Soil is indispensable for the sustenance of life. Soil provides essential resources for human activities such as agriculture, buildings, and industries (Brussaard, 1997). Several biological processes are continuously active in the soil and play an important role in the replenishment of soil resources and ecosystem maintenance (Young and Crawford, 2004). Biological processes in the soil are due to the dynamic interactions of diverse assemblages of living organisms including unicellular bacteria and protozoa to multicellular nematodes, earthworms, and arthropods (Giller
Nematodes play a pivotal role in organic matter decomposition (Freckman, 1988; Beare
Agricultural activities affect soil structure, biological activity, and processes such as decomposition, mineralization, and nutrient cycling by altering the physicochemical properties of soil (Stinner
Surface litter is essential for energy flow in the soil food webs (Attiwill and Adams, 1993). Removal of surface litter affects the dynamics of decomposition, which has a significant effect on the soil C pools (Wu
The effect of different types of physical disturbances on nematode communities has been previously investigated in agricultural ecosystems, i.e., those previously tilled or disturbed (Lenz and Eisenbeis, 2000; Okada and Harada, 2007; Rahman
A field experiment was conducted from April 2017 to May 2018 in a secondary mixed deciduous forest ecosystem dominated by nut-bearing oak and hickory species of trees in Farragut, TN, USA (35 º 54¢32N, 84 º 11¢372W; 311 m elevation). The experimental site is located in a temperate and seasonal climate with a mean annual temperature of 15.3°C and a mean annual precipitation of 1,224 mm. The soil at this site is classified as Minvale-Bodine-Fullerton complex. The experimental site had not been disturbed for at least 50 yr before the experiment was laid out. An understory was absent, and groundcover was negligible. The site sloped slightly toward the northwest.
The experiment included four treatments with increasing levels of physical disturbance. The first treatment was a control with no disturbance; the second treatment was surface litter removed (SLR) with no litter and no vegetation; the third treatment was tilling 15 cm deep with a rototiller every 2 mon after surface litter removal (R2M); the fourth treatment was tilling 15 cm deep every 2 wk after surface litter removal (R2W). Litter and vegetation were cleared every 2 wk from all the treatments except for the control. Each treatment was replicated thrice. Each plot was 2 m ´ 2 m and separated by a 2-m distance. The design of the experiment was a completely randomized design with repeated measures. The experiment was started in April 2017 and concluded in May 2018.
Soil samples were collected from all the plots in April 2017 before starting the experiment and subsequently samples were collected every 2 mon: June 2017, September 2017, November 2017, January 2018, and May 2018. The interval between the tillage and sampling was 2 mon for R2M treatment and 2 wk for R2W treatment. At each sampling time, five soil cores, each having a 2-cm diameter and a 20-cm depth, were randomly collected from each plot. Soil samples from each plot were pooled into a plastic bag to prevent drying of soil, and then transported in a cooler to the laboratory where they were subsequently stored at 4°C before extracting nematodes on the next day.
Nematode extraction and identification were carried out as follows: Composite soil samples were thoroughly homogenized and 100 cm3 of each soil sample was used for extraction of nematodes through a sugar flotation-centrifugation method (Jenkins, 1964). Extracted nematodes from each sample were counted and the first 150 nematodes were identified to genus level using a differential interference contrast microscope. Proportions of each taxon were extrapolated to the entire sample. The identified nematode genera were assigned to their respective trophic groups: bacterial feeders (BF), fungal feeders (FF), plant feeders (PF), omnivores (OM), and predators (PR) (Yeates
The following ecological indices were calculated to assess the structure and functional role of nematode communities in soil food webs of increasing levels of physical disturbance: Simpson’s dominance index (λ), λ = ΣP2 (Simpson, 1949); Shannon–Weaver index (H´), H´ = –ΣiPilnPi, where P is the proportion of individuals in the ith taxon (Shannon, 1948); maturity index (MI) for free-living taxa were computed as MI = Σ[CP-value (i) × f(i)]/[total numbers of nematodes], where i is the individual taxon and fi is the frequency of taxa in the sample (Bongers, 1990). MI is used to evaluate the functioning and condition of a soil ecosystem as a consequence of environmental disturbance. MI values range from 1 to 5. A high MI suggests more abundant and diverse nematodes of higher c-p classes and a less disturbed ecosystem. A low MI suggests more abundant and diverse nematodes of lower c-p classes and a highly disturbed ecosystem. Plant-parasitic index (PPI) was calculated for plant-parasitic genera (Yeates and Bongers, 1999). Nematode channel ratio (NCR) indicates the decomposition pathway of the soil food web (Yeates and Bongers 1999). NCR is calculated as NCR = bacterial feeders/(bacterial feeders + fungal feeders) and ranges from 0 (fungi-dominated) to 1 (bacteria-dominated). Soil food web indices were calculated based on nematode functional guilds determined by the combination of c-p groups and trophic groups (Ferris
