Soil physicochemical properties and macroarthropod abundance across two segments of a temperate forest in Darma Valley, Kumaun Himalaya, India

The study focused on soil macroarthropods, exploring their characteristics, environmental interactions, and role in soil nutrient dynamics—a subject that has been relatively understudied. To fill this gap, research was conducted in a temperate forest of Darma Valley, District Pithoragarh, Kumaun Himalaya, India. The forest was divided into two segments: A (disturbed) and B (undisturbed), based on anthropogenic pressure related to tree felling and lopping. The study spanned three months (July to September) over two consecutive years, 2020 and 2021. Soil properties, including soil organic carbon (SOC), total nitrogen (TN), available nitrogen (AN), available phosphorus (AP), soil moisture content (SMC), and pH, were analyzed. Data analysis employed techniques such as analysis of variance, total abundance, relative abundance, and correlation. The findings revealed significant differences in SMC, SOC, pH and AP between the two forest segments. A total of 2871 soil macroarthropods were sampled, representing 5 classes and 14 orders, with higher abundance found in the undisturbed forest segment B. While the species richness of soil macroarthropods remained relatively consistent, noticeable variations were observed in terms of total abundance and relative abundance across different orders. The relative abundance of soil macroarthropods was primarily influenced by soil pH and soil temperature .


INTRODUCTION
The soil-litter system, considered a major biodiversity pool (André et al., 1994;Lee, 1994), serves as a crucial pathway for nutrient exchange between soil and plants (Swift et al., 1979;Vitousek and Sanford, 1986).The release of nutrients from decaying plant matter is influenced by the abundance and diversity of soil biota, including soil macroarthropods (Ghilarov, 1977;Pimm, 1994), which is essential for maintaining ecosystem function (Cuevas and Medina, 1986;Jordan, 1985).Soil macrofauna, including macroarthropods, are sensitive to habitat changes, and human-induced disturbances can significantly impact their diversity and functioning (Beare et al., 1995;Jones et al., 1994;Lavelle et al., 1997;Matson et al., 1997).Selective removal of vegetation, as reported by Martin and Sommer (2004), has shown changes in macroarthropod diversity and community structure.
Soil macroarthropods are a critical component of soil biodiversity, playing a fundamental role in regulating ecosystem functioning (Brown et al., 2001).They contribute significantly to soil biomass and influence various soil processes (Lavelle and Spain, 2001;Susilo et al., 2004), including organic matter decomposition, mineralization, and nutrient cycling.Their activities also influence carbon flux from the soil (Ganjegunte et al., 2004;Wardle et al., 2003).Furthermore, soil and litter arthropods contribute to physical soil rearrangement, improving water and nutrient infiltration, and gaseous emissions (Lavelle and Spain, 2001;Paoletti and Bressan, 1996;Susilo et al., 2004).Soil biota encompass a diverse array of organisms, fulfilling various functional roles such as decomposers, microbial regulators, soil engineers, and predators (Chapuis-Lardy et al., 2011;Lavelle et al., 2006).The biological functioning of soils, crucial for providing ecosystem goods and services, involves a wide range of biological processes carried out by soil organisms in conjunction with the physical and chemical components of the soil (Chapuis-Lardy et al., 2011).
A review of the literature highlights a predominant focus on soil microarthropods rather than macroarthropods, particularly examining aspects such as spatial or vertical distribution and abundance (Price, 1975;Sarkar, 1991).Studies on soil macroarthropods are often limited to specific insect groups, primarily collembolans and acarine (Alfred et al., 1991;Edwards and Lofty, 1974;Janetschek et al., 1976;Mukharji and Singh, 1970;Steinberger and Whitford, 1984).Despite their significance in natural and man-made habitats (Gillison et al., 2003), soil macroarthropods remain a poorly understood component of terrestrial ecosystems (Ruiz et al., 2008).
This study delves into the impacts of changes in the abundance of all soil macroarthropods on soil physicochemical characteristics across two forest segments of a temperate forest, differing in anthropogenic pressure magnitude.The investigation aims to understand the resultant effects on soil nutrient dynamics.Unfortunately, due to a lack of expertise, macroarthropods were identified only up to the order level.The identification of specimens down to the species level would have provided a more comprehensive assessment of the variation in abundance and richness across the two forest segments.

