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Article

Effect of Flat Planting Without Film Mulching and Phosphorus Fertilization on Soil Phosphorus Dynamics and Nutrient Uptake in Faba Bean in Alpine Cropping Systems

by
Weidi Zhou
1,
Qiuyun Xu
1,
Man Su
1,
Chenglong Han
2 and
Yanjie Gu
1,*
1
College of Agriculture and Animal Husbandry, Qinghai University, Xining 810016, China
2
State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xining 810016, China
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(9), 2037; https://doi.org/10.3390/agronomy15092037 (registering DOI)
Submission received: 26 June 2025 / Revised: 7 August 2025 / Accepted: 19 August 2025 / Published: 25 August 2025
(This article belongs to the Section Soil and Plant Nutrition)
Browse Figures
Versions Notes

Abstract

Rational agronomic practice enhances crop productivity and resource use efficiency. Plastic film mulching and phosphorus (P) fertilization are widely applied in alpine agriculture to improve soil water content, temperature, and P availability. However, their effects on soil P transformation and nutrient uptake in faba bean (Vicia faba L.) remain unclear. This study conducted a field experiment to explore the effects of mulching methods and P levels on soil P fractions and nitrogen (N), P uptake in faba bean. The experiment followed a randomized block design with three film mulching treatments—no-mulching with flat planting (NMF), double ridges and furrows mulched with one film (DRM), and three ridges and furrows mulched with one film (TRM) and three P levels—P0 (0 kg P ha−1), P1 (9.10 kg P ha−1), and P2 (18.2 kg P ha−1). The results showed that soil medium- and highly active P increased, while low-active P decreased with increasing P levels. Compared with DRM and TRM, NMF had lower low-active P and higher medium- and highly active P, particularly under P2. These changes contributed to increases in soil total P and available P. The aboveground N, P uptake and N/P ratio under NMF were significantly higher than under DRM and TRM. As P levels increased, the aboveground N, P uptake and N/P ratio increased in NMF and DRM, but decreased in TRM. In all treatments, the aboveground N/P ratio was below 14, indicating N limitation. NMF, especially with P2, alleviated N limitation to faba bean growth. Overall, NMF combined with about 18.2 kg P ha−1 P fertilizer is a sustainable practice for faba bean cultivation in alpine regions. However, attention should be paid to achieving a balanced supply of N and P fertilizers.

1. Introduction

Phosphorus (P) is an essential macronutrient for plant growth, playing crucial roles in photosynthesis, nucleic acid synthesis, energy metabolism, and signal transduction, etc. [1]. However, average soil P availability is low globally, primarily due to the P fixation by soil minerals, which limits the amount of P that plants can directly absorb [2]. To sustain crop yields, P fertilizers are commonly applied to ensure plant growth. However, the utilization efficiency of P fertilizers applied to soils typically ranges from 10% to 30%, with the majority being fixed and transformed in insoluble forms, reducing its bioavailability [3]. Long-term excessive P application leads to P accumulation in soil, aggravating environmental pollutions such as water eutrophication and soil acidification, and negatively impacting the agricultural ecosystems stability [4]. Therefore, improving the effective transformation of soil P and optimizing agronomic practices are crucial for saving P resource and promoting sustainable agricultural development.
In recent years, plastic film mulching has become widely used in alpine agriculture due to its significant role in improving soil moisture and temperature conditions and regulating crop growth environment [5,6]. The effect of film mulching on soil P transformation is mainly reflected in two aspects, namely soil microenvironment regulation and microbial mediation. On the one hand, film mulching enhances soil temperature and moisture, creating an environment which is favorable to soil microbial and enzyme activities, thereby promoting soil organic P mineralization and inorganic P release [7,8]. Additionally, reduced soil pH due to film covering [9] may promote the dissolution and release of inorganic P, particularly insoluble P forms that combine with metal ions like Ca, which can undergo partial desorption under film covering conditions, thus increasing soil available P concentration [10]. On the other hand, film mulching influences soil aeration and redox conditions, regulating soil microbial community structure and enzyme activity related to soil P cycle (such as phosphatases), which promotes low-active P transform into medium- and highly active P [11]. Previous studies have shown that soil highly active P fractions (Resin-P and NaHCO3-Pi (NaHCO3-extracted inorganic P)) significantly promote P uptake and plant growth, while medium-active P fractions (NaOH-Pi (NaOH-extracted inorganic P) and NaOH-Po (NaOH-extracted organic P)) are constrained by soil microbial activity and associated enzymatic processes [12]. For instance, Zhu et al. [12] found that in nitrogen-fixing plants soil, promoting soil P fractions significantly increased soil P availability, especially increasing plant improvable P, while decreasing non-labile P content. This effect was attributed to enhanced microbial and alkaline phosphatase activity in soil. However, the effect of film mulching on soil P fraction content and transformation has not been systematically clarified, especially in alpine agricultural areas.
To enhance crop nutrient use efficiency and ensure the sustainable management of P resources, exploring soil P transformation processes have become particularly crucial. Soil P exists in multiple chemical forms, and its various fractions, with different activity levels, can transform into each other, and under certain conditions can participate in the process of plant uptake [13]. P application is an important factor to affect soil P fraction composition. For example, it can significantly increase soil highly active P and promote transformation from medium- and low-active P, thus improving soil P bioavailability, which benefits maize and soybean growth [13]. In terms of soil P fertility, although total phosphorus (TP) is an important indicator for evaluating soil P reserves, it cannot reflect P availability [14]. During crop growth periods, available phosphorus (AP) can more directly reflect the available P level that crops can access [15]. Soil AP transformed from TP is the main P sources for plants, and the extent of this transformation process largely determines soil P supply capacity [13]. However, AP is not a fixed source; it depends not only on direct fertilizer input but also on continuous transformation from medium- and low-active P, driven by microbial and enzyme activity [16]. Therefore, AP accumulation is largely limited by the degree and direction of transformations among different soil active P fractions [15]. In contrast, TP variation mainly depends on long-term P input and dynamic balance among P fractions [14]. On this basis, investigating transformations among different soil active P fractions and relationships between P fractions, TP and AP under P fertilization can provide a guidance for reasonable fertilization in faba bean production. Moreover, interactions between P fertilization and film mulching may have a complex effect on the transformation among different active P fractions, due to the changes in soil environmental conditions and management practices.
Faba bean (Vicia faba L.) is one of the most widely cultivated leguminous crops worldwide, characterized by strong resistance, making it particularly well adapted to the growing conditions of cold areas [17]. As a crop for both food and forage values, faba bean not only plays a crucial role in ensuring food security, but can also achieve biological N fixation by forming a symbiotic relationship with rhizobia, thereby enhancing soil N supply and reducing dependence on chemical fertilizer [18,19]. However, compared with its self-regulating ability for N, faba bean’s demand for P is more dependent on external inputs. Adequate P is not only necessary for the N fixation process, but also directly affects the biomass accumulation and grain yield of faba bean [19]. In agricultural production, the plant N/P ratio is often used to assess crop nutrient status [20]. This index reflects the balance between N and P uptake and allocation within the plant, and can be used as an effective test method for determining whether the plant is experiencing N or P limitation [20]. When the N/P ratio is higher than 16, it usually indicates that the plant may be limited by P, while a value below 14 may indicate insufficient N supply [21]. Therefore, exploring the contribution of the soil P fraction transformation to P uptake in faba bean, combined with the dynamic changes in N and P uptake and the N/P ratio, can not only evaluate the effect of soil P availability on faba bean growth, but also offer important guidance for optimizing faba bean nutrient management.
Based on this, the present study examines soil P transformation and N, P uptake by faba bean under different mulching methods and P fertilization levels in alpine agricultural areas. The objectives are to explore (1) the effects of different mulching methods and P fertilization levels on various active P fraction content in soil; (2) the impacts of different mulching methods and P fertilization levels on soil TP and AP; and (3) the relationships between soil P fractions and N, P uptake in the aboveground of faba bean.

