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Article

Characteristics of Soil Nematode Communities in Pure Populus hopeiensis Forests in the Loess Hilly Region and Their Responses to Precipitation

by
Yani Hu
,
Jiahao Shi
,
Fangfang Qiang
,
Changhai Liu
* and
Ning Ai
*
Shaanxi Key Laboratory of Research and Utilization of Resource Plants on the Loess Plateau, College of Life Sciences, Yan’an University, Yan’an 716000, China
*
Authors to whom correspondence should be addressed.
Agronomy 2025, 15(6), 1341; https://doi.org/10.3390/agronomy15061341
Submission received: 29 April 2025 / Revised: 26 May 2025 / Accepted: 27 May 2025 / Published: 30 May 2025
(This article belongs to the Special Issue Soil Health and Properties in a Changing Environment)
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Abstract

To clarify the response mechanisms of soil nematodes as bioindicators of ecosystem health to precipitation variations in loess hilly forests, this study investigated soil nematodes in pure Populus hopeiensis forests across different precipitation gradients in Wuqi County. Through soil physicochemical analysis and high-throughput sequencing of soil nematodes, we analyzed the characteristics of soil nematode communities and their responses to precipitation variation. The results demonstrated the following: (1) Dominant genera and trophic groups of soil nematodes were significantly influenced by precipitation, with Acrobeloides prevailing across all gradients while Paratylenchus reached maximum abundance (26.8%) in moderate precipitation zones. (2) Bacterivorous nematodes prevailed in both low- and high-precipitation zones, while herbivorous nematodes constituted the highest proportion in moderate precipitation zones. The abundances of herbivorous and fungivorous nematodes exhibited an initial increase followed by a decrease with rising precipitation, whereas predatory–omnivorous nematodes displayed the opposite trend. (3) The Chao1 and Shannon indices of soil nematodes initially increased and then decreased with increasing precipitation, reaching a peak in the Jinfoping site. Moreover, there were significant differences in nematode community structure among different precipitation gradients. (4) Redundancy analysis and PLS-PM modeling identified soil water content (SWC), total nitrogen (TN), and capillary water holding capacity (CWHC) as key drivers of nematode communities. Precipitation indirectly regulated nematode functionality by modifying soil physicochemical properties and microbial activity. (5) Ecological function analysis revealed bacterial-dominated organic matter decomposition (Nematode Channel Ratio, NCR > 0.75) in the Changcheng and Baibao sites, contrasting with fungal channel predominance (NCR < 0.75) in Jinfoping. This research elucidates the mechanism whereby precipitation drives nematode community divergence through regulating soil physicochemical properties and microbial activity. The findings provide scientific basis for soil biodiversity conservation and ecological restoration benefit assessment in regional ecological restoration projects, and soil health management and sustainable land use in agricultural ecosystems.

