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Article

Organic Materials and AMF Addition Promote Growth of Taxodium ‘zhongshanshan’ by Improving Soil Structure

by
Jingyi Zeng
1,†,
Shilin Ma
1,†,
Jing Liu
2,
Shenghua Qin
3,
Xin Liu
1,
Tao Li
1,
Yi Liao
1,
Yuxuan Shi
1 and
Jinchi Zhang
1,*
1
Co-Innovation Center for Sustainable Forestry in Southern China, Jiangsu Province Key Laboratory of Soil and Water Conservation and Ecological Restoration, Nanjing Forestry University, 159 Longpan Road, Nanjing 210037, China
2
China Forestry Group Holding Co., Ltd., Beijing 100084, China
3
Dafeng Forest Farm, Yancheng 224111, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2023, 14(4), 731; https://doi.org/10.3390/f14040731
Submission received: 19 February 2023 / Revised: 22 March 2023 / Accepted: 30 March 2023 / Published: 3 April 2023
(This article belongs to the Section Forest Soil)
Browse Figures
Versions Notes

Abstract

:
Soil salinization is considered a type of global-scale soil degradation, whereby excessive salinity severely diminishes soil health, which is primarily manifested through disrupted soil structures and reduced fertility. Furthermore, plant growth capacity is inhibited, and productivity is diminished. Consequently, the improvement of saline soils is regarded as a particularly important aspect of enhancing land production. To elucidate the roles of organic amendments and mycorrhizal fungi in the improvement of saline soils, seven treatments were set up, including biochar alone (B), straw alone (S), arbuscular mycorrhizal fungi alone (A), biochar in combination with arbuscular mycorrhizal fungi (BA), straw in combination with arbuscular mycorrhizal fungi (SA), and a control (CK). The results revealed that the seedling height growth rate under the BA treatment was significantly higher than that of the CK by 31.66%. The capillary porosity of the soil under the addition of BA was significantly higher than the CK by 3.17% in the 0–20 cm soil layer. The BA treatment reduced the fractal dimension of soil aggregates considerably by 19.06% in the 0–20 cm soil layer, and 13.71% in the 40–60 cm soil layer in contrast to the CK, respectively. In addition, the positive effects of the BA treatment were significant in the 20–40 cm soil layer. Overall, the application of biochar alone promoted the water stability of soil aggregates. The combination of arbuscular mycorrhizal fungi and biochar promoted plant growth, improved soil pore structures, promoted agglomerate water stability, and led to improved microbial activities. The results showed that organic amendments applied in conjunction with AMF improved the environment of salinized soil, which is a key factor in the promotion of plant growth and the long-term stability of soil health. This study provides a key technical basis for remediation of salinized soil.

