Nitrogen cycling in two 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 erosion/sedimentation during floods. In turn, the proportion of fine sediments fractions differs by location within a given community of riparian forest succession. In this paper we tested the effect of geomorphic features of floodplains, through soil grain size sorting, on the nitrogen cycling in riparian forest soils. Two typical riparian forests formed by vertical accretion deposits from repeated addition of sediments from overbank flow have been chosen along the River Garonne, southwest France. These riparian forests had equivalent vegetation, flood frequency and duration, differing only in soil grain size composition: one riparian forest had sandy soils and the other had loamy soils. The evolution of the main soil physical and chemical parameters as well as denitrification (DNT), N uptake (NU) and mineralization (NM) rates were measured monthly over a period of 13 months in the two study sites. The loamy riparian forest presented a better physical retention of suspended matter during floods. Moreover,in situ denitrification rates (DNT) and N uptake by plants (NU) measured in the loamy riparian forest soils were significantly greater than in the sandy soils. Although DNT and NU could be in competition for available nitrogen, the peak rates of these two processes did not occur at the same period of the year, NU being more important during the dry season when DNT was minimum, while DNT rates were maximum following the spring floods. N retention by uptake (NU) and loss by DNT represented together the equivalent of 32% of total organic nitrogen deposited during floods on the sandy riparian forest soils and 70% on the loamy ones. These significant differences between the two sites show that, at the landscape level, one should not estimate the rates of NU and DNT, in riparian forests soils only on the basis of vegetation, but should take also into account the geomorphic features of the floodplain.

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