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Article

Dynamics of Soil N and P Nutrient Heterogeneity in Mixed Forest of Populus × Euramercana ‘Neva’ and Robinia pseucdoacacia in Coastal Saline–Alkali Land

by
Shumei Wang
1,
Changxiao Lv
2,
Bingxiang Tang
1,
Mengxiao Wang
3,
Banghua Cao
1,* and
Ke Wu
2,*
1
Key Laboratory of State Forestry and Grassland Administration Silviculture of the Lower Yellow River, College of Forestry, Shandong Agricultural University, Tai’an 271018, China
2
Shandong Provincial Institute of Land and Space Planning, Jinan 250199, China
3
Shandong Academy of Forestry, Jinan 250014, China
*
Authors to whom correspondence should be addressed.
Forests 2024, 15(12), 2226; https://doi.org/10.3390/f15122226
Submission received: 6 November 2024 / Revised: 28 November 2024 / Accepted: 13 December 2024 / Published: 17 December 2024
(This article belongs to the Special Issue Forest Vegetation and Soils: Interaction, Management and Alterations—Second Edition)
Browse Figures
Versions Notes

Abstract

:
The mixing of poplar and robinia in coastal saline land is a useful attempt at difficult site afforestation. Investigating the long–term mixing effects of nitrogen–fixing and non–nitrogen–fixing tree species on the spatial heterogeneity of N and P nutrients and their ecological stoichiometric characteristics in the coastal saline–alkali soil can provide a scientific basis for soil improvement and plantation management in the coastal saline–alkali soil. By replacing time with space, poplar and robinia mixed forests and corresponding pure forests with the ages of 3, 7 and 18 years were selected, and soil profiles of 0–20 cm, 20–40 cm and 40–60 cm were dug up to determine the contents of total nitrogen, hydrolyzed nitrogen, total phosphorus and available phosphorus, the activities of soil urease and phosphatase and the number of soil bacteria, fungi and actinomycetes in rhizosphere soil. The mixture of poplar and robinia and the increase in planting years led to the heterogeneity of soil N and P in a coastal saline–alkali forest, which could significantly increase the contents of soil total nitrogen, hydrolyzed nitrogen, total phosphorus and available phosphorus between soil layers. Compared with the pure forest of poplar and robinia at the same age, the soil urease activity in the 0–20 cm soil layer of an 18a poplar and robinia mixed forest increased by 94.75% and 73.36%, and the soil phosphatase activity increased by 30.36% and 70.27%. The mix of poplar and robinia significantly increased the abundance of soil microorganisms in saline–alkali soil. The number of bacteria, fungi and actinomycetes in the 0–20 cm soil layer of the 18a poplar and robinia mixed forest was the highest, which were 703,200, 31,297 and 1903, respectively. Redundancy analysis showed that there was a significant positive correlation between soil N and P nutrient contents, soil enzyme activities and microbial abundance. The soil depth of N and P nutrient decomposition and transformation in the mixed poplar and robinia plantation was expanded. The soil N and P nutrient contents, enzyme activities and microbial abundance in the 40–60 cm soil layer of the mixed forest were higher than those of the pure forest. With the increase in plantation years, the depth of soil that can be used in the forest land is increasing. The mixture of poplar and robinia plantation is an excellent choice for the construction of coastal saline–alkali land plantation, which has a significant mixed gain for the decomposition and transformation of N and P nutrients and increases the depth of the available soil layer in the forest land in coastal saline–alkali land. However, the coastal saline–alkali land soil N/P is < 14 and is still restricted by nitrogen, so the application of nitrogen fertilizer can be increased during the afforestation process.

1. Introduction

As an important reserve land resource in China, coastal saline–alkali land has great potential, and its economic, social and ecological value cannot be ignored [1]. However, the problem of soil salinization has caused the ecological environment of the Yellow River Delta to be very fragile, and the artificial forest ecosystem has poor self–regulation and recovery ability, resulting in great difficulties in ecological construction and forestry development [2,3]. In coastal saline–alkali land, the soil development time is short, the soil layer changes are complex, and the soil spatial distribution is different, so soil N and P nutrients also have such spatial heterogeneity and are coupled with a single tree species in plantation forests, resulting in a reduction in the productivity and ecological benefits of many planted forest stands [4].
A large number of studies have shown that mixed forests in coastal saline–alkali land have better resistance to salt [5,6]. The complex stand structure formed by mixed forests increases the net release rate of soil nutrient elements, which is conducive to the turnover and circulation of soil nutrients and improves soil fertility [7,8]. Studies have shown that the net release rate of nitrogen, phosphorus and potassium elements in a mix of Larix kaempgeri and Alnus tinctoria plantation can reach 48%~51%, while the pure forest is only 21.8%~31.2%. The mix of Robinia pseudoacacia and Amorpha fruticosa was planted in coastal saline–alkali land for 6 years, which made the forest stand early closed, and had an obvious salt suppression effect. The organic matter content of the 0–10 cm topsoil in the Robinia pseudoacacia and Amorpha fruticosa mixed forest land is 3 times that of the outside forest, and the total nitrogen content of the soil is more than 4–times higher [9].
The mixing of poplar and robinia is a successful example of mixed forests [10]. Poplar trunk has a straight shape, rapid growth and high timber rate. Robinia tillers have strong shoot ability and nitrogen–fixing root nodules, which can increase the nitrogen content in the soil. This typical combination of nitrogen–fixing and non–nitrogen–fixing tree species can significantly promote stand growth, increase biomass and improve resistance to pests and diseases [11,12]. Compared with a pure poplar forest under the same site condition, the DBH and height of poplar in the mixed poplar and robinia plantation increased by 253% and 187%, respectively. The storage volume of poplar alone in the mixed forest exceeded 42.7% of the pure forest, and the total storage volume exceeded 96.0% of the pure forest [13]. In recent years, many scholars have focused on the salinity tolerance [7,14,15], dynamics of fine roots [16,17] and nutrient transfer complementarity of poplar and robinia mixed forest species in coastal saline–alkali land [18,19,20]. There are few in–depth studies on the dynamic improvement in soil heterogeneity in the perennial poplar and robinia mixed forest in coastal saline–alkali land. Therefore, in this study, the poplar and robinia mixed forest and the corresponding pure forest with ages 3a, 7a and 18a in a coastal saline–alkali area were selected as the research objects by using the method of replacing time with space. This study explored the spatial distribution dynamic characteristics of soil N, P nutrients (total nitrogen, hydrolyzed nitrogen, total phosphorus, available phosphorus), soil enzyme activities (urease and phosphatase) and soil microbial numbers (bacteria, fungi and actinomycete) of poplar and robinia mixed at different forest ages, revealed the interannual benefits and mixed gains of soil N and P nutrient spatial heterogeneity and ecological stoichiometry characteristics of mixed nitrogen fixation and non–nitrogen fixing tree species in coastal saline–alkali land, provided a reference for soil improvement and scientific fertilization in coastal saline–alkali land, and better optimized the operation and management of plantations in coastal saline–alkali land.

2. Materials and Methods

2.1. Overview of the Research Area

The experimental site is located in Dongying City, Shandong Province, which is a typical coastal saline–alkali land with a warm temperate continental monsoon climate. The annual average temperature is 12.8 °C, the average relative humidity is 65%, the annual average precipitation is 555.9 mm, the annual average sunshine duration is 2428.6 h, the frost–free period is 206 days and the water table is about 1.5 m. The soil in the study area is salinized and aquic, intermingled with sand and clay, and does not form a good soil structure. The soil is salinized fluvial soil, which is deposited by the impact of the loess parent material on the seawater erosion parent material; for the sand and clay phases, the development time is short, and a good structure is not formed; the vertical direction is from top to bottom, the aquifer particles change from fine to coarse, the horizontal direction changes from south to north, the aquifer particles change from coarse to fine and the structure changes from single to layered.

