial Composition

As shown in Figure 5, among the top 10 microorganisms with the highest relative abundance in each group under different DRW intensities and frequencies at the phylum classification level, Proteobacteria, Chloroflexi, and Acidobacteriota were recorded as the dominant phyla. The rest of the items were as follows: Verrucomicrobiota, Firmicutes, Bacteroidota, Crenarchaeota, and Actinobacteriota.
Under normal DRW, no notable difference was observed on day 17. Multiple DRW alternations changed the analogy of the bacterial community. After 49 days of DRW, the relative abundance of Proteobacteria increased by 17.6%, 9%, and 19.7% in normal rainfall conditions, moderate drought conditions, and extreme drought conditions, respectively. Chloroflexi relative abundance decreased by 17.7%, 14.3%, and 22.3%, respectively. Under moderate drought conditions, the relative abundance of Acidobacteriota increased significantly by 6.2%, while there was no significant change between normal rainfall and extreme drought conditions.

3.3.2. Changes in the Structure of Soil Bacterial Communities

In this study, observed species and Shannon indices were used to assess the richness and diversity of bacterial communities in the soil after DRW (Figure 6a,b). We found that the bacterial richness in the soil under normal rainfall, moderate drought, and extreme drought treatments significantly increased after 47 days of DRW conditions. The diversity and abundance of soil bacteria under moderate drought were higher than those under normal rainfall and extreme drought. As shown in Figure 6b, we observed that 17 days before the DRW conditions, there was no significant change in soil bacterial diversity. However, after 47 days of DRW conditions, the diversity of soil bacteria in the extreme drought treatment significantly decreased (p < 0.05).
Permutational Multivariate Analysis of Variance analysis indicated that the soil bacterial community was affected by DRW conditions (Figure 6b). In the PCA analysis, the first axis (PC1) and the second axis (PC2) explained 21.92% of the changes in the bacterial community structure. The Adonis analysis results of the bacterial community indicated that there were significant differences in the changes in soil bacterial communities between normal DRW conditions and extreme DRW conditions, suggesting that the bacterial community structure of the restored wetland soil underwent obvious changes after DRW treatment. There was no significant difference in soil bacteria in the moderate DRW condition, indicating that the difference in the effect of moderate DRW conditions on soil bacteria was not obvious.

4. Discussion

4.1. The Relationship Between Microbial Community Structure and P Fractions

The structure of microbial communities is intricately linked to the transformation and distribution of soil P fractions. Different microbial phyla contribute to various processes within the P cycle through specific physiological and metabolic pathways. The Pearson correlation analysis between different P fractions and microbial community types at the phylum level under DRW conditions is presented in Figure 7. Proteobacteria exhibited a highly positive correlation with PMA (p < 0.01) and a significantly negative correlation with PMA (p < 0.01). Chloroflexi demonstrated a significantly negative correlation with PMA (p < 0.01) and a significantly positive correlation with PMA (p < 0.01). Acidobacteriota showed a significantly negative correlation with PMA (p < 0.05). Proteobacteria, Chloroflexi, and Acidobacteriota are actively involved in the soil C, N, and P cycles, playing crucial roles in organic matter decomposition and nutrient cycling [23]. In this study, following 49 days of normal rainfall and extreme drought treatments, the soil PMA content increased. This may be attributed to the direct impact of DRW on microbial community structure and activity by altering soil moisture conditions and redox environments.
Proteobacteria represents one of the most dominant bacterial phyla in soil and plays a pivotal role in P fraction transformation. Under flooded and low redox potential conditions, it promotes the denitrification of certain Proteobacteria, indirectly influencing the adsorption and desorption processes of P through the reduction of Fe3+ to Fe2+ and the release of Fe-P [24]. Additionally, some Proteobacteria secrete organic acids such as citric acid, which dissolve Ca-P through acidolysis [25], thereby reducing PMA content. Meanwhile, some members of Proteobacteria carry the phoD gene and directly participate in the mineralization of Porg [26]. Chloroflexi experiences stress from DRW, and its species abundance progressively decreases with an increasing frequency of DRW. Chloroflexi is the main decomposer of recalcitrant organic matter and is difficult to degrade [27]. A decrease in its content may delay the decomposition of recalcitrant organic matter containing organophosphorus, thereby indirectly reducing the mineralization of Porg. Acidobacteriota exhibits resilience in acidic environments and Porg slowly mineralizes through low metabolic rates [28].
In this study, moderate DRW enhanced the activity of soil microorganisms and promoted microbial diversity. This may have been due to the burst growth of previously low-abundance microbial populations upon the restoration of water conditions following drought stress, enabling them to rapidly occupy vacant ecological niches in the microbial food web, thereby inducing changes in microbial community composition and soil functions. For instance, the OTU abundances of α-Proteobacteria in grassland ecosystems and Gemmatimonas in farmland ecosystems rapidly increased after DRW, becoming dominant populations in the microbial community [29,30]. Consequently, distinct adaptive strategies by various microbial groups evolving in response to water condition changes directly influence the dynamics of various P fractions in the soil and play a critical role in the transformation of soil P fractions.

