Species specific earthworm mediated nutrient dynamics in riparian buffer zones

To halt further water quality deterioration, riparian buffer zones have been constructed to reduce diffuse nutrient inputs from agriculture. The efficiency of buffer zone nutrient removal is, however, highly variable. Here, we argue that this may be due to the poorly understood contribution of earthworms to buffer zone nutrient dynamics. This study aimed therefore to elucidate species-specific and combined effects of earthworm species on nutrient dynamics in riparian buffer zones. To this end, a mesocosm study was performed using riparian buffer zone soil, and containing either no earthworms, Lumbricus rubellus (epigeic), Aporrectodea limicola (endogeic) or both species. To assess the effects on the vertical distribution of nutrient dynamics, the upper (10 cm) and lower (10 cm) half of the soil (20 cm) of each mesocosm was sampled and analysed separately for worm density and nutrient concentrations (NO 3 − , NO 2 − , NH 4 + , SO 4 2− and PO 4 3− ). Both worm presence and their effects on nutrient concentrations occurred mainly in the upper soil layer. Species-specific increases in nitrate concentrations in the porewater were observed. For sulphate, a decrease in the presence of worms was observed, albeit without species-specific effects. It is concluded that the effect of worms on nutrients dynamics in riparian buffer zones is species and nutrient specific. Therefore, to better understand earthworm effects on nutrient dynamics in buffer zones, it is crucial to elucidate species identity and distribution, as well as their activity and feeding strategy. The presently obtained insights into species-specific effects on nutrient concentrations in porewater, may aid to improve buffer zone design, to better protect aquatic ecosystems from excess agricultural nutrient inputs.