1. Introduction

The loess hilly region is one of the most severely water-eroded areas in China and a core implementation zone for forestry ecological engineering projects such as the Grain-for-Green Program [1]. After over two decades of vegetation restoration and reconstruction, this region has experienced a significant increase in vegetation coverage and a marked reduction in soil erosion [2,3]. As a native tree species, Populus hopeiensis has been widely distributed and planted in the study area due to its ecological adaptability, playing a vital role in regional soil water conservation and vegetation restoration. The maintenance and stability of its ecosystem functions are critical safeguards for regional ecological security [4,5]. Therefore, comprehensive research on this species is of great significance. However, previous studies have primarily focused on the vegetation community structure of Populus hopeiensis, with limited investigations addressing its understory soil biotic communities, particularly soil nematodes.
Soil nematodes are key components of soil micro-food webs, maintaining ecosystem functions by regulating organic matter decomposition, nutrient mineralization, and energy transfer [6]. Their community structure and functionality exhibit high sensitivity to environmental disturbances, making them ideal bioindicators for soil health assessment [7]. For instance, the Nematode Channel Ratio (NCR) between bacterivorous nematodes (BFs) and fungivorous nematodes (FFs) can indicate the bacterial or fungal dominance in organic matter decomposition pathways [8]. The Maturity Index (MI) and Plant-Parasitic Index (PPI) reflect soil disturbance levels and ecosystem stability [9]. Furthermore, as soil nematodes inhabit water films within soil pores, their locomotion and feeding activities depend on soil moisture, rendering them susceptible to precipitation-induced variations in soil humidity [10]. Precipitation, as a critical climatic driver of soil biological processes, directly influences nematode survival strategies and trophic group dynamics by altering soil water content (SWC), pore structure, and microbial activity [11]. For example, drought stress inhibits bacterial activity by reducing the continuity of soil water films and disrupts fungal hyphal networks, leading to an imbalance in the ratio of BF to FF. In contrast, moderate precipitation activates fungal-mediated organic matter decomposition pathways, thereby enhancing carbon sequestration efficiency [12,13]. Existing research on soil nematodes primarily focuses on single environmental factors or vegetation types, with limited systematic exploration of the interactive mechanisms between precipitation gradients and soil physicochemical properties or microbial activity [10,11]. However, elucidating precipitation-driven regulatory mechanisms on soil biota has become a scientific imperative for optimizing vegetation restoration strategies, enhancing ecosystem resilience, and systematically evaluating regional ecological restoration outcomes. Wuqi County, as a representative area for forestry ecological engineering construction such as the Grain-for-Green Program in the loess hilly region, exhibits a distinct northwest-to-southeast increasing trend in annual precipitation: from Changcheng (400~410 mm) and Jinfoping (440~445 mm) to Baibao (460~470 mm). This spatial pattern provides an ideal research platform for revealing cascading effects of precipitation changes on soil biotic communities. Previous studies have identified significant differences in soil microbial communities across precipitation gradients [14,15], yet the response patterns of soil nematode communities and their interaction mechanisms with environmental factors remain unclear, warranting further investigation. Therefore, this study focuses on soil nematodes in pure Populus hopeiensis forests in Wuqi County, located in the loess hilly region. Experimental demonstration areas were established across three precipitation gradients (Changcheng, Jinfoping, and Baibao) to investigate the effects of precipitation on soil nematode community composition, diversity, and ecological functions. The study aims to elucidate the driving mechanisms behind the structural and functional divergence of soil nematode communities. The findings are expected to provide theoretical support for the sustainable management of agroforestry systems and a scientific basis for soil biodiversity conservation and ecological restoration benefit evaluation in regional ecological rehabilitation projects.

2. Materials and Methods

2.1. Study Area Description and Site Selection

The study area is located in Wuqi County (107°38′57″–108°32′49″ E, 36°33′33″–37°24′27″ N) within the loess hilly region, with elevations ranging from 1233 to 1809 m above sea level. The topography is dominated by loess ridge-hill and gully landforms, and the primary soil type is loessial soil, characterized by a homogeneous texture and a slightly alkaline pH (8.0~8.3). The county experiences a semi-arid temperate continental monsoon climate, with a mean annual temperature of 7.8 °C, cold winters, hot summers, and significant diurnal temperature variations (Figure 1). Precipitation is concentrated from July to September, with an average annual rainfall of 483.4 mm, displaying a decreasing trend from the southeast to the northwest [16]. This study selected three Populus hopeiensis pure forests under distinct precipitation gradients (Changcheng, Jinfoping, and Baibao) in Wuqi County as research sites [17]. Detailed field surveys and data collection were conducted in April, July, and September 2023. Basic information of the sampling sites is presented in Table 1.

2.2. Soil Sample Collection and Analysis

This study was conducted in a 20 m × 20 m standard plot of pure Populus hopeiensis forest. Soil samples from 0–10 cm depth were randomly collected from 10 sampling points using a soil auger. After removing plant roots and stones, the samples were homogenized and stored at 4 °C for transportation to the laboratory. Each sample was divided into two portions: one air-dried for chemical property analysis, and the other stored at 4 °C for nematode detection. Additionally, soil cores (100 cm3) were collected for physical property measurements. Soil physicochemical properties were determined following the methods described in “Soil Physicochemical Analysis” [18], “Soil Agrochemical Analysis” [19], and “Soil Enzymes and Research Methods” [20]: soil bulk density (BD) [21], maximum water holding capacity (MaxWHC), capillary water holding capacity (CWHC), capillary porosity (CP), total porosity (TPR), and non-capillary porosity (NCP) were measured using the soil core method [22]. Soil water content (SWC) was determined via the oven-drying method. Soil pH was measured with a PHS-320 high-precision pH meter (soil-to-water ratio of 2.5:1). Electrical conductivity (EC) was analyzed using a DDS-608 multi-functional conductivity meter (soil-to-water ratio of 5:1) [23]. Soil organic matter (SOM) was quantified by the potassium dichromate oxidation–external heating method. Total nitrogen (TN) and total phosphorus (TP) were measured with a fully automated discrete chemical analyzer (CleverChem Anna) [24]. Alkali-hydrolyzable nitrogen (AN) was determined by the alkali diffusion method [25]. Available phosphorus (AP) was assessed via the molybdenum-antimony anti-spectrophotometric method [26]. Available potassium (AK) was extracted using the NH4OAc extraction method and quantified by flame photometry. Ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3−-N) were analyzed by the indophenol blue colorimetric method [27]. Alkaline phosphatase (ALP) activity was measured using the disodium phenyl phosphate colorimetric method; urease (URE) activity was assessed via the indophenol blue colorimetric method; catalase (CAT) activity was determined by the spectrophotometric method [28,29].

