Magnetized water irrigating influences the growth and quality characteristics of Capsicum annuum L. under greenhouse cultivation in Shouguang city

doi:10.21203/rs.3.rs-1645759/v1

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Magnetized water irrigating influences the growth and quality characteristics of Capsicum annuum L. under greenhouse cultivation in Shouguang city

https://doi.org/10.21203/rs.3.rs-1645759/v1

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Background: Magnetic techniques have been reported as a feasible technology to improve the plant growth environment and help to promote soil restoration. Hence, it we applied these techniques, aiming to explore the effect of the magnetic treatment of irrigation water on promoting soil microecological conditions and seedling growth and production. A field experiment was performed under the cultivation conditions of a solar greenhouse. The crop Capsicum annuum L. was cultured in season during 2017 and 2018. A magnetic device was used to treat water, and two irrigation treatments were carried out. The physicochemical properties in the soil and the characteristics of seedling growth and parameters of fruits in long hot green pepper were analyzed.

Results: Magnetized water irrigation (MWI) had the potential to promote bacterial and enzyme activities through increases in alpha-diversity indexes, abundance and multiple species, whereas mineral nutrient contents were enhanced. MWI had positive effects on promoting the physical characteristics in soil, which reduced the pH and bulk density and increased the total porosity and moisture content. MWI promoted morphological characteristics in both leaves and fruits and increased the fresh weight per fruit. MWI improved mineral nutrient contents and fruit quality. Moreover, the results of a correlation analysis between mineral nutrients, alpha-diversity parameters and bacterial communities showed that the relative abundance value was more than 10%, revealing that the dominant bacterial group of Proteobacteria was the opposite of Firmicutes and Bacteroidetes, whereas the latter two groups were significantly negatively correlated with the total contents of nitrogen and phosphorus and the contents of nitrate and ammonium nitrogen and available phosphorus but positively correlated with pH.

Conclusions: In conclusion, The above results showed that irrigation with magnetically treated water could have helpful influences on soil quality; additionally, the improved soil conditions could be beneficial for the formation of bacterial groups, crop growth and output.

magnetic treatment

irrigation water

mineral nutrient

soil bacterial community

crop growth and output

Capsicum annuum L.

Soil physiochemical properties could be affected by the land-use type and had an obvious influence on organism activities and plant growth [51]. In particular, greenhouse-based farming is already popular and produces tomatoes, hot or sweet peppers, cucumbers, eggplants, herbs and other crops. The greenhouse vegetable cultivation area in Shouguang covers more than 50 000 ha, accounting for over 80% of the total planted vegetables. Moreover, greenhouse cultivation is being efficiently utilized all year round and has become essential for the economy of local farmers. Notably, excessive irrigation and fertilization have been applied to contribute to rapid growth and high yield in crops. Simultaneously, a low use efficiency of water and fertilizer could be induced by overirrigation and fertilization [53]. Therefore, improving the use efficiency of water and fertilizer is critical for greenhouse agriculture, which is beneficial for food security and the soil microenvironment. In view of this, the appropriate methods and techniques must be supplemented to enhance the soil quality by improving the utilization efficiency of water and fertilizer, thus promoting crop production.

Various methods have been applied to elevate soil quality and crop output and quality, one of which is magnetism, whose application in agriculture and forestry has attracted more attention from many researchers in recent years. Research results have indicated that magnetism has the potential to conserve water, improve soil conditions and improve product quality [10] and has characteristics of being nontoxic and nonpolluting and having simple and low-cost operations. Some of the physicochemical properties in water, such as surface tension, conductivity and pH, are changed after magnetic treatment, which could be conducive to improving the solubility of soil minerals, increasing the effectiveness of soil nutrients, and protecting soil from secondary salinization [68]. Moreover, magnetized water irrigation (MWI) should improve nutrient supplies [17, 60], physical and chemical properties and water-salt transportation [23, 55, 61] in the soil, as well as seed germination, seedling growth and development [1]. Although soil quality can impact plant growth and yield, there are few studies on the relationship between the soil microecological environment and plant production when exposed to magnetic fields under long-term greenhouse cultivation.

As mentioned above, Shouguang city is one of the major vegetable-producing areas in China. For perennial greenhouse planting, soil ecological environmental problems such as secondary salinization occur in vegetable greenhouses soil due to long-term continuous planting coupled with poor scientific management. MWI is a new method that can increase soil fertility, reduce the amount of fertilizer applied and enhance crop output and quality. The present work aimed to explore the soil conditions and crop yield and quality, and the seasonal crop used was long hot green pepper (Capsicum annuum L. cv. ‘Huangtaiji’). The soil bacterial community structure and diversity after MWI and its relations to soil physical and chemical properties were investigated by using a new generation of high-throughput sequencing technology. Meanwhile, pepper growth and yield as well as the nutrient contents were measured. These results could help to analyze the influences of MWI on the soil microenvironment and crop production under greenhouse conditions.

Experiment site

The experiment was carried out in a commercial crop of Capsicum annuum L. cv. ‘Huangtaiji’ in DongJiayingzi village from 2017 to 2018; this village is in Shouguang city in Shandong province (118°32'-119°10' E, 36°41'-37°19' N), with the Bohai Sea nearby in the north. The experimental area has a warm-temperate continental monsoon climate with a mean annual sunshine duration of 2607.4 h, a relative sunshine duration of 495 h, a frost-free period of 195 days, a mean annual temperature of 12.4°C, an average annual precipitation of 592 mm and a relative humidity of 66%. Winter wheat and summer maize are the main crops; before the solar greenhouses were built, these crops were grown in the open air with rotation. The greenhouses have since been used to culture solanaceous vegetables. The cultivated soil type is a moist soil with the basic characteristics listed in Table 1. The study area is 800 m², the cultivating age is 10 years, and the base fertilizer is rice husk and superphosphate.

