Soil biota response to raised water levels and reduced nutrient inputs in agricultural peat meadows

In the Netherlands, peatlands are drained for agricultural purposes, resulting in CO₂ emissions, soil subsidence and biodiversity loss. Raising groundwater levels and reducing nutrient inputs are potential solutions, but their effects on soil biota in agricultural peat soils remain unclear. Therefore, we conducted a mesocosm experiment in which we exposed 40 intact fen meadow peat cores (80 cm, 20 cm in diameter) to four different water levels (0, 20, 40 and 60 cm below peat surface) and two nutrient application levels (50 and 250 kg N ha⁻¹ year⁻¹ with a N:P₂O₅:K₂O of 16:5:12), mimicking various rewetting degrees and land use options. After 15 months, we determined the bacterial, fungal and protozoan PLFA, and abundance and community composition of nematodes and earthworms. Our results show a significantly higher bacterial and saprophytic fungal PLFA abundance at high water levels (20 and 0 cm respectively) compared to the lower water levels, whereas nematodes and earthworms were significantly more abundant under lower water levels (60 and 40 cm respectively). Overall, water level influenced soil biota more strongly than nutrient levels, although nutrient effects became more prominent with increasing water levels. In the fully water saturated treatment with high nutrient application levels, no earthworms and fewer nematode taxa were found than under low nutrient levels. We conclude that wet conditions combined with a high nutrient application negatively affect soil food web stability. Furthermore, raising the water level results in a different soil biota composition, with potential implications for ecosystem functioning.