2.3. Soil Nematode Analysis

Soil nematodes were extracted using the Baermann funnel method. Briefly, 100 g of soil sample (stored at 4 °C for no more than 48 h) was placed on double-layered gauze in the funnel [30]. An appropriate amount of deionized water was added to submerge the sample, followed by incubation at room temperature for 24 h. The bottom liquid was then collected and centrifuged at 1902 rpm for 10 min. After discarding the supernatant, the suspension was transferred to 2 mL centrifuge tubes and stored at −80 °C before being shipped on dry ice every other day to Personal Biotechnology Co., Ltd. (Shanghai, China) for DNA extraction and identification. The company extracted DNA using the FastDNA™ Spin Kit for Soil (MP Biomedicals, Solon, OH, USA, Cat.116560200), performed PCR amplification with primers NF1 (5′-GGTGGTGCATGGCCGTTCTTAGTT-3′) and 18Sr2b (5′TACAAAGGGCAGGGACGTAAT-3′) [31], and implemented an amplification protocol consisting of pre-denaturation, 30 cycles (denaturation at 94 °C, annealing at 55 °C, extension at 72 °C), and final extension. The PCR protocol included initial denaturation at 98 °C for 3 min, followed by 30 cycles of denaturation at 98 °C for 30 s, annealing at 55 °C for 30 s, and extension at 72 °C for 45 s, with a final extension at 72 °C for 5 min. The PCR products were recovered, purified, and electrophoresed on a 2% agarose gel for quality control. Finally, paired-end sequencing was conducted on the Illumina platform by Personal Biotechnology Co., Ltd. (Shanghai, China).

2.4. Data Analysis

Soil nematode taxonomic composition was obtained via sequencing on the Paisennuo GeneCloud platform (https://www.genescloud.cn, accessed on 16 February 2025). Alpha diversity was analyzed using QIIME2 (2019) [32], while community structure differences across precipitation gradients were investigated through PCoA and NMDS analyses. Data processing utilized Excel 2021 and SPSS 26.0 [33], with one-way ANOVA (LSD post hoc test) applied to assess significant differences (p < 0.05) in nematode community characteristics and soil physicochemical factors among precipitation gradients. The figures were created using Origin 2021. Redundancy analysis (RDA) was performed using Canoco 5.0 [34] to analyze the relationships between soil nematode trophic groups and soil physicochemical factors. The “pls-pm” package in R 4.3.2 was employed to construct a partial least squares path model (PLS-PM) for elucidating precipitation gradient-driven regulatory mechanisms on soil nematode communities [35].

3. Results

3.1. Soil Nematode Community Composition

The study revealed distinct proportional distributions of dominant genera and relative abundances in soil nematode communities across precipitation gradients in Populus hopeiensis pure forests (Figure 2). High-throughput sequencing analysis (97% sequence similarity threshold for OTU clustering) identified 42 nematode genera in the study area. The top 10 genera collectively accounted for over 98% of the total abundance, indicating their dominance in the soil nematode communities. In the Changcheng site, the dominant genera of soil nematodes were Acrobeloides, Acrobeles, Microdorylaimus, and Hexamermis, among which Acrobeloides had the highest relative abundance (27.7%). The common genera included seven types such as Aporcelaimellus, Rotylenchulus, and Filenchus, accounting for 31.3% of the total abundance. At the Jinfoping site, the dominant genera were Acrobeloides, Aporcelaimellus, Paratylenchus, and Rotylenchulus. Among them, Paratylenchus had the highest relative abundance (26.8%). The common genera were composed of five types such as Filenchus, Aphelenchus, and Acrobeles, accounting for 23.8% of the total abundance. In the Baibao site, the dominant genera were Acrobeloides, Aporcelaimellus, and Acrobeles, and the common genera were Octomyomermis, Paratylenchus, and Aphelenchoides, accounting for 39.5% of the total abundance.
The relative abundance of soil nematode trophic groups exhibited significant responses to precipitation variations (Figure 3). In the Changcheng and Baibao sites, BFs exhibited significantly higher relative abundance than other functional groups, demonstrating dominant. In the Jinfoping site, PPs had the highest proportion, followed by BFs. Trophic group abundances displayed divergent trends along the precipitation gradient: PPs and FFs initially increased and then decreased with rising precipitation, whereas omnivorous-predatory nematodes (OPs) and BFs exhibited an opposite trend.