2.2. Experimental Design and Sample Collection

The sample plots were screened in Dongying, and through the verification of afforestation year, three representative sample plots were selected, which were located in the Swan Lake Scenic Area (118°49′15′′ E, 37°24′31′′ N), the 9th Branch of the Second Regiment of the Yellow River Delta Production Base of Jinan Military Region in Gudao Town (118°46′47′′ E, 37°49′46′′ N) and the No. 1 Branch of Guangbei (118°16′01′′ E, 37°25′41′′ N) (Figure 1).
The forest was divided into robinia pure forest (R), 107–poplar pure forest (P) and poplar and robinia mixed forest (P × R), with forest ages of 3a, 7a and 18a, respectively. The experimental afforestation was afforested by 1–year–old seedlings and supplemented with same–age seedlings in the following year, and no other management was carried out after the establishment of the stand. Each test forest land was set up with 3 20 × 30 m tree sample plots for each wood ruler. Three tree plots of 20 × 30 m were set up in each test forest land and each tree was measured among each plot. The growth status of poplar and robinia in each plot is shown in Table 1.
In the main growing season of forest trees (22nd of August), five sites were selected in each sample plot, each of which was dug to take three layers of soil samples of 30 cm in length and width and 20 cm in height and sampled at vertical heights of 0–20 cm, 20–40 cm and 40–60 cm, and the rhizosphere soil was obtained by the shaking method. After digging the soil profile, the root system of poplar and robinia with a diameter of less than 1 cm was carefully dug at random at multiple points in the soil layer at different vertical depths. The large chunks of soil that were attached to the root system with a root diameter of less than 0.5 cm were removed. The soil that clung to the root system (less than 0.5 cm from the root surface) was gently shaken off, so as to obtain poplar and robinia rhizosphere soil. The rhizosphere soil was brought back to the laboratory in a sterile plastic bag and stored in a refrigerator at 4 °C to determine soil nutrient content (total nitrogen, total phosphorus, hydrolyzed nitrogen, available phosphorus), soil enzyme activity (urease, phosphatase) and soil microbial number (bacteria, fungi, actinomycetes).

2.3. Analysis Methods of Soil Nutrients, Enzyme Activity, and Microorganisms

The total nitrogen of the soil was determined by a distillation and boiling method with Kjeldahl nitrogen analyzer. The total phosphorus of the soil was determined by the alkali molten molybdenum antimony anti–colorimetric method. The soil hydrolyzed nitrogen was determined by the alkaline hydrolysis diffusion method. The soil’s available phosphorus was determined by the sodium bicarbonate method.
The soil urease activity was determined by indophenol blue colorimetric; The soil phosphatase activity was determined by benzene disodium phosphate colorimetric.
The number of soil microorganisms was determined by the dilution plate method. The number of bacteria was determined by using beef paste peptone agar medium with a dilution concentration of 10−4 and incubating at 37 °C for 5 d. The number of fungi was determined by using Martin–Bengal red agar medium with a dilution concentration of 10−2 and incubating at 28 °C for 3 d. The number of actinomycetes was determined by using a modified Gao No. 1 agar medium with a dilution concentration of 10−2 and incubating at 28 °C for 7 days.

3. Results

3.1. Changes in Total N and P Nutrient Contents

As shown in Table 2, the total nitrogen content of the soil in the three forest stands decreased with the increase in soil depth. The highest total nitrogen content in the 0–20 cm soil layer of the 18a poplar and robinia mixed forest was 1.55 mg·kg−1. When the forest age was 3a, the total phosphorus content in the 40–60 cm soil layer was slightly higher than that in the 0–20 cm and 20–40 cm soil layers. When the forest age was 7a and 18a, the changes in the soil total phosphorus content of the three forest stands were the same as that of total nitrogen, and the highest soil total phosphorus content in the 0–20 cm soil layer of the 7a poplar and robinia mixed forest was 1.28 mg·kg−1. Multiple comparative analyses showed that compared with pure poplar forest and pure robinia forest, the soil total nitrogen and total phosphorus contents of the poplar and robinia mixed forest were significantly increased, and the variation was greater between different forest ages and different soil layer positions.
The total nitrogen content of the soil in each soil layer of the mixed forest increased with the increase in forest age, and the total nitrogen content of the soil in the 7a and 18a mixed forests was significantly higher than that in the 3a forest stand, and the soil total nitrogen content in 40–60 cm soil layer of 18a mixed forest was significantly higher than that in other mixed forests. It could be seen that the planting period had a significant effect on the increase in soil total nitrogen content, and with the increase in planting years, the depth of the soil layer where soil total nitrogen content could be used continues to increase. Different from the total nitrogen content of the soil, the total phosphorus content of the soil in the mixed soil layer increased first and then decreased with the increase in forest age, and the 7a mixed forest was significantly higher than that of the 3a and 18a (Table 2).

3.2. Changes in the Contents of Hydrolyzed Nitrogen and Available Phosphorus

According to Table 2, the contents of soil hydrolyzed nitrogen and available phosphorus decreased with the increase in soil depth, and the content of available nutrients was the highest in the 0–20 cm soil layer. With the increase in forest age, the contents of hydrolyzed nitrogen and available phosphorus in each soil layer increased. The contents of soil hydrolyzed nitrogen and available phosphorus in different soil layers were significantly increased by the mixture of poplar and robinia. Multiple comparison analyses showed that with the increase in planting years, the difference in soil hydrolytic nitrogen content between the poplar and robinia mixed forest and pure forest increased significantly. The content of the soil hydrolytic nitrogen in the three soil layers of poplar and robinia mixed forest at 18a were 61.85 mg·kg−1, 52.90 mg·kg−1 and 17.43 mg·kg−1, respectively, which increased by 69.08%, 73.05% and 67.44% compared with the forest age of 3a. The decomposition and transformation of soil hydrolytic nitrogen content in the 20–40 cm soil layer was the most active. The soil content of the available phosphorus of the poplar and robinia mixed forest at 18a was higher than that of other forest ages.
The available phosphorus content in the 0–20 cm soil layer of the 18a poplar and robinia mixed forest was 25.07 mg·kg−1, which was 30.37% higher than that of pure poplar forest at the same age, but the difference was not significant, and it was significantly higher than that of the pure robinia forest at the same age. The soil available phosphorus content of poplar and robinia mixed forest in the 20–40 cm soil layer was 18.58 mg·kg−1, which increased significantly by 18.42% and 44.03%, respectively, compared with that in the pure poplar and robinia forests at the same age (Table 2). The mix of poplar and robinia forest significantly improved the utilization of soil hydrolytic nitrogen and available phosphorus, which realized the benefits from both parties.

