The Negative Effects of Tourist Trampling on the Soil Physical Properties and Microbial Community Composition in a Natural Oak Forest

Abstract

Tourist trampling is a serious disturbance affecting the soil structure and microbial community in forests. However, it is still unclear whether the response of soil microorganisms to trampling is attributed to the alterations in soil physical (soil bulk density and total porosity) or soil chemical (total nitrogen and soil organic carbon) properties. To determine the response and mechanism of soil microbial community composition to tourist trampling, we conducted a field experiment including four levels of trampling intensity (control, mild, moderate, and severe) at the Baotianman forest ecotourism area. With increasing trampling intensity, soil bulk density showed a substantially increasing pattern, whereas soil total porosity, total nitrogen, and soil organic carbon showed a decreasing trend. Compared to the insignificant change under mild trampling, moderate and severe trampling significantly decreased soil bacterial PLFAs (phospholipid fatty acids) by 46.6% and 57.5%, and fungal PLFAs by 36.3% and 61.5%, respectively. Severe trampling showed a significantly negative effect (−4.37%) on the proportion of soil bacterial PLFAs. Changes in soil bulk density and porosity induced by trampling, rather than total nitrogen and soil organic carbon, played a greater role in regulating soil microbial community composition. These findings suggest that soil microbial community composition and biomass are significantly influenced by the changes in soil texture and aeration conditions caused by tourist trampling.

1. Introduction

With the development of the economy and society, tourism has rapidly developed into the world’s largest industry [1]. While improving financial incomes for tourism destinations and delivering economic benefits to local communities, it may also induce serious threats to the ecological environment [2,3]. Tourism activities, such as tourist trampling [4], the construction of tourist infrastructure, and waste management [5], exert strong impacts on soil physical, chemical, and biological variables [6]. While engaging in tourism, tourists often deviate from the designated path due to activities such as photography or wildlife observation, or due to personal preferences, leading to significant ecological disturbance to the natural environment of the scenic spot. Tourist trampling disturbance always occurs within a range of 1–3 m along the periphery of the tourism trails, with substantial variations across different ecosystems.

Among various tourist activities, tourist trampling is the most prevalent and serious problem, leading to significant disturbance to the vegetation and soil [7,8,9]. Tourist trampling can result in the subsequent alteration of the soil microenvironment, such as soil physical and chemical properties. For example, tourist trampling in the Baligou scenic spot in China resulted in a reduction in soil porosity and soil pH [9]. Numerous studies have consistently demonstrated that tourist trampling can result in a reduction in soil water content and soil organic carbon (SOC), potentially leading to severe soil erosion [3,8]. Various biotic and abiotic factors such as vegetation [10], soil physicochemical properties [11], temperature, moisture [12,13], and seasonal variations [14] exert profound impacts on soil microbial community composition [9]. The change in soil compactness caused by trampling may be one of the most crucial factors for quantifying the extent of soil degradation, which exerts a strong influence on soil microorganisms. Soil compaction and changes in above-ground vegetation induced by mild and medium trampling showed negative effects on soil microbial activities in beech forests [15]. Moreover, tourist activity has a remarkable impact on plants and vegetation [6,7], which serve as a vital food source for soil microorganisms and animals. Consequently, tourist trampling may affect soil microorganisms by changing the composition of above-ground species [16]. A study conducted in a Swiss sub-alpine pasture found cattle trampling resulted in a 30% decrease in the concentration of phospholipid fatty acids (PLFAs), which is considered an indicator of soil microbial abundance [17]. The responses of soil microorganisms to trampling may vary with trampling intensity. For example, moderate trampling led to an increased diversity of soil microbes, whereas severe trampling resulted in a reduced value [18]. In alpine meadows, trampling showed substantial negative effects on both bacterial and fungal communities due to the changes in soil chemical properties [19]. Although the effects of trampling on the soil physicochemical properties and microbial community have been reported in some ecosystems, the influence of soil physical and chemical properties on the changes in the soil bacterial and fungal communities under trampling remains unclear.