Long hot green pepper was cultured for eight months from seedling time to harvest in a greenhouse, with a temperature of 15 ~ 30°C, relative humidity of 60 ~ 70%, and natural light conditions. The seedling time of long hot green pepper was November 2017, and the planting time was February 2018. There were two rows of pepper in each border with triangle planting. The pepper seedling distance was 35 cm, the row spacing was 40 cm, and the planting depth of the soil was generally as deep as the roots of the original seedlings. Therefore, a total of 2800 seedlings were cultivated in 667 m². The rows of pepper were planted in a north-south direction.

Experimental design, irrigational treatment and samples collection

Two treatments were performed: magnetic (MWI) and nonmagnetic water irrigation (NMW). The sequencing test design was applied in our experiment. All the procedures were performed with tree replicates. The seasonable vegetables were irrigated every ten days with unified field management.A magnetizer was used for the MWI. It was installed and began use in the beginning of 2017. The model was PP-60-ADS-600 (Laiwu Agriculture and Forestry Science and Technology Co. Ltd, China), with a diameter of 60 mm, magnetic intensity of 600 Gs and irrigated area of 1200 ~ 1800 m².

After irrigation treatment for six months, the seasonal vegetables in May 2018 and soils were sampled in five locations. Mature leaves were collected at the same position in the middle-upper part, and ripe fruits were harvested at the same maturity and preserved at 4°C for analysis of morphological characteristics, output indexes and fruit quality. The soil was gathered at a depth of 5 ~ 15 cm from the topsoil with litter layer removal. Part of the soil sample was preserved at 4°C to investigate the physical parameters, enzyme activities and mineral nutrient contents. Another part of the soil sample, including the undisturbed sample, was stored at -80°C for the analysis of bacterial community structure.

Determination of soil properties and enzyme activity

Two soil samples were collected using a cutting ring (50.46 mm diameter, 50 mm height, 100 cm³) and a dedicated aluminum box (50 mm diameter, 30 mm height) per plot before crop harvesting. In particular, undisturbed soil was applied to bulk density and total porosity.

In the measurement of the moisture content, the dedicated aluminum box (G1) was weighed, and the data were recorded and sequenced. Then, the samples of wet soil plus the box were weighed and recorded after soil collection (G2). Third, the wet samples were dried to achieve contrast weight using an oven at 105°C, and the data were recorded (G3). Finally, the moisture content was calculated according to the following formula:

$$\text{M}\text{o}\text{i}\text{s}\text{t}\text{u}\text{r}\text{e} \text{c}\text{o}\text{n}\text{t}\text{e}\text{n}\text{t} \left(\text{%}\right)=\frac{G2-G3}{G3-G1}\times 100%$$

To investigate the bulk density, first, the diameter (D), height (H), volume (V) and weight (W1) were measured, and the parameters were recorded before soil collection. Then, the wet soil plus cutting ring was weighed after sampling and recording (W2). Third, the samples were saturated with water by capillarity in a water container and weighed every 1 h until the absorbed water in the soil reached a contrast weight for approximately six hours (W3). Finally, the watered soil was dried to a contrast weight at 105°C and weighed (W4).

The soil bulk density was calculated using the following equation: $\text{S}\text{o}\text{i}\text{l} \text{b}\text{u}\text{l}\text{k} \text{d}\text{e}\text{n}\text{s}\text{i}\text{t}\text{y} \left(g{ cm}^{-3}\right)=\frac{W4-W1}{V}$.

The total porosity was calculated according to the following formula: $\text{T}\text{o}\text{t}\text{a}\text{l} \text{p}\text{o}\text{r}\text{o}\text{s}\text{i}\text{t}\text{y}\left(\text{%}\right)=\frac{W3-W4}{V}$×100%

The soil was air-dried and sifted by a mesh with a size of 0.125 mm, and the pH value was measured by the potentiometric method. The air-dried soil (10 g) was homogenized in 50 ml of double-distilled water (soil to water ratio of 1:5) and then allowed to stand for 30 min. The supernatant was used for pH determination using an adjusted pH device (Thermo Eutech PC700, Thermo Fisher Scientific).

The percentages of sand, silt and clay in the soil were analyzed using a Laser Particle Size Analyzer (LS13320, Beckman Coulter)[36]. The air-dried soil of 0.5 g samples was sieved by a screen with a size of 2 mm and homogenized in 10 ml of 10% hydrogen peroxide (H₂O₂), favoring the removal of soil organic matter. Then, 10 ml of 10% hydrochloric acid (HCl) was added to the soil solution, and the solution was boiled to remove the carbonate. Deionized water was added at 50 ml to tubes and allowed to stand for 12 h. The supernatant was collected, and 10 ml of 0.1 mol L^− 1 sodium hexametaphosphate was applied as the dispersant. Finally, the tubes were oscillated by an ultrasonic cleaning machine for 30 s. The soil particle size was graded according to international standards

The air-dried soil samples sieved through a mesh of 0.125 mm were used to determine the soil organic matter content by potassium dichromate oxidation according to the method of Bao [3]. The total nitrogen in soil and plants was determined by the Kjeldahl method written by Bao [3]. The total phosphorus content in the soil and plants was determined by the molybdenum antimony-ultraviolet spectrophotometric (UVS) method at 700 nm [31]. Nitrate and ammonium nitrogen, as well as available phosphorus, were analyzed by an AA3-continuous flow analytical system (Bran + Luebbe, GmbH, Germany). Nitrate and ammonium nitrogen were extracted by 1 mol·L^− 1 potassium chloride (KCl), and the ratio of soil to water was 1:20. Available phosphorus was extracted by sodium bicarbonate (NaHCO₃) at 0.5 mol·L^− 1 with a ratio of soil to water of 1:20 [70].

