Effects of individual macrofauna groups

The presence of endogeic earthworms (in three-species mixture or represented by one species with other macrofauna species) induced a strong decrease in nematode density, notably of fungivore nematodes. It can be due to three mechanisms: nematodes consumption, selective feeding on fungi, and competition for organic matter and microbial communities. First,

147 endogeic earthworms create vast burrow systems and ingest larger quantities of soil than anecic earthworms (Capowiez et al. 2015). In our case, Aporrectodea icterica may have play a particular role in soil bioturbation as this particular species produced more numerous and longer burrows resulting in higher bioturbation than the other two species Aporrectodea caliginosa and Allolobophora chlorotica (Capowiez et al. 2015). This intense burrowing activity may lead to higher passive nematode ingestion by earthworm with strong consequences for soil micro food web (Dash et al. 1980; Monroy et al. 2008; Dionísio et al. 2018). On the opposite, anecic earthworms create fewer, vertical and (semi)permanent burrows (Capowiez et al. 2015), which leaves larger volume of soil undisturbed, and allow nematodes to establish themselves at vicinity of the burrow walls (Tiunov et al. 2001; Savin et al. 2004; Andriuzzi et al. 2016). Anecic earthworms therefore have smaller functional domain volume, and this may explain the absence of effect of this group on nematodes in our study, especially since we never sampled soil with anecic burrows in it, while it happened that some endogeic earthworm individuals were found in the samples. Secondly, decrease in fungivore nematodes may be explained by negative effect of earthworm on fungi communities. Fungi can constitute a major food source for earthworms (Brown 1995; Bonkowski et al. 2000) and be digested during the passage through earthworm gut (Wolter and Scheu 1999). In addition, soil bioturbation may break fungi hyphae and limit their development (Brown et al. 2004a). Future PLFA and DNA analysis on the soil samples will inform us on whether the fungal communities were affected or not. Finally, as earthworm ingest microorganisms, they can compete for food with bacterivore and fungivore nematodes and negatively affect their populations (Elliott et al. 1980; Räty and Huhta 2003).

The presence of millipedes increased predator and omnivore proportion over bacterivore nematodes. Millipedes can not only improve organic matter quality through their feeding activity, but they can also change soil structure by producing faecal pellets of different particle size than leaf litter and incorporate them passively in the soil while burrowing in the first few centimeters (Anderson 1988b; Dangerfield and Chipfunde 1995). This may improve soil structure, increase soil porosity, nutrient availability and favor nematode trophic interactions (Erktan et al. 2020). If predator and omnivore nematodes benefited from improved habitat characteristics, they likely exerted a stronger top-down control on bacterivore nematodes,

148 reducing their proportion in the community (Allen-Morley and Coleman 1989; Mikola et al.

1998; Bastow 2011).

Conclusion

Both prolonged drought and macrofauna community composition affected soil micro food web with potential consequences for ecosystem functioning. The increase in bacterivore proportion in response to drought may exert top-down control on bacterial communities and alter the response of microbial communities and associated soil processes to prolonged drought. The action of different soil macrofauna group induced a strong decrease in nematode density or restructuration of nematode communities. Our result first confirms the importance of macrofauna for the microfauna communities, but also show that community composition rather than functional diversity influence soil food web. Biotic interactions are therefore underpinning soil processes and should be further considered in studies investigating the role of soil organisms and their diversity on ecological processes under different environmental conditions (Eisenhauer et al. 2019b).

149

Supplementary information

Table S5.1 - Soil properties at the different depth L1 (5-15cm), L2 (15-35cm) and L3 (35-85cm).

Table S5.2 – Biological material sampling site information

Site of collection Coordinates Method of collection Tree species

Acer monspessulanum Mas d’Arnaud 43˚60’N, 3˚59’E Litter traps Arbustus unedo Montarnaud 43˚39’N, 3˚40’E Ground collection

Quercus ilex Puechabon 43˚44’N, 3˚35’E Litter traps

Quercus pubescens CEFE-TE 43˚63’N, 3˚86’E Litter traps

Detritivore species

Armadillidum vulgare Montarnaud 43˚39’N, 3˚40’E Litter hand sorting Armadillo officinalis ECOTRON 43˚68’N, 3˚88’E Litter hand sorting Porcellio laevis CEFE-TE 43˚44’N, 3˚35’E Litter hand sorting Glomeris marginata Montarnaud 43˚39’N, 3˚40’E Litter hand sorting Cylindroiulus caerulocinctus Montarnaud 43˚39’N, 3˚40’E Litter hand sorting Ommatoiulus sabulosus Marseille 43˚36’N, 3˚42’E Litter hand sorting Aporrectodea caliginosa Avignon 43°9'N, 3.88'E Soil hand sorting Aporrectodea icterica Avignon 43°9'N, 3.88'E Soil hand sorting Allolobophora chlorotica Avignon 43°9'N, 3.88'E Soil hand sorting Aporrectodea nocturna Avignon 43°9'N, 3.88'E Soil hand sorting Lumbricus terrestris Avignon 43°9'N, 3.88'E Soil hand sorting Scheroteca gigas ECOTRON 43˚68’N, 3˚88’E Soil hand sorting

150 Figure S2 – Watering events in the control and drought treatment mesocosms.

Figure S5.2 – Watering events in the control and drought treatment mesocosms.

Appendix 5.1 – Free-living soil nematodes extraction procedure.

~70g of fresh soil was weight to the nearest 0,1g and mixed into 1L of tap-water by gently disagregating soil particles with a stick during 1min and then let sediment for 10min. The solution was mixed again for 30 sec, let sediment for 30 sec and then passed through two seives of 180 and 38µm mesh size. The content retained by the 38µm seive was collected in 50mL Falcon tubes. Each falcon was then cetrifugated at 3000rpm, 25°C for 10minutes.

Supernant water was removre and replace by a sugar-solution at 45%. Tubes were then re-centrifugated at 3000rpm for two minutes, supernatent was pased through a 25µm seive, rinsed with tap water, and retrieved in a 10mL tubes. After nematodes counting with a stereoscope, samples were stored over night at 4°C to let nematodes sediment. The day after, 8 of the 10 mL were removed from the tube and replaced by a 4% formaldéhyde solution at 60°C and stored again at 4°C before identification under microscope.

151 Table S5.3 - PermANOVA table of nMDS ordination of nematod dissmiliraty matrix

Variable Df SS F R² pval

Table S5.4 - Nematode family occurrence in the different treatments. Plus signs indicate family occurrence while empty cells indicate family absence.

Family Guild % Drought Control Anecic Endogeic Isopod Diplopod

FD- FD+ FD- FD+ - + - + - + - +

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Discussion générale