Sequestration of microbial, aged and pyrogenic N during paddy soil development

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Although paddy soils exhibit low N use efficiency, the results from the 1st phase of this Research Unit suggested that they may sequester significant amounts of N in microbial residues and aged amino acids. In addition, we observed an accumulation of charred organic matter with yet unknown contribution to total N content. We suggest that mainly bacteria sequester N in their cell walls. While this process is efficient in the surface soils, in subsoils an effective mechanism for N storage seems to be missing. To which degree such processes also hold true for paddy soils formed from different parent materials and under different climatic regimes is unclear. Subproject will thus continue to use amino sugars and amino acid enantiomers in combination with compound-specific isotopic measurements to elucidate the microbial transformation and natural ageing of these hydrolysable SON forms during paddy soil formation and preservation. Besides, we will try to identify black N forms. In particular we aim at identifying (i) characteristic SON decomposition patterns within the different paddy soil profiles, (ii) pools of different amino acid, non-hydrolyzable N and microbial residue N accumulation, and (iii) the transformation rates of hydrolysable N in dependency of soil type. The N-cycling using biomarkers is assessed (iv) in the short-term using the 15N content of soil amino sugars after fertilizing with 15N urea, and (v) in the medium – to long-term by Δ14C tracing of bomb C in microbial residues and amino acids in the paddy soils of different mineralogy.