1. Introduction

Salinization of soil is a major form of land degradation worldwide [1]. Natural and anthropogenic factors such as rising temperatures and rapid global economic development have steadily increased the extent of saline soils [2,3]. Soil condition is the main factor affecting plant survival [4]. Soil salinization hinders plant growth and even leads to death due to the deteriorating soil structure and physiological and biochemical environment [5], then eventually restricts the development of coastal protection forests [6]. Therefore, it is of great importance to optimize saline soil.
Plant growth depends on viable soil structures to facilitate the extension of roots [7,8]; however, the presence of abundant sodium ions in the soil leads to the dispersion of soil particles [9], which translates to the hardening of the soil and the deterioration of its structure. The deterioration of soil structures is manifested primarily by the decreased content of large aggregates and the reduction in soil capillary pore space [10]. The pore structure consists of zones to store water, air, nutrients, etc., and its formation is determined largely by soil aggregates. [11]. Plant roots permeate the soil through pore networks to absorb nutrients to support plant growth and development [12]. Furthermore, soil pores provide sites for microbial activities, which in turn promote nutrient release and the mineralization of organic matter [13] and indirectly contribute to the cementation of soil particles [4]. However, the deterioration of the soil structure limits the space required for microbial activities, thus hindering this reciprocal effect. Thus, researchers have undertaken intense research to remediate the harsh conditions brought about by salinized soil.
Chemical [14], physical [15], and biological [16] measures have the capacity to improve salinized soil; however, long-term practical studies have found that chemical and physical additives produce residues that contaminate the soil. Thus, in recent years, environmentally compatible organic materials (e.g., plant residues, garbage, biochar [17,18,19], and straw [20,21,22]) are mostly used. As an important source of soil organic matter, the application of straw mulch has been considered to be an effective measure to block the upward return of soil salts [23]. Meanwhile, straw has been reported to affect soil physiology, and the release of nutrients during its decomposition also affects plant physiology and promotes plant growth in several ways [24]. Biochar is known as an amendment that is rich in organic matter and that is obtained by pyrolyzing organic wastes (e.g., straw, fallen matter, crop residues) at high temperatures [25]. It has been shown to promote plant growth through multiple pathways [26]. On the one hand, biochar has a large specific surface area, a property that largely affects the ability of soil particles to adsorb water and nutrients, and its addition directly provides growth-promoting nutrients for plants [27]. On the other hand, indirectly, it improves the physical and chemical properties and biological characteristics of the soil. In addition, biochar and straw possess an abundant porous structure [28] that can be added to the soil to enhance air permeability. Furthermore, the decomposition of biochar and straw has been shown to increase the input of organic matter, accelerate the aggregation of soil particles, and optimize the pore structure [29]. Arbuscular mycorrhizal fungi (AMF) are the most common plant root symbionts among soil micro-organisms [30] and can establish symbiosis with 80% of terrestrial plants [31]. AMF have been shown to improve the tolerance of plants [32] through colonization. It has been found that AMF can mitigate the threat of salt stress to plants by improving antioxidant capacity and increasing polyamine contents and their metabolic enzymes [33]. At the same time, the mycelial network that is established by AMF and plant roots [34] enlarges the extension space of roots [35] and facilitates root penetration through the soil, thus improving soil pore systems and optimizing their structure [35].
Even though the impact of organic materials and AMF on soil structure and microbial environment has received much attention in recent years [18,20,22,36], the synergistic impacts of organic materials combined with AMF have rarely been investigated. In summary, to investigate the effect of organic materials and AMF, five amendments (biochar, straw, AMF, biochar mixed with AMF, and straw mixed with AMF) were applied to the soil to establish field tests. The outcomes of the different amendments were compared by studying the differences in plant growth, soil structures and microbial activities, with the aim of selecting the most effective additive before providing theoretical and practical support for the construction and development of coastal protection forests. We proposed the following hypotheses in the pre-experimental stage: (1) the addition of organic material and AMF can promote plant growth, (2) the application of organic material and AMF can improve soil porosity, aggregate structure and soil microbial activities, and (3) the combined application of biochar and AMF can promote plant growth by improving soil structure and soil microbial activities.

2. Materials and Methods

2.1. Study Area

The trials for this study were conducted in Jiangsu, China (32°56′~33°36′ N, 120°13′~120°56′ E). The study area is home to a subtropical monsoon climate with warm winters and hot and humid summers, with an average annual temperature of 14.1 °C, average annual rainfall of 1042.2 mm, a frost-free period of ~230 d, and 2238.9 d of sunshine. The electrical conductivity of the native soil in the study area ranged from 1.6 to 5 mS/cm [37]. Local plants include Phragmites australis [38], Suaeda salsa [39], Solidago canadensis L., Spartina anglica [40], Scirpus mariqueter, Picris hieracioides L., etc. [41].

2.2. Sample Plot Setting

Plant residues, such as root stubble, weeds, etc. were removed from the field prior to the start of the experiment. The experimental plot was divided into six plots, with the different plots separated by soil ridge once land leveling was completed. Two organic materials (biochar and straw) and fungicides (arbuscular mycorrhizal fungi) were selected as test materials.
A total of six treatments were established, with three replications for each treatment, namely, biochar (B), biochar and arbuscular mycorrhizal fungi mixture (BA), straw and arbuscular mycorrhizal fungi (SA), straw (S), arbuscular mycorrhizal fungi (A), and a control (CK).

