Geomorphic control of denitrification in large river floodplain soils

Abstract

In this manuscript we investigated the relationshipsbetween the microbiological denitrification process inriver alluvial soils with structures and patterns ofthe floodplain visible at a larger scale. Wehypothesised that both topography and soil grain sizerepresent pertinent environmental factors to forecastdenitrification activity in river floodplain. Thestudy was conducted in 15 alluvial sites along a 30 kmlong stretch of the Garonne River, a seventh-orderstream of the south west of France. Sites wereselected to encompass the widest range possible ofaverage annual flood duration (0.04 to 29 days) andfrequency (return period from 0.6 to 7 years). On anannual basis, we found that average denitrificationrates did not show any significant trend along theflood frequency gradient. Although during the studythe flood frequency and duration was higher than thecalculated average, we did not find any relationshipbetween flood duration and denitrification enzymeactivity. If flood events do not last long enough tomaintain waterlogging conditions conducive to sustaindenitrification activity for long periods, theyindirectly affect the spatial distribution ofdenitrification activity through the sorting out ofsediment deposits. Indeed, we found a significantrelationship between denitrification rates in thefloodplain soils and their texture; highest rates weremeasured in fine textured soils with high silt + claycontent. Below a threshold of 65% of silt and claycontent, the floodplain soils did not present anysignificant denitrification rates. Above thatthreshold denitrification increased linearly. Theseresults demonstrate that alluvial soil texture is alandscape scale factor which has a significant effecton denitrification in floodplains.