3.3. Ecological Stoichiometry of N/P in Forest Land

The N/P between each soil layer of the three forest stands was less than 14 (Figure 2). The forest stands of pure poplar, robinia and their mix in coastal saline–alkali land were severely restricted by nitrogen. Soil N/P decreased with the increase in soil depth, and N/P was highest in the 0–20 cm soil layer. In the 0–20 cm soil layer, the soil N/P of the three forest stands increased significantly with the increase in forest age. The poplar and robinia mixed forest at 3a was significantly higher than the pure poplar and robinia forests by 56.00% and 46.25%, respectively. Soil N/P was highest in the pure robinia forest at 7a, and soil N/P was the highest in the pure poplar forest at 18a. In the 20–40 cm soil layer, the soil N/P of the poplar and robinia mixed forest was significantly higher than that of the pure poplar forest and robinia forest. The soil N/P of the pure poplar forest and poplar robinia mixed forest increased significantly with forest age. The soil N/P of pure robinia forest showed a trend of first increasing and then decreasing with the increase in forest age and reached the highest level at 7 a, which was 1.29. In the 40–60 cm soil layer, the N/P of the pure poplar forest was the highest when the forest age was 3a and 7a, which were 0.58 and 0.86, respectively. There were no significant differences in soil N/P among the three forest stands.

3.4. Changes in Urease and Phosphatase Activities

According to Figure 2, the soil enzyme activity of the forest stands was mainly phosphatase, and the phosphatase activity accounted for more than 79% of the total soil enzyme activity. Soil phosphatase activity increased with the increase in forest age and decreased with the increase in soil depth. The phosphatase activity of the poplar and robinia mixed forest at 3a was significantly higher than that of the pure poplar and robinia forest by 130.63% and 74.30%. The phosphatase activity of the poplar and robinia mixed forest at 7a was significantly higher than that of the pure poplar and robinia forests by 40.74% and 66.72%. The phosphatase activity of the poplar and robinia mixed forest at 18a was significantly higher than that of the pure poplar and robinia forests by 41.72% and 34.38%. The soil phosphatase activity of the poplar and robinia mixed forest was the highest in 0–20 cm and 20–40 cm soil layers at 18a, which were 40.83 mg·kg−1·h−1 and 30.26 mg·kg−1·h−1, respectively, were 30.36% and 18.39% higher than that of the pure poplar forest at the same age, and 70.27% and 37.42% higher than that of the pure robinia poplar forest at the same age. The soil phosphatase activity was significantly increased in the mix of poplar and robinia forest, especially in the soil layer of 0–40 cm root accumulation area.
Soil urease activity increased with the increase in forest age, and the soil urease activity was significantly increased by the mix of poplar and robinia forest among the three soil layers. The soil urease activity of the poplar and robinia mixed forest in 0–20 cm and 20–40 cm soil layers at 18a was the largest, which were 7.42 mg·kg−1·h−1 and 6.35 mg·kg−1·h−1, respectively, which were significantly increased by 94.75% and 80.68% compared with the pure poplar forest at the same age, and significantly increased by 73.36% and 64.08% compared with the pure robinia forest at the same age. The soil urease activity of the poplar and robinia mixed forest in the 40–60 cm soil layer at 7a was significantly higher than that of the mixed forest at 18a, which was 5.12 mg·kg−1·h−1, and it was significantly increased by 86.18% and 56.10% compared with the pure poplar and robinia forests at the same age (Figure 3).

3.5. Changes in Soil Bacteria, Fungi and Actinomyces

As Table 3 shows, the number of soil bacteria, fungi and actinomycetes in the 0–20 cm soil layer of the three forest stands was generally higher than that of the soil layer of 20–40 cm and 40–60 cm. Compared with the pure poplar and robinia forests, the poplar and robinia mixed forest significantly increased soil microbial numbers. The soil of the poplar and robinia mixed forest in 0–20 cm soil at 18a had the largest number of bacteria, actinomycetes and fungi, which were 70.32 × 104, 312.97 × 102 and 19.03 × 102, respectively. In the 20–40 cm soil layer, the number of soil bacteria and actinomycetes in the poplar and robinia mixed forest at 18a was the highest, which were 59.80 × 104 and 289.55 × 102, respectively, and the number of soil fungi in the pure robinia forest at 18a was the largest, which was 15.37 × 102, but there was no significant difference between the pure robinia forest and the mixed forest at 18a. In the 40–60 cm soil layer, the number of soil bacteria and actinomycetes in the poplar and robinia mixed forest was significantly higher than that in the pure poplar and robinia forests. The number of soil fungi in the poplar and robinia mixed forest at 3a was significantly higher than that in the pure poplar and robinia forests by 22.81% and 16.09%. The number of soil fungi in the pure robinia forest at 7a and 18a was slightly higher than that in the pure poplar forest and the mixed forest, but the difference between the poplar and robinia mixed forest was not significant. In the 0–20 cm and 20–40 cm soil layers, the number of soil bacteria, actinomycetes and fungi in the three forest stands showed an increasing trend with the increase in forest age. In the 40–60 cm soil layer, the number of bacteria in the pure poplar forest increased first and then decreased with the increase in forest age, but there was no significant difference in the number of bacteria at 7a and 18a. The number of actinomycetes decreased first and then increased with the increase in forest age. The number of actinobacteria at 18a was significantly higher than that at 3a and 7a by 73.51% and 82.10%, respectively. The number of fungi decreased with the increase in forest age, but the difference between forest ages was not significant. In the 40–60 cm soil layer, the number of bacteria and actinomycetes in pure robinia forest and the mixed forest increased significantly with the increase in forest age.
The number of fungi in the pure robinia forest and the mixed forest showed a trend of increasing first and then decreasing with the increase in forest age and reached the highest at 7a, which was 12.56 × 102 and 11.63 × 102, respectively. There was no significant difference in the number of fungi in the 40–60 cm soil layer among the three stands.

3.6. Analysis of RDA Redundancy

A two–dimensional ranking analysis was carried out on the soil nutrients, enzyme activities and microbial quantity in the poplar and robinia forest stands in coastal saline–alkali land. As shown in Figure 4, the blue arrow represents the number of soil microorganisms and enzyme activity, and the red arrow represents soil nutrients. The RDA1 axis interpretation amount was 70.6%, the RDA2 axis interpretation amount was 4.1% and the cumulative interpretation amount was 74.7%. There was a positive correlation between the measured indexes, among which the arrows of soil total nitrogen, available phosphorus, hydrolyzed nitrogen and phosphatase, urease, bacteria and actinomycetes were longer, indicating that the RDA had a large amount of explanation that included soil available phosphorus and phosphatase activity, the number of bacteria and total nitrogen content, and the angle between hydrolyzed nitrogen and urease activity were small, indicated a strong positive correlation between them.

4. Discussion

4.1. The Mixed of Populus × Euramercana ‘Neva’ and Robinia pseucdoacacia Significantly Increased Soil N and P Nutrient Contents