Description of the study site
The study site is situated above Baling village in Darma Valley, District Pithoragarh, Kumaun Himalaya, India.It encompasses an old-growth forest spanning 4,527.585km 2 , located between 30°12'37" N latitude and 80°32'5" E longitude, within an altitudinal range of 3200 to 3400 meters (Figure 1).The current study was conducted over three months, from July to September, for two consecutive years in 2020 and 2021.The present forest is dominated by West Himalayan blue fir (Abies spectabilis), which is represented by thick litter, characteristic of a temperate coniferous forest.The forest was divided into two segments; that is, disturbed (forest segment A) and undisturbed (forest segment B), based on the magnitude of the anthropogenic pressure, primarily in terms of tree felling, lopping, and relative exploitation of the fuelwood (Figure 2).A bridle path effectively demarcates the two forest segments.To minimize other environmental variables, apart from tree felling, lopping and exploitation of fuelwood in the two forest segments, care was taken so that other features were commonly shared between the two segments.For instance, distinctive characteristics of the forest segments include the absence of atypical tree species, a near homogeneous vegetation cover, and the prevalence of common dominant tree species, particularly Abies spectabilis.The extent of anthropogenic disturbance, identified through visual observation, can be attributed to the fact that the disturbed forest segment is in close proximity to and above the village.Therefore, it experiences relatively higher human activities compared to the farflung and relatively undisturbed forest segment.The latter is situated above the temple, surrounding the water source, and is considered 'sacred,' resulting in enrichment with leaf litter content.
Throughout the study period, ambient temperatures ranged between 10 and 21°C, with recorded relative humidity ranging from 72 to 92%.The minimum and maximum soil temperatures in forest segment A were 13 and 20°C, respectively, while in forest segment B, these values were 11 and 17.3°C.These findings highlight distinct temperature profiles within the two forest segments.The average soil temperature was 16.87 and 14.59°C, and the average humidity was 80.5 and 83.67% in forest segments A and B, respectively (Table 1).

Experimental design and soil sampling
Soil analysis was done by using quadrat methods (Kent and Coker, 1992).Within each forest segment, 4 randomly chosen rectangular plots measuring 5m x 10m were established.Within each large plot, 5 sub-plots measuring 1m x 1m were established for soil sampling.Soil samples taken from five sub-plots were clumped together into one lot.In other words, 4 soil samples constituted the total number per forest segment per visit.Since the study was carried out fortnightly (every 2nd week, twice a month), the total number of soil samples would translate into 24 per forest segment, or collectively 48 soil samples (24 samples x 2 segments) from both the forest segments, for the duration of the 3-month study.Soil sampling was conducted by collecting soil samples from a depth of 0-10 cm after removing above-ground debris, using sickles, shovels, digging hoes, and bucket auger.Soil samples were then stored in airtight plastic bags and labeled.The soil samples were then air-dried and subsequently homogenized by passing through a 2 mm sieved filter to eliminate any visible roots or plant remains.Soil temperatures were recorded by a steel-tipped digital soil thermometer at 10 cm depth.

Sampling of arthropods, identification, and classification
As with soil samples, within each 4 larger plots (5 x 10 m), 5 subplots measuring 1m x 1m were established.While soil samples taken from five sub-plots were clumped together into one lot; macroarthropods were studied separately using a quadrat method (Kent and Coker, 1992).In other words, altogether 20 separate plots were sampled per forest segment per visit.Again, since the study was carried out fortnightly, the total number of soil macroarthropod samples would translate into 120 soil macroarthropod samples (40 sample plots x 3 months) per forest segment, or collectively 240 macroarthropods samples (120 x 2 segments) from both of the forests, for the duration of the 3-month study.The soil macroarthropods were collected using the hand sorting method.To clear above-ground vegetation, sickles, shovels, and/or digging hoes were resorted to.The macroarthropod specimens were photographed in their natural habitat and subsequently preserved in 70% alcohol.Identification of the collected specimens was chiefly carried out through a literature review, while still unidentified samples were referred to the experts.Arthropods were classified using identification keys, as outlined by Duyar (2014).To ascertain the dominant soil macroarthropods, an evaluation was conducted on abundance, relative abundance, and abundance per meter square.While abundance refers to the total number of individuals in the sample or population, relative abundance represents the proportion of individuals (macroarthropods) of each species/taxon relative to the total number of all species or taxa (x 100).Abundance per square meter refers to the average number of individuals per square meter based on the total number of individuals sampled in both forest segments A and B.

Statistical analyses
All the statistical analyses were carried out using IBM SPSS 23 software.Soil parameters and macroarthropods abundance data were log-transformed, where necessary.Soil samples were averaged within each plot to obtain representative data.The least significant difference (LSD) test with a significance level of (p < 0.05) was used to compare the means among soil variables.For  assessing relationships between soil macroarthropods and different soil parameters, sample t-tests and correlation analyses were conducted.