2. Materials and Methods

2.1. Site Description

The field experiment was conducted in Xiliangqi Village, Huangzhong County, Qinghai Province, China (36°33′ N, 101°36′ E, 2493 m above sea level). The study area has an average annual precipitation of approximately 542 mm and an average annual temperature of 5.0 °C [5]. The region is located in the eastern agricultural region of Qinghai Province, characterized by a continental plateau climate. The soil type in the experimental field is classified as chestnut soil, with a bulk density of 1.44 g cm−3, pH 8.01, organic carbon content of 10.2 g kg−1, total nitrogen content of 1.02 g kg−1, total phosphorus content of 0.77 g kg−1, and available phosphorus content of 4.51 mg kg−1. Bulk density was measured using the core method, by collecting undisturbed soil cores (100 cm3) and drying them at 105 °C to constant weight using a drying oven (DHG-9420B, Shanghai Langxuan Experimental Equipment Co., Ltd., Shanghai, China). Soil pH was determined in a 1:2.5 soil-to-water suspension using a PHS-3E pH meter (INESA Scientific Instrument Co., Ltd., Shanghai, China) [22]. Organic carbon was measured by the Walkley–Black dichromate oxidation method [5]. Total nitrogen was analyzed using the Kjeldahl digestion method [5]. Total phosphorus was measured by the molybdenum–antimony anti-colorimetric method after HClO4-H2SO4 digestion [23]. All digestions were performed using a digestion furnace (SKD-20S2, Shanghai PEO Analytical Instrument Co., Ltd., Shanghai, China). N analysis was conducted using an automatic Kjeldahl nitrogen analyzer (K9840, Haineng Future Technology Group Co., Ltd., Jinan, China), and P was analyzed using a multifunctional microplate reader (Model 3020, Thermo Fisher Scientific, Vantaa, Finland). Soil available phosphorus was determined using the Olsen-P method [23].

2.2. Experimental Design and Field Management

From 2020 to 2023, a field experiment was conducted using a randomized block design with faba bean (Vicia faba L.) cultivar “Qinghai 13”. The study included three mulching treatments [no-film mulching with flat planting (NMF), double ridges and furrows mulched with one plastic film (DRM), and three ridges and furrows mulched with one plastic film (TRM)], and three P levels [0 kg P ha−1 (P0), 9.1 kg P ha−1 (P1), and 18.2 kg P ha−1 (P2)]. The second P level (P1) represents the typical P fertilization rate for local faba bean production, while the third P level (P2) was designed to explore the optimal P fertilization rate for faba bean. The P fertilizer used was CaP2H4O8 (P2O5 ≥ 12%), which was applied to the top 20 cm of soil in each plot using a rotary tiller in early April each year. The P content in P fertilizer is determined annually by the vanadium–molybdenum yellow colorimetric method [24]. The actual P content is shown in Table 1. No additional fertilizers were applied. For mulching treatments, transparent plastic film (0.008 mm thick, 1.2 m wide) was used to cover the ridge surfaces. Approximately one week after mulching, faba bean seeds were sown at a spacing of 15 cm within rows using a dibbler. A total of 27 plots were established, each measuring 16.7 m in length and 4.2 m in width, with three biological replicates for each treatment. Faba bean harvesting was performed in late August each year, after which the faba bean residues were removed, leaving the plastic film place on the field surface. Weeds were manually controlled, and no irrigation was applied; all water supply relied entirely on precipitation.

