The nematode fauna from the top soil to the vadose zone in a forested groundwater recharge area

Soil nematodes are major microfaunal grazers that drive the turnover of organic matter as they foster the activity of microorganisms. The latter are an essential component for water purification processes. The present study is the first study that investigates the nematode community along an entire depth transect from top soil to the vadose zone at a forested groundwater recharge area, the “Lange Erlen”, which provides drinking water to the city of Basel (Switzerland). Vertical core drills were performed from 0 – 450 cm depth at two locations in the study area. The vertical transect was divided into 30 cm thick soil sections. Nematodes were extracted, counted, identified and divided into five trophic groups (i.e. plant feeders, fungal feeders, bacterial feeders, omnivores, predators). Based on the classification of functional groups the Maturity Index (MI), Plant Parasitic Index (PPI), as well as the Shannon-Weaver Index (H’) were assessed. A total of 67 taxa were identified comprising 26 nematode families. The nematode population density was low with an average of 6.26 and 0.85 ind./10 g DW soil across depths at the sampling sites HST and VW, respectively. Density decreased strongly with depth, with on average 46% of the total nematode density located in the uppermost soil layer (0 – 30 cm). Although soil samples were taken down to a depth of 450 cm, no nematodes were found below 240 cm, except for Cephalobus persegnis Bastian, 1865, which was the only species present in the lower vadose zone (220 – 450 cm). Plant feeders were the dominant trophic group (65%) throughout the entire depth transect. Decomposition was mainly mediated by the bacterial carbon and energy channel as indicated by the low number of fungal feeders. The general low MI, PPI and H’ were neither depth nor site dependent, suggesting similar environmental conditions at the two investigated locations due to frequent flooding. SIMPER analysis revealed that the dissimilarity in nematode community patterns at HST and VW increased with depth.  Plant feeders contributed to the community dissimilarity in the upper soil layers, while the impact of bacterial feeders increased with depth, indicating that the main resource changes along the depth profile.