Importance of the physical soil conditions on nematodes
A main finding of our study is the pronounced “site” effect on all nematode parameters, such as diversity, abundance, distribution of trophic groups, and structure, and on their temporal changes following rubber tree cutting. The level of soil nematode community resilience following rubber tree cutting was highly dependent on the local site context. A strong difference between the two sites was the soil texture, that is, the soil texture was sandy–silty at the sandy site (10% clay) and silty-sandy at the clay site (23% clay). Soil texture is known to strongly affect soil nematode composition at both global (van den Hoogen et al. 2020) and local scales (Koenning et al. 1998; Olabiyi et al. 2009; Quist et al. 2019), with a stronger influence on nematodes than climate or land management (Renčo et al. 2020). We thus suppose that the between-site changes in soil texture could explain this significant site effect. However, little is known about the extent to which soil texture alters the temporal changes in soil nematodes following perturbation. In addition, we can exclude other soil properties involved in these nematode differences, such as total P content or pH. Thus, more studies are needed.
Resistance of the soil nematodes communities to clear-cutting and land preparation
Most of the nematode parameters decreased significantly at both sites following rubber tree cutting and land preparation. This biological loss is thought to be related to plantation clearing, which involves soil compaction and habitat degradation by the heavy machinery used for mechanical cutting and logging residue extraction (Hartmann et al. 2012; Hartmann et al. 2014; Ranius et al. 2018). This hypothesis is supported by the well-known negative effects of soil compaction on nematode trophic groups (Bouwman and Arts 2000).
MI is considered as a measure of environmental disturbance of nematode communities (Bongers, 1990); the higher the index, the higher the soil stability. In our study, the fluctuation in the MI under all practices over time could be explained by the sensitive nematode persisters (K-strategy species with high c-p value=3-5) disappearing and the number of colonisers (r-strategy species with high c-p value=1-2) increasing (Bongers, 1990; Porazinska et al. 1999). After rubber tree cutting, colonisers could occupy the niches of the disappeared persisters, which explains the increase in the maturity index at 12 months under all practices at both sites. Nematode persisters were the last group of nematodes to colonise a soil ecosystem after a disturbance due to their sensitivity (Villenave et al. 2018). These trends are similar to those reported by (Ewald et al. 2020), who found similar low MI values across all practices and depths, reflecting the soil disturbance in the arable soil of maize crops (Zea mays). The texture effect appeared to have a greater impact than compaction, because the nematode abundance decreased when the soil became less compact (Tables S9 and S10). The clay texture retained more moisture (Table S10) and allowed the nematodes to resist soil disturbance following clear-cutting.
Resilience of the soil nematodes communities to logging residues
We found that adding organic matter, such as legumes and logging residues, after clear-cutting and land preparation affected both the alpha and beta diversity of the soil nematode community. A strong negative effect on nematode abundance was observed under the practice without residues (R0L0) at the sandy site and for all sampling dates. Accordingly, the absence of cover crops is known to decrease the abundance of soil microbial populations (Leroy et al. 2009) and bacterivore and fungivore nematodes (Leslie et al. 2017; Kim et al. 2020). The absence of crops on these plots without residues could generate greater variability in soil temperature and moisture (Tables S9 and S10), which negatively affects nematode populations (Bakonyi et al. 2007). In contrast, the practices with logging residues (R1L1 and R2L1) had higher total nematode and trophic group abundances between 12 and 18 months. This positive contribution of organic matter application to the soil nematode abundance was previously reported along organic amendment gradients in annual crops (Pan et al. 2020) or following organic fertiliser application, but has never been reported in the context of tree plantations.
At sandy sites, the low resilience of the total nematode abundance observed under the practices without logging residues (R0L0 and R0L1) was related to the low level of organic matter input (Liu et al. 2016). In contrast, for practices with logging residues, the low resilience values of nematodes could be attributed to the fast decomposition of logging residues under tropical conditions, because almost 80% of residues were decomposed after 24 months (data not shown), as previously reported in tropical rainforests (Krashevska et al. 2018). At the clay site, the low nematode abundance in plots without residues (R0L0 and R0L1) could be related to both the low amount of residue input and carbon loss due to erosion, given the high slopes prevailing at this site (Guillaume et al. 2015).