2.3. Sampling and Measurement

2.3.1. Aboveground N, P Uptake and N/P Ratio

During the flowering and pods formation stage of faba bean from 2021 to 2023, two rows were randomly selected for sampling, excluding the edge rows and a 1 m boundary at both ends of each row. The aboveground plant parts were harvested and oven-dried at 65 °C until a constant weight was achieved to determine the dry weight of the aboveground biomass, using a drying oven (DHG-9420B, Shanghai Langxuan Experimental Equipment Co., Ltd., Shanghai, China) [24]. The aboveground biomass per unit area was calculated on a dry matter basis. Both nitrogen and phosphorus samples were digested using a digestion furnace (SKD-20S2, Shanghai PEO Analytical Instrument Co., Ltd., Shanghai, China). The N content in faba bean was determined by the Kjeldahl N determination method using an automatic Kjeldahl N analyzer (K9840, Haineng Future Technology Group Co., Ltd., Jinan, China), while the P content was measured by the molybdenum–antimony anti-colorimetric method after digestion [25]. The aboveground N, P uptake was calculated as the product of N, P content and aboveground biomass [26]. The aboveground N/P ratio was determined as the ratio of N content to P content [20].

2.3.2. Soil Available Phosphorus and Total Phosphorus

Soil samples from the planting rows were collected during the flowering and pods formation stage of faba bean, including three randomly selected subsamples (each with a diameter of 4 cm and a depth of 20 cm). The samples were sieved through a 2 mm mesh to remove plant debris, root fragments, and stones. The soil samples were air-dried at room temperature until constant weight to measure AP, TP, and P fractions. Soil AP was determined using the Olsen-P method [23]. Soil TP was measured by colorimetry after digestion with HClO4-H2SO4 using a digestion furnace (SKD-20S2, Shanghai PEO Analytical Instrument Co., Ltd., Shanghai, China), and the absorbance was read using a multifunctional microplate reader (Model 3020, Thermo Fisher Scientific, Finland) [23].

2.3.3. Soil Phosphorus Fractions

Soil P fractions were determined using the modified Hedley P fractionation method by Tiessen and Moir [27], which divides soil P into highly active, medium-active, and low-active P components [28]. The highly active P includes Resin-P, NaHCO3-extracted inorganic P (NaHCO3-Pi), and NaHCO3-extracted organic P (NaHCO3-Po). The medium-active P consists of NaOH-extracted inorganic P (NaOH-Pi) and NaOH-extracted organic P (NaOH-Po). The low-active P includes dilute hydrochloric acid-extracted inorganic P (D.HCl-Pi), concentrated hydrochloric acid-extracted inorganic P (C.HCl-Pi), concentrated hydrochloric acid-extracted organic P (C.HCl-Po), and Residual-P. The content of each organic P fraction was calculated as the total P content in the respective extract minus the inorganic P content [28]. All P contents were determined by the molybdenum–sulfur antimony colorimetric method [25].

2.4. Statistical Analysis

A two-way analysis of variance (ANOVA) with randomized block design was performed to test the effects of mulching methods and P fertilization levels on the response variables. Significant differences were determined at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001 for all tests. The Mantel test was performed using the mantel function in the linkET package of R software (version 4.3.2) to analyze the relationships between soil P fractions and TP, AP, and aboveground N, P uptake and N/P ratio. All reported measurements are the averages of three replicates. IBM SPSS Statistics 26 (IBM Corp., USA) was used for statistical analysis, and graphs were created using Origin 9.8 (OriginLab OriginPro 2021, Northampton, MA, USA).

3. Results

3.1. Soil Phosphorus Fraction Content

The average Resin-P content from 2021 to 2023 was in the order of NMF > DRM > TRM (Figure S1a), with TRM having similar or lower soil Resin-P compared to DRM. As the P fertilizer application level increased, the Resin-P content significantly increased from 2021 to 2023, with the largest increase under P2 conditions, which was significantly higher than P0 by 85.6%. Except for 2022, the NaHCO3-Pi content in NMF treatment was consistently higher than that in DRM and TRM in 2021 and 2023 (Figure S1b), and NaHCO3-Pi content significantly increased with increasing P levels across all treatments. The NaHCO3-Po content in 2021 and 2022 was in the order of NMF > DRM > TRM (Figure S1c), with the average NaHCO3-Po content in NMF being significantly higher than DRM and TRM by 8.39% and 12.1%, respectively. As P levels increased, the NaHCO3-Po content significantly increased in 2021.
Figure S2 illustrates the effects of different mulching treatments and P fertilizer application levels on soil NaOH-Pi and NaOH-Po content from 2021 to 2023. In terms of NaOH-Pi, from 2021 to 2023, the average NaOH-Pi content in NMF treatment was in the order of NMF > DRM > TRM (Figure S2a), with TRM showing similar or higher soil NaOH-Pi than DRM, and significantly increasing with higher P levels. Under P2 treatment, the content of NaOH-Pi increased significantly, with a much larger increase compared to P0. In terms of NaOH-Po, the average content over the three years showed a significant change in TRM treatment compared to NMF and DRM (Figure S2b), with P1 and P2 treatments showing significant increases compared to P0. The NaOH-Po content significantly increased with higher P levels in 2021 and 2023.
The average content of D.HCl-Pi from 2021 to 2023 was in the order of DRM > NMF > TRM (Figure S3a), and the three-year average content shows that D.HCl-Pi significantly decreased with increasing P levels. The average content of C.HCl-Pi from 2021 to 2023 was in the order of NMF > TRM > DRM (Figure S3b), with TRM showing similar or higher soil C.HCl-Pi than DRM. P levels significantly affected the C.HCl-Pi content, and it significantly decreased with increasing P levels. The average content of C.HCl-Po in 2023 was in the order of DRM > TRM > NMF (Figure S3c), with TRM showing similar or lower soil C.HCl-Po than DRM, and NMF having the lowest C.HCl-Po content. P levels significantly affected the C.HCl-Po content, and it significantly decreased with increasing P levels. The mulching method significantly influenced the soil Residual-P content, with the ranking as DRM > NMF > TRM (Figure S3d), and NMF treatment having significantly higher or similar soil Residual-P content compared to TRM. The Residual-P content in NMF, DRM, and TRM treatments significantly decreased with increasing P levels.