Functional metabolic footprints (FMF) of nematode communities in soil food webs of increasing levels of physical disturbance were calculated using the Nematode Joint Indicator Analysis tool (Sieriebriennikov
Overall richness and abundance of nematodes were estimated for each sample. In addition, nematode richness and abundance for each trophic group and each c-p class at each time point were estimated. Statistical analyses were performed to compare the overall nematode richness and abundance as well as the richness and abundance of each trophic group and each c-p class across different treatments at different time points. Normality of residuals and equal variance were assessed using the Shapiro–Wilk statistic and visual observation of histograms and data were ln(x + 1)-transformed prior to statistical analysis. Analysis of variance with repeated measures was conducted with SAS (Glimmix procedure, SAS Institute, Cary, NC) and least square means were compared with Tukey’s LSD with a significance level of
Changes in community structure with increasing levels of physical disturbance over time were visualized by nonmetric multidimensional scaling (NMDS) ordination with the Bray–Curtis distance matrix. Permutational multivariate analysis of variance (PERMANOVA; Anderson, 2001) was used to assess the significance of the differences among nematode community composition of the four treatments. The similarity percentage analysis (SIMPER) was used to determine the contribution of nematode genera to dissimilarities between treatments with a significance level of
In total, 56 genera were identified at different levels of physical disturbance across different sampling times. Of the 56 genera, 26 most abundant genera are listed in Supplementary Table 1. The nematode genera with zero abundance in most of the treatments at different sampling times were not considered for individual nematode analysis. Rhabditidae
The effect of increasing levels of physical disturbance on nematode abundance was statistically significant during January 2018 and May 2018 (
Effect of increasing levels of physical disturbance on genus-level nematode abundance. Box plots representing the number of nematodes per 100 cm3 of soil in control, SLR, R2M, and R2W at each sampling time. Lower and upper box boundaries represent 25th and 75th percentiles, respectively; line inside the box indicates median; and lower and upper error lines represent 10th and 90th percentiles, respectively. Letters indicate significant differences among treatments at each sampling time at
Effect of increasing levels of physical disturbance on genus-level nematode richness. Box plots representing the number of genera per 100 cm3 of soil in control, SLR, R2M, and R2W at each sampling time. Lower and upper box boundaries represent 25th and 75th percentiles, respectively; line inside the box indicates median; and lower and upper error lines represent 10th and 90th percentiles, respectively. Letters indicate significant differences among treatments at each sampling time at
Among 56 genera, 20 taxa were bacterial feeders, 18 taxa were plant feeders, 7 taxa were fungal feeders, 6 genera were omnivores, and 5 taxa were predators. The effect of increasing levels of physical disturbance on nematode abundance and richness of each trophic group was analyzed. Increasing levels of physical disturbance significantly affected the abundance of bacterial feeders, fungal feeders, predators, and omnivores during the last two samplings (January 2018 and May 2018) (
Increasing levels of physical disturbance significantly affected the abundance of 10 genera,
The effect of increasing levels of physical disturbance on nematode abundance and richness of each c-p class was also analyzed. Increasing levels of physical disturbance did not affect the abundance of c-p 1 and c-p 3 class nematodes whereas the impact was significant in the cases of c-p 2, c-p 4, and c-p 5 classes (
A significant effect of increasing levels of physical disturbance was observed on the values of λ, H´, EI, and SI (
A significant effect of increasing levels of physical disturbance was observed on efoot and sfoot (
FMF of nematode communities subjected to different levels of physical disturbance: control, SLR, R2M, and R2W. The vertical axis represents the efoot, and the horizontal axis represents the sfoot. The FMF is depicted by sequentially joining points: (SI– 0.5Fs/k, EI); (SI+ 0.5Fs/k, EI); (SI, EI– 0.5Fe/k); and (SI, EI+ 0.5Fe/k). Fs represents sfoot and Fe represents efoot (Ferris, 2010). The adjusted k value is 4. FMF, functional metabolic footprints; R2M, rototill for every 2 mon; R2W, rototill for every 2 wk; sfoot, structure footprint; SLR, surface litter removed.