3.2. Soil Nematode Alpha Diversity Indices

As shown in Figure 4, all alpha diversity indices of soil nematodes in Populus hopeiensis pure forests exhibited an initial increase followed by a decrease with rising precipitation. Significant differences (p < 0.05) were observed in Chao1 and Shannon indices among the Changcheng, Jinfoping, and Baibao sites. However, no significant differences (p > 0.05) were detected in Simpson indices across the three sites. The Pielou index in the Jinfoping site significantly differed from those in Changcheng and Baibao (p < 0.05). The Jinfoping site displayed the highest values for all four diversity indices.

3.3. Soil Nematode Beta Diversity Indices

Principal coordinate analysis (PCoA) revealed that the first two principal coordinates (PC1 and PC2) explained 29.5% and 18.0% of the soil nematode community structure variation, respectively (Figure 5a). Nematode communities in the Changcheng and Jinfoping sites were distinctly separated from those in the Baibao site, with partial overlap observed between the Changcheng and Jinfoping samples. Non-metric multidimensional scaling (NMDS) yielded a stress value of 0.00316, indicating highly reliable ordination results that further validated the structural differences in nematode communities (Figure 5b).

3.4. Soil Nematode Ecological Indices

The analysis of responses of soil nematode ecological function indices to precipitation under Populus hopeiensis forests showed (Figure 6) that the PPI in the Jinfoping sample area was higher than other regions. The PPI values across different precipitation regions followed the following order: Jinfoping > Baibao > Changcheng, i.e., PPI initially increased and then decreased with rising precipitation. The MI in the Baibao sample area showed significant differences compared with both the Changcheng and Jinfoping areas (p < 0.05), with MI decreasing as precipitation increased. No significant differences were observed in PPI/MI among the three regions (p > 0.05), with mean values showing Baibao > Changcheng > Jinfoping. The NCR in the Changcheng and Baibao sample areas significantly differed from the Jinfoping area (p < 0.05), where NCR values in Changcheng and Baibao were greater than 0.75, while Jinfoping’s NCR was below 0.75. Both PPI/MI and NCR exhibited a pattern of first decreasing then increasing with precipitation augmentation.
As shown in Table 2, no significant differences (p > 0.05) were observed in SOM, NH4+-N, AP, TP, or CAT across the three precipitation gradients, while significant differences (p < 0.05) were detected in pH, BD, MaxWHC, and TPR. In the Changcheng site, SWC, CWHC, CP, ALP, and NO3−-N showed significant differences (p < 0.05) compared to Jinfoping and Baibao. Additionally, AK in Changcheng differed significantly from Jinfoping (p < 0.05). The Baibao site exhibited significant differences (p < 0.05) in AN, EC, TN, and NCP relative to the other two sites. URE activity in Jinfoping significantly differed from the other sites (p < 0.05).
Redundancy analysis (RDA) revealed that the first and second ordination axes in the Changcheng site explained 74.14% and 1.44% of the community variation, respectively, collectively accounting for 75.58% of the variability, with the first axis dominating the ordination (Figure 7a). SWC, TN, and CWHC were the primary physicochemical drivers of nematode communities in Changcheng. PPs and BFs clustered on the positive side of the first axis, showing positive correlations with pH and AN, while OPs were positively correlated with SWC, CWHC, and TN. PPs exhibited a negative correlation with TN. In the Jinfoping site, the first and second ordination axes explained 64.49% and 1.04% of community variation, respectively (Figure 7b). ALP governed nematode community distribution, displaying a positive correlation with PPs but a negative correlation with OPs. FFs and BFs were positively associated with TPR, CAT, and pH. For the Baibao site, the first ordination axis (53.27%) and second axis (22.3%) jointly explained 75.57% of community variation (Figure 7c). CWHC, EC, and BD emerged as key drivers. PPs showed positive correlations with CWHC, TP, ALP, and EC, but a negative correlation with BD. OPs were negatively correlated with NH4+-N, whereas BFs and FFs exhibited positive correlations with NH4+-N and negative correlations with NO3−-N.
To further elucidate the regulatory mechanisms of precipitation gradients on soil nematode communities, a PLS-PM model was constructed (Figure 8). The results demonstrated that precipitation exerted a direct positive effect on soil physicochemical indicators (path coefficient = 0.936, p < 0.001) and microbial activity indicators (path coefficient = 0.874, p < 0.05), significantly influencing both soil environments and microbial activity indices. Nematode communities exhibited a significant negative effect on nematode functional indices (p < 0.001), with a path coefficient of −0.723. Precipitation indirectly affected nematode functional indices through soil physicochemical properties and microbial activity indicators, improving soil conditions to indirectly support ecosystem functioning. However, its direct effect on nematode functional indices was non-significant (p > 0.05).