2.3. Experimental Design

Two-year-old seedlings of Taxodium ‘zhongshanshan’ were planted in the spring of 2018 with a planting density of 3 m × 3 m. Each plot was planted with 18 Taxodium ‘zhongshanshan’ for a total of 108 trees, with an average seedling height of 2.06 m and a basial diameter of 2.90 cm.
The test materials were applied in a one-time application in the spring of 2018, with an added weight of 0.5 kg per seedling in all cases. The biochar was comprised of rice husk charcoal, which was carbonized via pyrolysis at 800 °C and was applied around the seedlings following repeated tillage and thorough mixing with the soil. The straw was provided by Dafeng Forestry, and a layer was mulched 40 cm below the surface in the main root zone of the seedlings. The arbuscular mycorrhizal fungi (AMF) from Funneliformis mosseae were provided by the Institute of Plant Nutrition and Resources, Academy of Agricultural and Forestry Sciences, Beijing, China. The AMF strain was trapped by using maize and clover as carriers and autoclaved yellow sand (0.14 MPa, 121 °C, 2 h) as a substrate. After three months of propagation, a spore mixture consisting of root fragments, mycelium and yellow sand was obtained. This mixture was then applied to the seedling roots as an inoculant [33].

2.4. Sample Collection

A five-point sampling method was employed to collect the soil samples in October 2021. Three Taxodium ‘zhongshanshan’ with relatively uniform growth were randomly selected in the experimental plots (corresponding to the six treatments) to measure seedling height and basal diameter and collect the soil around the trees. The soil was divided into 3 layers (0–20 cm, 20–40 cm and 40–60 cm, respectively) below the land surface. The soil samples were extracted from each soil layer with ring knives for the determination of their basic physical indices. Then, the samples were excavated from the corresponding layers for the determination of soil aggregates.

2.5. The Determination of Samples

Wet sieving was performed with reference to the method of Elliot [42], where 50 g of dried soils was placed at the top of a nest of sieves (2, 1, 0.5, 0.25, 0.106 and 0.053 mesh). The sieves were placed into a bucket of water and nested such that the largest mesh was at the top. The soil was spread evenly on the metal mesh of the 2 mm sieve, after which, distilled water was added to the bucket, and the water level was raised until the soil could be wetted for 10 min. The sieve was then shaken at 30 times min−1 for 30 min. After sieving, the sieves were left undisturbed in a bucket of water for 5 min to allow the fine particles to settle out. The soil aggregates remaining on each sieve were collected by using aluminum cassettes, which were then placed on a low-temperature electric plate to dry and be weighed (accurate to 0.01 g).
The calculation formula of each soil aggregate indexes is as follows:
Mean weight diameter:
M W D = ∑ i = 1 n x i − ω i
Geometric mean diameter:
G M D = e x p ⁡ ( ∑ i = 1 n ω i ln ⁡ x i − )
Fractal dimension:
( ( 3 − D ) log 10 ⁡ ( x i − / x m a x ) = log 10 ⁡ ω δ < x i − / ω 0
log 10 ⁡ ( x i − / x m a x ) and log 10 ⁡ ω δ < x i − / ω 0 were used as horizontal and vertical coordinates, respectively, to draw the plot. The slopes of the experimental lines of log 10 ⁡ ( x i − / x m a x ) and log 10 ⁡ ω δ < x i − / ω 0 were 3-D, and the soil D value was calculated via the equation. x i − is the average diameter of soil aggregate with particle size; ω i is the mass percentage of soil aggregate particle size; x m a x is the average diameter of soil aggregate with maximum particle size; ω δ < x i − is the mass of soil smaller than i particle size (g). ω 0 is the total soil mass (g).
The soil capillary porosity and non-capillary porosity were determined via the ring knife method, while the soil pH and conductivity were determined by using the 1:5 soil–water ratio potentiometric method. The soil peroxidase activity was determined by using the potassium permanganate titration method. The phosphatase activity was determined via the colorimetric method by using p-nitrophenyl disodium phosphate, while the urease activity was determined via the phenol-sodium hypochlorite colorimetric method. The sucrase activity was determined colorimetrically by using 3, 5-dinitrosalicylic acid. Referring to the method of Qiu et al. [43], the soil peroxidase, phosphatase, urease and sucrase activities measured by the experiment were normalized and integrated into the soil microbial activity.

