Microbial cell-envelope fragments and the formation of soil organic matter: a case study from a glacier forefield

Genesis of soil organic matter (SOM) during pedogenesis is still a matter of controversy in soil science. Recently, it was hypothesized that microbial cell-envelope fragments contribute significantly to SOM formation. We tested the relevance of this process during pedogenesis by evaluating the development of SOM along a chronosequence of a glacier forefield (Damma glacier). Samples of increasing soil age collected along the forefield were analyzed for C and N contents, phospholipid and total fatty acids (PLFA and tFA), water contact angle, micro-hydrophobicity and surface coverage by microbial cell-envelope residues. The surface coverage was visualized and quantified by analysis of representative, equally-scaled scanning electron micrographs (SEM). Increasing SOM contents were accompanied by increasing coverage and overall abundance of microbial cell-envelope fragments as evaluated on the basis of scanning electron microscopy; this is also reflected in the amounts of tFA and PLFA, the trend of C/N ratios, and the increasing hydrophobicity and water contact angles of the soil samples. Using SEM and the image analysis approach, we can provide a process-based description of the development of SOM in the newly developing ecosystem of the glacier forefield. The majority of small-sized SOM visible with scanning electron microscopy appears to consist of bacterial cell envelope fragments that remain stable after cell death, such that their shape does not change with soil age. Our results show the importance of microbial processing of SOM, and highlight the existence of microbial necromass as a significant part of the fine-particulate SOM even in later stages of soil development.