The exchangeable cations Ca²⁺, Mg²⁺, K⁺ and Na⁺ were investigated by atomic absorption spectrophotometry (AAS) and recorded at 317.9 nm, 2855.2 nm, 766.4 nm and 589.5 nm, respectively. The meshed soil samples were extracted by 1 mol·L^− 1 neutral ammonium acetate according to the method of Bao [3]. Samples of leaves and fruits were dried to contrast weight, ground and sieved through a mesh of 0.125 mm. Samples (0.5 g) were digested with H₂SO₄-H₂O₂. The contents of iron (Fe), manganese (Mn), zinc (Zn) and copper (Cu) were measured by AAS at 248.3 nm, 279.5 nm, 213.9 nm and 324.8 nm, respectively, in accordance with the method described by Song et al. [47].

The enzyme activities of urease, sucrose, neutral phosphatase and catalase in air-dried soil sieved through 1 mm mesh were tested using the UVS method according to Bao [3] with some modification. Samples (2.0 g) were applied to analyze the catalase activity. They were homogenously mixed in 40 ml of distilled water, and 5 ml of 0.3% H₂O₂ was added and vibrated for 30 min. After oscillation treatment, a saturated solution of aluminum potassium alum (1 ml) was introduced, and then the supernatant was filtered into a conical flask with 5 ml of 1.5 mol·L^− 1 H₂SO₄ solution. The filtrate was assayed at 240 nm [63]. Ten grams of soil samples were used to analyze the activities of urease, sucrose, and neutral phosphatase cultured in a biochemical incubator at 37°C. The solutions were investigated after 48 h, 24 h and 24 h at absorbances of 527 nm, 508 nm and 578 nm, respectively.

Determination of the soil bacterial community structure

The total DNA of soil bacteria (wet soil) was extracted using a soil DNA extraction kit (E.Z.N.A. oil DNA Kit, Omega Bio-Tek, Inc., Norcross, GA, USA) with three biological replicates and dissolved in 50 µl of TE buffer. The extracted DNA was then quantified through a NanoDrop spectrophotometer (Nanodrop Technologies, Oxfordshire, UK). In addition, 1% agarose gel electrophoresis (AGE) was used to analyze the integrity and relative concentration of the extracted DNA, which was used to amplify gene fragments of 16S rRNA.

The highly variable area of V3 ~ V4 of 16S rRNA was used for real-time PCR (polymerase chain reaction) amplification. The primer pair 341F (CCTACGGGNGGCWGC A) and 805R (GACTACHVGGGTATCTAATCC) was used. The conditions of the PCR were as follows: initial denaturation at 95°C for 2.0 min, followed by 25 cycles of denaturation at 95°C for 30 s, 55°C for 30 s and 72°C for 30 s. Signals were collected during the elongation step at 72°C for 5 min. Then, 2% AGE was applied to detect the independent PCR product extracted by the AxyPrep DNA (AXYGEN) gel recovery kit and elution with Tris-HCl solutions. The QuantiFluorTM-ST system (Promega) was utilized for the quantification of PCR products. The high-throughput sequencing technique of Illumina MiSeq PE300 was applied to sequence the extracted DNA samples [30].

The raw DNA sequence data were first filtered by Mothur (V.1.36.1) to remove the chimera. Then, the alpha diversity indexes and bacterial community composition were calculated in the optimized sequence [69].

Measurement of leaf and fruit morphological parameters and fruit quality

Healthy pepper plants with consistent growth were selected as the test materials according to the method written by Zhou et al. [71]. Leaf and fruit samples were collected before harvest. The morphological parameters of individual leaves and fruits were analyzed by Vernier calipers and measuring tape. After data recording, some of the samples were used for drying treatment, and the others were preserved at 4°C℃ and − 80°C for further analysis. The weight of a single fruit was measured using an electronic balance of 1/1000.

The ascorbic acid content was determined using the method of Zhao et al. (2010) with some modifications. Fresh fruits (0.5 g) were assayed by UVS at 265 nm. The grinding tissues were homogenized in 50% chilled metaphosphoric acid and centrifuged at 12 000×g for 20 min at 4°C, and the supernatant was used for absorbance measurement.

The organic acid content was determined using a high-performance liquid chromatography (HPLC) method consistent with Jia et al. [20] with some modifications. Methanol and 0.05 mol·L^− 1 KH₂PO₄ (2%) with a pH of 2.8 was used as the mobile phase with a flow velocity of 0.8 ml·min^− 1, column temperature of 17°C, injection volume of 5 µl, and measuring wavelength of 214 nm. A Hypersil-ODS2 chromatographic column (4.6 mm×200 mm, 5 µm) was used in the processing.

The anthocyanin content was assayed according to the method of Pirie and Mullins [43] with some modifications [5]. Two grams of fresh fruit (peel and pulp) was homogenized in 1% HCl-CH₃OH solution at 4°C, and the supernatant was assayed at an absorbance of 530 nm using a TU-1900 UVS (Beijing Purkinje General Instrument Co., Ltd, China).

Samples of 1.0 g of fresh whole fruits were ground well. Then, the homogenate was centrifuged at 5000 rpm for 10 min. Four milliliters of anthranone was added to 1 ml of supernatant in an ice bath. The solution was exposed to a water bath for 10 min and cooled for measurement by UVS at an absorbance of 620 nm [5].

Total soluble sugar was measured by a colorimetric analysis of anthrone reagent in accordance with Yemm and Willis [65] with some revision. Samples of 0.5 g of fresh whole fruits were ground in 50 mM potassium phosphate buffer with a pH of 7.5 at 4°C. The homogenates were first filtered by four layers of cheesecloth and then centrifuged at 3500×g for 15 min at 4°C. The supernatant was collected and measured at 620 nm using UVS. The contents of total soluble sugar were calculated from a calibration curve using glucose as a standard.

