The Contribution of Fungal Necromass to Soil Organic Matter Storage

Saprotrophic fungi have the ability to degrade the three most important biopolymers: cellulose, lignin, and chitin, and therefore are key moderators of a globally important flow of carbon. However, little is known about how that carbon is transformed and/or stored in soil organic matter (SOM). Fungi are also known to produce a variety of biopolymers, such as chitin, melanin, glucan, and mucus-like exudates, and it is likely that these compounds contribute to long-term storage of SOM. In fact, recent work with ectomycorrhizal fungi has shown that a portion of the fungal necromass survives after degradation times of a few weeks to one month (e.g. Drigo et al. 2012, Clemmenson et al. 2013). Until now, the potential contribution of other abundant fungi to recalcitrant SOM has been unknown. Soil incubations have been performed with the common saprotrophic fungus, Fusarium avencum. Approximately 80% of the fungal material was found to turnover over on a time period of days, but 15% of the original biomass was left over at the end of the two-month degradation experiment in both laboratory experiments and in situ in the Dixon Prairie of the Chicago Botanic Garden. In both experiments, degradation was performed by a natural soil microbial consortium. Residual fungal material at each point in the decomposition sequence was analyzed using FTIR and thermochemolysis-GCMS with tetramethyl ammonium hydroxide. The recalcitrant fraction contained carbohydrate and amide-linked functional groups, which is consistent with the chitin or chitosan biopolymer. The breakdown of more labile organic carbon (including proteins and ester-linked groups) appears to occur on a more rapid time scale. Additionally, lipid biomarker analyses revealed a succession of microbial degraders in the degradation process. This is the first time a study of this kind has been performed using a widely distributed saprotrophic fungus, and indicates that these fungi are potentially important in long-term C-sequestration in SOM. Clemmensen, K.E., A. Bahr, O. Ovaskainen, A. Dahlberg, A. Ekblad, H. Wallander, J. Stenlid, R.D. Finlay, D.A. Wardle, and B.D. Lindahl (2013) Roots and associated fungi drive long-term carbon sequestration in boreal forest. Science 339: 1615-1618. Drigo, B., I.C. Anderson, G.S.K. Kannangara, J.W.G. Cairney, and D. Johnson (2012) Rapid incorporation of carbon from ectomycorrhizal mycelial necromass into soil fungal communities. Soil Biology and Biochemistry, 49: 4-10.