Soil characteristics
Forest segments, A and B demonstrated comparable soil texture, with silt being the prevailing component, accounting for 70.76 and 70.43%, respectively, and thus could be defined as loam soil (Table 1).The disturbed forest segment A, exhibited SMC ranging from 37.23 to 51.26% while B showed a variation from 53.15% to 72.78%.Statistical analysis revealed significant differences in soil moisture content between the two forest segments (p < 0.0001), (Table 1).
A notable difference (p < 0.05) was observed in soil temperature between the two forest segments (Table 1).Segment A exhibited soil temperature within the range of 14.23 to 19.05 °C, while in forest segment B, the values varied from 12.2 to 14.67°C.Additionally, a significant difference at (p < 0.05) was observed in humidity levels across the two forest segments (Table 1).In segment A, humidity values ranged from 74.5 to 88%, whereas in segment B, the values ranged from 79 to 92%.
The disturbed forest segment A displays a soil pH ranging from 4.8 to 5.1, while in forest segment B, the pH varies from 6.2 to 6.3.The difference in soil pH between the two forest segments was significant (p < 0.05).These findings underscore the distinct soil acidity levels in the respective forest segments (Table 1).
The soil organic carbon (SOC) content within forest segment A fluctuates between 7.11% and 8.06%, while in forest segment B, it ranges from 8.08% to 11.40% (Table 1).A significant difference in soil organic carbon content exists between the two forest segments (p < 0.0001) (Table 1).
The soil total nitrogen (TN) values within forest segment A, exhibit a range from 0.31 to 0.35 g kg -1 , while in forest segment B, they vary from 0.35 to 0.46 g kg -1 .No significant variation in soil TN was observed between the two forest segments (p > 0.05) (Table 1).These findings indicate comparable levels of total nitrogen in the soils of both forest segments.
The soil available nitrogen (AN) values within forest segment A range from 85.52 to 207.95 kg ha -1 while in forest segment B, the values range between 103.14 and 192.61 kg ha -1 .The observed differences in soil AN value between the two forest segments is statistically not significant (p > 0.05), (Table 1).
The available phosphorus (AP) values within forest segment A, exhibit a range from 8.56 to 15.51 mg kg -1 , while in forest segment B, they vary from 14.17 to 22.99 mg kg -1 .The observed variation in AP values across the two forest segments is statistically significant (p < 0.05), (Table 1).

Macroarthropod abundance
During the study period, a total of 2871 soil macroarthropods were collected, comprising 5 different classes and 14 orders.The abundance m -2 for disturbed forest segment A and forest segment B, was 47.95 ind.m -2 , and 95.6 ind.m -2 , respectively.Notably, the disturbed site (forest segment A) exhibited a lower number of macroarthropods compared to the undisturbed forest segment B (Figure 3).In forest segment A, Diptera (17%) and Hemiptera (16%) were more abundant.Conversely, in forest segment B, Araneae (28%) and Coleopterans (15%) were more prevalent.However, some macroarthropod taxa, such as Geophilomorpha, Lithobiomorpha, Opiliones, Trichoptera, and Scutigeromorpha, showed low abundance in both forest segments (Figure 3).

Soil texture and soil moisture content
The soil texture observed in both the forest segments can be described as loam.Since the soil texture is similar across the two segments of the forest (Table 1), it can be inferred that the differences in soil moisture content were not influenced by soil texture.However, soil moisture content varies significantly, across the two forest segments A and B (Table 1).The higher soil moisture content in forest segment B could be attributed to intact vegetation cover, its sacredness, and consequent relatively more moisture content.Conversely, forest segment A, experiencing anthropogenic pressure led to variable air and soil temperatures, elevated evapotranspiration rates, and greater vapour pressure deficits caused by increased solar radiation and wind, all these factors may contribute to a relatively lower soil moisture content in forest segment A, as indicated by various studies (Chen et al., 1995;Didham and Lawton, 1999;Herbst et al., 2007).

