NITROGEN CYCLING IN 2 RIPARIAN FOREST SOILS UNDER DIFFERENT GEOMORPHIC CONDITIONS

At the floodplain scale, spatial pattern and successional development of riparian vegetation are under the control of geomorphic processes. The geomorphic and hydraulic characteristics of stream channels affect the sorting of organic material and inorganic sediment through erosio n/sedimentation during floods. In turn, the proportion of fine sedimen ts fractions differs by location within a given community of riparian forest succession. In this paper we tested the effect of geomorphic fe atures of floodplains, through soil grain size sorting, on the nitroge n cycling in riparian forest soils. Two typical riparian forests forme d by vertical accretion deposits from repeated addition of sediments f rom overbank Row have been chosen along the River Garonne, southwest F rance. These riparian forests had equivalent vegetation, hood frequenc y and duration, differing only in soil grain size composition one ripa rian forest had sandy soils and the other had loamy soils. The evoluti on of the main soil physical and chemical parameters as well as denitr ification (DNT), N uptake (N-U) and mineralization (N-M) rates were me asured monthly over a period of 13 months in the two study sites. The loamy riparian forest presented a better physical retention of suspend ed matter during floods, Moreover, in situ denitrification rates (DNT) and N uptake by plants (N-U) measured in the loamy riparian forest so ils were significantly greater than in the sandy soils. Although DNT a nd N-U could be in competition for available nitrogen, the peak rates of these two processes did not occur at the same period of the year, N -U being more important during the dry season when DNT was minimum, wh ile DNT rates were maximum following the spring floods. N retention by uptake (N-U) and loss by DNT represented together the equivalent of 3 2% of total organic nitrogen deposited during floods on the sandy ripa rian forest soils and 70% on the loamy ones. These significant differe nces between the two sites show that, at the landscape level, one shou ld not estimate the rates of N-U and DNT in riparian forests soils onl y on the basis of vegetation, but should take also into account the ge omorphic features of the floodplain.