Statistical analysis

The results are shown as the mean of three biological replicates to confirm the reproducibility of the data. The means of four different experiments were subjected to analysis by one-way ANOVA, with a probability level of 0.05 defined as significant according to Duncan’s multiple range test. A correlation analysis of the nutrients was conducted using SPSS 16.0.

R software was used to manage the statistical analysis of bacterial community structure. The alpha-diversity parameters of the Shannon index, Chao1 index, Simpson index and ACE index were calculated by QIIME (Version 1.7.0) software to evaluate species richness and species diversity in samples.

Rationality analysis of sequencing of the soil samples

Illumina MiSeq high-throughput sequencing was performed on long hot green pepper soil samples from MWI and NMW soil. According to the sequencing results, 7,8674 and 75,549 original sequences were obtained from long hot green pepper MWI and NMW soil, respectively, among which 5,9215 and 58,167 were effective sequences. When the sequence was 28738, the dilution curves of soil samples tended to be flat, indicating that the sequencing depth of bacteria was almost saturated (Fig. 1). The coverage of the soil bacterial DNA library for MWI and NMW was 97.6% and 98.2%, respectively, indicating that the probability of gene sequence detection in soil samples was very high. Therefore, the sequencing results can represent the real situation of the bacterial community in MWI and NMW soil.

Changes in the alpha-diversity indexes of the bacteria in soil

Table 2 shows the correlation index richness and diversity of the bacterial community between MWI and NMW soils. The Chao1 and ACE indexes of bacterial communities in MWI soil were all higher than those in NMW soil. The Chao1 and ACE indexes can reflect the richness of the bacterial community, so this result indicated that the richness of the bacterial community in MWI soil is higher than that in NMW soil. The Shannon index and Simpson index of MWI soil were both higher than those of NMW soil, indicating that irrigation with magnetized water increased the diversity of the soil bacterial community. The number of OTUs, Chao1, ACE, and Shannon index were significantly different between MWI and NMW soils (P < 0.05).

Changes in the microbial communities at the phylum level in soils

In terms of phylum classification level, a total of 44 bacteria were detected in MWI and NMW soils: 43 for MWI soil and 39 for NMW soil (Fig. 2). As shown in Fig. 2, there were 9 bacterial phyla (relative abundance > 1%) in the MWI and NMW soils: Proteobacteria, Firmicutes, Bacteroidetes, Actinobacteria, Chloroflexi, Acidobacteria, Gemmatimonadetes, Planctomycetes, and Thermomicrobia. Their relative abundance was approximately 93.7%~94.6%. The relative abundance of Proteobacteria in MWI soil (31.6%) was significantly higher than that in NMW soil (20.4%). The relative abundance of Firmicutes and Bacteroides in NMW soil was significantly higher than that in MWI soil (18.7%, 28.1%). The relative abundance of Actinobacteria and Planctomycetes in MWI soil was significantly higher than that in NMW soil by 1.12 and 0.41 times, respectively. The relative abundance of Acidobacteria, Gemmatimonadetes, and Thermomicrobia in MWI soil was significantly higher than that in NMW soil. Among these bacteria, GAL15, Gracilibacteria, Spirochaetes, RBG-1 and WWE3 were only in the MWI soil, and FCPU426 was only in the NMW soil. Bacteria whose relative abundance was less than 1% in the MWI and NMW soils were represented by Others, whose total relative abundance was approximately 5.0%~5.8%. For the Unclassified bacteria in terms of phylum, there were 0.48% in MWI soil and 0.40% in NMW soil.

Changes in the microbial communities at the phylum level in soils

In terms of genera classification, there were 485 genera of bacteria detected in the MWI and NMW soils: 448 genera for the MWI soil and 424 genera for the NMW soil. According to Fig. 3, the dominant genera of bacteria were Bacillus, Chryseolinea, Truepera, Luteimonas, Steroidobacter, Streptomyces, Nocardioides, Gemmatimonadaceae, and Planctomyces. Their relative abundance was 5.5%, 3.4%, 1.9%, 1.6%, 1.6%, 1.4%, 1.3% and 1.1%, respectively. Chryseolinea (17.6%), Bacillus (8.9%), and Steroidobacter (1.7%) were the dominant bacteria in the NMW soil (Fig. 3). The relative abundance of Chryseolinea, Bacillus, and Steroidobacter in the NMW soil was greater than that in the MWI soil. The relative abundance of Truepera, Gemmatimonadaceae, Streptomyces Nocardioides, Luteimonas, and Planctomyces in the MWI soil was greater than that in the NMW soil. The sum of the relative abundances of bacteria with lower abundances in the MWI and NMW soils were 20.0% and 15.7%, respectively. Hyphomicrobium with denitrification had a significant difference in relative abundance between the MWI and NMW soils. Bacillus with phosphorus dissolution and nitrogen fixation was significantly more abundant in the NMW soil than in the MWI soil, while Pseudomonas with phosphorus dissolution was significantly more abundant in the NMW soil than in the MWI soil. The relative abundance of Planctomyces is the highest in MWI soil. The relative abundance of Sphingomonas in the MWI soil was higher than that in the NMW soil. In addition, there were approximately 48.8% and 47.9% unclassified bacteria in the MWI and NMW soils, respectively.

Changes in the physicochemical properties of soil

The physical and chemical properties of long hot green pepper MWI and NMW soils are shown in Table 3. The total phosphorus, nitrate nitrogen, ammonium nitrogen and available phosphorus in the MWI soil were 2.37 g·kg^− 1, 21.90 mg·kg^− 1, 62.37 mg·kg^− 1 and 73.87 mg·kg^− 1, respectively, which were significantly (1.03, 1.42, 1.39 and 1.39 times) higher than those in the NMW soil, respectively. The pH of the NMW soil was significantly higher than that of the MWI soil. There was no significant difference in soil moisture, soil bulk density, soil total porosity, organic matter or total nitrogen content between the MWI and NMW soils.