4. Discussion

4.1. Soil Nematode Community Characteristics

The composition of dominant nematode genera and trophic groups exhibited marked differences across precipitation gradients. In the low-precipitation Changcheng site, Acrobeloides dominated with a relative abundance of 27.7%. As a typical r-strategist, it adapts to resource competition under drought stress through rapid proliferation. The increased abundance of Microdorylaimus (a predatory genus) reflects cascading effects from simplified soil food web hierarchies in low-precipitation areas: drought suppresses plant root development, leading to reduced PP populations, which consequently forces predators to shift their feeding preference toward BFs. [36,37]. In the moderate-precipitation Jinfoping site, Paratylenchus exhibited the highest abundance (26.8%). This genus, adapted to stable environments, demonstrated significant dominance under moderate soil moisture, likely associated with improved soil aeration and alleviated drought stress [38]. In the high-precipitation Baibao site, the concentration of dominant nematode genera reflects the screening effect of low-oxygen conditions on communities. As precipitation increases, the soil oxygen diffusion rate decreases. This inhibits the survival of aerobic species. However, the anoxia-tolerant genera Acrobeloides and Aporcelaimellus gain ecological dominance through metabolic adaptations.
Regarding trophic groups, BFs exhibited consistently high relative abundances across all sites [39], which is closely associated with the high-pH environments in the loess region [40]. The abundances of PPs and FFs initially increased and then decreased with rising precipitation [41]. Moderate precipitation promotes the release of plant root exudates and microbial growth, indirectly supporting PP reproduction and providing resources for microbivorous nematodes. However, excessive precipitation reduces soil aeration and suppresses microbial activity [42]. The abundances of OPs and PPs declined under drought conditions due to prey resource scarcity and constrained plant growth but gradually recovered with improved moisture conditions. PPs declined again in high-precipitation areas due to root hypoxia [43].

4.2. Soil Nematode Diversity Indices

In this study, Chao1 and Shannon indices exhibited an initial increase followed by a decrease with rising precipitation, peaking in the Jinfoping site. This suggests that moderate precipitation maximizes community diversity by providing stable microenvironments and diversified resources [44], while diversity decline in high-precipitation areas may stem from suppressed microbial activity due to soil pore water saturation, thereby reducing nematode food resources [45]. The lack of significant differences in Simpson indices indicates persistent dominance of key genera across all sites, diminishing spatial divergence in community dominance [46]. The high diversity in Jinfoping implies that moderately humid conditions offer more stable microhabitats and heterogeneous food resources for nematodes [47]. Its significantly higher Pielou index compared to other sites further demonstrates that moderate precipitation promotes species evenness, where optimal environmental stress maximizes community diversity. The results from PCoA and NMDS analyses corroborated precipitation-driven impacts on nematode community structure in Populus hopeiensis forests. Partial overlap between Changcheng and Jinfoping reflects structural similarities in their nematode communities, suggesting continuity in adaptive strategies across low-to-moderate precipitation gradients. The distinct separation of Baibao’s nematode community likely relates to precipitation-induced alterations in soil physical properties [48]. High precipitation accelerates nutrient leaching and modifies organic matter composition, thereby reshaping nematode functional proportions [49].

4.3. Soil Nematode Ecological Function Indices

The results demonstrate significant precipitation-driven effects on soil nematode ecological function indices. The marked decline in MI in the Baibao site indicates a high-frequency disturbance regime in its soil ecosystem, likely linked to physical perturbations or nutrient leaching under elevated precipitation [50]. The higher PPI in Jinfoping reflects stability in plant–nematode interaction systems. NCR > 0.75 in Baibao suggests that high soil moisture induces capillary pore saturation, suppressing fungal growth while promoting hypoxia-tolerant bacterial proliferation, thereby establishing bacterial-dominated organic matter decomposition pathways. Similarly, in Changcheng, NCR > 0.75 implies bacterial-channel dominance in decomposition despite drought-induced diversity constraints [8]. The alkaline pH of loessial soils further facilitates ammonifying bacteria while inhibiting fungal hyphal expansion. In contrast, NCR < 0.75 in Jinfoping, coupled with PP dominance, implies fungal-channel-driven decomposition, potentially influencing soil carbon sequestration efficiency [51].