2.6. Statistical Analyses

One-way ANOVA followed by Duncan’s test (SPSS 26.0 Inc., Chicago, IL, USA) was used to analyze differences in plant seedling height and ground diameter growth rate between treatments with different additions, as were differences in soil porosity, aggregate particle size distribution and water stability, and soil microbial activity between soil layers and treatments under different additions. Correlation and boosted regression tree analysis were performed by using the R 4.0.5 statistical program to examine the correlations between soil layers, soil capillary porosity, and non-capillary porosity, as well as the mean weight diameter, geometric mean diameter, fractal dimension of the aggregates, soil microbial activity, and plant growth. Furthermore, the contribution of these indicators to plant height and ground diameter growth rates was assessed.

3. Result

3.1. Plant Growth Rate

The application of additives increased the seedling height and basal diameter growth rate of Taxodium ‘zhongshanshan’. When treated with BA, the seedling height growth rate was significantly higher by 31.66% than that of CK (Figure 1A). Consistent with the seedling height growth rates, the basal diameter growth under the BA treatment was 60% and 132% higher than the B treatment and CK, respectively (Figure 1B). In addition, the basal diameter growth rate under the B treatment was also significantly higher than that of CK (Figure 1B).

3.2. Soil Porosity

The capillary porosity of soil under the BA treatment was significantly higher than the CK by 3.17% in the 0–20 cm soil layer (Figure 2A). However, no significant difference was observed in capillary porosity and non-capillary porosity between all treatments for the 20–40 cm and 40–60 cm soil layers.

3.3. Soil Aggregate Stability

Two-way ANOVA revealed that the soil layer had significant effects on the mean weight diameters and geometric mean diameters, as well as the fractal dimension of the aggregates. (p < 0.05). The treatments had a significant effect on the fractal dimension of the aggregates. (p < 0.05).
Both the mean weight diameter and geometric mean diameter showed a decreased trend within the deeper soil layers. For the 20–40 cm soil layer, the addition of B increased the mean weight diameter significantly (by 58.08%) compared to the CK. (Figure 3A). The geometric mean diameter was increased by 55.80% under the BA treatment in the 0–20 cm soil layer, while it was significantly increased (by 159.55%) for the B treatment in the 20–40 cm soil layer (Figure 3B).
The fractal dimension tended to increase in the deeper soil layers. The BA treatment significantly reduced the fractal dimension of soil aggregates by 19.06% compared with the CK in the 0–20 cm soil layer. Similarly, the BA treatment resulted in a significantly lower fractal dimension of aggregates than the CK by 13.71% in the 40–60 cm soil layer (Figure 3C).

3.4. Soil Microbial Activity

The average of the z-score of the activities of each enzyme was calculated to assess the soil microbial activity (Figure 4). CK treatment has a negative effect on soil microbial activity, whereas the BA treatment and A treatment had the opposite effect. Specifically, the positive effect of the BA treatment was significant in the 20–40 cm soil layer.

3.5. Correlation Analysis

The correlation calculated by R revealed that the growth rates of plant seedling height and ground diameter were correlated with both the stability of soil aggregates and microbial activities (Figure 5). The seedling height growth rate had a significantly positive correlation with the mean weight diameter and geometric mean diameter of aggregates in the 0–20 cm and 40–60 cm soil layer (Figure 5). On the contrary, the seedling height growth rate was significantly negatively correlated with the fractal dimension of aggregates and non-capillary porosity in the 40–60 cm soil layer (Figure 5). The basal diameter growth rate was significantly positively correlated with the MWD, GMD, and capillary porosity in the 0–20 cm and 40–60 cm soil layers (Figure 5) and was significantly negatively correlated with the fractal dimension of aggregates in the 20–40 cm soil layer (Figure 5). Moreover, both the seedling height and basal diameter growth rates were significantly positively correlated with the microbial activity in the 0–20 cm soil layer (Figure 5), while only the basal diameter growth rate showed a significantly positive correlation with microbial activity in the 20–40 cm layer (Figure 5).