NMDS analysis of nematode communities showed a significant differentiation in nematode communities with increasing levels of physical disturbance during January 2018 (Fig. 4; PERMANOVA:
Biplot representing the NMDS performed on nematodes communities subjected to different levels of physical disturbance: control, SLR, R2M, and R2W in April 2017 (PERMANOVA:
Nematodes play a key role in maintaining and regulating several biological processes, crucial for soil and plant health (Yeates and Coleman, 1982; Liang
Tillage significantly reduced the overall abundance and overall richness of nematode communities over time in R2M and R2W, which was attributed to the decrease in the abundance of bacterial feeders, fungal feeders, predators, and omnivores and a decrease in the richness of bacterial feeders, predators, and omnivores. Tillage directly affects nematode communities by abrasion and indirectly by changing the food web, temperature, moisture, and aeration of soil in tillage treatments compared to the control, which was undisturbed (Kladivko, 2001; Holland, 2004; Rahman
Among nematode trophic groups, tillage significantly reduced the abundance and richness of bacterial feeders over time. Many studies conducted in agricultural fields have reported that tillage stimulated the bacterial feeding nematodes due to the probable increase in bacterial biomass with the incorporation of organic matter (Andren and Lagerlof, 1983; Parmelee and Alston, 1986; Ettema and Bongers, 1993; Lenz and Eisenbeis, 2000; Liphadzi
Tillage significantly reduced the abundance but not the richness of fungal feeders. There is a discrepancy in reports on the response of fungal feeding nematodes to tillage practices. Some studies have reported that tillage increased the fungal feeding nematode communities (Parmelee and Alston, 1986; Liphadzi
Among nematodes belonging to the higher hierarchy of the soil food web, tillage significantly reduced the abundance and richness of predators and omnivores, which are sensitive to disturbances (Bongers 1990; Ferris
The effect of tillage disturbances on nematode communities according to c-p classes was also assessed. The abundance and richness of c-p 2, c-p 4, and c-p 5 class nematodes were significantly decreased by tillage. Nematodes belonging to lower c-p classes are r-strategists, which are characterized by a high fecundity rate, short generation time, and tolerance to disturbances (Bongers, 1990; Ferris
The significant differences between the control and SLR were not reflected in trophic and c-p group analyses. However, individual nematode analyses revealed that removal of surface litter resulted in a significant decrease in the abundance of
The nematode ecological indices are often used to assess the condition of the soil food web. l and H´ indicate the diversity of nematode communities. Increasing levels of physical disturbance increased the Simpson index (l) and decreased the Shannon diversity index (H´). This agreement indicates the decrease in diversity of nematode communities with the increase in the level of physical disturbance. The NCR value decreased with increasing levels of physical disturbance, indicating the shift of the decomposition pathway from bacterial to fungal dominated. The higher value of NCR in the control treatment suggested the predominance of the bacterial decomposition pathway. Our results are in agreement with Zhang
Nematode FMF were calculated to indicate the structure and function of soil food webs with different levels of physical disturbance. The nematode trophic footprints suggested the changes in the metabolic activity and flow of C into the soil food web through their respective trophic channels (Ferris, 2010). The value of efoot is considered as an indicator of the flow of C and energy through r-strategists, which are lower c-p values (1–2) (Ferris
The FMF area of nematode communities decreased with increasing levels of physical disturbance. FMF with a larger area in the control treatment during May 2018 indicated a higher metabolic activity and inflow of C, which was used for nematode production (Ferris, 2010). The high availability of organic matter in the control treatment increased the abundance of predators and omnivores and activated a stronger pathway through the predator channel, which may promote the stronger metabolic process and stability of the soil food web (Ferris, 2010; Thakur and Geisen, 2019; Kou
Soil nematodes have been used as bioindicators to assess the effect of physical disturbances (Yeates, 2003). Our study revealed that different nematode genera had varying sensitivities to the physical disturbances. NMDS analysis of nematode communities revealed that soil nematode genera were clearly separated by increasing levels of physical disturbance during January and May 2018. The dissimilarity between the treatments indicates the progressive decrease in the abundance of nematode communities at R2M and R2W treatments. These declining trends generated significantly different nematode assemblages at all treatments during January and May 2018, as emphasized by PERMANOVA. All the nematodes belonging to higher c-p classes (c-p 4 and 5) and bacterial feeders of the c-p 2 class were negatively affected by the R2M and R2W treatments, while nematodes of lower c-p classes including bacterial feeders except c-p 2 class, fungal feeders, and plant feeders were not impacted by the tillage treatments. Nematodes of higher c-p classes such as Dorylaimida,
Overall, this study gives an insight into the effect of increasing levels of physical disturbance on nematode communities in an undisturbed forest ecosystem, indicating that tillage reduced the abundance and richness of nematode communities, which was consistent with previous studies in the literature that were conducted in agricultural ecosystems. However, in this study, bacterial feeding nematodes belonging to the c-p 2 class responded differently compared to those of agricultural ecosystems. Tillage significantly reduced the abundance and richness of bacterial feeding nematodes of the c-p 2 class along with predators and omnivores, which belong to higher c-p classes. Moreover, tillage significantly reduced the FMF of nematodes, which indicates decreased metabolic activity, reduced C inflow, and poorly structured soil food webs. Unlike tillage, minimal disturbance such as removal of surface litter resulted in a significant reduction of very few nematode genera. Previous studies conducted in agricultural ecosystems determined that