4.2. Effect of DRW on Soil P Mineralization

In the restored wetland soil of this experiment, Piorg accounted for a large proportion (about 75%), and the transformation of Piorg to Porg fluctuated with changes in water conditions. When DRW frequency increased during the experiment’s initial stage, soil Porg content initially increased and subsequently stimulated soil P mineralization. After 49 days, P mineralization tended to level off, indicating that Porg and Piorg reached a balanced state, which is closely related to the mineralization fixation process mediated by microorganisms and chemical adsorption–desorption reactions [31]. Microorganisms can absorb and utilize Piorg in the environment, reducing its content in the environment. During the process of DRW, some microorganisms die and release Porg. The surviving microorganisms will accelerate the mineralization of P, while soil minerals will adsorb Piorg [32]. When the concentration of Piorg drops below the requirement for microbial growth, the P adsorbed by minerals will desorb and be released [33]. Consequently, when the rates of Piorg absorption by surviving microorganisms and the release of Porg by dead microorganisms are similar, and the desorption and adsorption rates of P by minerals are also similar, the mineralization process tends to reach a stable state.
The conversion of Piorg and Porg was dominated by the hydrological conditions of wetlands. Porg was more stable under flooding conditions, and soil oxidation accelerated the mineralization of Porg under drought conditions. While Piorg is more stable under drought conditions and stimulates the release of mineral-binding P during floods [34], Novair et al. [35] indicated that the alternating process of soil dryness and wetness enhances the activity of alkaline phosphatase and inorganic pyrophosphatase, while reducing the content of Porg. This suggests that an increase in enzyme activity can promote the mineralization of Porg [36]. This necessitates bacterial and fungal activity to mineralize and immobilize additional Porg [29]. It also explains that under normal moisture conditions, Piorg gradually increases as DRW proceeds (Figure 4d). Following periods of flood stress, surviving microorganisms can absorb and utilize dissolved Piorg to convert it into their own Porg [37]. This is consistent with the research of other scholars. Alori et al. [38] suggested that degrading bacteria (such as Proteobacteria) dissolve Piorg through secreting organic acids and converting it into cellular components (such as nucleic acids and phospholipids), thereby increasing Porg components. Meanwhile, changes in osmotic pressure can lead to the rupture and dissolution of certain microbial cells during rehydration, resulting in the release of microbial P and an increase in Porg content [18]. Helfenstein et al. [39] conducted a study which indicated that after DRW alternation, the turnover rate of microorganisms increased by 30%, and the Porg released from dead organisms accounted for 10% of the increase in Porg.
However, different conclusions have been drawn regarding alterations in the soil P pattern during drought. The content of Porg initially increased and then decreased under the moderate drought condition, which might be attributed to the fact that organic matter and metal ions formed a Porg–heavy metal–organic matter complex in the early stage of the experiment. This complex inhibited the enzymatic hydrolysis of organic matter and, thus, promoted the accumulation of Porg in the soil [40]. Long-term drought will disrupt aggregate structures, resulting in the dispersion of the soil organic matter and the release of Piorg adsorbated on organic matter. This explains the decrease in Piorg (Figure 4e).