Correlation between soil bacteria and soil properties

Table 4 shows the richness and diversity index of soil bacterial communities in soil by magnetized and by nonmagnetized water, as well as a correlation analysis of the dominant bacterial communities in the soil and the soil physical and chemical properties. The richness and diversity index of the soil bacterial community was significantly negatively correlated with soil pH and significantly positively correlated with total phosphorus, nitrate nitrogen, ammonium nitrogen and available phosphorus. The bacteria with relative abundances greater than 10% in the soil were Proteobacteria, Bacteroides and Firmicutes. The relative abundance of Proteobacteria was negatively correlated with soil pH and positively correlated with total phosphorus, nitrate nitrogen, ammonium nitrogen and available phosphorus. The relative abundance of Bacteroidetes was positively correlated with soil pH and negatively correlated with total phosphorus, nitrate nitrogen, ammonium nitrogen and available phosphorus. The relative abundance of Firmicutes was positively correlated with soil pH and negatively correlated with total phosphorus, nitrate nitrogen, ammonium nitrogen and available phosphorus.

Magnetized water irrigation influences the bacterial community in the soil

In the present study, we found that the structure, richness and diversity of the bacterial community in soil were significantly changed after long-term MWI in a greenhouse compared to NMW. Notably, the dominant bacterial communities in the soil were Proteobacteria, Firmicutes and Bacteroides in both the NMW and MWI, and the latter induced an increase in the diversity and richness of soil bacteria. For example, the relative abundance of Proteobacteria was significantly higher than that of the NMW soil, while the relative abundance of others, such as Firmicutes and Bacteroides, decreased significantly. Many studies have shown that magnetic fields can affect some physiological activities of microorganisms, thus affecting the growth and reproduction of microorganisms [27]. Moreover, the osmotic pressure of the aqueous solution can increase when water becomes magnetized [58], whereas it can promote the dissolution of nutrients. Meanwhile, magnetized water can influence the physical and chemical properties of soil, which provides a favorable habitat for bacterial growth [40, 42]. Correspondently, in this study, irrigation with magnetically treated water improved the soil moisture and total porosity and increased the contents of organic matter, total nitrogen, total phosphorus, nitrate nitrogen, ammonium nitrogen and available phosphorus in the soil, whereas irrigation decreased the pH in accordance with Chang and Weng [7]. In general, the improved physicochemical properties in the soil caused by magnetized water could be the reason for changes in the community composition and diversity of the bacteria in soil. Meanwhile, MWI could regulate the abundance of a variety of bacterial species, for which bacteria with strong competitiveness will inhibit bacteria with weak competitiveness, thereby enhancing their abundance [29].

Long-term fertilizer and/or organic fertilizer application leads to a decrease in the pH of farmland soil [13]; simultaneously, fertilization increases biological yield and improves the soil environment [33]. Furthermore, magnetic fields have promoted the mobilization and transportation of soil nutrients [37, 54]. A correlation analysis between soil bacterial community richness and diversity and soil physical and chemical properties showed that the soil pH value, total phosphorus, nitrate nitrogen, ammonium nitrogen and available phosphorus had a high correlation with soil bacterial community diversity and richness. Soil physical and chemical properties are not only the environmental conditions for soil bacteria survival but also the food source for these bacteria. The influence of MWI on soil physical and chemical properties also affects soil bacteria indirectly. Li et al. [28] studied the diversity of soil bacterial communities with different salinities and alkalinities in Inner Mongolia and found that the richness and diversity of soil bacterial communities were related to the total nitrogen content. Some studies have found that soil bacterial community diversity changed with changes in NH₄⁺-N in soil [64]. Maheshwari and Grewal [37] showed that magnetized water decreased the pH in soil-grown snow pea and celery and increased the available phosphorus in the soil. Soil nitrogen and phosphorus have an important effect on soil bacterial abundance, especially on Proteobacteria. Studies have shown that the contents of total phosphorus and nitrate nitrogen have a significant impact on Proteobacteria [24]. Xu et al. [62] suggested that a nutrition-rich soil was more conducive to the growth and reproduction of Proteobacteria. Our study also showed that NO₃⁻-N soil and total phosphorus were significantly positively correlated with Proteobacteria. This is because magnetized water could increase the contents of nitrogen and phosphorus in soil, thus increasing soil nutrients and providing favorable conditions for the growth and reproduction of bacteria.

Different microorganisms have different adaptive ranges for soil pH. Higher or lower pH is not conducive to the survival of microorganisms. Chu et al. [8] studied the microbial diversity of arctic soil and found that soil pH was the main factor affecting the composition of the bacterial community in soil. Acidobacteria are eosinophilic and oligotrophic bacteria [41], so a partially acidic soil environment is conducive to their growth. Sui et al. [52] reported that the abundance of Acidobacteria was 53.0% in wetland soil of the Sanjiang Plain. Liu et al. [32] found that the abundance of Acidobacteria was within 53.0 ~ 68.0% in the soil of a Dinghushan forest. Zhang et al. [70] indicated that Acidobacteria was closely related to soil pH. Liu et al. [35] also found that soil pH was an important factor affecting the community structure of Acidobacteria in the black soil of northeast China. Fu et al. [11] studied the microbial community of the soil under rice-barley cultivation in coastal saline soil and found that the soil pH and salinity decreased and the number of soil bacteria increased significantly under the condition of rice-wheat rotation. In our study, the relative abundance of Acidobacteria was higher in the MWI soil than the NMW soil. This means that the decreased soil pH induced by magnetized water was conducive to the growth and reproduction of Acidobacteria.