4.4. Environmental Drivers of Soil Nematode Communities

This study revealed significant differences in soil physicochemical properties across precipitation gradients in Populus hopeiensis pure forests, reflecting precipitation-driven remodeling of soil microenvironments. In the low-precipitation Changcheng site, distinct disparities in SWC and CWHC compared to Jinfoping and Baibao highlighted dual limitations of soil moisture and nitrogen under drought conditions. Low precipitation-induced water scarcity suppressed microbial activity and organic matter mineralization, leading to reduced NO3−-N and AN levels [52]. Conversely, elevated precipitation in Baibao accelerated nitrogen leaching and transformation, resulting in significant differences in AN and TN compared to other sites, consistent with precipitation-mediated regulation of soil nitrogen cycling [53].
Both redundancy analysis and PLS-PM modeling identified SWC, TN, and CWHC as primary drivers of nematode communities. In Changcheng, dual constraints of SWC and TN correlated BFs positively with pH, reflecting alkaline soil-enhanced ammonifying bacterial activity that supported BF proliferation. In Jinfoping, ALP dominates the distribution of soil nematode communities. By promoting organic phosphorus mineralization, ALP increased soil phosphorus availability, and its positive correlation with PPs indicates that increased phosphorus availability under moderate precipitation conditions promotes the release of plant root exudates, indirectly promoting PP proliferation [54,55]. The positive FF–TPR correlation further validated improved soil aeration fostering fungal growth. Elevated EC in Baibao accelerated nitrogen leaching, driving negative FF–NO3−-N correlations, suggesting high nitrogen suppresses mycorrhizal fungal activity and FF food resources [56].
The PLS-PM model elucidated precipitation’s indirect regulation of nematode communities via soil properties. Model outputs aligned with SWC data from Changcheng and Baibao, demonstrating precipitation reshapes soil microenvironments through SWC to indirectly modulate nematode structure and function. The significant negative effect of nematode communities on functional indices confirmed trophic composition’s regulatory role in organic matter decomposition pathways [3], consistent with NCR > 0.75 and diversity decline in Baibao. The model revealed that the total effect of precipitation on microbial activity indices was significantly positive. Specifically, alkaline phosphatase (ALP) activity in the Jinfoping sample area showed an increasing trend with higher precipitation. These findings collectively demonstrate that under the baseline environmental conditions of the study area, moderate precipitation may enhance microbial activity characterized by phosphorus-metabolizing enzymes (particularly ALP), thereby promoting the relative contribution of the fungal channel and positively influencing soil carbon sequestration efficiency [56]. Additionally, the non-significant direct precipitation→functional index effect underscores a hierarchical “precipitation–soil–biota–function” regulatory mechanism mediated entirely through indirect pathways.

5. Conclusions

This study revealed that precipitation significantly influenced nematode community structure and soil physicochemical environments. The moderate precipitation zone exhibited the highest nematode diversity and fungal-dominated organic matter decomposition channels due to optimal moisture conditions. The low-precipitation zone developed simplified communities dominated by r-strategist nematodes under water limitation, while the high-precipitation zone showed reduced diversity and bacterial-driven decomposition pathways owing to diminished soil aeration. SWC, TN, and CWHC were identified as key drivers of nematode communities. The PLS-PM model further confirmed that precipitation indirectly regulates nematode functional indices by altering soil physicochemical properties and microbial activity. These findings emphasize a “precipitation–soil–biota–function” multi-level cascading mechanism, providing a scientific basis for soil biodiversity conservation and ecological restoration efficacy evaluation in regional rehabilitation projects.

Author Contributions

Conceptualization, Y.H., N.A. and F.Q.; methodology, Y.H. and N.A.; software, Y.H.; validation, Y.H., N.A. and C.L.; formal analysis, Y.H.; investigation, Y.H. and N.A.; data curation, Y.H.; writing—original draft preparation, Y.H.; writing—review and editing, Y.H. and N.A.; visualization, Y.H. and N.A.; supervision, F.Q., N.A., J.S. and C.L.; project administration, N.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (32060297); the National Key Research and Development Plan of China (2023YFF130510401); the Shaanxi Natural Science Basic Research Project (2025JC-YBMS-204); and the Research Project of Yan’an University (2022YDZX12, 2021ZCQ009, 2023JBZR-20).