4.3. Effect of DRW on Soil P Migration

The movement of soil P can be affected by the physical and chemical properties of soil. The alternating process of DRW can affect the physical and chemical properties of soil [41]. Under normal rainfall and extreme drought conditions, an increase in DRW frequency leads to an increase in PMA content. The reason for this change may be related to the decrease in pH due to drought treatment (Figure 3a). The decomposition and precipitation of soil Fe/Al-P are susceptible to the influence of environmental factors such as soil pH and redox potential [42]. This is manifested as follows: when the pH value is less than 7, a higher amount of iron and aluminum oxides can combine with P, resulting in an increase in Fe/Al-P content [43], which is consistent with our research results. Zhang et al. [44] found a similar phenomenon over four years of drought treatment in temperate forests, where a decrease in pH drove the dissolution of P in calcium phosphate, while the amount of P bound to secondary minerals (Fe/Al oxides) also increased. Under acidic conditions, when the pH value decreases, the bound Piorg will transform into free phosphate ions and diffuse into the interstitial water for migration [45]. Furthermore, changes in water levels have a potential impact on the migration of P. Previous studies have found that as the Eh potential increases, the Fe2+ in bound Fe-P undergoes oxidation reactions and partially or completely transforms into Fe3+ precipitates. Conversely, through reduction processes, the solubility of iron minerals is increased, activating and releasing the Fe-P in the sediment [46]. After drying and then flooding, the soil redox potential is significantly reduced [47]. Therefore, under the conditions of this study, this may lead to the reduction of Fe3+ to Fe2+, promoting the release of P from Fe-P, weakening the soil’s P fixation capacity and increasing the release of P in the soil [48].
In this study, the DOC content gradually decreased as the frequency of DRW increased (Figure 3b). This might have been related to decomposition and consumption by soil microorganisms, thereby reducing the competition between DOC and phosphate ions (PO43−) for the adsorption sites on the surface of Fe3+/Al3+ oxide, resulting in more Pi being fixed as Fe-P or Al-P [49]. The negatively charged functional groups (such as carboxyl group, phenol, etc.) in organic matter can interact with positively charged minerals (such as iron oxide) to change the adsorption of P [50]. Additionally, the process of soil changing from wet to dry conditions is conducive to the massive reproduction of microorganisms [51], enhancing the nitrogen mineralization process, thereby increasing the NH4+-N content in the soil (Figure 3c). The nitrification of ammonia nitrogen may release H+, enhancing the solubility of iron ions and aluminum ions, fixing more Piorg [52], but some studies have indicated that in anaerobic conditions of wetlands, NH4+ accumulation often accompanies the decomposition of organic matter, providing electron donors to accelerate the reduction of Fe3+ to Fe2+, causing the dissolution of Fe-P and the release of soluble phosphate [53], and increasing the ecological risk of PMA.
DRW alternation of submerged soil is conducive to the release of P. Reddy et al. [54] posited that the longer the sediment remains dry, the higher the possibility of oxidation and mineralization of the sediment, and the faster the release rate after rewetting. The study results explain that soil P mobility was reduced by promoting the transformation of Pavail to PMA and PUA with an increase in drought frequency. However, since Fe-P is regarded as the most readily mobile and transformable Piorg [47], close attention should be paid to changes in soil P fractions and the risk of its release by DRW.

