Spatial variation of denitrification and key controlling factors in streams and ponds sediments from a critical zone (southwestern France)

Numerous ponds were constructed along streams for local water supply in agricultural areas of south-western France. Although the water quality of streams and ponds has been surveyed for nitrate (NO3−), the role of ponds in NO3− mitigation at a catchment scale has rarely been investigated. Since denitrification in sediments is a key NO3− pathway, the denitrification variability in stream and pond sediments and its controlling factors, particularly the pond position and characteristics are important aspects to investigate. Potential denitrification rate (PDR), physico-chemical factors, and denitrification functional genes were quantified in sediments from nine ponds located in three adjacent agricultural catchments in spring period. PDR showed a great spatial variation (both horizontal and vertical) and was mainly driven by the availability of water NO3− and sediment organic carbon. Stream discharge and pond hydraulic retention time can also affect PDR in stream and pond sediments, respectively. Although high PDR was observed in stream sediments, stream discharge diluted NO3− and can indirectly lower PDR downstream. The function of denitrifiers was revealed by the abundance ratio of nirK/16S rRNA, but no significant relationship was found between PDR and abundance of functional genes. N2O emission rate was positively related to PDR, which should be aware of in the upstream PDR hotspot ponds. Simple empirical models based on pond and sediment characteristics were good predictors of PDR, which could also help a better management of ponds by policy makers with the aim to improve NO3− mitigation and water quality.