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The Research Unit Biogeochemistry of Paddy Soil Evolution is focussing on the identification of main processes responsible for paddy soil evolution. To understand paddy soil evolution, we have to relate the dynamics of hydrologically and microbially mediated redox processes to the dynamics of soil minerals and soil organic matter. This is strongly related to the question of the microbial accessibility of organic carbon (OC) and nitrogen (N), but also of iron (Fe). These elements form stable organo-mineral associations. The research approach for this question includes the quantitative study of effects of redox conditions on production and stabilization of organic matter, mineral transformations and the related consequences for the geochemistry of the soil solution in paddy soils. A major focus is on the availability of iron species in paddy soils for microbially mediated redox processes. These investigations are closely linked to investigations on the stabilization of dissolved organic matter by sorption on iron (hydr)oxides and clay minerals and  (co-)precipitation with Fe in dependence on redox conditions. Paddy soils are characterized by preservation of organic matter predominantly derived from the rice plant itself, from manure or straw combustion residues, and from associated microbial consortia under at least periodically anaerobic conditions due to submergence.

At present, a general understanding of the stabilization of organic matter (both organic carbon and organic nitrogen) in paddy soils is lacking. A major topic of the Research Unit is to assess the cycling and storage of organic matter at different stages of paddy soil evolution. Linking soil science to microbiology, we focus on redox-sensitive processes covering mineral transformations and changes in soil organic matter. Particular attention is paid to microbially mediated processes in the C, N, and Fe cycles, and to the populations involved. With respect to organic C (OC) and organic N (ON) accumulation it is of major importance to elucidate quantitatively if this is mainly due to the lack of oxygen during decomposition, resulting in the accumulation of recalcitrant OC/ON or due to the stabilization of OC and ON in organo-mineral associations (OC/ON associated with strongly and weakly dispersed clays as well as attached to different Fe [and Al] oxides), in charred organic matter or in phytoliths. The approach generally implies to study the complete paddy soil profile, including relevant soil properties, such as type and content of pedogenic minerals and OC/ON. Due to the specific management practices, paddies additionally receive input via the sediment load derived from flooding. Different molecular markers of rice, other plants, markers for microbial and anthropogenic input and for aged organic matter residues within paddy soil will be investigated to differentiate these contributions. Isolation and 14C-AMS dating of individual components will provide an insight into the mobility of such markers and the development of paddy soil biogeochemistry through time.


  Chronosequence (Zheijang Province, PR China)

During the first phase (2008-2011), the Research Unit focussed on two chronosequences derived from
uniform parent material in the coastal region of the Zhejiang Province (PR China).
One chronosequence consisted of differently aged paddy soils (50 - 2000 years),
characterized by a yearly cropping sequence of rice cultivation under flooded conditions alternated by a non-inundated upland crop.
The adjacent upland chronosequence was exclusively used for non-inundated crop production for 50 - 700 years.
The investigation of these chronosequences provided the unique opportunity to understand the effect of soil redox conditions
on the evolution of soil microbiology, mineralogy, and soil organic matter properties and their interactions.
This approach allowed us to identify the influence of agricultural management at different stages of pedogenesis.
Results of these investigations are already published: <Publications>


During the second phase (2012-2014), the aim of the Research Unit is to transfer and to expand the obtained knowledge on other soil types that are typically used for rice cultivation.
Vertisols, Ferralsols and Andosols play a major role for wetland rice production in Asia and are therefore included in the investigations. The three soil types represent the major range of soil properties to be expected in Asian paddy fields.
In addition, they characterize a different initial status of paddy soil evolution which started from weathered soil and not from fresh marine/brackish sediment substrates as in the chronosequence studied so far.
Thus, the second phase of the Research Unit focusses on different parent materials and correlated climatic regions, resulting in different main soil types (Ferralsols, Andosols, Vertisols), and how these soils are influenced by paddy management.

The Indonesian island of Java has an extremely high population density. Rice is the major staple crop and high yields are achieved on fertile soils like Vertisols and Andosols, but also less fertile soils like Feralsols are used for paddy rice production.
For a better link to our previous research area, we additionally selected sites dominated by low-activity clays in the Jiangxi province (China). They are either Acrisols or Fluvisols derived from eroded Acrisol material. Acrisols are the dominant soils in southern China and with that the dominant soils of the whole traditional rice-growing area of China.