4.4. The Critical Factors of P Fractions in the Soil

In order to study the effects of soil chemical properties and microbial communities on the changes in soil P fractions, we used redundancy analysis to reveal that the contribution of soil bio-chemical parameters [including pH, DOC, SOC, NH4+-N, and Bacterial diversity (Shannon index)] to dynamic changes in P fractions, SOC, and bacterial diversity explained 67.7% and 18.4% of the change in P fractions (Table 2), respectively. This is smilar to the results of Hedley’s study [55].
The dynamic change in SOC is closely related to the participation of microorganisms in P cycling. In this study, DRW conditions significantly altered the distribution of soil P fractions, and the composition of soil P components changed significantly. After 49 days of DRW conditions, the percentages of soil PMA and PUA increased by 7% and 3%, respectively, under normal rainfall conditions. Under moderate drought conditions, the content of soil Porg increased. On the one hand, DRW conditions led to a series of physical and chemical changes in SOC. The large surface area of organic matter makes it easy to adsorb P, which is released after decomposition by microorganisms [50]. In addition, as an active component of organic carbon, DOC can compete with phosphorus on the surface of soil colloids for adsorption and further stimulate a change in P fractions [56]. On the other hand, it is generally believed that DRW conditions accelerate the fragmentation and disintegration of larger aggregates in the soil, during which the P that was previously in the aggregates or bound to humus is exposed [57]. At the same time, an increase in soil P fractions is related to the release of P from the bodies of microorganisms after their death under DRW conditions [32]. DRW events affect the abundance and composition characteristics of soil microorganisms. The extent of this effect generally depends on the number of DRW cycles [40]. In the early stages of the experiment, this process leads to the expansion of microorganisms, increasing their abundance and diversity, which explains the increased Piorg (Figure 4b). However, the Porg is converted to Piorg as the degree of drought increases, because drought may kill soil microorganisms [58], resulting in the release of P-rich cellular fractions, promoting soil P mineralization and migration.
Although the current measures for protecting wetland ecosystems have become increasingly sophisticated and the wetland environment has been effectively restored and improved [59], environmental issues such as endogenous pollution and water body eutrophication still cannot be ignored. In the early stage of wetland restoration, accumulated P from fertilizer will cause new biogeochemical cycles. The return of plant litter will lead to a slow increase in organic matter. In this study, it was found that DRW promoted the fixation of soil P and reduced its biological availability. However, the accumulation of insoluble P may form a stable P pool, affecting the future P cycle. Moreover, the accumulation of insoluble P may be reactivated in future extreme hydrological events [60]. In particular, in reducing conditions (such as wetland restoration), the reduction of Fe3+ to Fe2+ may lead to the release of adsorbed P, affecting the future P cycle [32]. With global warming, the regional temperature rises, and the DRW process in wetlands becomes more active, possibly exacerbating this process [61]. Phosphate is one of the main factors causing water body eutrophication. Therefore, it is necessary to pay attention to the issue of restoring the P migration and transformation in wetland soil and closely monitor and evaluate the short-term retention and long-term release of P in restored wetlands.
This study systematically evaluated the impact of DRW conditions on the transformation and mobility of soil P fractions during the initial stage of wetland restoration. However, there are still several limitations that need to be addressed. In the data analysis method, we used the one-way analysis of variance to illustrate the differences among the treatments. In subsequent studies, we will enhance field verification and adopt repeated measures analysis of variance or mixed-effects models to evaluate the dynamic effects of DRW treatments on soil properties. This study did not monitor the dynamic changes in key enzymes such as phosphatases, nucleases, and the microbial P, which, to some extent, limits a comprehensive understanding of the P biogeochemical process. The breakthrough of these limitations will help establish more complete wetland P cycling models and provide more scientific theoretical basis for the ecological restoration of degraded wetlands.

5. Conclusions

This study focused on determining the impact of DRW alternation on P fractions from the perspectives of soil chemical properties and microbial community changes. The process of DRW contributed to the conversion of labile Piorg to Porg and promoted the conversion of Pavail to PMA under normal rainfall and extreme drought conditions, thus decreasing the movement risk of soil Pavail and increasing the movement risk of soil PMA. Normal and extreme drought conditions resulted in decreases in bacterial diversity. Despite the varying levels of DRW alternation, the overall composition of the bacterial community remained relatively stable. SOC and bacterial diversity were the main explanations for the changes in P fractions. Therefore, close monitoring and release risk assessments of P fractions in restored wetland are needed.

