Chairs Comments The Nitrogen Cycle

D. Werner, D. Ingendahl, E. ter Haseborg, H. Kreibich, P. Vinuesa

Fachgebiet Zellbiologie und Angewandte Botanik, Fachbereich Biologie der Philipps-Universitaet, D-35032 Marburg, Germany

In contrast to general belief, the largest nitrogen pool on our planet is not in the atmosphere (4 x 1015 tons) but in the primary rocks (1.9 x 1017 tons). Compared to these pools, the N2 dissolved in the sea is only around 2.2 x 1013 tons. The other nitrogen pools (of nitrate, ammonia, organic nitrogen in the soil and in the sea and the nitrogen fixed in plants and animals) are in the order of 2 x 1012 tons. Compared to the pools of N2, this is a tiny fraction. However, these pools are by four orders of magnitude larger, compared to N2, fixed annually by organisms in the soil and in the sea, which is in the range of 200 to 300 million tons per year (Burns, Hardy 1975).

The carbon pool on this planet has a similar scale with, of course, very interesting differences in the various pools and turnover rates. The total carbon on this planet is around 1017 tons, from which more than 70% are in the lithosphere, the second largest pool is the oceans with 3.8 x 1013

tons C. The carbonate fraction in the upper soil layers contains around 1.7 x 1012 tons C and the organic soil carbon is in the range of 1.6 x 1012 tons C. The atmosphere contains only about 0.75 x 12 12 10 tons C and the biomass of the plants with around 0.6 x 10 tons C has a similar size. It is also very interesting to compare the annual turnover rate produced by humans from combustion of fossil energies (6.3 x 109 tons C) with the turnover rate by photosynthesis of land plants (120 x 109 tons

C), the transfer rates from plant biomass to the soil (60 x 109 tons C) and the respiration rate from the soil into the atmosphere (62 x 109 tons C) per year. Also the equilibrium between atmosphere and the ocean is apparently not complete, with a small surplus of uptake versus transfer in the atmosphere (Werner 2000).

Surveys on various aspects of the nitrogen cycle in the biochemistry and ecophysiology of the different components of the cycle have been published by Zumft (1997) on denitrification, on atmospheric trace gases such as N20 and NO by Conrad (1996), on nitrogen fertilizer use by Kawashima et al. (2000), on nitrogen fixation in agricultural systems by Peoples and Herridge (2000), on nitrogen mineralization and nitrification by Van Veen (2000) and on nitrogen use efficiency by Simek and Cooper (2001).

The four contributions of this session contribute to various aspects of the nitrogen cycle. Daniel Arp and co-workers report on "A gene to genome look at Nitrosomonas europaea" (this volume). Hermann Bothe and co-workers study "The distribution of dinitrogen fixing and denitrifying bacteria in soils assessed by molecular biological methods" (this volume). Denitrification is studied with Bradyrhizobium japonicum genes by Socorro Mesa and co-workers (this volume) and the plant aspects of plant ureases is contributed by Joe Polacco et al. (this volume).

From our laboratory some results on "Population shift of denitrifying bacteria in interstitial and surface waters downstream of a purification plant documented by amplified 16S ribosomal DNA restriction analysis (ARDRA) patterns" are presented. Sixteen different genotypes of denitrifying bacteria have been isolated from the various river sediments before and after the purification plant. Some genotypes were only present in the free flowing water while other genotypes were isolated only from the hyporheic interstitial. Other genotypes had a ubiquitous occurrence. A further group of genotypes could be only detected in the hyporheic interstitial and in the surface waters upstream of the purification plant (ter Haseborg et al. 2001). Altogether 37 ARDRA types were isolated in the interstitial and in the surface water of the Lahn River in Germany. Sixteen of them were only obtained in one sampling period. This means that the variation in the various years in the structure of the bacterial community is rather high. Five ARDRA types were found in continuing years, representing stable denitrifying groups in the interstitial as well as in the surface water. From agricultural soils, 26 ARDRA types have been isolated (Cheneby et al. 2000) compared to the 37 groups from the aquatic environment. The temporal and spatial diversity of denitrifying bacteria in the aquatic environment may therefore be larger than in agricultural systems.

Nitrosomonas europaea and Nitrosomonas eutropha are able to nitrify and denitrify at the same time, growing under oxygen limitation (Bock et al. 1995). These simultaneous processes form large quantities of N2O and N2, which causes a significant nitrogen loss. Nitrite is another important intermediate, which can occur under certain temperature conditions in surface waters (von der Wiesche, Wetzel 1998) and also in agricultural soils (Burns et al. 1995). The concentrations in the soil were highly variable and ranged from 0 to 2.7 pg N x g"1 soil. An increase in soil pH produced large nitrite flushes (Burns et al. 1995). The known biodiversity of nitrifying bacteria has increased over the last years. Nine species of ammonia oxidizers and seven species of nitrite oxidizers have been described (Spiek, Bock 1998).

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