Geomorphic control of denitrification in large river floodplain soils

In this manuscript we investigated the relationships between the microbiolo gical denitrification process in river alluvial soils with structures and p atterns of the floodplain visible at a larger scale. We hypothesised that b oth topography and soil grain size represent pertinent environmental factor s to forecast denitrification activity in river floodplain. The study was c onducted in 15 alluvial sites along a 30 km long stretch of the Garonne Riv er, a seventh-order stream of the south west of France. Sites were selected to encompass the widest range possible of average annual flood duration (0 .04 to 29 days) and frequency (return period from 0.6 to 7 years). On an an nual basis, we found that average denitrification rates did not show any si gnificant trend along the flood frequency gradient. Although during the stu dy the flood frequency and duration was higher than the calculated average, we did not find any relationship between flood duration and denitrificatio n enzyme activity. If flood events do not last long enough to maintain wate rlogging conditions conducive to sustain denitrification activity for long periods, they indirectly affect the spatial distribution of denitrification activity through the sorting out of sediment deposits. Indeed, we found a significant relationship between denitrification rates in the floodplain so ils and their texture; highest rates were measured in fine textured soils w ith high silt + clay content. Below a threshold of 65% of silt and clay con tent, the floodplain soils did not present any significant denitrification rates. Above that threshold denitrification increased linearly. These resul ts demonstrate that alluvial soil texture is a landscape scale factor which has a significant effect on denitrification in floodplains.