3.2. Soil Phosphorus Pool

3.2.1. Soil Available Phosphorus and Total Phosphorus

From 2021 to 2023, the AP content in NMF treatment was significantly higher than in DRM and TRM (Figure 1), especially under P2 conditions, where the increase in AP content in NMF was most pronounced, while TRM treatment had lower AP content with no significant difference compared to DRM. As the P application level increased, the AP content significantly increased in all treatment groups.
The effect of mulching treatments on soil TP had little impact over the three years (Figure 2). In 2022 and 2023, the soil TP content significantly increased with higher P application levels. In 2022 and 2023, the average soil AP in NMF, DRM, and TRM for P0 and P1 treatments was 6.78% and 3.28% lower than for P0, respectively.

3.2.2. Composition of Soil Active Phosphorus Fractions

In 2021 and 2022, mulching methods significantly influenced the soil highly active P fractions (Figure 3a), with the order as NMF > TRM > DRM, and TRM and DRM having similar or higher soil highly active P fractions. From 2021 to 2023, as P levels increased, the highly active P fractions significantly increased in all treatments (NMF, DRM, and TRM). In 2022 and 2023, the soil medium-active P fractions were in the order of TRM > DRM > NMF (Figure 3b), with TRM having an average medium-active P fraction significantly higher by 49.3% and 35.3% compared to NMF and DRM, respectively. The medium-active P fractions significantly increased with increasing P levels in 2021 and 2023. In 2021 and 2022, the average soil low-active P fractions were in the order of DRM > NMF > TRM (Figure 3c), with NMF having similar or lower soil low-active P fractions compared to DRM. The low-active P fractions significantly decreased with increasing P levels in NMF, DRM, and TRM treatments.

3.2.3. The Proportion of Soil Phosphorus Fractions

From 2021 to 2023, the study area was dominated by low-active P fractions (D.HCl-Pi, C.HCl-Pi, C.HCl-Po, and Residual-P) (Figure 4), which on average accounted for 70.3% of TP. From 2021 to 2023, the average proportions of Resin-P and NaHCO3-Pi in NMF were significantly higher than those in DRM and TRM. The P1 and P2 treatments significantly increased the proportions of highly active P fractions (Resin-P and NaHCO3-Pi) while reducing the proportion of D.HCl-Pi. For the soil low-active P fractions, D.HCl-Pi was the major contributor to low-active P (45.2%), followed by C.HCl-Pi (30.8%) and NaOH-Po (24.0%). From 2021 to 2023, the proportions of NaOH-Po and C.HCl-Po in TRM were significantly higher than in NMF and DRM. The P1 and P2 treatments significantly increased the proportion of NaOH-Po, but reduced the proportion of C.HCl-Po. The NMF treatment promoted the proportion of highly active P fractions (Resin-P and NaHCO3-Pi), while TRM had a more significant effect on increasing the proportion of low-active P (C.HCl-Po). The P1 and P2 treatments significantly increased the proportions of Resin-P, NaHCO3-Pi, and NaHCO3-Po, indicating that high P levels help in the accumulation and transformation of active P. High P levels also reduced the proportions of D.HCl-Pi and C.HCl-Po, suggesting that some P fractions may have transitioned from a stable state to an active form.

3.3. Faba Bean N, P Uptake and N/P Ratio

From 2021 to 2023, the N and P uptake of faba bean in NMF treatment was significantly higher than in DRM and TRM (Figure 5a,b). The average annual N and P uptake was highest in NMF, followed by TRM and DRM. In the P0, P1, and P2 treatments, the N and P uptake in TRM was significantly higher than in DRM. In both NMF and DRM, N and P uptake increased with higher P levels, while in TRM, N and P uptake decreased. The N/P ratio in NMF was significantly higher than in DRM and TRM in all years. In the P0 and P1 treatments, TRM had a significantly higher or similar N/P ratio compared to DRM, but in the P2 treatment, TRM had a lower N/P ratio than DRM. The increase in P levels led to a higher N/P ratio in NMF and DRM in 2022, but a decrease in the N/P ratio in TRM. In all treatments, the aboveground N/P ratio was below 14, indicating N limitation in plant growth, which was effectively alleviated with the increase in P levels in 2022.

3.4. Relationships Between Aboveground N, P Uptake, Plant N/P Ratio, and Soil P Fractions

According to the Mantel test, overall, there is a significant correlation between soil P fractions and faba bean N, P uptake and N/P ratio, particularly with highly active P fractions, which show a strong relationship with faba bean N uptake and N/P ratio. In contrast, faba bean N, P uptake and N/P ratio are strongly negatively correlated with C.HCl-Po. Additionally, medium- and highly active P fractions have a strong relationship with soil TP and AP (Figure 6).

