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Dynamic Modeling of Soil Nitrogen Budget and Vegetation Colonization in Sediment Bars of a Regulated River

Author(s): Takashi Asaeda; Md H. Rashid; Rabi’Atul A. Abu Baker

Linked Author(s): Takashi Asaeda

Keywords: Salix spp.; Robinia pseudoacacia; Succession; Flood

Abstract: Biomass of vegetation, and soil and plant nutrient status were observed during plant colonization before and after the flood events on sandbars of several gravelly rivers in Japan. The observational results provided the location and rates of tree species recruitment as functions of flood and sedimentation characteristics, allometric relations of tree biomass with leaves and woody components, tree height, crown diameters, and flushing rate of trees. Herbaceous plant biomass was also obtained and formulated as the decreasing functions of soil nitrogen content, sediment particle size, and shading by neighboring trees. Organ specific nitrogen content of major plant species and atmospheric nitrogen fixation rate by symbiotic plants were determined. Accordingly, nitrogen budget of the sandbar sediment was formulated in relation to plant biomass. From these formulations, a numerical model for nutrient dynamics and vegetation succession was developed. At first, the developed model was applied to simulate the observed vegetation biomass and soil nitrogen data. Simulations were conducted for several cases; related to the processes of two tree species, Robinia pseudoacacia (a nitrogen fixing species) and Salix sp. (a non-nitrogen fixing species). Satisfactory agreements between observed and simulated results were obtained. It was found that herbaceous plant biomass is substantially suppressed by tree colonization. At unshaded areas, however, the herb biomass increases quickly during the early stage when the rhizome system is establishing. After the rhizome system is fully established, the biomass follows the increasing rate of nitrogen in the soil. Under tree canopy, on the other hand, following the increasing shading due to the growth of trees, the herbaceous plant biomass initially decreases. With increasing nitrogen concentration in the soil and the mortality of trees, the herb biomass increases again. The amount of nitrogen taken by herbaceous plants follows the similar trend of its biomass. However, in the case of R. pseudoacacia colonization, as a large fraction of nitrogen is supplied by nitrogen fixation at low nitrogen concentration of the soil, its nitrogen uptake rate is much smaller than that of herbaceous plants. Since the amount of nitrogen released through their defoliation is much larger than the uptake rate, the soil nitrogen gradually increases. Whereas, after the elevation of soil nitrogen concentration, nitrogen fixation rate declines and the nitrogen in the soil gradually decreases accordingly due to the storage of the same by trees in the woody organs. The patterns of nitrogen decrease in soil with the increase of tree growth due to organ storage are almost similar in R. pseudoacacia and Salix.


Year: 2013

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