The Baotianman scenic spot is located in the southwest of Henan Province in China, which is a famous mountain range belonging to the east side of Funiu Mountain. Baotianman serves not only as an important biodiversity reserve, but also as an irreplaceable destination for leisure tourism in central China. We conducted a study at the Baotianman scenic spot to examine the impact of tourist trampling on the soil microbial community composition. We hypothesize that tourist trampling could decrease the biomass of total soil microbes, as well as for each group (bacteria and fungi), and changes in soil physical properties caused by trampling may show a greater contribution to the altered soil microbial community. The objectives of this study were as follows: (1) to determine how tourist trampling affects the soil bacterial and fungal biomass and proportion and (2) to clarify which factors (soil bulk density, total porosity, TN, and SOC) regulate the changes in the soil microbial community induced by tourist trampling.

2. Materials and Methods

2.1. Site Description

This study was conducted at the Forest Ecological Research Station (30°20′–33°36′ N, 111°47′–112°04′ E) in Baotianman Natural Reserve, China. The study area has a warm temperate continental climate [20]. Mean annual temperature and precipitation are 15.1 °C and 886 mm, respectively. Nearly 60% of precipitation falls in the growing season (from May to September). The soils are dominated by Haplic Luvisol (FAO classification) with sand, silt, and clay contents of 57%–62%, 11%–13%, and 27%–30% [21], respectively. The typical forest community in this region is natural secondary forests mainly dominated by different species of oak trees (Quercus aliena var. acuteserrata Maximowicz ex Wenzig, Q. variabilis Blume, and Q. acutissima Carruth.). Soil organic C varied from 30.1 to 62.3 g kg⁻¹ and soil total nitrogen ranged from 1.56 to 3.15 g kg⁻¹ based on our early investigation. However, we did not have the data of the background soil microbial community. The Baotianman ecotourism scenic spot is renowned for its picturesque forest and stream landscape. Local managers had built a variety of well-designed tourist routes and elevated viewing platforms within the scenic spot. Although there were well-defined trails within the scenic spot, many tourists preferred to explore the adjacent stands surrounding the main road, thereby resulting in the formation of forest paths within the scenic spot.

2.2. Experimental Design

In April 2021, we conducted a preliminary survey along the cement-hardened roads to identify 31 forest paths suitable for further investigation and research. Trampling intensity is closely related to the duration (time) of trampling and the number of persons involved in the trampling. Using the records of local managers, we obtained the data on the formation time of each path and the approximate number of tourists walking through each path (

Table 1. Basic information on the plots with different trampling intensities (mean ± S.E., n = 4).

Treatment	Soil Penetration Resistance (Mpa)	Litter Biomass (g m−2)	Trampling History	Trampling Intensity (Person/Day)	Path Width (m)
Control	1.03 ± 0.13 a	491.2 ± 26.7 d	never	0	0
Mild	1.76 ± 0.36 b	236.6 ± 16.9 c	Since 2018	10–20	0.65 ± 0.05 a
Moderate	2.28 ± 0.22 c	117.8 ± 9.1 b	Since 2015	25–30	1.25 ± 0.11 b
Severe	2.79 ± 0.31 d	13.8 ± 4.2 a	Since 2012	35–45	2.05 ± 0.17 c

Data labeled with different lowercase letters indicate a significant difference among different intensities at 0.05 levels.

). Paths that have an older formation time and greater tourist loading were defined as “severe trampling”. In contrast, paths with a more recent formation time and less tourist loading were defined as “mild trampling”. Other paths that experienced a level between severe and mild trampling were classified as “moderate trampling” (Table 1). Based on the rules proposed above, 12 paths with similar vegetation and topographic conditions but different trampling intensities were selected from the 31 paths to conduct our study. The 12 paths were further classified according to 3 trampling intensities (mild, moderate, and severe). We selected four completely untrampled sites adjacent to the path as controls. The distance between all paths was more than 20 m. There were four replications for each trampling intensity. Three 60 cm × 60 cm plots with at least a 5 m distance were set up on each path and control to perform our field investigation. All paths and plots were situated at the same elevation.

With the increased trampling intensity, path width showed an increasing trend due to the increased tourist number. In addition, trampling resulted in a reduction in litter biomass (Table 1). A soil penetration resistance (SPR) of 0–20 cm was measured using a soil penetration resistance meter (PV6.08, Eijkelkamp, Giesbeek, The Netherlands) to assess the extent of the tourist trampling. SPR values showed significant differences under different trampling intensities, with 1.03 Mpa for the control, 1.76 Mpa for mild, 2.28 Mpa for moderate, and 2.79 Mpa for severe, respectively (p < 0.05, Table 1). In addition, litter biomass values showed significant differences between trampling intensities (Table 1).