4. Discussion

4.1. Effects of P Fertilization and Mulching on Soil P Fractions

P fertilization significantly affects different soil P fraction contents and their biological availability. With increasing P application levels, soil medium- and highly active P fraction contents increased significantly, while low-activity P fraction contents decreased markedly, especially P2 (18.2 kg P ha−1). This indicates that P fertilization not only provides plants with more available P [15], but also promotes low-activity P transformation in soil to medium- and highly activity P, enhancing the availability of P in soil [11]. P transformation depends not only on physicochemical mechanisms in soil, especially the adsorption/desorption processes [22], but also is closely related to biological processes in soil [29]. In terms of biological mechanisms, soil microorganisms and plant roots participate in P activation and transformation by secreting various extracellular enzymes, especially acid and alkaline phosphatases, as well as various organic acids, which can promote Po mineralization and insoluble P mobilization, thereby increasing medium- and highly active P fraction accumulation [30,31]. Meanwhile, specific P solubilizing microorganisms can also effectively enhance low-active P bioavailability by regulating rhizosphere pH and releasing solubilizing substances [31]. With increasing P application levels, these microorganisms’ activity is also enhanced accordingly, thereby accelerating low-active P transformation into medium- and highly active P [32]. In addition, P fertilization can also affect the structure and diversity of soil microbial communities [33,34,35]. Studies have shown that long-term P application can increase the microbial group proportion with phosphatase gene functions and enhance the P cycling potential of soil systems [32]. Meanwhile, plant roots also regulate the migration and release of P in soil through rhizosphere exudation and symbiosis with arbuscular mycorrhizal fungi, further promoting medium- and highly active P fraction accumulation [35,36]. In other legume crops, solubilizing P microorganisms related to P cycle in the rhizosphere have also been discovered [35,36]. All of these microorganisms are associated with the transformation of P [36]. In addition, soil environmental conditions, particularly pH changes, play a key regulatory role in the P transformation process [37]. The soil in this study site is alkaline; P tends to combine with Ca2+ to form calcium phosphate precipitate, which limits P availability [38]. After applying P fertilizer, the soil pH may decrease, which helps reduce P fixation by calcium and releases more P available for plant uptake [37]. Meanwhile, organic acids and H+ ions secreted by plant roots and microorganisms can also lower the rhizosphere pH, further promoting P desorption and dissolution [39].
Although film mulching is generally believed to improve the crop growth environment by increasing soil temperature and moisture [40], the medium- and highly active P fraction contents in NMF treatment were higher than those in DRM and TRM, indicating that film mulching is not superior to no-mulching in enhancing P availability in the present study. This difference may be related to the excessive intervention of mulching in the soil micro-ecological environment [41]. Film mulching significantly increases soil moisture and temperature, but it may also cause excessively high soil moisture and temperature, thereby inhibiting soil microbial activity [5], especially the metabolic capacity of P solubilizing microorganisms and functional microbes related to phosphatase secretion [32], ultimately reducing the effective transformation of soil P. In contrast, although soil moisture and temperature under no-mulching are relatively low, which is closed to the natural conditions, but it is conducive to maintaining soil aeration and microbial communities stability [42]. This condition also promotes soil organic matter decomposition and extracellular enzymes secretion [43]. As a result, it facilitates a stronger transformation from low-active P into medium- and highly active P. Although the soil pH was generally higher under mulching [9], this was usually considered to be due to the fact that mulching reduced organic acid accumulation and other acidic metabolite generation, thereby limiting the release of H+ ions and weakening the soil acidification process [44]. Our present study found that soil nitrogen availability was insufficient under mulching relative to no-mulching [5]. N is an essential component for protein synthesis [45], and phosphatase is one of the key enzymes promoting phosphate mineralization, N deficiency will directly limit phosphatase synthesis and activity [46]. In addition, when plants absorb NH4+, they need to release H+ to maintain the charge balance [47]. This process not only helps plants absorb N but also creates an acidic environment in soil, promoting P desorption and transformation. However, due to the decreased soil available N content under mulching [5], this process may be inhibited, thereby limiting P conversion efficiency, especially in terms of phosphatase activity and H+ secretion, ultimately leading to the inhibition of P bioavailability. In addition, the closed environment and anaerobic conditions induced by film mulching are more likely to trigger metal ions redox reactions and induce the P refixation process [38], particularly insoluble substance formation such as calcium phosphate, iron phosphate, and manganese phosphate. In contrast, no-mulching facilitates more sufficient exchange between soil and the external environment, with better aeration. Although soil temperature and moisture are relatively low, no-mulching supports more sustained microbial metabolic activity and a more balanced release of phosphatase and H+ ions, which facilitate the continuous low-active P transformation into medium- and highly active P. This also reduces precipitation risk caused by P combination with metal ions such as Ca2+, thereby enhancing P biological availability [38,39].