Soil physicochemical properties
The measurement of soil pH, which indicates its acidity or alkalinity, is associated with various soil characteristics such as ion hydrolysis equilibrium (Tyler and Olsson, 2001), microbial populations (Kooijman and Cammeraat, 2010), and the amount of organic matter (Lambkin et al., 2011).The current study highlights a significant distinction in soil pH between the two forest segments, A and B (Table 1).Forest segment A, experiencing greater anthropogenic disturbance (tree felling, lopping), leads to relatively less vegetation cover, higher temperature, and relatively more compact soil, exhibiting more acidic soil, due to the faster decomposition of the organic matter that is known to release acids (de Hann, 1977;Gairola et al., 2012;Groffman et al., 2009;Gupta and Sharma, 2008) (Table 1).
Soil organic matter is the most widely used indicator of soil quality (Wander and Drinkwater, 2000).Soil carbon plays a crucial role as both a source and a sink for carbon dioxide in the atmosphere (Fisher and Binkley, 2000;Froberg, 2004;Hogberg et al., 2002), acting as a biogeochemical connection between major carbon reservoirs, such as the biosphere, atmosphere, and hydrosphere (Wilding et al., 2001).Comparing the soil organic carbon content in the current forest segments, A and B, a significant difference was observed (Table 1).In the present study, the undisturbed forest B, treated sacred and characterized by relatively more vegetation cover, and a higher litter mass, exhibited relatively more storage of carbon in the soil (Sheikh et al., 2009).Consequently, lower temperature and a higher moisture content may in turn affect the soil organic carbon values, as compared to the disturbed forest segment A, as SOC is known to be positively influenced by humidity, and precipitation, and negatively influenced by the temperature (Post et al., 1982), vegetation and the soil environment (Ravindranath and Ostwald, 2008;Baldock and Nelson, 2000).
As concerns total nitrogen (TN) and available nitrogen (AN), there was no significant difference observed between the two forest segments (Table 1).This could be attributed to the intricate processes of microbial immobilization, cation exchange capacity, and soil organic matter absorption, as highlighted by Silver et al. (2005) and Templer et al. (2008).However, there was a significant difference in soil available phosphorus (AP), comparatively higher in the undisturbed forest segment B (Table 1).This variation in AP values could be attributed to lower pH, as is exhibited by forest segment A (pH 4.95), which results in the leaching of the essential nutrients, including phosphorus (Gairola et al., 2012;Larcher, 1980).