Data Availability Statement

Data are available on request from the corresponding authors.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Sample plot map.
Figure 1. Sample plot map.
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Figure 2. The relative abundance distribution of the 10 dominant nematode genera shared among the three sites. Note: CC: Changcheng; JFP: Jinfoping; BB: Baibao; the same applies to the following text.
Figure 2. The relative abundance distribution of the 10 dominant nematode genera shared among the three sites. Note: CC: Changcheng; JFP: Jinfoping; BB: Baibao; the same applies to the following text.
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Figure 3. Response of soil nematode trophic taxa to precipitation. Note: PPs: herbivorous nematodes; OPs: omnivorous-predatory nematodes; BFs: bacterivorous nematodes; FFs: fungivorous nematodes.
Figure 3. Response of soil nematode trophic taxa to precipitation. Note: PPs: herbivorous nematodes; OPs: omnivorous-predatory nematodes; BFs: bacterivorous nematodes; FFs: fungivorous nematodes.
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Figure 4. Response of soil nematode diversity index to precipitation (mean ± standard deviation). Note: Different English lowercase letters indicate significant differences (p < 0.05) among different study areas for the same vegetation.
Figure 4. Response of soil nematode diversity index to precipitation (mean ± standard deviation). Note: Different English lowercase letters indicate significant differences (p < 0.05) among different study areas for the same vegetation.
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Figure 5. Principal coordinate analysis of soil nematode communities (a) and non-metric multidimensional scaling (b). Note: CChby: Changcheng (400~410 mm) Populus hopeiensis; JFPhby: Jinfoping (440~450 mm) Populus hopeiensis; BBhby: Baibao (460~470 mm) Populus hopeiensis; the same applies to the following.
Figure 5. Principal coordinate analysis of soil nematode communities (a) and non-metric multidimensional scaling (b). Note: CChby: Changcheng (400~410 mm) Populus hopeiensis; JFPhby: Jinfoping (440~450 mm) Populus hopeiensis; BBhby: Baibao (460~470 mm) Populus hopeiensis; the same applies to the following.
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Figure 6. The response of soil nematode ecological functional indices to precipitation (mean ± standard deviation). Note: PPI: Plant-Parasitic Index; MI: Maturity Index; NCR: Nematode Channel Ratio. Different English lowercase letters indicate significant differences (p < 0.05) among different study areas for the same vegetation.
Figure 6. The response of soil nematode ecological functional indices to precipitation (mean ± standard deviation). Note: PPI: Plant-Parasitic Index; MI: Maturity Index; NCR: Nematode Channel Ratio. Different English lowercase letters indicate significant differences (p < 0.05) among different study areas for the same vegetation.
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Figure 7. Redundancy analysis of soil nematode trophic groups and soil physicochemical factors under different precipitation gradients. Notes: Hollow dashed arrows indicate soil physicochemical factors, and solid arrows indicate nematode trophic groups. (a) RDA analysis of soil nematode trophic groups and soil physicochemical factors at Changcheng. (b) The same analysis at Jinfoping. (c) The same analysis at Baibao.
Figure 7. Redundancy analysis of soil nematode trophic groups and soil physicochemical factors under different precipitation gradients. Notes: Hollow dashed arrows indicate soil physicochemical factors, and solid arrows indicate nematode trophic groups. (a) RDA analysis of soil nematode trophic groups and soil physicochemical factors at Changcheng. (b) The same analysis at Jinfoping. (c) The same analysis at Baibao.
