LOESSFEST'09 | Aug. 31st – Sept. 3rd, 2009 |Novi Sad-Serbia
Paleovegetational Implications for Rhizoliths From Nussloch (SW Germany)
Gocke, M.1, Wiesenberg, G.L.B.1, Pustovoytov, K.2, Kuzyakov, Y.1
1Dep. of Agroecosystem Research, University of Bayreuth, Germany
2Institute of Soil Science and Land Evaluation, University of Hohenheim, Stuttgart, Germany
Under arid and semiarid conditions, secondary carbonate precipitates from Ca2+, which is abundant in soil solutions, with dissolved CO2 originating from root and rhizomicrobial respiration. Usually, hundreds to thousands of years are necessary for the formation of secondary CaCO3. δ 13C of pedogenic carbonates reflects the photosynthetic pathway of the predominant local vegetation, because secondary CaCO3 is formed in isotopic equilibrium with soil CO2 released by root and rhizomicrobial respiration. Therefore, δ 13C of pedogenic carbonates is used for paleoenvironmental reconstructions.
Rhizoliths are a special form of pedogenic carbonate formed by encrustation of plant roots. While the organic part of the root is mainly degraded during decomposition, carbonatic, bone-like structures remain in the terrestrial sediments. They occur locally abundant in calcareous sediments like loess. These calcified roots are built probably within a few decades or even years. We elucidated the origin of rhizoliths present in loess on a molecular level in combination with stable C isotopic composition (δ13C) of carbonatic and organic parts of the rhizoliths. The comparison of isotopic values and the carboxylic acid composition in rhizoliths and in the adjacent loess should provide information on the sources of both materials.
δ13C values indicated that rhizoliths originate from C3 vegetation. Carboxylic acid composition of the rhizolith matter confirmed the C3 photosynthetic pathway of former vegetation. Moreover, distribution patterns of carboxylic acids and alkanes from rhizolith and loess disproved the prevalent opinion about rhizolith formation taking place simultaneously with loess sedimentation: Loess organic matter reflected former steppe conditions with predominant grass vegetation. In contrast, rhizolith OM indicated more likely roots from shrub or tree vegetation. This suggests that roots entered the loess later under changed climatic conditions and consequently derived from savannah-like vegetation.