4. Discussion

4.1. Effects of Organic Materials and AMF Addition on Plant Growth

Since plants depend on the soil as their main source of nutrients, the growth of plants is intricately related to soil conditions [7]. High salinity can cause disruption of soil health and triggers ionic imbalance in plants, causing osmotic stress and oxidative stress and leading to various physiological disorders which then cause growth retardation and yield reduction [5].
The response of plant growth to organic material and AMF was investigated by conducting experiments, and the results showed that both biochar and straw additions increased plant growth rates in saline soil. Consistent with other studies, the present study showed that the addition of biochar [44] and straw [20] helped to maintain plant growth under extreme conditions in saline [45], acidified [46], and heavy-metal-contaminated soils [47]. Meanwhile, our experimental results showed that the application of biochar significantly promoted plant growth compared with the straw. On the one hand, studies have demonstrated that biochar can improve plant survival [48] and reduce the toxic effects of heavy metals on plant growth and physiology [49] and have shown great potential in promoting plant growth [47,50]. On the other hand, biochar can enhance plant growth by improving nutrient utilization [51]. It provides organic matter for plants and can promote the enrichment of the nutrient nitrogen [51] and increase the effectiveness of the nutrient phosphorus [52], which may be the reason of our experimental results [28].
Many studies have demonstrated that AMF may serve as a plant growth promoter [53,54,55], which was also confirmed by our results, for which the application of AMF promoted both plant seedling height and ground diameter growth rates. Liang et al. also revealed that inoculation with AMF increased the biomass of plants in salt-stressed soil [32].
In particular, we found that the biochar combined with AMF promoted plant growth significantly, compared with other treatments in this study, which is consistent with previous experiments [56]. Gujre et al. [57] reviewed studies on the use of biochar combined with AMF and indicated that it was effective in promoting plant growth. The advantageous effects of biochar and AMF were attributed to the improvement of soil by biochar and the enhancement of plant tolerance through AMF colonization. Consequently, plant growth was more effectively enhanced by the synergistic action of biochar and AMF, which supports our first hypothesis.

4.2. Effects of Organic Materials and AMF Additions on Soil Structure

It was believed that soil structures impact the propagation of plant roots [8], the exchange of water and gas in the soil [58], and soil fertility, and these soil structures provide habitats for soil micro-organisms and drive changes in microbial communities [59]. Conversely, biological and abiotic processes can modify and even reshape soil structures [60]. According to earlier studies [35,61], soil structure can be characterized by pore networks and the stability of aggregates. High capillary porosity indicates a good water and air permeability of the soil [62], whereas the high geometric mean diameter and mean weight diameter of aggregates indicate good soil stability [63,64]. On the contrary, the stability of aggregates is improved when the fractal dimensions are low [65].
We experimentally investigated the contribution of the addition of organic materials and AMF to soil structure and found that the application of biochar combined with AMF increased the capacity porosity of soil. Earlier studies have shown that soil physical properties may be influenced by the addition of biochar. This influence was mainly due to the fact that biochar has large pores, which is a property that largely determines its outstanding ability in the improvement of the soil pore system [66]. In addition, AMF inoculation was shown to promote plant root growth [34]. Therefore, we suggested that biochar addition improved soil structure by providing more extension space for plant roots inoculated with AMF, which accounted for the superior ability of biochar in promoting plant growth in this study.
The water-stability of the soil aggregate is an important indicator of soil structure. The results of this study showed that the soil layer significantly influenced the water stability of aggregates (p < 0.05), and the addition of biochar increased the MWD and GMD of the aggregate. Compared with CK, the MWD and GMD of soil aggregate were increased by 58.08% and 159.55%, respectively. At the same time, the fractal dimension of the aggregate was reduced. These results proved that the addition of biochar was effective in altering the soil aggregates. Studies have found that biochar could improve the availability of soil nutrients and increase the content of soil organic matter through its own properties [28] and decomposition [67]. Moreover, biochar is an important source of soil organic carbon, a substance closely related to soil structure [68]. Its addition can lead to soil particle aggregation by increasing soil organic carbon [69] and accelerating the rate of soil particle aggregation [70], increasing the content of large aggregates and improving aggregate stability, which may be one reason for our experimental result. Studies have also suggested that biochar can be used as a supplement to modifiers and can be applied in combination with other organic materials to improve the stability of aggregates [71,72]. Meanwhile, we noticed that the mean weight diameter and geometric mean diameter of aggregates were increased through the application of AMF alone. This result was also explained in the research of Cho et al. [73]. They found that AMF inoculation increased the range of 1–2 mm and 1–0.5 mm aggregates, which were similar in our study (Figure S1).
In this study, the influence of addition treatments on the fractal dimension of soil aggregates was significant (p < 0.05), such that the combined application of biochar and AMF significantly reduced the fractal dimensions of aggregates, indicating that biochar and AMF enhanced the aggregation of soil particles and reduced the degree of dispersion of soil particles. This result verified that biochar and AMF synergistically co-operate to enhance soil improvements in contrast to their independent effects alone. This may be attributed to biochar, which provides a safe habitat for AMF, thus creating greater mycelial root symbiosis while promoting root aggregation and the segregation of soil particles [74]. Furthermore, it has been found that different plant growth stages and biochar amounts may influence AMF colonization in plant roots by affecting soil physical properties, which may explain the differences between the separate use of biochar and AMF and their combination in this study [27]. In addition, one study investigated the utility of biochar and AMF under low temperature stress and found that the combined application of biochar and AMF reduced malondialdehyde content but had no significant effect on proline content, which also demonstrated that the mechanism of biochar in combination with AMF was diverse but not comprehensive [75].
Correlation analysis showed that plant growth was positively correlated with MWD and GMD, negatively correlated with fractal size, and positively correlated with capillary porosity. This indicates that plant growth in this study was greatly influenced by the physical structure of the soil. This finding confirmed our second hypothesis, namely, that the combined application of biochar and AMF has a more superior effect in improving soil structure.