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Figure 8. PLS-PM path model of precipitation gradient on soil nematode community and ecological functions. Notes: Gray arrows indicate negative impacts, and green arrows indicate positive impacts. The numbers beside the arrows are path coefficients (p-values) representing the effect size of the relationships. *** indicates a significant correlation at the 0.001 level, and * indicates a significant correlation at the 0.05 level. JSL: precipitation gradient; SPCs: soil physicochemical indices; MAIs: microbial activity indices; Nem: nematode community; NFIs: functional indices of nematodes.
Figure 8. PLS-PM path model of precipitation gradient on soil nematode community and ecological functions. Notes: Gray arrows indicate negative impacts, and green arrows indicate positive impacts. The numbers beside the arrows are path coefficients (p-values) representing the effect size of the relationships. *** indicates a significant correlation at the 0.001 level, and * indicates a significant correlation at the 0.05 level. JSL: precipitation gradient; SPCs: soil physicochemical indices; MAIs: microbial activity indices; Nem: nematode community; NFIs: functional indices of nematodes.
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Table 1. Basic information of sampling sites.
Table 1. Basic information of sampling sites.
Sampling SiteAnnual
Precipitation (mm)		Slope AspectStand Age (a)Altitude (m)Slope
Gradient (°)		Canopy
Density (%)
Changcheng400~410Shady slope301505.72330
Jinfoping440~445Shady slope301331.62075
Baibao460~470Shady slope301484.12585
Table 2. Soil physicochemical indicators across precipitation gradients.
Table 2. Soil physicochemical indicators across precipitation gradients.
Soil Physicochemical
Indicator		Changcheng
(400–410 mm)		Jinfoping
(440–445 mm)		Baibao
(460–470 mm)
NH4+-N (mg kg−1)0.46 ± 0.14 a0.58 ± 0.1 a0.55 ± 0.12 a
NO3−-N (mg kg−1)0.43 ± 0.19 b0.63 ± 0.05 a1.84 ± 1.31 a
AN (mg kg−1)38.55 ± 10 a72.3 ± 38.86 a68.85 ± 26.44 b
SOM (g kg−1)13.41 ± 4.87 a11.98 ± 4.58 a11.46 ± 2.33 a
pH8.27 ± 0.02 a8.06 ± 0.03 c8.16 ± 0.07 b
EC (μS cm−1)68 ± 6.2 a73 ± 0.38 a100.13 ± 12.61 b
AP (mg kg−1)12.34 ± 9.54 a17.99 ± 12.39 a14.11 ± 12.06 a
AK (mg kg−1)97.94 ± 11.53 a170.61 ± 65.83 b127.06 ± 51.92 ab
TN (g kg−1)0.63 ± 0.25 b0.97 ± 0.6 b2.31 ± 0.45 a
TP (g kg−1)0.7 ± 0.14 a0.84 ± 0.44 a0.76 ± 0.19 a
CAT (ml g−1)2.76 ± 1.12 a2.8 ± 0.87 a2.91 ± 0.43 a
ALP (mg g−1 d−1)0.33 ± 0.1 b0.43 ± 0.07 a0.49 ± 0.07 a
URE (mg g−1 d−1)0.3 ± 0.14 a0.21 ± 0.04 b0.41 ± 0.1 a
SWC (%)8.13 ± 0.58 a15.74 ± 4.26 b19.54 ± 1.48 b
BD (g·cm−3)1.4 ± 0.04 a1.29 ± 0.06 b1.19 ± 0.09 c
MaxWHC (%)32.26 ± 0.76 c37.78 ± 3.24 b44.73 ± 6.12 a
CWHC (%)28.59 ± 0.46 b34.15 ± 2.08 a37.01 ± 2.73 a
NCP (%)5.12 ± 0.68 b4.62 ± 1.69 b8.95 ± 3.67 a
CP (%)39.9 ± 1.15 b44.06 ± 1.28 a43.31 ± 1.6 a
TPR (%)45.02 ± 1.38 c48.68 ± 1.96 b52.27 ± 3.55 a
Note: Different English lowercase letters indicate significant differences (p < 0.05) among different study areas for the same vegetation.
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Hu, Y.; Shi, J.; Qiang, F.; Liu, C.; Ai, N. Characteristics of Soil Nematode Communities in Pure Populus hopeiensis Forests in the Loess Hilly Region and Their Responses to Precipitation. Agronomy 2025, 15, 1341. https://doi.org/10.3390/agronomy15061341

AMA Style

Hu Y, Shi J, Qiang F, Liu C, Ai N. Characteristics of Soil Nematode Communities in Pure Populus hopeiensis Forests in the Loess Hilly Region and Their Responses to Precipitation. Agronomy. 2025; 15(6):1341. https://doi.org/10.3390/agronomy15061341

Chicago/Turabian Style

Hu, Yani, Jiahao Shi, Fangfang Qiang, Changhai Liu, and Ning Ai. 2025. "Characteristics of Soil Nematode Communities in Pure Populus hopeiensis Forests in the Loess Hilly Region and Their Responses to Precipitation" Agronomy 15, no. 6: 1341. https://doi.org/10.3390/agronomy15061341

APA Style

Hu, Y., Shi, J., Qiang, F., Liu, C., & Ai, N. (2025). Characteristics of Soil Nematode Communities in Pure Populus hopeiensis Forests in the Loess Hilly Region and Their Responses to Precipitation. Agronomy, 15(6), 1341. https://doi.org/10.3390/agronomy15061341

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