Author Contributions

M.R.: writing—original draft, formal analysis, data curation, methodology, software, validation. Y.L.: resources, data curation, investigation, methodology, software. L.W.: conceptualization, funding acquisition, supervision, writing—review and editing. N.H.: conceptualization, writing—review and editing. B.B.: software. J.Z.: investigation. Y.R.: writing—review and editing, Funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (42401458 and 41771505).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

We are grateful to the managing editors and the anonymous reviewers who provided insightful comments and suggestions.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
DRWDrying and rewetting e
PPhosphorus
CCarbon
NNitrogen
DOCDissolved organic carbon
NH4+-NAmmonium nitrogen
NO3−-NNitrate nitrogen
TPTotal phosphorus
SOCSoil organic carbon
PavaiAvailable phosphorus
PMAModerately available phosphorus
PUAUnavailable phosphorus
PorgOrganic phosphorus
PiorgInorganic phosphorus
PVCPolyvinyl chloride

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Figure 1. Schematic diagram of the drying–rewetting experiment. Red arrows indicate sampling times of days 0, 17, 33, and 49.
Figure 1. Schematic diagram of the drying–rewetting experiment. Red arrows indicate sampling times of days 0, 17, 33, and 49.
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Figure 2. Scheme of phosphorus fractionation procedure. Note: TP, Total phosphorus; Pi, Inorganic phosphorus; L-Pi, Available inorganic phosphorus; L-Po, Available organic phosphorus; Fe/Al-P, Iron–aluminum-bound phosphorus; Hu-P, Humic acid phosphate; Ca/Mg-P, Calcium–Magnesium-bound phosphorus; Ml-Po, Organic phosphorus extracted with HCl; Re-P, Residual phosphorus.
Figure 2. Scheme of phosphorus fractionation procedure. Note: TP, Total phosphorus; Pi, Inorganic phosphorus; L-Pi, Available inorganic phosphorus; L-Po, Available organic phosphorus; Fe/Al-P, Iron–aluminum-bound phosphorus; Hu-P, Humic acid phosphate; Ca/Mg-P, Calcium–Magnesium-bound phosphorus; Ml-Po, Organic phosphorus extracted with HCl; Re-P, Residual phosphorus.
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Figure 3. Changes in chemical properties of soil under simulated DRW. (a) pH; (b) DOC, Dissolved organic carbon; (c) NH4+-N, Ammonium nitrogen; and (d) NO3−-N, Nitrate nitrogen. Error bars represent the standard deviation (SD). The zigzags with different colors represent different drought intensity treatments.
Figure 3. Changes in chemical properties of soil under simulated DRW. (a) pH; (b) DOC, Dissolved organic carbon; (c) NH4+-N, Ammonium nitrogen; and (d) NO3−-N, Nitrate nitrogen. Error bars represent the standard deviation (SD). The zigzags with different colors represent different drought intensity treatments.
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Figure 4. Effect of DRW on P (a–c) mobility and (d–f) mineralization under different treatments. (a) Effect of DRW on P mobility under normal rainfall condition; (b) effect of DRW on P mobility under moderate drought condition; (c) effect of DRW on P mobility under extreme drought condition; (d) effect of DRW on P mineralization under normal rainfall condition; (e) effect of DRW on P mineralization under moderate drought condition; (f) effect of DRW on P mineralization under extreme drought condition.
Figure 4. Effect of DRW on P (a–c) mobility and (d–f) mineralization under different treatments. (a) Effect of DRW on P mobility under normal rainfall condition; (b) effect of DRW on P mobility under moderate drought condition; (c) effect of DRW on P mobility under extreme drought condition; (d) effect of DRW on P mineralization under normal rainfall condition; (e) effect of DRW on P mineralization under moderate drought condition; (f) effect of DRW on P mineralization under extreme drought condition.
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Figure 5. The relative abundance of soil bacterial phyla species under different treatments. The numbers in the grey squares represent the number of sampling days. In the vertical coordinate, CK stands for initial soil sample, A represents the normal rainfall condition, B represents the moderate drought condition, C represents the extreme drought condition.