Relationships between soil properties and soil macroarthropods
Variations in soil physicochemical properties have a significant influence on the diversity of soil-inhabiting macroarthropods.In the present study, forest segment B is a sacred forest, protection thus afforded, has resulted in relatively more intact vegetation cover, and consequently, lower temperature and higher humidity at the ground surface (Table 1).Undoubtedly, these differences exert a significant influence on the assemblages of ground-dwelling arthropods (Pinzon et al., 2012;Work et al., 2004).Furthermore, diversity and arthropod assemblages are strongly affected by vegetation composition and forest structure (Schowalter and Zhang, 2005;Work et al., 2004).In contrast, anthropogenic pressure in terms of tree felling, lopping, and mushroom harvesting, as experienced by forest segment A, consequently leading to less soil moisture content, negatively impacts the soil macroarthropods richness and abundance (Barros et al., 2002;Curry et al., 2002).Zhao et al. (2014) observed that the abundance and distribution of soil and litter macroarthropods were significantly influenced by the physical and chemical characteristics of the soil and litter.These factors included temperature, moisture, pH, and others within temperate forest ecosystems.In the present study, soil moisture content exhibited a positive correlation with macroarthropod abundance in both forest segments.Soil moisture has been implicated in regulating diversity, distribution and abundance of macroarthropods (Abrahamsen, 1971;MacKay et al., 1986).Increased soil moisture creates a favourable condition for soil macroarthropods by providing the necessary moisture for their survival and reproduction (Johnson et al., 1995).However, it is known that certain insect orders such as Hymenoptera, Hemiptera, and Diptera can be negatively affected by both low and high soil moisture values (Adis and Junk, 2002;Ausden et al., 2001;Kajak, 1987;Tajovsky, 1999).Also, excessive moisture can sometimes lead to oxygen stress or waterlogging, negatively influencing the macroarthropod populations (Cárcamo et al., 2000;Hättenschwiler and Jørgensen, 2010).Additionally, spiders (Araneae) being predatory, are directly influenced by changes in the habitat structure and prey availability (Wise, 1993).
The existence of arthropods was significantly affected by the temperature and moisture levels in the soil.According to Medianero et al. (2007), an elevation in temperature, coupled with sufficient moisture, led to an increased abundance of arthropods.Conversely, a decrease in arthropod abundance was observed when both litter and soil experienced water loss due to heightened evaporation at higher temperatures.In the present study, a negative correlation was observed between soil temperature and the abundance of soil macroarthropods, which correlates well with the findings by Medianero et al. (2007) and Sharon et al. (2001).Forest segment A, experienced tree felling which leads to higher soil temperatures, and relatively lower SMC.Thus, it exhibited lower macroarthropod abundance, similar to observations made by Peña-Peña and Irmler (2016).
Soil pH levels varied between the two forest types.It is known that the preferences of ground-dwelling arthropods for specific habitats can be directly affected by soil pH, as is demonstrated by certain species of carabid beetles (Coleoptera) (Paje and Mossakowski, 1984), or can be indirectly influenced by the habitat preferences of their prey, such as mites (Straalen and Verhoef, 1997).
Out of the 14 macro arthropod orders, Diptera, Hymenoptera, Hemiptera, Geophilomorpha, Julida, and Tomoceridae, prefer relative lower pH, as exhibited in Forest segment A, while Aranea and Coleoptera show higher abundance in forest segment B, exhibiting relatively higher pH.Since different insect orders exhibit varying preferences for pH levels (Hyvönen and Persson, 1990;Loranger et al., 2001;van Straalen, 1998;van Straalen and Verhoef, 1997), this could explain the variations in macroarthropod abundance observed between the two forest segments.Overall, a positive correlation was exhibited between soil pH and the relative abundance of soil macroarthropods, underscoring the significance of soil pH as a determinant of macroarthropod abundance (Augusto et al., 2002;Loranger et al., 2001).A positive correlation was found between macroarthropod abundance and SOC in the undisturbed forest segment B. SOC invariably remains a source of energy and nutrients for the soil macroarthropods, which in turn contribute to the decomposition of the soil organic matter, and thus soil nutrient cycling (Ganjegunte et al., 2004;Kaczmarek et al., 1995;Wardle et al., 2003).This relationship is mutually beneficial, as SOC enhances soil structure, moisture retention, and nutrient availability, thus providing favourable conditions for the proliferation of macroarthropod populations (Ayuke et al., 2011;Suthar, 2009;Tiunov and Scheu, 2004).However, the soil parameters, as outlined above, are sensitive to disturbance, inclusive of anthropogenic pressure, and may thus influence the abundance of the soil macroarthropods (Bergeron et al., 2017;Swanson et al., 2011).For example, Apigian et al. (2006) and Sileshi and Mafongoya (2006), have reported lower species richness and population density of annelids, chilopods, arachnids, and some insects in disturbed forest patches.
Nitrogen modulates plant biomass and community structure, thereby influencing the dynamics of soil macroarthropods (Zhou et al., 2022).Several studies have exhibited a positive correlation between SOC, soil nitrogen, and the abundance of soil macroarthropods (Ayuke et al., 2011;Suthar, 2009).This is in contrast to our findings, where a negative correlation was observed between soil macroarthropods and soil nitrogen.Notwithstanding the fact, the top predator orders-Araneae and Coleopterans, were significantly more abundant in the undisturbed forest segment B. This relatively greater abundance of the predator macroarthropods remains a good indicator of ecosystem productivity and thus, biodiversity, since energy constraints are known to impact both the abundance and the diversity of the top predators (Borer et al., 2003;Arim et al., 2006).Similarly, Badji et al. (2007) and Maribie et al. (2011) highlighted that soil macro-arthropods could act as important bioindicators; these include ants (Hymenoptera), springtails (Collembola), termites (Blattodea), and Acari (mites) from the present study.Among these groups, ants and springtails were extensively studied due to their high abundance and diversity.These organisms play a crucial role in essential biological processes, such as catalyzing the decomposition of organic matter and serving as central figures in the soil food web.

Conclusion
In the present study, we observed significant differences in soil physicochemical parameters, particularly in soil pH, SMC, and SOC, between the two forest segments (A and B) that differ in the magnitude of anthropogenic pressure resulting from tree felling and lopping.These findings reinforce the observation that changes in soil physicochemical properties, induced by anthropogenic disturbance, have a substantial impact on the relative abundance of soil macroarthropods, and vice versa.Although the species richness of soil macroarthropods remained relatively consistent, noticeable variations were observed in terms of total abundance and relative abundance across different orders.The relative abundance of soil macroarthropods was primarily influenced by soil pH, SMC, soil total nitrogen, and soil temperature.In conclusion, this study underscores the significance of even minor and subtle changes in soil macroarthropod abundance, resulting from anthropogenic disturbance, in influencing soil nutrient dynamics and, consequently, forest productivity.

Figure 1 .
Figure 1.Map of the study site.

Figure 2 .
Figure 2. Tree felling combined with extensive lopping has resulted in lesser canopy cover in forest segment A (left), in contrast to dense cover in forest segment B (right).

Figure 3 .
Figure 3. Variation in abundance of different orders of soil macroarthropods between disturbed (Forest segment A) and undisturbed (Forest segment B) that were sampled from July to September; error bars indicate the standard error.

Table 1 .
The average value of soil characteristics in the study sites (mean ± S.E.; N = 24).

Table 2 .
Pearson's correlation coefficient of soil macroarthropods' relative abundance and different soil parameters between two forest segments (disturbed A and undisturbed B).