2.3. Soil Sampling and Analysis

In July 2021, a soil sample at a 0–10 cm depth was collected from each plot using a soil drill (5 cm diameter). Three fresh soil samples from each path were mixed and sieved by a 2 mm mesh to remove all plant tissue and stones. The fresh soils sieved through 2 mm mesh were used for further physicochemical analysis. Soil water content (mass, %) was determined by the oven-drying method [22]. After sieving with mesh <250 μm, the air-dried soil sample was used to measure SOC (g kg⁻¹) by the potassium dichromate-vitriol oxidation method [23] and total nitrogen (TN, g kg⁻¹) content by Kjeldahl digestion [24].

A soil bulk density (BD, g cm⁻³) of 0–10 cm was measured by dividing the dry weight of the soil by the volume of the stainless steel cutting ring. Soil total porosity (TP, %) was calculated using Equation (1) based on the BD and assuming a soil particle density (PD) of 2.65 g cm⁻³.

TP = (1 − BD/PD) × 100

(1)

Soil microbial biomass C (MBC, mg kg⁻¹) was investigated by the chloroform fumigation-extraction method [25]. Briefly, extractable C of filtered K₂SO₄ extracts from both fumigated and unfumigated soil samples were analyzed by a TOC analyzer (TOC-VCPH Shimadzu Corp., Kyoto, Japan). Soil MBC was calculated from the differences in extractable C contents between the fumigated and the unfumigated samples using a conversion factor of 0.45 [26].

Phospholipid fatty acid (PLFA, nmol g⁻¹ dry weight) analysis was used to evaluate microbial community composition. As one of the most common approaches, the chemotaxonomic method could bypass the requirement for cultivation and enable the analysis of nearly 99% of natural populations [27,28]. Following the protocols described by Bossio and Scow (1998) [29], a fresh soil sample equivalent to 8 g dry weight was used for PLFA extraction. Extracted fatty acid methylesters were re-dissolved in 200 μL hexane containing 19:0 as an internal standard, and were analyzed using a Hewlett-Packard 6890 Gas Chromatograph (MINI, Inc., Newark, DE, USA) equipped with an Ultra 2-methylpolosiloxane column. All PLFA concentrations were calculated based on the 19:0 internal standard concentrations. The abundance of individual fatty acids was quantified as relative nmol per g of dry soil, and standard nomenclature was used [30]. As reported by Bossio and Scow (1989) [29], ‘A:BωC’ was used to denote the fatty acids. Fungal biomass was estimated from the concentrations of the biomarkers 18:1ω9c, 18:2ω6c, and 18:2ω9c [31]. Gram-positive bacteria were considered to be represented by the following PLFAs: i14:0, i15:0, a15:0, i16:0, i17:0, and a17:0 [29,32,33]. Gram-negative bacteria were identified by the following PLFAs: 16:1ω7c, cy17:0, 18:1ω7t, and cy19:0 [31,34,35,36]. Non-specific bacteria were identified by the saturated straight-chain PLFAs: 14:0, 15:0, 16:0, and 17:0 [35,36]. The PLFA value was used to represent the biomass of each group of soil microbes.

2.4. Statistical Analysis

Data of soil variables were first tested for normality and homogeneity of variance. Data presented in the paper are expressed as mean ± S.E. One-way ANOVA and multi-comparisons were performed to test the effects of trampling intensity on soil properties. Differences with statistical analysis reaching 0.05 level (p < 0.05) were considered to be significant. Pearson correlation analysis was used to assess the relationships between the soil microbial community and soil physicochemical properties. All statistical analyses were performed using the SPSS 26.0 software package for Windows (SPSS Inc., Chicago, IL, USA). Structural equation modeling (SEM) was used to determine the direct (with direct correlation) and indirect (closely related to the direct factors) effects of soil physicochemical properties on soil microbial community composition. Trampling intensity was regarded as an exogenous variable. A priori model was developed based on the literature review and our knowledge of these related predictors. The SEM analysis was performed using AMOS 21.0 software (IBM, SPSS, New York, NY, USA). Several tests were used to evaluate model fit, including the χ²-test, adjusted goodness of fit index, and root square error of approximation (RMSEA).