Magnetized water irrigation influences the physical properties and enzyme activities in soil

The present findings indicated that MWI improved the physical properties plants cultured in greenhouses. MWI led to a decrease in soil bulk density, followed by increases in soil total porosity and moisture content, thereby enhancing the physical characteristics of greenhouse soil. These discoveries were consistent with Zhang et al. [67], who found that rice straw biochars could promote a higher fine particle content and total porosity and increase water retention in soil.

The analysis of soil texture (Fig. 4), in which particle size was divided according to the international system, revealed that the percentages of silt and clay, with particle sizes less than 0.02 mm in the soil, were enhanced by 2.1 ~ 25.3% after long-term irrigation with magnetically treated water, and the percentage of silt was the highest at 91.2% in the soil. Moreover, clay showed the largest increase, 25.3%, in soil, while sand, with a particle size over 0.02 mm, decreased by 25.6% compared to that in the NMW soil. A previous study demonstrated that the correlation coefficient between the fractal dimension of soil and organic matter is remarkably positive; consequently, the elevated clay and silt contents are beneficial to the retention of organic matter in soil [12], thus improving the content of organic matter affected by the magnetic field.

The promotion of soil physical characteristics and fertility conditions is good for microbial and enzyme activities. Correspondingly, the stimulated enzyme activities could help increase soil metabolism and enhance the nutrient transportation of soil. Previous findings indicated that the activities of urease and phosphatase are closely related to microbial diversity, and their activities increase with increasing biomass. Consistently, MWI resulted in increases in the activities of urease (Fig. 5), phosphatase and sucrase by 24.8%, 17.7% and 16.6%, respectively, relative to those of the NMW soil, whereas catalase showed obvious changes. These results have many causes. On the one hand, MWI could contribute to microbial diversity and multiply the advantageous bacterial groups, such as Proteobacteria, Firmicutes, and Bacteroidetes, causing the alpha diversity indexes in soil to be promoted, thus strengthening urease activity. On the other hand, the magnetic field resulted in better water conditions, as the improved moisture content and total porosity were accompanied by a decreased volume weight in the soil; meanwhile, the content of available phosphorus was improved during long-term MWI. As the available phosphorus in soil can improve the diversity of the bacterial community because the high available phosphorus in the soil provides phosphorus needed for the growth of bacteria [52], phosphatase showed higher activity under MWI than that of the controls. Moreover, the activated sucrase affected by MWI was associated with organic matter, humidity and particle-size distribution in soil, and it could change the transformation and utilization of carbohydrates [57], thus improving soil fertility. The mentioned findings were consistent with the positive effects of no-till on soil fertility reported by Hepp et al. [15].

Magnetized water irrigation impacts the seedling growth and fruit yield and quality of Capsicum annuum L.

The positive effects induced by MWI on seed germination and seedling growth and output have been reported in tomato, long hot green pepper, cucumber and wheat [18, 48, 50]. Consistent with the above conclusions, MWI led to a 4.0% increase in the fresh weight per fruit with a promoted diameter of 7.1% (Table 5). Simultaneously, MWI caused beneficial effects on leaf morphological parameters such as length, width, petiole length and fresh weight, with increasing percentages of 19.2%, 20.2%, 25.3% and 63.7%, respectively, relative to those in the NMW soil. Moreover, MWI increased the nutrient content to 46.97 mg·g^− 1 with an elevated ratio of 10.3% compared to the NMW. However, the improved leaf area in green capsicum may lead to a higher interception of light and improved photosynthetic capacity [9]. Consistently, Souza et al. [49] found that the seed germination, leaf area and seedling growth of onion (Allium cepa L.cv. Red Creole) could be accelerated by magnetic fields under controlled conditions. The promotion of seedling growth induced by MWI could be attributed to increased leaf characteristics and photosynthetic area, which could help in the accumulation of photoproducts, thus benefitting higher yields [26].

It is worth mentioning that MWI has the potential to promote the accumulation of nitrogen and phosphorus, as well as the contents of mineral nutrients in iron (Fe), manganese (Mn), zinc (Zn) and copper (Cu), by 3.0 ~ 54.0% in the leaves and fruits of long hot green pepper compared to that in the NMW soil (Fig. 6). Accordingly, Grewal and Maheshwari [14] found that MWI led to increases in the nutrient content of both snow peas (Pisum sativum L.) and chickpeas (Cicer arietinum L.). The mean maximal Fe content in leaves was 57.2%, followed by changes in phosphorus content, whereas there was no obvious variation in Mn content. The promoted Fe content could benefit the synthesis and renewal of chlorophyll [44], which resulted in an elevated content in leaves caused by the magnetic field under controlled conditions. In particular, MWI led to an increase in the blade nitrogen content. Hence, more nitrogen was allocated to chloroplasts as the nitrogen content in leaves increased, so it became involved in photosynthesis, and the plants grew faster [25]. Notably, leaf morphological indexes were promoted under MWI. Moreover, the photosynthetic capacity of leaves is related to the phosphorus content [4], and an increased phosphorus content in the blade could contribute to the essential substances used to synthesize adenosine triphosphate (ATP) and diphosphoribulose carboxylase (Rubisco), thus stimulating the accumulation of ribonucleic acid (RNA), which is necessary for transcription protein [16]. However, the changes in the leaf nitrogen and phosphorus contents of C. annuum resulting from magnetic treatment would affect plant metabolism, energy and protein synthesis, and a greater absorption of water would be conducive to nutrient mobilization and transportation from leaves to fruits. As a result, the changes in the fruit mineral contents in fruits were enhanced. The findings in the first half of the Discussion indicated that employed magnetized water could improve the microenvironment in the soil, which provided supplemental exchangeable cations such as calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na), with an elevated percentage of 1.0 ~ 5.0% (Fig. 7), and NH₄⁺-N at 38.5% (Table 3), also with an increasing content of available phosphorus of 39.1% (Table 3) relative to that under NMW. Similar results were found with the mineral nutrient contents of date palm (Phoenix dactylifera L.) when organic manures were added [39] or combined with mineral sources [2]. The changes in mineral contents revealed the same variation trend both in leaves and in fruits, as the interrelationship of the mineral nutrients in the leaves and in the roots needs further study.