Due to the short development time of soil in coastal saline–alkali land, the spatial distribution of soil is different, so soil N and P nutrients also have such spatial heterogeneity. At the same time, in the ecosystem, soil and vegetation influence and restrict each other, and the degree of utilization of soil nutrients by vegetation growth or the difference in the biomass of vegetation litter and its decomposition rate further leads to the heterogeneity of soil nutrients [13]. The study of poplar and robinia mixed forest litter found that the decomposition rate of poplar forest litter was slower, and the release time of N and P nutrients was long, while the litter of the robinia forest decomposed faster and it was easy to release N and P elements, so mixed could improve the decomposition ability of poplar forest, shorten the nutrient enrichment time and accelerate the cycle of N and P elements in the forest land [14,15]. In this study, it was found that the mixed forest of poplar and robinia had significant differences in improving the total N, total P, hydrolyzed nitrogen and available phosphorus in the forest land compared with the corresponding pure forests. The nutrient content of N and P increased with the increase in forest age. Studies have shown that the poplar and robinia mixed forest on barren sandy land can greatly improve the organic matter, long–acting nutrients such as total N and total P in the topsoil, available nutrients such as NH4+ –N, NO3––N, available P and available K [16]. Due to the nitrogen fixation ability of robinia in mixed forests, it can provide a high nitrogen environment for the activity of microorganisms and related enzymes in the soil [17], thereby the decomposition rate of litter is accelerated in the forest [18], the ammonification and nitrification intensities of forest soil are enhanced, the level of N elements in the soil is improved and the enhancement of soil nutrient mineralization is promoted [19,20], which is consistent with the results of this study. In this study, it was found that the urease and phosphatase activities in the soil of the mixed poplar and robinia forest were significantly higher than those in the corresponding pure forests. Compared with the pure forests of poplar and robinia at the same age, the soil urease in the 0–20 cm soil layer of the poplar and robinia mixed forest at 18a increased by 94.75% and 73.36%, and the soil phosphatase activity increased by 30.36% and 70.27% compared with the pure forests of poplar and robinia at the same age. The number of soil bacteria, actinomycetes and fungi in the 0–20 cm soil layer of the poplar and robinia mixed forest at 18a was the largest, which was 70.32 × 104, 312.97 × 102 and 19.03 × 102, respectively. Redundancy analysis showed that there was a significant positive correlation between soil N and P nutrient content, soil enzyme activity and the number of soil microorganisms [21]. Under the joint action of soil enzymes and microorganisms, the decomposition rate of total nitrogen and total phosphorus in the mixed forest soil of poplar and robinia was accelerated, and the total nitrogen and total phosphorus were rapidly converted into hydrolyzed nitrogen and available phosphorus that could be directly absorbed and utilized by plants. The mixed forest increased the nutrient cycle rate of N and P, increased the nutrient reserves of forest land, improved the growth conditions of forest trees and promoted the continuous improvement in forest productivity [22,23,24].

4.2. Populus × Euramercana ‘Neva’ and Robinia pseucdoacacia Mixed to Realize the Dynamic Complementation of Soil N and P Nutrients Between Soil Layers

This study found that the mixed forest of poplar and robinia expanded the soil depth of N and P nutrient decomposition and transformation. The nutrient content, enzyme activity and microbial number of soil in the 40–60 cm soil layer in the mixed forest sample were higher than those in the corresponding pure forests. With the increase in plantation years, the depth of soil that could be used in the forest land was increased. Since robinia is a shallow–rooted tree species and poplar is a deep–rooted tree species, this mixed combination of “shallow–deep–rooted species” can continuously expand the horizontal area and longitudinal depth of the underground soil [24,25,26]. Due to the increase in expansion area and depth, the interaction between roots and soil makes the number of bacteria, fungi and actinomycetes in the soil increase [27,28], while improving the activities of soil urease and phosphatase, accelerating the decomposition of insoluble nutrients in the soil and the absorption of soluble nutrients [29]. We cannot ignore the role of the fine roots of forest trees in the accumulation, decomposition and return of litter in the entire forest ecosystem [30].
Studies have shown that in the mixed forest of poplar and robinia, poplar can transfer the P elements in the body to adjacent robinia through root contact [31]. At the same time, robinia can transfer the N elements that are fixed and absorbed by itself to poplars through root contact, and the intensity of N and P nutrients transfer is very large, and finally complete the complementary N and P nutrients between poplar and robinia species [32]. After the mix of poplar and robinia, the demand for the P element in young (3a) poplar is less than that of robinia, which can greatly promote the transfer of the P element by poplar, benefit the P element nutrition of robinia in the mixed forest and significantly improve the nitrogen fixation ability of robinia in the mixed forest, thereby releasing a large amount of N element in the soil to supply poplar and directly transfer its fixed N element to poplar through root contact, so as to realize the complementary N and P nutrients between poplar and robinia in the mixed forest. After the mixed forest reaches adulthood (7a), it is mainly achieved by improving the level of soil N content. Robinia can increase the level of N element nutrition in poplars, and poplars can promote the absorption of P elements by robinia by reducing the absorption of soil P nutrients and transferring a large amount of P nutrients absorbed into the body to robinia through the root system [14,19]. The results found that with the increase in forest age, the root systems of poplar and robinia continued to grow and overlapped with each other. The soil enzyme activity (urease, phosphatase) and the number of microorganisms (bacteria, fungi, actinomycetes) in the mixed forest of poplar and robinia showed a changing trend of 18a > 7a >3a, and the soil hydrolyzed nitrogen and available phosphorus content also showed significant differences. The development time of the 3a poplar and robinia mixed forest is short, and the soil root system is generally concentrated at a depth of about 40 cm. With the increase in forest age, the root system continues to grow and develop, the 7a poplar and robinia root system has been rooted in the soil at a depth of 60 cm, and the root system of the 18a poplar and robinia mixed forest is densely distributed in the soil at a depth of 60 cm.
Since robinia is a shallow root tree with horizontal stretching of the root system and poplar is a deep–rooted tree with a vertical distribution of roots, the root system is densely and evenly distributed in the soil under the forest of poplar and robinia mixed forest, and the root system is staggered and connected, which is more conducive to the return of poplar’s P nutrients. The P element is transmitted to robinia through fine root contact [33], which promotes the nitrogen fixation of robinia [34]. Under the turnover and decomposition of fine roots, robinia can return a large amount of N elements to promote the growth of poplar.

4.3. Ecological Stoichiometry Variation Characteristics of Soil N/P in Coastal Saline–Alkali Land

This study found that the nutrient content of N and P in coastal saline–alkali land was low. The content of N elements decreased significantly with the increase in soil depth and increased significantly with the increase in forest age. The change in the P element was consistent with the N element, but with the trend of forest age increasing first and then decreasing, the difference in soil total P was not significant. This is consistent with the findings of the study on the ecological stoichiometry characteristics of nitrogen and phosphorus in the natural secondary forest of Quercus liaotungenensis and Robinia pseudoacacia plantation in the hilly area of the Loess Plateau [35]. The analysis concluded that the decomposition and synthesis of plant residues in soil is the main source of the formation of total N [36]. The activities of soil enzymes (urease, phosphatase) and the number of microorganisms (bacteria, fungi, actinomycetes) in soil are the main reasons for the uneven distribution of N elements [37,38,39], which is consistent with the results of this study. The 0–20 cm soil layer is the enrichment area of soil N elements. However, the soil P element is a sedimentary mineral with low mobility in the soil, so the vertical change in soil total P in the soil layer is relatively stable [40]. Previous studies have shown that the heterogeneity of soil N and P in Chinese fir plantations may begin to emerge after about 20 years [41]. During the 0~25 years of vegetation restoration in Robinia pseudoacacia plantations on the Loess Plateau, soil total phosphorus (TP) gradually decreased, while soil organic carbon, total nitrogen (TN) and enzyme activities showed upward trends [42]. The patterns of soil nutrient uptake, utilization and return of soil nutrients by different tree species were very different, and the decomposition of litter and the N and P cycles began to change significantly after about 10 months in the mixed poplar and Robinia pseudoacacia forest [43], and the litter decomposition rate was higher in the mixed forest compared with the pure forest [44].
This study found that the soil N content of the poplar and robinia mixed and its pure forests of different forest ages showed a trend of 18a > 7a > 3a. As a nitrogen–fixing tree species, the nitrogen–fixation effect of robinia was very significant with the increase in forest age. The soil N/P increased significantly with the increase in poplar and robinia forest age, but the variation range of soil N/P was 0.7~2.5, which was far lower than the average level of N/P = 3.9 in China and the average level of N/P = 5.9 in the world [45]. It has been shown that N/P < 14 is the restriction site of the N element, N/P > 16 is the restriction site of the P element and N/P is restricted by both N and P elements when it is between 14 and 16 [46]. This study found that due to the harsh soil conditions of coastal saline–alkali land, even if poplar and robinia as a combination of nitrogen–fixing and non–nitrogen–fixing species, the soil N nutrition in the forest land was very deficient. When the forest age reached 18 years, the mixed forest of poplar and robinia was still limited by N elements. Therefore, in the process of plantation construction and management of coastal saline–alkali land, the creation of a nitrogen–fixing tree species combination tries to increase the content of the N element in the soil through mixed forest nitrogen fixation. In the management of forest land, nitrogen fertilizer is added to increase the content of N elements in the soil to ensure the normal growth and development of forest trees [47].

