Geomorphic control of denitrification in large river floodplain soils

In this manuscript we investigated the relationships between the microbiological denitrification process in river alluvial soils with structures and patterns of the floodplain visible at a larger scale. We hypothesised that both topography and soil grain size represent pertinent environmental factors to forecast denitrification activity in river floodplain. The study was conducted in 15 alluvial sites along a 30 km long stretch of the Garonne River, 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 annual basis, we found that average denitrification rates did not show any significant trend along the flood frequency gradient. Although during the study the flood frequency and duration was higher than the calculated average, we did not find any relationship between flood duration and denitrification enzyme activity. If flood events do not last long enough to maintain waterlogging 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 soils and their texture; highest rates were measured in fine textured soils with high silt + clay content. Below a threshold of 65% of silt and clay content, the floodplain soils did not present any significant denitrification rates. Above that threshold denitrification increased linearly. These results demonstrate that alluvial soil texture is a landscape scale factor which has a significant effect on denitrification in floodplains.