4.2. Drivers of Soil TP and AP Accumulation

P fertilization has a significant impact on soil P fraction composition. Soil medium- and highly active P fractions have high bioavailability, making them the most easily absorbed forms by plant roots [11], and they also constitute the main components of soil AP [48]. In the present study, with increasing of P application levels, medium- and highly active P contents in soil increased significantly, which was consistent with the changing trend of AP. This indicates that medium- and highly active P fraction accumulation is a direct driving factor for the increase in AP [11], and P2 (18.2 kg P ha−1) is the most significant. Mantel test also indicated that all P forms within the medium- and highly active P fractions were significantly positively correlated with AP, verifying their central role in regulating soil AP [11,48]. In addition, P fertilization not only increases medium- and highly active P contents, but also promotes the dynamic transformation process among soil P fractions [48,49]. The transformation from low-active P to medium- and highly active P is a fundamental pathway for P activation and utilization [24], and also an internal process for continuously maintaining soil AP supply [48]. It is worth noting that although the transformation between P fractions does not directly change the total amount of soil TP, our study still observed that TP significantly increased with P fertilization levels, indicating that TP accumulation not only depends on external input [50], but may also be closely related to the plant uptake–return process [51,52]. After available P has been absorbed by plants, most P is returned to the soil in organic residues or mineralizable organic P form through root exudates, litterfall, and root turnover [51]. These returned substances are gradually transformed and stabilized in the soil by soil microorganisms, and may re-accumulate as TP partly in the form of difficult-to-absorb Po, Pi, or microbial residue P, especially under multi-year planting conditions [52,53].
In this study, DRM and TRM treatments promoted medium- and highly active P fraction accumulation to a certain extent, indicating that film mulching can activate low-active P partly by improving soil temperature and moisture, thereby enhancing soil P availability [54]. Affected by this, the AP content may be relatively increased under mulching. However, from the comparison results, there were significant differences in soil AP among mulching methods, and soil AP content in NMF treatment was significantly higher than that in DRM and TRM. This phenomenon indicates that although film mulching can improve soil environment conditions and promote partial P activation [24], its effect on AP formation and accumulation is inferior to NMF under natural ventilation conditions. In addition, the effect of film mulching on soil TP content is not significant. This suggests that the role of P cycling process within plant–soil system for TP accumulation is limited under film mulching, and the contribution is relatively small compared to direct addition of external P fertilization [55]. Furthermore, although different mulching methods promoted soil P fraction transformation to some extent, this transformation belongs to P redistribution among soil P fractions and does not result in a net increase in soil TP content [56,57]. Therefore, the mulching method played a certain role in regulating soil P availability, but had a relatively small impact on soil phosphorus pool expansion.

4.3. The Relationships Between Soil P Transformation and Aboveground N, P Uptake

With increasing P fertilization levels, the aboveground N, P uptake and N/P ratio of faba bean both increased under NMF and DRM treatments, whereas those showed decline trends under TRM. This difference may not be due to the change in soil P fraction content [58]. Since soil medium- and highly active P fraction contents in DRM and TRM treatments are similar, the different N, P uptake and N/P ratio might be caused by the different response mechanisms of faba bean to P fertilization in two mulching treatments. Our previous study found that soil available nitrogen (AN) significantly decreased by P application under TRM treatment, while soil microbial biomass nitrogen (MBN) significantly increased during the early growth stages of faba bean [5]. This indicates that P application significantly stimulates the growth of soil microorganisms under TRM treatment, enhances AN absorption and fixation by microorganisms, and forms a distinct “microbes-plant N competition” pattern [59]. This shows that, apart from nutrient supply itself, the competitive relationship between plants and microorganisms is also a key factor to determine crop nutrient uptake capacity and N, P balance. This competition limited soil microorganism activity and soil N release, and then affected N absorption by plants [60,61]. Therefore, even with adequate P supply, insufficient N nutrition limited faba bean growth, which resulted in the decreased N and P uptake under TRM treatment. This phenomenon may be related to the limited root space structure under the TRM. The insufficient soil N availability in TRM may restrict the expansion of roots [5,62]. This not only affects the root’s ability for P uptake but may also limit root nodule development and nitrogenase activity [63].
As a result, P plays a key role in ATP synthesis [64], which is essential for the N fixation process [65]. Root nodule development is a key for plant N acquisition [63]. However, under TRM treatment, root nodule development may be restricted, preventing the P-driven biological N fixation process, further weakening soil N availability and N, P uptake by plants [65,66]. In addition, the aboveground N/P ratios under three mulching methods were all lower than 14, indicating that the faba bean was in a N-limited status. However, the N/P ratio in NMF treatment was significantly higher than that in DRM and TRM, suggesting that no-mulching was helpful for alleviating plant N limitation [67].
Under all P fertilization levels, the N, P uptake and N/P ratio of faba bean in NMF treatment were significantly higher than those in DRM and TRM, demonstrating stronger nutrient acquisition capacity and nutritional coordination in NMF than in mulching treatments. This advantage may be closely related to the opened soil environment provided by NMF [42]. NMF treatment has better soil aeration, which facilitates root system development and expansion, and enhances soil nutrient absorption capacity [65,68]. The improvement of N availability can promote P transformation, and thereby enhance the P uptake of faba bean. Additionally, adequate oxygen supply in the root zone is conducive to rhizobia and the nitrogen fixation process, which not only increases the N uptake but also indirectly improves the P uptake [65,68]. In addition, soil medium- and highly active P fraction contents under NMF treatment were higher, which improved soil P supply capacity. Higher soil P availability effectively supported the ATP synthesis required for biological N fixation, and then strengthened the P-N synergistic mechanism [64,69]. In contrast, although DRM and TRM treatments also resulted in P transformation from low-active P to medium- and highly active P, the uptake response of faba bean to N and P was weaker, resulted in the lower N, P uptake and N/P ratio compared with NMF. On the one hand, the mulching structure may affect root distribution and functional area development [65]. On the other hand, the increased soil temperature and moisture caused by film mulching may inhibit soil microorganism metabolic activity and phosphatase secretion, which reduced soil P mineralization and transformation [70].
The issue of degradation relating to the extensive use of plastic film differs from that of residual film. Residual film accumulation reduces soil microorganism activity and hinders soil organic matter decomposition, and thereafter affects soil nutrient cycling [71,72]. In addition, residual film in soil not only reduces water permeability, causing water retention, but also affects soil aeration and oxygen supply, leading to an anaerobic environment in the rhizosphere [73], and limiting farmland ecosystem sustainability [74,75]. The restricted soil moisture and air circulation make it difficult for crop root systems to penetrate deeper into the soil layers, which results in limited root growth [76]. This not only affects the supporting role of root systems for soil microorganisms, but it reduces the efficiency of root system in absorbing water and nutrient elements, and then influences crop growth [76,77]. Therefore, it conforms to the Sustainable Development Goals through reducing the use of agricultural plastic film and minimizing the environmental burden, which is conducive to developing environmentally friendly agriculture. In conclusion, no-mulching with P fertilization (about 18.2 kg P ha−1) not only effectively enhances soil P availability and promotes N-P synergistic uptake by faba bean, but also avoids the potential negative impacts on soil quality and crop growth associated with plastic film mulching.