5. Conclusions

The mix of poplar and robinia and the increase in planting years caused the heterogeneity of forest land soil, which significantly increased the contents of total nitrogen, hydrolyzed nitrogen, total phosphorus and available phosphorus in the soil interlayer, increased the number of soil bacteria, fungi and actinomycetes and increased the activities of soil urease and phosphatase, which was conducive to the decomposition and utilization of soil nitrogen and phosphorus. At 18 years, the contents of hydrolyzed nitrogen and available phosphorus in the 0–20 cm soil layer increased by 69.08% and 49.14% compared with the mixed forest with a forest age of 3 years. There was a strong positive correlation between microbial and enzyme activities and soil nutrients, and the high microbial content and enzyme activities accelerated the decomposition process of organic matter in the soil, enhanced the decomposition and transformation efficiency of N and P elements, significantly increased the number of bacteria, fungi and actinomycetes in the 18–year–old mixed forest and the contents of the N and P elements were also significantly higher than those in pure forests. However, due to the low N content in the soil of coastal saline–alkali land and its N/P <14, it can be seen that the growth of poplar and robinia forest stands in coastal saline–alkali land is still restricted by nitrogen, so the application of nitrogen fertilizer can be increased during the silvicultural process. This study only involved the number of microorganisms in the soil, there was a lack of investigation of the key microbial species that affect plant growth and soil element transformation, only N, P and other elements were measured and the organic carbon content affecting plant growth was not measured.