4.3. Effects of Organic Materials and AMF Addition on Soil Microbial Activities

Soil enzyme activities correlate with those of soil microbes, and the normalization of microbial activities by using enzyme activity indices can reflect the overall microbial activity status of the soil [43]. In this study, we integrated soil urease, catalase, phosphatase, and sucrase activities into soil microbial activity, thus providing a unified description of micro-organisms that are associated with soil nutrients. The analysis results showed that AMF alone conveyed a positive effect on soil microbial activities compared with the CK, which was similar with the results of other studies [76]. The reasons for this result, we suspected, may be complex. On the one hand, AMF is considered to be a beneficial fungus that can symbiotically grow with most plants [54], and it can not only promote the plant growth but also contribute to the adjustment of the soil microbial environment [77]. It interacted with soil micro-organisms to promote microbial enrichment and compensate for deficiencies [77,78]. On the other hand, AMF altered the structures of the soil microbial communities between plant roots through proliferation and colonization [79]. While supporting plants to absorb soil nutrients, conducting water [80], and improving plant tolerance, the presence of AMF affects nutrient-related micro-organisms [81]. Plant photosynthesis products can be transferred into soil by mycorrhizal symbionts [82]. During the process of the nutrient cross-conduction, soil nutrients such as carbon, nitrogen, and phosphorus change, which in turn affects the activity of nutrient-related enzymes [83]. In general, this may explain the positive feedback of soil microbial activities in our study.
The addition of biochar was found to have an inhibitory influence on microbial activities in this study. It was consistent with some research, in which biochar was found to produce harmful substances during decomposition [84], which subsequently leads to the inhibition of microbial activities. This may have accounted for the result of our research. A particular feature was that the combination of biochar and AMF exhibited positive effects on soil microbial activities. On the basis of the result that a positive effect of AMF was found above, we can speculate that the presence of AMF weakened the toxic effects caused by biochar. At the same time, AMF itself possesses the ability to enrich micro-organisms [78], and this ability counteracts the inhibitory effect of biochar.
Correlation analysis showed that there was a significant positive correlation between plant growth and soil microbial activity, indicating that plant growth would be regulated by soil microbial. On contrary, soil microbial activities showed a positive response to AMF and the combined addition of biochar and AMF, which supported our second hypothesis that the combined addition of organic material and AMF could enhance soil microbial activity, especially the combination of biochar and AMF.

