Indication of rapid soil food web recovery by nematode-derived indices in restored agricultural soil after open-cast lignite mining

•Only four individuals of Cephalobidae and Plectidae neamatodes after soil substrate deposition.•Thornenematidae indicated food web structure and stability already four years after restoration.•Nematodes indicated fast development of the soil food web structure after recultivation with alfalfa.•This structural recovery apparently was not paralleled by an increase of its N retention capacity.

Open-cast lignite mining consumes arable land, which has to be restored and recultivated afterwards. This process strongly dilutes the organic carbon and nitrogen pool as well as soil biological parameters of the former soil due to mixing with the underlying subsoil and parent material before redeposition. Cultivation of alfalfa is commonly used to restore agricultural land and to refill diluted C and N pools and re-establish biologic functions. Based on nematode-derived indices, we here evaluate the development of the food web during the early recovery period with N-fixing alfalfa on post-mining soil substrates in the lignite mining district west of Cologne (Germany). Nematode-derived indices revealed a fast recovery of the soil food web during this initial alfalfa cultivation. We found evidence that the applied recultivation procedure lowers the stress and disturbance level in the soil-microbial food web and improves the trophic complexity. The fast maturing of the food web was indicated by Thornenematidae nematodes, which indicated a highly structured and stable food web already after three years of alfalfa cultivation. A declining δ15N signal of the soil indicated a strong impact of N-fixation by alfalfa. Microbial and mineral N content increased during the alfalfa cultivation period. We concluded that the rapid recovery of the soil food web might not be paralleled by an equal increase of its capacity to retain N in the soil food web. This might have implications for nitrate leaching, nitrous oxide emission, and a later agricultural recultivation with common field crops.