Author Contributions

Conceptualization, S.W. and B.C.; Methodology, C.L. and M.W.; Software, C.L. and B.T.; Validation, S.W. and M.W.; Writing—original draft preparation, S.W.; Writing—review and editing, K.W. and B.C.; Visualization, S.W. and B.T.; Supervision: B.C. and K.W.; Funding acquisition, B.C. and K.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Shandong Province Agricultural Breeding Project: Breeding of New Varieties of Salt Alkali Tolerant Ecological Economic Forest Trees (2023LZGC012–2) and the Major Scientific and Technological Innovation in Shandong (025–381885).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Cao, B.H.; Wu, L.Y. Studies on Soil Enzyme Activity and Soil Nutrient Content of Mixed Stands with Robinia pseudoacacia and Fraxinus velutina in Coastal Saline Soil. J. Soil Water Conserv. 2008, 22, 128–133. [Google Scholar] [CrossRef]
  2. Lin, S.J.; Zhang, Z.H.; Zhang, D.W.; Fu, S.C.; Yang, C.Y.; Xie, P.; Zhao, S.S.; Liu, J.B.; Zhang, X.H.; Tain, F.C. Research Progress on the Trees Species Choice in Saline–Alkali Land and Drought Resistant Forestation Technology. Chin. Agric. Sci. Bull. 2012, 28, 1–5. [Google Scholar]
  3. Sun, J.X.; Kang, Y.H.; Wan, S.Q.; Hu, W.; Zhang, T.B. Soil Salinity Management with Drip Irrigation and Its Effects on Soil Hydraulic Properties in North China Coastal Saline Soils. Agric. Water Manag. 2012, 115, 10–19. [Google Scholar] [CrossRef]
  4. Du, Z.Y.; Liu, F.C.; Ma, B.Y.; Ma, H.L.; Xing, S.J. Evaluation of Ecosystem Service Value for Long–Term Plantations in Saline–Alkali Soils of the Yellow River Delta. Chin. Agric. Sci. Bull. 2013, 29, 17–23. [Google Scholar]
  5. Cao, B.H.; Li, F.M.; Qin, Y.J.; Zhang, Y.J.; Liu, Y.Y.; Zhao, T. Growth Characteristics and Volume Growth Mathematical Simulation of Mixed Robinia pseudoacacia Forests in Coastal Saline–Alkali Soil. J. Shandong Agric. Univ. 2012, 43, 201–205. [Google Scholar]
  6. Mao, P.L.; Tang, Q.; Cao, B.H.; Liu, J.M.; Shao, H.B.; Cao, Z.Y.; Hao, M.Z.; Zhu, Z.B. Eco–Physiological Adaptability in Mixtures of Robinia pseudoacacia and Fraxinus velutina and Coastal Eco–Engineering. Ecol. Eng. 2017, 106, 109–115. [Google Scholar] [CrossRef]
  7. Cao, B.H.; Wu, L.Y.; Song, A.Y.; Yu, X.N.; Hao, M.Z.; Hu, M.M. The Soil Water and Soil Salt Distribution in Coastal Saline–Alkali Area: A Comparison of Pure and Mixed Plantations. Acta Ecol. Sin. 2008, 28, 939–945. [Google Scholar]
  8. Cao, S.G. Ecological Function of Mixed Conifer–Broadleaved Forest. Prot. For. Sci. Technol. 2015, 6, 12–13, 23. [Google Scholar] [CrossRef]
  9. Fang, Y.; Jiang, N.N.; Liang, Y. Effects of Afforestation on Secondary Salinization Sites in the Yellow River Delta. J. For. Eng. 2009, 23, 15–19. [Google Scholar]
  10. Wang, S.M.; Pang, Y.X.; Song, A.Y.; Cao, B.H.; Zhu, Z.B.; Li, Y.Y. Soil Physiochemical Properties and Diversity of Herbaceous Plants Dynamic on the Different Ages Mixed Forests of Populus × Euramercana ‘Neva’and Robinia pseucdoacacia in Coastal Saline–Alkali Area. Acta Ecol. Sin. 2018, 38, 6539–6548. [Google Scholar] [CrossRef]
  11. Cui, W.S.; Zhang, S.Y. A Study on the Mixed Poplar Forest. J. Shenyang Agric. Univ. 1993, 4, 347–349. [Google Scholar]
  12. Fan, J.G. Biochemical Compositions in Rhizospheric Soil of Robinia pseudoacacia and Sophora japonica and Their Influence on Nutrition of Associated Poplar. Chin. J. Appl. Ecol. 1994, 5, 349–354. [Google Scholar] [CrossRef]
  13. Liu, C.L.C.; Kuchma, O.; Krutovsky, V.K. Mixed–Species Versus Monocultures in Plantation Forestry: Development, Benefits, Ecosystem Services and Perspectives for the Future. Glob. Ecol. Conserv. 2018, 15, e00419. [Google Scholar] [CrossRef]
  14. Jia, L.M. The Review of Mixtures of Nitrogen–Fixing and Non–Nitrogen–Fixing Tree Species. World For. Res. 1998, 11, 20–26. [Google Scholar] [CrossRef]
  15. Yu, Z.J.; Wang, K.B.; Li, J.W.; Zhouping, S.G.; Lei, D. Mixed Plantations have More Soil Carbon Sequestration Benefits than Pure Plantations in China. For. Ecol. Manag. 2023, 529, 120654. [Google Scholar] [CrossRef]
  16. Du, Z.Y.; Ma, H.L.; Ma, B.Y.; Liu, F.C.; Xing, S.J. Effects of Mixed Forests in Coastal Saline Soil. J. Northwest For. Univ. 2015, 30, 144–149. [Google Scholar] [CrossRef]
  17. Guo, J.H.; Feng, H.L.; McNie, P.; Liu, Q.; Xu, X.; Pan, C.; Yan, K.; Feng, L.; Goitom, E.A.; Yu, Y.C. Species Mixing Improves Soil Properties and Enzymatic Activities in Chinesefir Plantations: A Meta–Analysis. Catena 2023, 220, 106723. [Google Scholar] [CrossRef]
  18. Qin, Y.J.; Niu, Q.L.; Jia, B.; Mao, P.L.; Cao, B.H.; Qin, Y.G. Dynamic Characteristics of Soil Properties in a Robinia pseudoacacia Precipitation on Coastal Saline Soil. J. Northwest For. Univ. 2017, 32, 13–17. [Google Scholar] [CrossRef]
  19. Zhai, M.P.; Jiang, S.N.; Jia, L.M. Study on Fine–Root Nutrient Dynamics in the Mixed Plantation of Poplar and Black Locust. Sci. Silvae Sin. 2004, 40, 46–51. [Google Scholar]
  20. Xie, X.F.; Pu, L.J.; Wang, Q.Q.; Zhu, M.; Xu, Y.; Zhang, M. Response of Soil Physicochemical Properties and Enzyme Activities to Long–Term Reclamation of Coastal Saline Soil, Eastern China. Sci. Total Environ. 2017, 607–608, 1419–1427. [Google Scholar] [CrossRef]
  21. Bai, Y.X.; Wei, H.; Ming, A.A.; Shu, W.W.; Shen, W.J. Tree Species Mixing Begets Admixture of Soil Microbial Communities: Variations along Bulk Soil, Rhizosphere Soil and Root Tissue. Geoderma 2023, 438, 116638. [Google Scholar] [CrossRef]
  22. Kemal, G.; Thomas, V.; Noland, T.; Mccaughey, H.; Morrison, I.; Treitz, P. Mapping Continuous Forest Type Variation by Means of Correlating Remotely Sensed Metrics to Canopy N:P Ratio in a Boreal Mixedwood Forest. Appl. Veg. Sci. 2015, 18, 143–157. [Google Scholar] [CrossRef]
  23. Bastin, J.F.; Finegold, Y.; Garcia, C.; Mollicone, D.; Rezende, M.; Routh, D.; Zohner, C.M.; Crowther, T.W. The Global Treerestoration Potential. Science 2019, 365, 76–79. [Google Scholar] [CrossRef] [PubMed]
  24. Zhai, M.P. The Survey of Mixed Forests and the Interaction of Tree Species. World For. Res. 1993, 6, 39–45. [Google Scholar] [CrossRef]
  25. Du, Z.-Y.; Wang, Q.-H.; Xing, S.-J.; Liu, F.-C.; Ma, B.-Y.; Ma, H.-L.; Liu, D.-X. Fine Root Distribution, Characteristics and Rhizosphere Soil Properties in a Mixed Stand of Robinia pseudoacacia and Fraxinus velutina in a Saline Soil. Silva Fenn. 2013, 47, 970. [Google Scholar] [CrossRef]