MWI promoted the fresh weight of a single fruit, which could increase the fruit yield per plant. Simultaneously, it could improve the plant nutrient status, thus impacting fruit quality. In the present study (Fig. 8), the characteristics of flavor components, including ascorbic acid, reducing and soluble sugar, as well as anthocyanin, were enhanced in long hot green pepper fruits by 1.0 ~ 47.0% under MWI, while it inhibited the synthesis of organic acid by 1.8% compared to that in the NMW soil. Particularly, the reducing sugar had the largest proportion, 46.2%, followed by the soluble sugar at 16.9%, and these findings were consistent with the fact that the application of a bioformulation had positive effects on the quality attributes of pomegranate (Punica granatum L.) fruit [38]. In addition, the sugar-acid ratio (ratio of soluble sugar and organic acid) was slightly higher (17.4%) under MWI than in the NMW soil. Likewise, Wada et al. [59] reported that the sugar-acid ratio is a critical index to evaluate fruit quality: the higher the sugar-acid ratio is, the better the fruit quality is. These findings indicated that MWI could improve the fruit quality of long hot green pepper, which is consistent with El-Zawily et al. [10], who found that the relative yield, total chlorophyll, and mineral contents in the leaves and the contents of ascorbic acid and total soluble solids in the fruits of tomato (Solanum lycopersicon) were all promoted when a magnetic field was applied to the water treatment.

Anthocyanins are the products of secondary metabolism in plants [45], and they are vital bioactive substances and main pigments in fruits [19] that could affect the flesh color in the ripening period [22]. Carbone et al. [6] revealed that environmental factors can impact anthocyanin concentrations in various ways. The magnetic treatment of irrigation water could contribute to the synthesis of anthocyanins, which is in accordance with Liu et al. [34], who found that exogenous abscisic acid (ABA) treatment could effectively promote the fruit anthocyanin content. Additionally, anthocyanin accumulation was stimulated, and fruit maturity was promoted when exposed to exogenous sugar treatment [21]. At the same time, magnetic treatment caused a promotion percentage in reducing and soluble sugars, which is closely related to the synthesis of anthocyanins in fruits [46]. Therefore, the increasing the contents of anthocyanins and sugars in long hot green pepper could change the flesh color to promote fruit ripening, in accordance with Teribia et al. [56].

In this experiment, the effects of MWI on the soil microecosystem, crop growth and output, as well as the fruit quality of C. annuum cultivated in a greenhouse, were analyzed. The results indicated that MWI improved soil physical properties and also contributed to the formation and abundance of the bacterial community, promoted enzyme activities and enhanced mineral nutrients in the soil. These changes could provide an ideal environment for long hot green peppers to grow and develop. Accordingly, MWI could promote leaf morphological parameters and chlorophyll content, and these changing characteristics were of benefit to raising photosynthesis capacity and enhancing average weight in fresh fruit. In addition, MWI induced an improvement in long hot green pepper quality, which was attributed to the evaluated mineral nutrients in leaves and nutrient components in fruits with early ripening. Due to different soil types leading to various chemical and physical characteristics, the influences of magnetized water on different crop species may be different. Therefore, to comprehensively understand the impact of magnetized water on edaphic conditions and crop growth and output in greenhouses during cultivation processing, further studies on different types of soil and crops are needed. Additionally, this study can supply a theoretical basis of magnetic techniques for improving the microecological conditions of facilities culturing soil and has potential value for increasing crop quality and production.

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Availability of data and materials

The datasets generated and/or analysed during the current study are available in the [https://pan.baidu.com/s/1CYpo5JLsPgxcqEombwx0TA] repository. Password: ej95.

All aspects of the study comply with institutional, national, and international guidelines.

Competing interests

The authors declare that they have no competing interests.

Funding

The fundings were provided by the Funds for Agricultural Science and Technology of Shandong Province (Forestry Technological Innovation Project, 2019LY004) and the Agricultural Major Application Technology Innovation Program of Shandong Province (Financial and Agricultural Indicators, 2016, No. 36).

Authors' contributions

XML, LW and FYM conceived and designed the study. XML, LW, LGK, JYG, SYM and SSB, performed the experiments. XML, FYM and LW analyzed the data. XML, LW, and FYM wrote the manuscript. XML, FYM, LW share the first authorship.

Not applicable.

Affiliations

Key Laboratory of State Forestry Administration for Silviculture of the Lower Yellow River/ forestry College of Shandong Agricultural University Tai’an 271018, China; Shanghai Environment College, Shanghai 200135, China

Xiumei Liu, Fengyun Ma, Lu Wang

Jinan Forest-Fruit Manufacture Technologist Extension and Service Center, Jinan 250000, Shandong, China

Linggang Kong

Shandong Forestry Protection and Development Center, Jinan 250000, Shandong, China

Jianyao Guo

Guangzhou Institute of Forestry and Landscape Architecture, Guangzhou 510000, Guangzhou, China

Shiyuan Meng

Shandong Academy of Forestry, Jinan 250000, Shandong, China

Sisheng Bi

Corresponding author

Correspondence to Fengyun Ma.