Interestingly, organic management affected the total abundance more directly; for example, we found ~100 fold more bacterivore nematodes at 18 months in plots with logging residue than in plots with no residue at the sandy site. We observed a similar trend at the clay site, but at a lower level (21 fold). Several studies have reported a significant impact of field organic matter management practices on the nematode trophic composition (Freckman and Ettema 1993; Renčo et al. 2010). The high proportion of bacterivorous nematodes under practices with logging residues (R1L1 and R2L1) at the sandy site at 18 months underlines their active involvement in organic matter decomposition (Neher 2001). Furthermore, the high proportions of bacterivorous nematodes in all sites and on each sampling date suggested a significant bacterial-energy channel contribution to logging residue decomposition.
Generally, after anthropogenic disturbance of soil, the communities present, such as bacterivorous nematodes, are dominated by rapid growth (Ferris and Matute 2003). The low proportions of omnivores and predators under all practices at both sites are in accordance with the findings of (Wang and Hooks 2011), who reported that omnivorous and predatory nematodes were typically the last groups of nematodes to colonise a soil ecosystem after a disturbance due to their sensitivity (Villenave et al. 2018). The high SI values under most practices (>80) between 6 and 24 months suggest a more structured community than that on the previous dates (Ferris et al., 2001). The impact of logging residues on the SI value in tree plantations has never been addressed; however, in annual crops, such as sorghum plantations in Burkina Faso, Villenave et al. (2010) observed similar trends in SI (>75) in both unamended and amended plots.
In this study, at both sites, the nematode indices before rubber tree cutting (T0) indicated a structured food web with moderate soil enrichment, a high C/N ratio, and fungal decomposition channels. At 24 months, the nematode profile under practices with residues R1L1 and R2L1 and with legumes only (R0L1) indicated a mature food web structure, high N concentration, low C/N ratio, bacterial decomposition channels, and low to moderate levels of disturbance to the soil environment(Ferris 2010a; Ferris 2010b; Wang et al. 2019). These conditions are mainly due to the high quantities of carbon and organic N in the soil resulting from the decomposition of logging residues and legume litter. The effects of logging residue input on soil food webs have never been addressed in RP, but similar results have been reported in tea and walnut plantations (Li et al. 2014; Song et al. 2020). The restitution of harvest residues, such as organic matter, increases the resources available to the soil food web and supplies the soil ecosystem (Liu et al. 2019). Zhang et al. (2013) and Zhang et al. (2016) argued that soil food web resilience and ecosystem functions are affected by agricultural management practices.
The ratio (nestedness/Sorensen) of the nematode communities in each practice over time was low (< 0.5) in this study (data not shown), indicating that turnover was the major contributor to beta diversity. This indicates that, among the different practices, nematode composition was mostly due to species turnover, and less importantly due to differences in richness.
The changes in the dominant taxa at both sites between 0 and 24 months were likely associated with major changes in life strategies and trophic group composition. Indeed, the life-cycles of nematodes with short life-cycles may not be as visible, because they can produce several generations in one year, such as Tylenchidae, Aphelenchidae, and Rhabditidae (Verschoor et al. 2001). Generally, these observations indicate a continuous succession of nematode communities under different practices over time, probably due to changes in environmental conditions (Villenave et al. 2010). Under different practices, nematode communities tended to be in a dynamic equilibrium provided by the interchangeability of particular taxa under particular habitat conditions. However, the magnitude of the current assessment highlights the need for more intensive sampling and better understanding of the present taxa.
Implication for management: are logging residues a sustainable crop management in rubber plantations?
The negative impacts of rubber tree logging on soil health reflected by nematode communities should challenge farmers in their choice of machinery type and frequency of traffic, depending on the soil type, weather conditions, and acceptable degree of compaction. Logging residues and/or legume inputs could result in pest pressure that may increase over time at sites according to the practices employed. Meloidogyne, Pratylenchus, Dolichodorus, and Helicotylenchus are the main parasitic nematodes in RP. The high abundance of Meloidogyne found at the clay site indicates possible high pest pressure at this site. Monitoring the soil nematode composition is necessary for identifying pest risks due to the input of rubber residues and legumes. As rubber trees are usually cultivated in association with cover crops, the infectivity of nematodes in the roots of these plants should be examined by visual macroscopic inspection. Soil texture, which determines soil compaction and porosity (and, therefore, the availability of moisture and aeration for nematodes), is likely one of the most important soil characteristics related to free-living nematode abundance and plant-parasitic nematode infestations in crops (Moore and Lawrence 2013). Therefore, soil characteristics (such as bulk density) must be considered to maintain healthy soil for a sustainable cropping system.