3. Results

3.1. Soil Physicochemical Properties

Severe trampling resulted in a significant decrease in soil moisture content (relative change 28.6%) compared to the control (p < 0.05, Figure 1a). However, mild and moderate trampling showed no effect on soil water contents (p > 0.05, Figure 1a). Compared with the control, three types of trampling significantly increased soil bulk density. Compared to the control, mild, moderate, and severe trampling led to a significant increase of soil bulk density by 32.8%, 65.5%, and 59.7%, respectively (p < 0.05, Figure 1b). Soil total porosity decreased significantly with increasing trampling intensity, and moderate and severe trampling resulted in significant reductions of 10.5% and 24.7% in total soil porosity compared to the control, respectively (p < 0.05, Figure 1c).

With increasing trampling intensity, soil TN, SOC, and MBC showed decreasing trends. In contrast to the control, severe trampling significantly reduced soil TN and SOC by 25.0% and 34.1% (p < 0.05, Figure 1d,e), respectively. Moderate trampling showed significant negative effects on SOC (24.8%, p < 0.05), whereas it showed no effect on TN (Figure 1). Mild, moderate, and severe trampling significantly reduced MBC by 19.9%, 36.1%, and 33.3%, respectively (p < 0.05, Figure 1f).

3.2. Soil Microbial Community Composition

Compared to the control, moderate and severe trampling significantly decreased soil bacterial PLFAs by 46.6% and 57.5% (p < 0.05), whereas mild trampling showed no effect on bacterial biomass (Figure 2). Similarly, moderate and severe trampling also showed a significant negative effect on fungal PLFAs (36.3% and 61.5%) (p < 0.05, Figure 2). Gram-positive and -negative bacterial PLFAs were significantly suppressed under moderate (38.4% and 34.9%) and severe (44.5% and 46.8%) trampling (p < 0.05, Figure 2).

Compared with the control, severe trampling significantly reduced the proportion of bacterial PLFAs (4.37%) (p < 0.05), whereas it showed no effect on the proportion of fungal PLFAs in the soil microbial community (Figure 3a,b). Three trampling treatments showed no effect on the ratio of bacterial PLFAs to fungal PLFAs when compared to the control (Figure 3c).

3.3. Relationships between Soil Microbial Composition and Soil Characteristics

Based on the Pearson correlation analysis, we found that soil total microbial PLFA content was positively correlated with soil porosity (r = 0.75), soil water content (r = 0.59), and TN (r = 0.51), but negatively correlated with soil bulk density (r = −0.71) (p < 0.05, Figure 4). Soil bacterial PLFA content also showed a positive dependence upon soil porosity (r = −0.75) and soil water content (r = −0.55), but a negative dependence upon soil bulk density (r = −0.72) (p < 0.05, Figure 4). Soil fungal PLFA content showed a strong positive correlation with soil porosity (r = 0.77), TN (r = 0.68), water content (r = 0.57), and SOC (r = 0.53) (p < 0.05, Figure 4).

The structural equation model analysis showed that the equation had a good fitness (Chi–square = 4.099, d.f. = 8, p = 0.79). Overall, soil porosity had a direct positive effect (R² = 0.53, p < 0.01) on soil microbial PLFA content, whereas changes in soil bulk density caused by trampling showed a direct negative effect (R² = −0.41, p < 0.05) on soil microbial PLFA content (Figure 5). Decreased soil water content induced by trampling showed an indirect effect on soil microbial PLFA content by increasing soil porosity (R² = 0.66, p < 0.01). Soil TN showed no effect on soil microbial PLFA content, although soil water content showed positive effects on soil TN (R² = 0.55, p < 0.01, Figure 5). The indirect effect of trampling intensity (−0.675) and the direct effect of soil bulk density (−0.411) led to a total negative effect on soil microbial PLFA content (Figure 6). Soil water content (0.348) and soil porosity (0.525) showed a total positive effect on soil microbial PLFA content through an indirect and a direct effect, respectively (Figure 6).

