With their various feeding types, soil nematodes play a crucial role in the soil food web. Here, we investigated if and how soil nematodes responded to a long-term irrigation in a drought-prone Scots pine (Pinus sylvestris) forest in southern Switzerland, applied to study whether greater soil water availability would improve tree health and reduce tree mortality. After 14 years of irrigation, tree vitality and soil development had improved significantly. However, morphological observations of the soil nematodes revealed a decrease in their total number in the irrigated plots. Overall, the irrigated plots had a lower nematode richness compared with the dry control plots, but the Shannon index did not differ between the two treatments. In addition, the nematode community shifted significantly as a result of the irrigation. Soil physical parameters, such as sand and silt contents and bulk density, were significantly positively correlated with the nematode community in the irrigation treatment. According to a DNA marker sequence analysis, a total of 43 genera of nematodes were assigned. Predatory nematodes were significantly less abundant in the irrigated plots than in the dry control plots, as the average number decreased to 74 in the irrigated plots compared to 3579 in the dry control plots, while non-predators were not significantly affected. A differential abundance analysis revealed that the genera Tripyla and Anatonchus were the predators that declined the most. Overall, marker sequence analysis of forest soil nematodes appears to be a suitable tool for assessing changes in nematode communities and taxa. The disappearance of predatory nematodes under irrigation, however, can perhaps only be explained if other predatory animal groups, such as predatory mites or millipedes, are also analyzed at the same time.
