FOR 995: Biogeochemistry of paddy soil evolution
Zusammenfassung der Projektergebnisse
The central topic of the Research Unit was a better understanding of the biogeochemistry of major processes involved in the formation of paddy soils. Special attention has been paid to redox-controlled processes during different stages of paddy soil evolution, in particular to mineral transformations, changes in amount and composition of soil organic matter, and to the associated activities and structure of microbial communities involved in C, N, and Fe cycling. During the first phase of the Research Unit “Biogeochemistry of Paddy Soil Evolution”, a chronosequence of paddy soils and of cropland soils that were not used for wetland rice production were investigated to identify main processes which are responsible for paddy soil development in a coastal region in the eastern part of China. Main processes of paddy soil development on marine/estuarine sediments (Fluvisols) occurring in different timeframes are (a) eluviations of soluble salts and carbonates, (b) shifts in the soil microbial community, (c) changes in accumulation rates of paddy-specific organic carbon (OC) and organic nitrogen (ON) compounds and biomarkers, (d) (re-)formation of oxide assemblages and (e) (co-)precipitation of dissolved OC. The dynamics of paddy soil development in direct comparison to soils not used for lowland rice cultivation clearly reveal accelerated soil formation processes under wetland rice production. We were able to outline three fundamental phases of paddy soil development in our chronosequence: (1) short-term processes like desalinization (specific for marine sediments) and development of a compacted plough pan form the basis for the next phase (2), which is mainly characterized by decalcification, differentiated in topsoil and subsoil time domains, and (3) long-term processes like OC accumulation, but also (trans-)formation of oxides, which occurred to a considerable extent after complete decalcification of the entire soil profile. But 2000 years of paddy soil formation (>1300 years of weathering after decalcification) on these terrigenous sediments was too short for a fourth phase of paddy soil formation, which would be characterized by silicate weathering and clay migration. The results obtained here thus relate to the evolution of young soils, with paddy management from the beginning of terrestrial soil formation. However, wetland rice is mostly produced on many different soil types that have undergone a certain (and often long-term) soil formation before paddy management started. These soils are characterised by a specific pedogenetic biogeochemistry and contrasting mineralogy. It is therefore not possible to simply generalise the results from the chronosequence where paddy use started at point zero of terrestrial soil formation. Thus, the second phase of the research unit focused on processes that may occur during paddy soil evolution on fully developed terrestrial soil groups to expand our knowledge on other soil types that play a major role for wetland rice production in Asia. We studied paddy soils that derived from three different soil types (Vertisols, Andosols, Alisols) on volcanic parent material on Java (Indonesia). To account for the variability in parent materials, we additionally sampled sandstone-derived Alisols in China. Adjacent non-paddy soils were sampled as references. The results showed that effects of paddy management on texture, clay mineral composition, and cation exchange capacity were little and initial differences between soil types were not overruled. Except for Vertisols, paddy management caused significant depletion in Fe oxides in the topsoils (puddled layer and plough pan) due to redox processes. The extent to which the reduced Fe was leached or re-oxidized as short range-ordered Fe oxides depended on the soil texture. In addition, paddy management does not necessarily enhance carbon sequestration. Rather, differences in organic matter and burned straw input seem to determine whether OC is accumulated under paddy management or not. Overall, paddy management-induced changes were partly influenced by the original soil and the parent material. In turn, the main characteristics of the initial soil type were preserved and not overridden by paddy management.
