The role of soil fungi and bacteria in plant litter decomposition and macroaggregate formation determined using phospholipid fatty acids

• Assess the role of soil fungi and bacteria to decomposition of maize residues and macroaggregate formation.
• We used PLFA and 13C in specific PLFA.
• Amount of and maize-derived C in fungal PLFA correlated with macroaggregate amount.
• Quality of plant residues determines composition and activity of the soil microbial biomass.
• Fungal activity is of importance in the formation of macroaggregates.
• Actinomycetes may take over the role of soil fungi in the decomposition of soil organic matter.

Although microbial-derived carbon (C) inputs to soil are increasingly acknowledged as an important source of soil organic matter (SOM), the contribution of different microbial compounds to soil C transformation and their role in aggregation remain poorly understood. This study assessed the contribution of soil fungi and bacteria to the decomposition of maize residues by means of extracted phospholipid fatty acids (PLFAs) and 13C in specific PLFAs and investigated the importance of soil fungi in the formation of macroaggregates. Sieved soil (<250 μm) was incubated for 28 days with and without addition of maize residues and fungicide. Our results show a significant relation between the amount of fungal PLFA 18:2ω6 and the amount of macroaggregates. Further, the amount of macroaggregates was higher in the treatment with the higher amount of maize-derived C in fungal PLFA, suggesting that fungal activity is important for macroaggregate formation. Based on an increased incorporation of maize-derived C into actinomycetes in fungicide treatments, we suggest that actinomycetes may take over the role of soil fungi in the decomposition of SOM. Our study underpins the important role of soil fungi in the decomposition of organic matter and structure formation in the soil, and shows that during inhibition of soil fungi other soil microorganisms are promoted and adopt their function in the soil food web.