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Table 1 Basic physico-chemical characteristics in the soil under greenhouse cultivation before irrigational treatment

Parameters

Organic matter

(g kg^-1)

Total nitrogen

(g kg^-1)

Available nitrogen

(mg kg^-1)

Available phosphorus

(mg kg^-1)

Available potassium

(mg kg^-1)

Moist soil

7.16

14.70

1.24

126.21

58.23

219.10

Table 2 Changes in the indexes of alpha diversity in moist soil when exposed to magnetic water irrigation (MW) and non-magnetic treatment (NMW) under greenhouse conditions. Values are shown as the means±SE of three biologically replicated determinations. Different lowercase letters in the same row indicate significant differences at the 0.05 level.

Treat-ment	Similar level at 97%
	OTUs	Chao1	ACE	Shannon index	Simpson index	Coverage (%)
	OTUs	index	index	Shannon index	Simpson index	Coverage (%)
MW	2563.67 ± 28.68 a	3020.54 ± 67.96 a	3147.61 ± 41.76 a	9.28 ± 0.11 a	0.994 ± 0.001 a	97.63
NMW	2110.00 ± 28.58 b	2435.61 ± 38.42 b	2483.76 ± 53.23 b	8.26 ± 0.14 b	0.975± 0.005 b	98.23

Table 3 The major physico-chemical parameters in the treated soils irrigating with magnetized (MW) and non-magnetized water (NMW) under greenhouse cultivation. Values are indicated as the means±SE of three biologically replicated determinations. Different lowercase letters in the same row reveal the significant differences at the 0.05 level.

Treatments	MW	NMW
pH	7.09 ± 0.14 b	7.47 ± 0.05 a
Soil moisture content (%)	12.76 ± 1.92 a	10.92 ± 2.69 a
Soil bulk density (g·cm^-3)	1.13 ± 0.04 a	1.18 ± 0.06 a
Soil total porosity (%)	57.47 ± 1.41 a	55.41 ± 2.37 a
Organic matter (g^.kg^-1)	13.20 ± 0.05 a	12.87 ± 0.54 a
Total nitrogen (g^.kg^-1)	1.91 ± 0.06 a	1.79 ± 0.07 a
Total phosphorus (g^.kg^-1)	2.37 ± 0.00 a	2.31 ± 0.01 b
Nitrate nitrogen (mg^.kg^-1)	21.90 ± 1.02 a	15.43 ± 1.31 b
Ammonium nitrogen (mg^.kg^-1)	62.37 ± 1.40 a	45.03 ± 2.24 b
Available phosphorus (mg^.kg^-1)	73.87 ± 2.11 a	53.10 ± 3.89 b

Table 4 Correlation analysis among the vital physicochemical properties, alpha-diversity indexes and bacteria with relative abundances over 10% at the phylum level in the moisture when subjected to MWI and NMW in a greenhouse experiment. ‘*’ indicates significant differences at the 0.05 level, and ‘**’ shows remarkable varieties at the 0.01 level of different measured parameters.

Factors	pH	Moisture content	Organic matter	Total nitrogen	Total phosphorus	Nitrate nitrogen	Ammonium nitrogen	Available phosphorus
OTU	-0.896	-0.43	0.564	0.849*	0.992**	0.985**	0.995**	0.950**
Chao1 index	-0.878*	-0.423	0.519	0.769	0.970**	0.966**	0.985**	0.933**
ACE index	-0.881*	-0.397	0.535	0.825*	0.983**	0.984**	0.995**	0.995**
Shannon index	-0.853*	-0.344	0.566	0.870*	0.972**	0.988**	0.987**	0.987**
Simpson index	-0.855*	-0.351	0.575	0.839*	0.950**	0.950**	0.952**	0.987**
Proteobacteria	-0.901*	-0.41	0.443	0.707	0.952**	0.917**	0.947**	0.947**
Firmicutes	0.8	0.32	-0.184	-0.536	-0.869*	-0.847*	-0.895*	-0.895*
Bacteroidetes	0.902*	0.423	-0.568	-0.827*	-0.974**	-0.953**	-0.965**	-0.965**

Table 5 Changes in the growth parameters in leaves and fruits of Capsicum annuum L. irrigating with magnetized (MW) and non-magnetized water (NMW) under greenhouse cultivation. Values are indicated as the means±SE of three biologically replicated determinations. Different lowercase letters in the same column show the obvious differences at the 0.05 level.

Treat- ments	Fruits			Leaves
Treat- ments	Vertical diameter (cm)	Diameter (cm)	Fresh weight (g)	Length (cm)	width (cm)	Petiole length (cm)	Fresh weight (g)
MW	26.47 ± 0.15 a	3.75 ± 0.05 a	132.56 ±2.26 a	17.80 ± 0.10 a	8.50 ± 0.17 a	7.93 ± 0.51 a	3.65 ± 0.12 a
NMW	26.40 ± 0.85 a	3.50 ± 0.37 a	127.51 ± 2.12 b	14.93 ± 0.25 b	7.07 ± 0.15 b	6.33 ± 0.40 b	2.23 ± 0.21 b

No competing interests reported.

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Magnetized water irrigating influences the growth and quality characteristics of Capsicum annuum L. under greenhouse cultivation in Shouguang city

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Abstract

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Introduction

Materials And Methods

Experiment site

Experimental design, irrigational treatment and samples collection

Determination of soil properties and enzyme activity

Determination of the soil bacterial community structure

Measurement of leaf and fruit morphological parameters and fruit quality

Statistical analysis

Results

Rationality analysis of sequencing of the soil samples

Changes in the alpha-diversity indexes of the bacteria in soil

Changes in the microbial communities at the phylum level in soils

Changes in the microbial communities at the phylum level in soils

Changes in the physicochemical properties of soil

Correlation between soil bacteria and soil properties

Discussion

Magnetized water irrigation influences the bacterial community in the soil

Magnetized water irrigation influences the physical properties and enzyme activities in soil

Conclusion

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References

Tables

Additional Declarations

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