Summary1. Nitrification plays a key role in the functioning of many natural ecosystems. It is directly involved in plant nitrogen nutrition and soil N losses through leaching and denitrification. The control of this process by plants is poorly understood, although modifications of nitrification would allow plants to manipulate competition for N and induce changes in ecosystem N balance. In a wet tropical savanna ecosystem (Lamto, Côte d'Ivoire), the soil N cycle is characterized by distinct high-and low-nitrification sites. Previous publications showed that nitrification was positively or negatively correlated with root densities of the dominant grass covering these sites. These contrasting sites were chosen to investigate the extent to which vegetation controls longterm nitrification. 2. In situ experimental plots were created where grass individuals originating from high-or low-nitrifying soils were transplanted into both soils. Nitrifying enzyme activity (NEA) was measured up to 24 months after transplanting. Grasses from both sites significantly modified NEA up to rates similar to those at their respective control sites. 3. The level of individual plant control (inhibition and stimulation) was correlated with grass biomass. The potential mechanisms of this control is discussed, along with its consequences for ecosystem N cycling (such as N losses), as the denitrifying enzyme activity (DEA) is much higher in the high-nitrification site. Such results suggest that plant species can have important consequences for N cycling at the population level.
Summary 1.Wet tropical savannas are characterized by strong environmental constraints-particularly low soil nutrient availability-associated with high plant productivity. Nitrogen recycling, and especially nitrification, is supposed to be a strong determinant of the balance between conservation and loss of nutrients at the ecosystem level. Savanna facies dominated by the grass Hyparrhenia diplandra (Andropogoneae) are known to exhibit low levels of nitrification and thus avoid nitrate losses. 2. By comparing two sites in the Lamto area (Côte d'Ivoire, West Africa) with similar soil physico-chemical characteristics and equally dominated by H. diplandra (80% of the grass cover), it was demonstrated that, within this facies, nitrification is highly heterogeneous, with a 240-fold variation in potential nitrification within a specific site. 3. In order to test whether these differences can be considered as permanent in this ecosystem, nitrate reductase activities were compared on H. diplandra plantlets from the two sites, cultivated under identical conditions in the presence of nitrate. The leaves of plants originating from the high nitrification site were always able to reduce nitrate at a significantly higher rate than those from the low nitrification site. This observation indicates a long-term adaptation of the plants and stable nitrification behaviour. 4. Lamto can thus be considered as a contrasted dual ecosystem relative to its nitrogen cycle. The two sites studied therefore constitute useful models to assess the determinism of nitrification in wet savannas and the role of this process on nitrogen retention in such ecosystems.
Aims: To investigate the relationships between the operation of the volatile organic compound (VOC) removal biofilter and the structure of microbial communities, and to study the impact on degradation activities and the structuring of microbial communities of biofilter malfunctions related to the qualitative composition of the polluted air. Methods and Results: A microbiological study and a measurement of biodegradation activities were simultaneously carried out on two identical peat-packed columns, seeded with two different inocula, treating polluted air containing 11 VOCs. For both reactors, the spatial structure of the microbial communities was investigated by means of single-strand conformation polymorphism (SSCP) analysis. For both reactors, stratification of degradation activities in function of depth was observed. Oxygenated compounds were removed at the top of the column and aromatics at the bottom. Comparison of SSCP patterns clearly showed a shift in community structure in function of depth inside both biofilters. This distribution of biodegradation activities correlates with the spatialization of microbial density and diversity. Although the operating conditions of both reactors were identical and the biodegradation activities similar, the composition of microflora differed for biofilters A and B. Subdivision of biofilter B into two independent parts supplied with polluted air containing the complex VOC mixture showed that the microflora having colonized the bottom of biofilter B retained their potential for degrading oxygenated compounds. Conclusions: This work highlights the spatialization of biodegradation functions in a biofilter treating a complex mixture of VOCs. This distribution of biodegradation activities correlates with the spatialization of microbial density and diversity. Significance and Impact of the Study: This vertical structure of microbial communities must be taken into consideration when dealing with the malfunctioning of bioreactors. These results are also useful information about changes in microbial communities following natural or anthropogenic alterations in different ecosystems (soils and sediments) where structuring of microbial communities according to depth has been observed.
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