Tuesday, 11 July 2006

Rhizosphere Effect under Oats and Maize Plants.

Ilya V. Yevdokimov1, Reiner Ruser2, Franz Buegger2, Marc Marx2, and Jean Charles Munch2. (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

The assimilation and translocation of 13C after the 5 h pulse-chase labeling in the 13CO2 enriched atmosphere were studied in the greenhouse experiments with plants of oats (Avena sativa L.) and maize (Zea mays L.). The dynamics of 13C in the rhizosphere microbial C (Cmic) and Dissolved Organic Carbon (DOC) were opposite to that for 13CO2 fluxes immediately after the pulse- labelling (in the first 50 h): the minima in Cmic and DOC coincided with the maxima in CO2 emissions, and vice versa. 13C values in the carbon pools oscillated periodically depending on day/night changes. Thus, close inter-relations between 13C dynamics in CO2 emissions, microbial biomass, and DOC were revealed in diurnal scale both for oats and maize plants. Along with pulse-chase labelling, a greenhouse experiment was conducted by growing oats in a continuously 13CO2 labeled atmosphere. The allocation of 13C labeled photosynthates in plants, microbial biomass in rhizosphere and root-free soil, pools of soil organic C, and CO2 emissions were examined. To isolate rhizosphere from root-free soil, plant seedlings were placed into bags made of nylon monofilament screen tissue (16 m meshes) filled with soil. Two peaks in the dynamics of 13C in rhizosphere pools of microbial biomass, DOC, and in CO2 emissions at the earing and ripeness stages were found during the experiment with continuous labelling in the periods of rapid root growth and starting the root decomposition. Development of rhizosphere effect was monitored using ratio between soil characteristics in rhizosphere and in bulk soil which was called rhizosphere factor (Rf). Minimal mean value of Rf was found for water- extractable N (0.9) while the maximal for 13 pool in DOC (4.3). We divided a number of soil characteristics into three groups group I with average Rf values equal to 2 and higher, group II with Rf ranged from 1 to 2, and group III with Rf values < 1. The group I included the most labile C pools: labelled 13C of photoassimilates in the pools of Cmic and DOC, active Cmic determined by substrate-induced growth response (SIGR) method, water-soluble sugars. The group II consisted of less sensitive soil indices - total microbial C and N, specific growth rate of microorganisms, and total DOC. The only constituent of the group III was water-extractable N. In the course of the experiment with continuous 13C labelling, maxima in rhizosphere factors were first revealed for the indices of the group I (period of rapid root growth), then for the group II (period of intensive root turnover and decomposition of dead roots after harvesting). The pool of water-extractable N (group III) had no prominent maxima in the course of the experiment. Thus, both short-term (day/night oscillations) and long-term (in the course of whole vegetation period) dynamics of microbial pools in rhizosphere were connected to the root activity. Dynamics of rhizosphere factor for different soil characteristics demonstrated the development of rhizosphere effect first for the labile C pools, and then for more conservative C and N pools and specific growth rate of microorganisms. 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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