5. Conclusions

No-mulching and P fertilization are effective in regulating the distribution pattern of soil P fractions, mainly manifested as an increase in medium- and highly active P and a decrease in low-active P. The input of P fertilizer improves soil available P by activating soil medium- and low-active P fractions, and further drives the increase in soil total P. Aboveground N, P uptake and N/P ratio increased with P fertilization levels in no-mulching and mulching with double ridges and furrows, but decreased with P fertilization levels in mulching with three ridges and furrows. Faba bean exhibited higher N, P uptake and a more balanced N/P ratio in no-mulching relative to mulching treatments. Therefore, no-mulching with high P levels was more effective in reducing the N deficiency limiting faba bean growth. In conclusion, no-mulching combined with about 18.2 kg P ha−1 P fertilizer can effectively promote soil P transformation and improve soil P availability, optimize plant N, P uptake and relieve plant N limitation, and thus enhance agricultural production efficiency and promote the sustainable development of faba bean planting in alpine agricultural areas.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy15092037/s1, Figure S1. Resin-P (a), NaHCO3-Pi (b) and NaHCO3-Po (c) in the 2021–2023 growing seasons and the corresponding 3-year averages for three mulching treatments. Figure S2. NaOH-Pi (a) and NaOH-Po (b) in the 2021–2023 growing seasons and the corresponding 3-year averages for three mulching treatments. Figure S3. D.HCl-Pi (a), C.HCl-Pi (b), C.HCl-Po (c), and Residual-P (d) in the 2021–2023 growing seasons and the corresponding 3-year averages for three mulching treatments.

Author Contributions

Conceptualization, Y.G.; Methodology, Y.G.; Validation, W.Z. and Q.X.; Investigation, W.Z., Q.X. and M.S.; Data curation, W.Z., Q.X. and M.S.; Writing—original draft, W.Z.; Writing—review and editing, Y.G.; Visualization, W.Z.; Supervision, C.H.; Project administration, C.H.; Funding acquisition, Y.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 31960625. The APC was also funded by the National Natural Science Foundation of China, grant number 31960625.

