Saturday, 15 July 2006

Microbial Community Structure in Soil as Dependent on the Rates of N Fertilization.

Ilya V. Yevdokimov1, Andreas Gattinger2, Michael Schloter3, and Jean Charles Munch3. (1) Institute of Physicochemical and Biological Problems in Soil Science, RAS, 142290, Institutskaya 2, Pushchino, Moscow region, Russia, (2) GSF National Research Center for Environment and Health, Institute of Soil Ecology, Ingolstaedter Landstr. 1, 85764, Neuherberg, Germany, (3) GSF National Research Center for Environment and Health, Institute for Soil Ecology, Ingolstaedter Landstr. 1, 85764, Neuherberg, Germany

The phenomenon of fast immobilisation and long-term retention of extremely high rates of 15N inorganic fertilisers in soil without additional C source applications was revealed earlier (Yevdokimov et al., 2005). The long-term retention of 15N added was probably connected to the formation of new microbial community tolerant to high concentrations of nitrogen in soil. The aim of our research was to test this hypothesis. Changes in microbial community structure were tested on Eutric Cambisol by PhosphoLipids Fatty Acid (PLFA) analysis, while the kinetic characteristics of microbial biomass were determined by substrate induced growth response (SIGR). 15N was applied as KNO3 at the rates of 0, 50, 100, 300, 1000 and 2000 mg N kg-1 soil. The N immobilisation on day 4 was as high as up to 73 % of 15N added (treatment 100 mg N kg-1). Even after 1 month incubation the percentage of 15N recovered in soil organic matter and microbial biomass was as high as up to 74% of 15N added (treatment 2000 mg N kg-1). 1 month incubation of soil with 2000 mg N kg-1 applied led to the formation of a microbial community with low specific growth rate, low respiration rate and prolonged period between C substrate addition and exponential growth. These changes in growth kinetics indicate formation of microbial community capable to survive by C starvation. Microbial communities under N rich conditions (2000 mg N kg-1) showed contrasting PLFA composition in comparison with those under low N (control and 100 mg N kg-1) on the basis of 98 analysed PLFA. Discriminations along PC1 and PC2 PLFA indicated shifts in the structure of microbial community to Gram-negative bacteria and fungi due to high N rates (2000 mg N kg-1). Accordingly the ratio of cyclic PLFA to their monounsaturated precursor, an indicator for C limitation, showed different values under varying N contents. It demonstrated that the phenotype of the soil microbial communities responded sensitively to excess N and depletion of C. Also, extremely high N fertilization decreased the bacteria-to-fungi ratio. Thus, different mechanisms of microbial community adaptation to C limiting and N rich conditions in soil were revealed. Acknowledgements: this research was supported by the Alexander von Humboldt Foundation, the Russian Foundation for Basic Researches, the Russian Ministry of Science and Education, and the Russian Academy of Sciences.

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