Figure 5. The relative abundance of soil bacterial phyla species under different treatments. The numbers in the grey squares represent the number of sampling days. In the vertical coordinate, CK stands for initial soil sample, A represents the normal rainfall condition, B represents the moderate drought condition, C represents the extreme drought condition.
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Figure 6. The changes in soil (a) richness and (b) diversity. (c) Changes in the structure of soil bacterial communities. One-way ANOVA was performed with Tukey’s HSD post hoc test to determine the statistical differences among different treatments for multiple comparisons (** p < 0.01; * p < 0.05). The variations explained by PC1 and PC2 are shown as percentage values along the axes. Different graphics represent different treatments. CK stands for initial soil sample; A represents the normal rainfall condition; B represents the moderate drought condition; C represents the extreme drought condition.
Figure 6. The changes in soil (a) richness and (b) diversity. (c) Changes in the structure of soil bacterial communities. One-way ANOVA was performed with Tukey’s HSD post hoc test to determine the statistical differences among different treatments for multiple comparisons (** p < 0.01; * p < 0.05). The variations explained by PC1 and PC2 are shown as percentage values along the axes. Different graphics represent different treatments. CK stands for initial soil sample; A represents the normal rainfall condition; B represents the moderate drought condition; C represents the extreme drought condition.
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Figure 7. Relationship diagram between soil P fractions and microorganisms at the phylum level (* p < 0.05, ** p < 0.01). Correlation coefficient is indicated by the color gradient, Positive and negative correlations are indicated by the blue and red, respectively. Pavai, Available phosphorus; PMA, Moderately available phosphorus; PUA, unavailable phosphorus; Porg, Organic phosphorus; Piorg, Inorganic phosphorus.
Figure 7. Relationship diagram between soil P fractions and microorganisms at the phylum level (* p < 0.05, ** p < 0.01). Correlation coefficient is indicated by the color gradient, Positive and negative correlations are indicated by the blue and red, respectively. Pavai, Available phosphorus; PMA, Moderately available phosphorus; PUA, unavailable phosphorus; Porg, Organic phosphorus; Piorg, Inorganic phosphorus.
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Table 1. Physical and chemical properties of the soil.
Table 1. Physical and chemical properties of the soil.
IndexValue
pH6.54 ± 0.14
Dissolved organic carbona (DOC)303.75 ± 8.40 mg/kg
Soil organic carbon (SOC)73.94 ± 0.51 g/kg
Total nitrogen (TN)7.70 ± 0.12 g/kg
Total phosphorus (TP)1.12 ± 0.003 g/kg
Ammonia nitrogen (NH4+-N)11.45 ± 2.07 mg/kg
Nitrate nitrogen (NO3−-N)24.73 ± 1.80 mg/kg
Note: Standard deviations represent standard errors for the corresponding mean values (n = 3).
Table 2. Main factors for P fraction changes in DRW.
Table 2. Main factors for P fraction changes in DRW.
NameExplains %Total Explanation %p
SOC67.7 **67.70.002 **
Shannon index18.4 *86.10.034 *
pH11.397.40.230
NH4+−N1.598.90.588
DOC1.11000.614
Note: An asterisk indicates statistical significance (* p < 0.05, ** p< 0.01), SOC, Soil organic carbon; Shannon index respects Bacterial diversity; NH4+-N, Ammonium nitrogen; DOC, Dissolved organic carbon.
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Ren, M.; Li, Y.; Wang, L.; Hussain, N.; Bai, B.; Zhou, J.; Ren, Y. Effect of Drying–Rewetting Alternation on Phosphorus Fractions in Restored Wetland. Agriculture 2025, 15, 1720. https://doi.org/10.3390/agriculture15161720

AMA Style

Ren M, Li Y, Wang L, Hussain N, Bai B, Zhou J, Ren Y. Effect of Drying–Rewetting Alternation on Phosphorus Fractions in Restored Wetland. Agriculture. 2025; 15(16):1720. https://doi.org/10.3390/agriculture15161720

Chicago/Turabian Style

Ren, Mingyue, Yingxin Li, Lixia Wang, Naseer Hussain, Bing Bai, Jie Zhou, and Yongxing Ren. 2025. "Effect of Drying–Rewetting Alternation on Phosphorus Fractions in Restored Wetland" Agriculture 15, no. 16: 1720. https://doi.org/10.3390/agriculture15161720

APA Style

Ren, M., Li, Y., Wang, L., Hussain, N., Bai, B., Zhou, J., & Ren, Y. (2025). Effect of Drying–Rewetting Alternation on Phosphorus Fractions in Restored Wetland. Agriculture, 15(16), 1720. https://doi.org/10.3390/agriculture15161720

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