Data Availability Statement

The datasets presented in this article are not readily available because they are part of an ongoing study. Requests to access the datasets should be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Soil available phosphorus in the 2021–2023 growing seasons and the corresponding 3-year averages for three mulching treatments [no film mulching with flat planting (NMF), double ridges and furrows mulched with one plastic film (DRM), and three ridges and furrows mulched with one plastic film (TRM)] and three P levels [0 kg P ha−1 (P0), 9.10 kg P ha−1 (P1), and 18.2 kg P ha−1 (P2)]. M: mulching treatments; P: phosphorus levels; M×P: interaction between mulching and phosphorus levels. * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001; n.s. indicates no significant difference between treatments. Error bars are standard deviations.
Figure 1. Soil available phosphorus in the 2021–2023 growing seasons and the corresponding 3-year averages for three mulching treatments [no film mulching with flat planting (NMF), double ridges and furrows mulched with one plastic film (DRM), and three ridges and furrows mulched with one plastic film (TRM)] and three P levels [0 kg P ha−1 (P0), 9.10 kg P ha−1 (P1), and 18.2 kg P ha−1 (P2)]. M: mulching treatments; P: phosphorus levels; M×P: interaction between mulching and phosphorus levels. * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001; n.s. indicates no significant difference between treatments. Error bars are standard deviations.
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Figure 2. Soil total phosphorus in the 2021–2023 growing seasons and the corresponding 3-year averages for three mulching treatments [no film mulching with flat planting (NMF), double ridges and furrows mulched with one plastic film (DRM), and three ridges and furrows mulched with one plastic film (TRM)] and three P levels [0 kg P ha−1 (P0), 9.10 kg P ha−1 (P1), and 18.2 kg P ha−1 (P2)]. M: mulching treatments; P: phosphorus levels; M×P: interaction between mulching and phosphorus levels. * p ≤ 0.05, *** p ≤ 0.001; n.s. indicates no significant difference between treatments. Error bars are standard deviations.
Figure 2. Soil total phosphorus in the 2021–2023 growing seasons and the corresponding 3-year averages for three mulching treatments [no film mulching with flat planting (NMF), double ridges and furrows mulched with one plastic film (DRM), and three ridges and furrows mulched with one plastic film (TRM)] and three P levels [0 kg P ha−1 (P0), 9.10 kg P ha−1 (P1), and 18.2 kg P ha−1 (P2)]. M: mulching treatments; P: phosphorus levels; M×P: interaction between mulching and phosphorus levels. * p ≤ 0.05, *** p ≤ 0.001; n.s. indicates no significant difference between treatments. Error bars are standard deviations.
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Figure 3. Highly active phosphorus fraction (a), medium-active phosphorus fraction (b), and low-active phosphorus fraction (c) in the 2021–2023 growing seasons and the corresponding 3-year averages for three mulching treatments [no film mulching with flat planting (NMF), double ridges and furrows mulched with one plastic film (DRM), and three ridges and furrows mulched with one plastic film (TRM)] and three P levels [0 kg P ha−1 (P0), 9.10 kg P ha−1 (P1), and 18.2 kg P ha−1 (P2)]. M: mulching treatments; P: phosphorus levels; M×P: interaction between mulching and phosphorus levels. * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001; n.s. indicates no significant difference between treatments. Error bars are standard deviations.
Figure 3. Highly active phosphorus fraction (a), medium-active phosphorus fraction (b), and low-active phosphorus fraction (c) in the 2021–2023 growing seasons and the corresponding 3-year averages for three mulching treatments [no film mulching with flat planting (NMF), double ridges and furrows mulched with one plastic film (DRM), and three ridges and furrows mulched with one plastic film (TRM)] and three P levels [0 kg P ha−1 (P0), 9.10 kg P ha−1 (P1), and 18.2 kg P ha−1 (P2)]. M: mulching treatments; P: phosphorus levels; M×P: interaction between mulching and phosphorus levels. * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001; n.s. indicates no significant difference between treatments. Error bars are standard deviations.
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Figure 4. The proportion of soil P fractions in the 2021–2023 growing seasons for three mulching treatments [no film mulching with flat planting (NMF), double ridges and furrows mulched with one plastic film (DRM), and three ridges and furrows mulched with one plastic film (TRM)] and three P levels [0 kg P ha−1 (P0), 9.10 kg P ha−1 (P1), and 18.2 kg P ha−1 (P2)].
Figure 4. The proportion of soil P fractions in the 2021–2023 growing seasons for three mulching treatments [no film mulching with flat planting (NMF), double ridges and furrows mulched with one plastic film (DRM), and three ridges and furrows mulched with one plastic film (TRM)] and three P levels [0 kg P ha−1 (P0), 9.10 kg P ha−1 (P1), and 18.2 kg P ha−1 (P2)].
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Figure 5. Aboveground N uptake (a), aboveground P uptake (b), and plant N/P ratio (c) in the 2021–2023 growing seasons and the corresponding 3-year averages for three mulching treatments [no film mulching with flat planting (NMF), double ridges and furrows mulched with one plastic film (DRM), and three ridges and furrows mulched with one plastic film (TRM)] and three P levels [0 kg P ha−1 (P0), 9.10 kg P ha−1 (P1), and 18.2 kg P ha−1 (P2)]. M: mulching treatments; P: phosphorus levels; M×P: interaction between mulching and phosphorus levels. * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001; n.s. indicates no significant difference between treatments. Error bars are standard deviations.
Figure 5. Aboveground N uptake (a), aboveground P uptake (b), and plant N/P ratio (c) in the 2021–2023 growing seasons and the corresponding 3-year averages for three mulching treatments [no film mulching with flat planting (NMF), double ridges and furrows mulched with one plastic film (DRM), and three ridges and furrows mulched with one plastic film (TRM)] and three P levels [0 kg P ha−1 (P0), 9.10 kg P ha−1 (P1), and 18.2 kg P ha−1 (P2)]. M: mulching treatments; P: phosphorus levels; M×P: interaction between mulching and phosphorus levels. * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001; n.s. indicates no significant difference between treatments. Error bars are standard deviations.
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Figure 6. Mantel test of aboveground N, P uptake, plant N/P ratio, soil P fractions, TP, and AP.
Figure 6. Mantel test of aboveground N, P uptake, plant N/P ratio, soil P fractions, TP, and AP.
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Table 1. Total phosphorus, available phosphorus, and their proportion in the applied fertilizer.
Table 1. Total phosphorus, available phosphorus, and their proportion in the applied fertilizer.
YearTotal P (% P2O5)Available P (% P2O5)Available P/Total P (%)
20214.963.2665.7
20226.845.4780.0
20235.593.9470.4
Average5.80 4.2272.0
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MDPI and ACS Style

Zhou, W.; Xu, Q.; Su, M.; Han, C.; Gu, Y. Effect of Flat Planting Without Film Mulching and Phosphorus Fertilization on Soil Phosphorus Dynamics and Nutrient Uptake in Faba Bean in Alpine Cropping Systems. Agronomy 2025, 15, 2037. https://doi.org/10.3390/agronomy15092037

AMA Style

Zhou W, Xu Q, Su M, Han C, Gu Y. Effect of Flat Planting Without Film Mulching and Phosphorus Fertilization on Soil Phosphorus Dynamics and Nutrient Uptake in Faba Bean in Alpine Cropping Systems. Agronomy. 2025; 15(9):2037. https://doi.org/10.3390/agronomy15092037

Chicago/Turabian Style

Zhou, Weidi, Qiuyun Xu, Man Su, Chenglong Han, and Yanjie Gu. 2025. "Effect of Flat Planting Without Film Mulching and Phosphorus Fertilization on Soil Phosphorus Dynamics and Nutrient Uptake in Faba Bean in Alpine Cropping Systems" Agronomy 15, no. 9: 2037. https://doi.org/10.3390/agronomy15092037

APA Style

Zhou, W., Xu, Q., Su, M., Han, C., & Gu, Y. (2025). Effect of Flat Planting Without Film Mulching and Phosphorus Fertilization on Soil Phosphorus Dynamics and Nutrient Uptake in Faba Bean in Alpine Cropping Systems. Agronomy, 15(9), 2037. https://doi.org/10.3390/agronomy15092037

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