4. Discussion

4.1. Effects of Trampling on Soil Physicochemical Properties

This study reveals that there was a gradual increase in soil bulk density but a decrease in soil porosity with elevated trampling intensity, which suggests that soil bulk density and porosity exhibited high sensitivity to tourist trampling. In line with our findings, decreased soil bulk density induced by trampling has also been reported in Oulanka Natural Reserve (Finland) [37] and in Ramat Hanadiv Park (Israel) [38]. However, a study conducted in a pasture found that animal trampling reduced soil porosity, whereas it showed no effect on soil bulk density [39]. The differential responses of soil bulk density and porosity to tourist trampling may be related to soil texture and particle composition. It has been found that loam soil is more susceptible to external forces compared to sand and clay soils. On the other hand, the response of soil physical structure to trampling is also related to the number of plants present, particularly the quantity of underground roots. The plant root system not only plays a crucial role in elevating soil porosity through its growth and penetration processes, but also increases soil resistance to external forces when the soil is subjected to trampling, thereby keeping the integrity of the soil physical structure [3,38].

The findings that severe trampling led to a substantial reduction in soil TN and SOC indicate that the response of soil chemical characteristics to trampling may be weaker than that of soil physical properties. Soil chemical properties generally showed a resistance or elasticity to disturbance, because their changes are closely associated with the shift of plant and soil microbial activity, both of which depend on time. Consistent with our observations, decreased soil TN and SOC were also found in other forests [15], grasslands [40], and desert scenic spots [16]. The decreased SOC and TN under tourist trampling can be attributed to the following two aspects: Firstly, tourist trampling could result in a significant reduction in litter biomass (Table 1), thereby reducing the conversion of litter C into soil C. Secondly, the destruction of the soil physical structure due to tourist trampling may lead to a decrease in fine root biomass and root secretions in the topsoil [41]. Tourist trampling showed a direct negative effect on the biomass of the understory plants [41].

4.2. Effects of Trampling on the Soil Microbial Community

It has previously been reported that trampling could alter the composition and diversity of soil bacterial and fungal groups [4,18]. For example, enhanced soil bacterial diversity, but reduced fungal diversity, under trampling have been found in a Pinus tabuliformis forest [42]. In our study, we observed a negative effect of trampling on the bacterial and fungal biomass, although we did not quantify the changes in microbial diversity. Consistent with our findings, studies conducted on Jiugong Mountain [43] and in sub-alpine pastures [17] also reported a decreased bacterial and fungal amount under tourist trampling. The decreased soil bacterial and fungal biomass under tourist trampling could be explained by the following: The reduced soil porosity caused by trampling can impede soil aeration, thereby suppressing the normal survival and metabolism of soil microorganisms, especially for the oxygen-dependent species. The declined SOC and TN, as well as the decreased above-ground litter biomass due to trampling, could hinder the input of soil C substrates.

In terms of the soil microbial community, we found that soil bulk density and porosity contributed to the change in soil microbial community induced by trampling, whereas soil chemical properties showed no effect. Inconsistent with our findings, however, SOC, ammonium, and acid phosphatase activity were regarded as the primary driving factors for the changes in the microbial community under trampling in an urban forest park [18]. Therefore, the shifted soil microbial community induced by tourist trampling may be regulated by different environmental factors due to the variations in vegetation types and soil conditions. Compared with the grassland ecotourism scenic spot, trampling effects on forest soil may be more severe because the understory plants in forests are generally scarce and the recovery process after trampling is comparatively difficult.

5. Conclusions

Taken together, our findings that tourist trampling decreased the biomass of soil microorganisms and that the changes correlated to soil physical properties support our hypothesis. Specifically, our study showed that tourist trampling exhibited negative effects on soil physicochemical and microbiological variables, with stronger responses observed under severe trampling. In comparison to soil physical properties, the response of soil chemical variables to trampling was relatively weak, with significant decreases under severe trampling. With increasing trampling intensity, soil bacterial, fungal, and total microbial PLFA content showed decreasing trends. Reduced soil microbial biomass induced by trampling was mainly attributed to the changes in soil bulk density and porosity, rather than soil chemical properties. This finding suggests that soil physical texture plays a greater role in regulating the soil microbial community under tourism disturbance.