  26. Rebola–Lichtenberg, J.; Schall, P.; Annighöfer, P.; Ammer, C.; Leinemann, L.; Polle, A.; Euring, D. Mortality of Different Populus Genotypes in Recently Established Mixed Short Rotation Coppice with Robinia pseudoacacia L. Forests 2019, 10, 410. [Google Scholar] [CrossRef]
  27. Gillespie, L.M.; Hättenschwiler, S.; Milcu, A.; Wambsganss, J.; Shihan, A.; Fromin, N. Tree Species Mixing Affects Soil Microbial Functioning Indirectly via Root and Litter Traits and Soil Parameters in European Forests. Funct. Ecol. 2021, 35, 2190–2204. [Google Scholar] [CrossRef]
  28. Pereira, A.P.A.; Santana, M.C.; Zagatto, M.R.G.; Brandani, C.B.; Wang, J.-T.; Verma, J.P.; Singh, B.K.; Cardoso, E.J.B.N. Nitrogen–Fixing Trees in Mixed Forest Systems Regulate the Ecology of Fungal Community and Phosphorus Cycling. Sci. Total Environ. 2021, 758, 143711. [Google Scholar] [CrossRef]
  29. Kooch, Y.; Bayranvand, M. Composition of Tree Species Can Mediate Spatial Variability of C and N Cycles in Mixed Beech Forests. For. Ecol. Manag. 2017, 401, 55–64. [Google Scholar] [CrossRef]
  30. Du, Z.; Liu, F.C.; Ma, B.Y.; Dong, H.F.; Ma, H.L. Root Distribution and Fine Root Growth in Mixed Plantation of Robinia pseudoacacia and Fraxinus velutina in Coastal Saline–Alkali Area. Sci. Silvae Sin. 2014, 47, 10–15. [Google Scholar]
  31. Hu, Y.J.; Zhai, M.P.; Wu, J.W.; Jia, L.M. Dynamics of Enzyme Activity in the Soil of Pure and Mixed Stands of Poplar and Black Locust. J. Beijing For. Univ. 2001, 23, 23–26. [Google Scholar] [CrossRef]
  32. Shen, G.F.; Zhai, M.P.; Jia, L.M. The Soil Amelioration Effect of Poplar–Black Locust Mixed Plantation on Sand Soil and the Interaction of Mutual Supplement of Nutrient Between Tree Species. Sci. Silvae Sin. 1998, 34, 12–20. [Google Scholar]
  33. Bi, B.Y.; Yin, Q.L.; Hao, Z.Q. Root Phosphatase Activity is a Competitive Trait Affiliated with the Conservation Gradient in Root Economic Space. For. Ecosyst. 2023, 10, 279–286. [Google Scholar] [CrossRef]
  34. Yang, H.J.; Gao, J.F.; Pan, C.; Qing, S.Q.; Yun, W.; Jiang, L.; Wang, Z.B.; Wang, D.H. Species Composition and Influencing Factors of Understory Woody Species in Robinia pseudoacacia Plantations on the Loess Plateau. J. For. Res. 2023, 34, 1693–1706. [Google Scholar] [CrossRef]
  35. Lü, J.L.; Yan, M.J.; Song, B.L.; Guan, J.H.; Shi, W.Y.; Du, S. Ecological Stoichiometry Characteristics of Soil Carbon, Nitrogen, and Phosphorus in an Oak Forest and a Black Locust Plantation in the Loess Hilly Region. Acta Ecol. Sin. 2017, 37, 3385–3393. [Google Scholar] [CrossRef]
  36. Chen, X.L.; Taylor, A.R.; Reich, P.B.; Hisano, M.; Chen, H.Y.H.; Chang, S.X. Tree Diversily Increases Decadal Forest Soil Carbon and Nitrogen Accrual. Nature 2023, 618, 94–101. [Google Scholar] [CrossRef]
  37. Peng, M.; Jia, H.B.; Wang, Q.Y. The Effect of Land Use on Bacterial Communities in Saline–Alkali Soil. Curr. Microbiol. 2017, 74, 325–333. [Google Scholar] [CrossRef]
  38. Bai, S.H.; Ma, F.Y.; Li, S.S.; Yao, X.F. Relational Analysis of Soil Enzyme Activities, Nutrients and Microbes in Robinia pseudoacacia Plantations in the Yellow River Dalta with Different Degradation Degrees. Chin. J. Eco–Agric. 2012, 20, 1478–1483. [Google Scholar] [CrossRef]
  39. Du, Z.Y.; Ma, B.Y.; Liu, F.C.; Dong, H.F.; Ma, H.L. Rhizosphere Soil Characteristics of Artificial Mixed Stands with Robinia pseudoacacia and Fraxinus velutina in Coastal Saline Soil. Soil Fertil. Sci. China 2013, 6, 1–5. [Google Scholar] [CrossRef]
  40. Mitran, T.; Mani, P.K.; Bandyopadhyay, P.K.; Basak, N. Effects of Organic Amendments on Soil Physical Attributes and Aggregate–Associated Phosphorus under Long–Term Rice–Wheat Cropping. Pedosphere 2018, 28, 823–832. [Google Scholar] [CrossRef]
  41. Zhang, Y.; Li, H.T.; Zhang, H.; Huang, B.B.; Liu, C.H.; Jiang, Z.K.; Ma, X.Q. Soil C: N: P stoichiometry and its Relationship with the Soil Physicochemical Properties of Different Aged Chinese Fir (Cunninghamia lanceolata) Plantations. Acta Ecol. Sin. 2019, 39, 2520–2531. [Google Scholar] [CrossRef]
  42. Zhang, K.; Li, J.J.; Wei, Z.H.; Fan, M.C.; Shangguan, Z.P. Revealing Nutrient Limitation Status of Microorganisms in the Soil of Robinia pseudoacacia Plantation Through Soil Stoichiometry and Enzyme Metrology. Chin. J. Appl. Ecol. 2024, 35, 1799–1806. [Google Scholar] [CrossRef]
  43. Jia, L.M.; Fang, L.M.; Hu, Y.J. Decomposition of Leaf Litter in Pure and Mixed Stands of Poplar and Black Locust. J. Integr. Agric. 1998, 9, 16–21. [Google Scholar]
  44. Su, Z.; Zhu, X.; Wang, Y.; Mao, S.; Shangguan, Z. Litter C and N Losses at different decomposition Stages of Robinia pseudoacacia: The weaker Effects of Soil Enzyme Activities Compared with Those of Litter Quality and the Soil Environment. Front. Environ. Sci. 2022, 10, 956309. [Google Scholar] [CrossRef]
  45. Chen, Y.P.; Xia, J.B.; Zhao, X.M.; Zhuge, Y.P. Effect of Different Plantation Types on Soil Ecological Stoichiometry in Yellow Delta. Chin. J. Soil Sci. 2017, 48, 392–398. [Google Scholar] [CrossRef]
  46. Wu, W.; He, X.D.; Zhou, Q.X. Review on N∶P Stoichiometry in Eco–System. J. Desert Res. 2010, 30, 296–302. [Google Scholar]
  47. Bai, J.S.; Zhang, S.Q.; Huang, S.M.; Xu, X.P.; Zhao, S.C.; Qiu, S.J.; He, P.; Zhou, W. Effects of the Combined Application of Organic and Chemical Nitrogen Fertilizer on Soil Aggregate Carbon and Nitrogen: A 30–Year Study. J. Integr. Agric. 2023, 22, 3517–3534. [Google Scholar] [CrossRef]
Forests 15 02226 g001
Figure 1. Location of the study area.
Figure 1. Location of the study area.
Forests 15 02226 g001
Forests 15 02226 g002
Figure 2. Differences in soil N–P ratios among plots. Note: the uppercase letters in the table represent the differences between different tree species at the same forest age, and the lowercase letters represent the differences between different forest ages at the same tree species, where the same letters represent no significant differences, and different letters represent significant differences, p < 0.05, the same as below.
Figure 2. Differences in soil N–P ratios among plots. Note: the uppercase letters in the table represent the differences between different tree species at the same forest age, and the lowercase letters represent the differences between different forest ages at the same tree species, where the same letters represent no significant differences, and different letters represent significant differences, p < 0.05, the same as below.