4.4. Combined Effect of Organic Materials and AMF on Soil Condition and Plant Growth

Many studies have been conducted on biochar, straw, and AMF, showing that biochar, straw, and AMF play key roles in heavy-metal-contaminated, saline, acidified, and drought-prone soil [20,32,33]. Based on the results of this study, we can affirm the previous hypotheses that the application of organic materials and AMF (1) promoted plant growth; (2) improved soil pore structure, aggregate stability, and soil microbial activities; (3) and then promoted plant growth by improving soil structure and microbial activity. According to our results, the combined application of biochar and AMF played a significantly prominent role in improving soil conditions and promoting plant growth. However, compared with the combination of biochar and AMF, the combined application of straw and AMF did not show outstanding effects. We speculate that one of the reasons may arise from the fact that straw plays a major role in blocking the upward return of salts rather than directly changing the soil pore and agglomeration structure. Meanwhile, in a study by Dai et al. [60], it was proposed that the promotion of plant growth by biochar showed a significant decrease in clay texture, and different organic material feedstocks and different soil types could lead to differences in the results, which could be another reason for the difference in the performance of biochar and straw in this study. Thus, by conducting comparisons and analyses, we found that the combined application of biochar and AMF was the most effective method that was used to improve the soil and promote plant growth in this study.

5. Conclusions

In our study, the application of biochar, straw, and AMF was found to promote plant growth in saline soils, with biochar and AMF playing a more prominent role. In particular, the combination of biochar and AMF improved soil pore structure and aggregate structure by increasing soil capillary porosity and positively affecting soil microbial activity, which was consistent with our hypothesis. Therefore, we summarize the results of our study and suggest that biochar and AMF can establish the combined effects on improving soil structure and mobilizing soil microbial activity to sustain plant growth. This finding could be effective technical support for saline land improvement in future practice. In addition, we hope to further investigate the mechanisms of the improvement of soil conditions via organic materials and AMF in order to enable the effective use of amendment materials in the processing of saline soils.
Overall, the combined application of biochar and AMF is a proven measure to promote plant growth in saline soils and can improve plant growth and productivity, which may have great significance for afforestation.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f14040731/s1, Figure S1. Variations in the aggregate size distribution under treatments. Lowercase letters indicate significant differences between various salt concentrations under the inoculation treatments (p ˂ 0.05).

Author Contributions

Conceptualization, J.Z. (Jingyi Zeng), S.M. and J.L.; data curation, J.Z. (Jingyi Zeng), S.M. and J.L.; formal analysis, J.Z. (Jingyi Zeng), S.M. and J.L.; funding acquisition, J.Z. (Jinchi Zhang); investigation, J.Z. (Jingyi Zeng), S.M., J.L., T.L., S.Q. and Y.S.; methodology, J.Z. (Jingyi Zeng), S.M., J.L., T.L. and Y.L.; project administration, J.Z. (Jinchi Zhang); resources, J.Z. (Jinchi Zhang); software, J.Z. (Jingyi Zeng); supervision, J.Z. (Jinchi Zhang); validation, J.Z. (Jingyi Zeng), S.M. and J.L.; visualization, J.Z. (Jingyi Zeng), S.M., J.L. and T.L.; writing—original draft, J.Z. (Jingyi Zeng) and S.M.; writing—review and editing, J.Z. (Jingyi Zeng), S.M., X.L. and J.Z. (Jinchi Zhang). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Jiangsu Science and Technology Plan Project [BE2022420]; the Innovation and Promotion of Forestry Science and Technology Program of Jiangsu Province [LYKJ (2021) 30]; and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

We thank Frank Boehm from Lakehead University for the language editing of this manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations/Nomenclature

AMF: arbuscular mycorrhizal fungi; SH: seedling height growth rate; BD: basal diameter growth rate; CP: capillary porosity; NCP: non-capillary porosity; SMA: soil microbial activity; MWD: mean weight diameter; GMD: geometric mean diameter; D: fractal dimension.