Forests 15 02226 g002
Forests 15 02226 g003
Figure 3. Differences in soil enzymatic activity among plots.
Figure 3. Differences in soil enzymatic activity among plots.
Forests 15 02226 g003
Forests 15 02226 g004
Figure 4. RDA analysis of soil nutrients, enzyme activity and microbial quantity.
Figure 4. RDA analysis of soil nutrients, enzyme activity and microbial quantity.
Forests 15 02226 g004
Table 1. Information of stand characteristics of various plots.
Table 1. Information of stand characteristics of various plots.
TreesAges/aPlant Spacing/m × mCanopy DensityHeight/mDBH/cmCrown Breadth/mpHSalt Content/%
R31 × 2free growing3.493.041.877.870.27
P3.643.250.94
P31 × 2free growing3.192.730.717.890.31
R31 × 2free growing3.122.400.887.950.30
R72 × 30.759.2710.574.288.060.23
P10.3310.612.86
P72 × 30.558.3511.751.808.140.24
R72 × 30.827.1312.582.328.130.24
R182 × 40.89.7010.703.997.700.19
P10.9922.603.11
P182 × 40.513.1411.322.577.670.21
R182 × 40.8511.2913.193.627.790.20
Note: R: Robinia Pseucdoacacia forest, P: Populus × Euramercana ‘Neva’ forest, the same as below.
Table 2. Changes in N and P nutrient contents in forest land.
Table 2. Changes in N and P nutrient contents in forest land.
Soil DepthForest StandForest AgeTotal Nitrogen/(mg·kg−1)Hydrolysis Nitrogen/(mg·kg−1)Total Phosphorus/(mg·kg−1)Available Phosphorus/(mg·kg−1)
0–20 cmP3a0.39 ± 0.02 Bc22.27 ± 0.95 Cb0.52 ± 0.00 Bc14.13 ± 1.18 Bb
7a1.24 ± 0.08 Bb29.37 ± 3.00 Ca0.96 ± 0.03 Ba16.07 ± 0.65 Bb
18a1.37 ± 0.06 Ba31.73 ± 0.25 Ca0.59 ± 0.00 Cb19.23 ± 2.26 ABa
R3a0.43 ± 0.02 Bc24.47 ± 1.03 Bb0.53 ± 0.00 Bc10.35 ± 0.03 Cb
7a1.34 ± 0.09 ABa34.80 ± 0.85 Ba0.94 ± 0.02 Ba13.56 ± 1.51 Cab
18a1.10 ± 0.03 Cb35.63 ± 0.68 Ba0.65 ± 0.00 Bb16.77 ± 5.09 Ba
P × R3a0.74 ± 0.05 Ab36.58 ± 1.04 Ab0.64 ± 0.02 Ac16.81 ± 0.22 Ab
7a1.45 ± 0.01 Aa61.13 ± 1.06 Aa1.28 ± 0.02 Aa22.97 ± 1.24 Aa
18a1.55 ± 0.08 Aa61.85 ± 0.22 Aa0.73 ± 0.01 Ab25.07 ± 2.29 Aa
20–40 cmP3a0.35 ± 0.01 Bb19.53 ± 0.85 Bb0.44 ± 0.03 Cc11.21 ± 0.05 Bc
7a0.96 ± 0.12 Ba26.73 ± 2.84 Ca0.86 ± 0.01 Aa14.19 ± 0.80 Bb
18a0.91 ± 0.05 Ba28.77 ± 0.84 Ca0.54 ± 0.00 Bb15.69 ± 0.80 Ba
R3a0.36 ± 0.02 Bc21.27 ± 0.35 Bb0.50 ± 0.00 Bb9.74 ± 0.04 Cb
7a0.85 ± 0.04 Ba31.77 ± 1.36 Ba0.66 ± 0.03 Ba10.67 ± 1.11 Cb
18a0.47 ± 0.04 Cb32.27 ± 0.55 Ba0.53 ± 0.01 Bb12.90 ± 0.56 Ca
P × R3a0.46 ± 0.01 Ab30.57 ± 1.98 Ab0.56 ± 0.01 Ac16.10 ± 0.09 Ab
7a1.21 ± 0.06 Aa52.05 ± 0.82 Aa0.82 ± 0.03 Aa17.70 ± 1.82 Aab
18a1.30 ± 0.06 Aa52.90 ± 0.72 Aa0.63 ± 0.01 Ab18.58 ± 0.84 Aa
40–60 cmP3a0.31 ± 0.01 Bc9.94 ± 0.59 Ab0.53 ± 0.00 Cb4.17 ± 0.10 Bb
7a0.80 ± 0.03 Aa13.23 ± 0.33 Aa0.93 ± 0.00 Ba4.47 ± 0.19 Cb
18a0.75 ± 0.02 Bb14.53 ± 1.35 Ba0.52 ± 0.01 Bb5.91 ± 0.27 Ba
R3a0.30 ± 0.02 Bb10.60 ± 1.31 Ab0.56 ± 0.01 Bb4.73 ± 0.57 Bb
7a0.69 ± 0.15 Aa10.75 ± 1.42 Bb0.84 ± 0.02 Ca5.18 ± 0.07 Bb
18a0.37 ± 0.08 Cb14.86 ± 0.59 Ba0.50 ± 0.01 Cc6.28 ± 0.09 Ba
P × R3a0.36 ± 0.03 Ac10.41 ± 0.04 Ac0.68 ± 0.01 Ab8.17 ± 0.17 Aa
7a0.73 ± 0.24 Ab14.29 ± 0.57 Ab1.05 ± 0.07 Aa8.07 ± 0.43 Aa
18a1.11 ± 0.04 Aa17.43 ± 0.48 Aa0.61 ± 0.00 Ab8.44 ± 0.21 Aa
Note: the uppercase letters in the table represent the differences between different tree species at the same forest age, and the lowercase letters represent the differences between different forest ages at the same tree species, where the same letters represent no significant differences, and different letters represent significant differences, p < 0.05, the same as below.
Table 3. Changes in the number of soil bacteria, fungi and actinomycetes.
Table 3. Changes in the number of soil bacteria, fungi and actinomycetes.
Soil DepthForest StandForest AgeBacterium/(104)Actinomycete/(102)Fungus/(102)
0–20 cmP3a40.03 ± 3.27 Ac94.88 ± 3.81 Bc9.93 ± 0.15 Cc
7a52.82 ± 3.72 Bb106.78 ± 6.40 Cb12.49 ± 0.73 Bb
18a64.90 ± 1.65 ABa166.73 ± 1.75 Ca16.23 ± 1.11 Ba
R3a28.34 ± 0.09 Bc96.58 ± 3.55 Bc12.34 ± 0.42 Bc
7a47.86 ± 3.39 Bb120.69 ± 1.33 Bb17.09 ± 0.82 Ab
18a58.04 ± 1.42 Ba193.35 ± 2.87 Ba19.53 ± 0.60 Aa
P × R3a43.86 ± 0.75 Ab175.66 ± 3.11 Ac15.92 ± 0.17 Ac
7a65.23 ± 5.66 Aa219.78 ± 4.33 Ab18.00 ± 0.27 Ab
18a70.32 ± 7.26 Aa312.97 ± 4.60 Aa19.03 ± 0.36 Aa
20–40 cmP3a37.82 ± 2.73 Ac86.33 ± 1.54 Cc9.09 ± 0.25 Cb
7a43.38 ± 1.72 Bb91.54 ± 1.71 Cb9.47 ± 0.32 Bb
18a54.41 ± 2.21 Ba155.28 ± 0.99 Ca10.81 ± 0.15 Ba
R3a26.07 ± 0.65 Bc104.25 ± 2.00 Bc11.17 ± 0.10 Bb
7a38.31 ± 0.74 Cb136.99 ± 10.09 Bb14.32 ± 0.27 Aa
18a46.78 ± 1.32 Ca178.60 ± 3.21 Ba15.37 ± 1.05 Aa
P × R3a38.68 ± 1.31 Ac153.06 ± 5.13 Ac12.82 ± 0.12 Ab
7a52.38 ± 0.15 Ab213.71 ± 6.44 Ab14.44 ± 0.36 Aa
18a59.80 ± 2.20 Aa289.55 ± 3.89 Aa14.46 ± 0.55 Aa
40–60 cmP3a17.90 ± 0.83 Cb79.88 ± 0.05 Bb8.46 ± 0.23 Ba
7a38.18 ± 0.77 Aa76.11 ± 1.77 Cb8.40 ± 0.46 Ba
18a37.48 ± 4.43 Ba138.60 ± 6.98 Ca8.32 ± 0.86 Ba
R3a22.31 ± 0.15 Bb81.81 ± 2.92 Bc8.95 ± 0.80 Bb
7a32.79 ± 1.11 Ba93.35 ± 6.36 Bb12.56 ± 1.28 Aa
18a37.50 ± 4.21 Ba153.02 ± 3.29 Ba11.51 ± 1.02 Aa
P × R3a32.32 ± 0.63 Ac130.50 ± 3.57 Ac10.39 ± 0.35 Aa
7a39.68 ± 1.85 Ab151.40 ± 3.01 Ab11.63 ± 0.84 Aa
18a45.90 ± 0.77 Aa197.35 ± 5.78 Aa10.39 ± 1.74 Aa
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Wang, S.; Lv, C.; Tang, B.; Wang, M.; Cao, B.; Wu, K. Dynamics of Soil N and P Nutrient Heterogeneity in Mixed Forest of Populus × Euramercana ‘Neva’ and Robinia pseucdoacacia in Coastal Saline–Alkali Land. Forests 2024, 15, 2226. https://doi.org/10.3390/f15122226

AMA Style

Wang S, Lv C, Tang B, Wang M, Cao B, Wu K. Dynamics of Soil N and P Nutrient Heterogeneity in Mixed Forest of Populus × Euramercana ‘Neva’ and Robinia pseucdoacacia in Coastal Saline–Alkali Land. Forests. 2024; 15(12):2226. https://doi.org/10.3390/f15122226

Chicago/Turabian Style

Wang, Shumei, Changxiao Lv, Bingxiang Tang, Mengxiao Wang, Banghua Cao, and Ke Wu. 2024. "Dynamics of Soil N and P Nutrient Heterogeneity in Mixed Forest of Populus × Euramercana ‘Neva’ and Robinia pseucdoacacia in Coastal Saline–Alkali Land" Forests 15, no. 12: 2226. https://doi.org/10.3390/f15122226

APA Style

Wang, S., Lv, C., Tang, B., Wang, M., Cao, B., & Wu, K. (2024). Dynamics of Soil N and P Nutrient Heterogeneity in Mixed Forest of Populus × Euramercana ‘Neva’ and Robinia pseucdoacacia in Coastal Saline–Alkali Land. Forests, 15(12), 2226. https://doi.org/10.3390/f15122226

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