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Figure 1. Variations in plant growth rate under different treatments. (A) Seedling height growth rate; (B) basal diameter growth rate. Different lowercase letters indicate significant differences between various treatments (p < 0.05). Error bars refer to standard errors.
Figure 1. Variations in plant growth rate under different treatments. (A) Seedling height growth rate; (B) basal diameter growth rate. Different lowercase letters indicate significant differences between various treatments (p < 0.05). Error bars refer to standard errors.
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Figure 2. Variations in soil porosity under different treatments. (A) Soil porosity in 0–20 cm soil layer; (B) soil porosity in 20–40 cm soil layer; (C) soil porosity in 40–60 cm soil layer. Different lowercase letters indicate significant differences between various treatments (p < 0.05). Error bars refer to standard errors.
Figure 2. Variations in soil porosity under different treatments. (A) Soil porosity in 0–20 cm soil layer; (B) soil porosity in 20–40 cm soil layer; (C) soil porosity in 40–60 cm soil layer. Different lowercase letters indicate significant differences between various treatments (p < 0.05). Error bars refer to standard errors.
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Figure 3. Variations in stability indicators of aggregates between treatments. (A) Mean weight diameter of aggregates; (B) geometric mean diameter of aggregates; (C) fractal dimension of aggregates. Different uppercase letters indicate significant differences between different soil layers with the same treatment (p < 0.05). Different lowercase letters indicate significant differences between treatments in the same soil layer (p < 0.05). Error bars refer to standard errors.
Figure 3. Variations in stability indicators of aggregates between treatments. (A) Mean weight diameter of aggregates; (B) geometric mean diameter of aggregates; (C) fractal dimension of aggregates. Different uppercase letters indicate significant differences between different soil layers with the same treatment (p < 0.05). Different lowercase letters indicate significant differences between treatments in the same soil layer (p < 0.05). Error bars refer to standard errors.
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Figure 4. Variation in soil microbial activity indices between treatments. Different uppercase letters indicate significant differences between different soil layers under the same addition of treatment (p < 0.05). Different lowercase letters indicate significant differences between additions in the same soil layer (p < 0.05). Error bars refer to standard errors. The average of the z-score of each soil enzyme activity was calculated to indicate soil microbial activities.
Figure 4. Variation in soil microbial activity indices between treatments. Different uppercase letters indicate significant differences between different soil layers under the same addition of treatment (p < 0.05). Different lowercase letters indicate significant differences between additions in the same soil layer (p < 0.05). Error bars refer to standard errors. The average of the z-score of each soil enzyme activity was calculated to indicate soil microbial activities.
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Figure 5. Abbreviations: SH—seedling height growth rate; BD—basal diameter growth rate; CP—capillary porosity; NCP—non-capillary porosity; MWD—mean weight diameter; GMD—geometric mean diameter; D—fractal dimension; SMA—soil microbial activity. * indicates significant correlation at 0.05 level; ** indicates significant correlation at 0.01 level.
Figure 5. Abbreviations: SH—seedling height growth rate; BD—basal diameter growth rate; CP—capillary porosity; NCP—non-capillary porosity; MWD—mean weight diameter; GMD—geometric mean diameter; D—fractal dimension; SMA—soil microbial activity. * indicates significant correlation at 0.05 level; ** indicates significant correlation at 0.01 level.
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MDPI and ACS Style

Zeng, J.; Ma, S.; Liu, J.; Qin, S.; Liu, X.; Li, T.; Liao, Y.; Shi, Y.; Zhang, J. Organic Materials and AMF Addition Promote Growth of Taxodium ‘zhongshanshan’ by Improving Soil Structure. Forests 2023, 14, 731. https://doi.org/10.3390/f14040731

AMA Style

Zeng J, Ma S, Liu J, Qin S, Liu X, Li T, Liao Y, Shi Y, Zhang J. Organic Materials and AMF Addition Promote Growth of Taxodium ‘zhongshanshan’ by Improving Soil Structure. Forests. 2023; 14(4):731. https://doi.org/10.3390/f14040731

Chicago/Turabian Style

Zeng, Jingyi, Shilin Ma, Jing Liu, Shenghua Qin, Xin Liu, Tao Li, Yi Liao, Yuxuan Shi, and Jinchi Zhang. 2023. "Organic Materials and AMF Addition Promote Growth of Taxodium ‘zhongshanshan’ by Improving Soil Structure" Forests 14, no. 4: 731. https://doi.org/10.3390/f14040731

APA Style

Zeng, J., Ma, S., Liu, J., Qin, S., Liu, X., Li, T., Liao, Y., Shi, Y., & Zhang, J. (2023). Organic Materials and AMF Addition Promote Growth of Taxodium ‘zhongshanshan’ by Improving Soil Structure. Forests, 14(4), 731. https://doi.org/10.3390/f14040731

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