Temperature and substrate controls on microbial phospholipid fatty acid composition during incubation of grassland soils contrasting in organic matter quality

Soil incubations are often used to investigate soil organic matter (SOM) decomposition and its response to increased temperature, but changes in the activity and community composition of the decomposers have rarely been included. As part of an integrated investigation into the responses of SOM components in laboratory incubations at elevated temperatures, fungal and bacterial phospholipid fatty acids (PLFAs) were measured in two grassland soils contrasting in SOM quality (i.e. SOM composition), and changes in the microbial biomass and community composition were monitored. Whilst easily-degradable SOM and necromass released from soil preparation may have fuelled microbial activity at the start of the incubation, the overall activity and biomass of soil microorganisms were relatively constant during the subsequent one-year soil incubation, as indicated by the abundance of soil PLFAs, microbial respiration rate (r), and metabolic quotient (qCO₂). PLFAs relating to fungi and Gram-negative bacteria declined relative to Gram-positive bacteria in soils incubated at higher temperatures, presumably due to their vulnerability to disturbance and substrate constraints induced by faster exhaustion of available nutrient sources at higher temperatures. A linear correlation was found between incubation temperatures and the microbial stress ratios of cyclopropane PLFA-to-monoenoic precursor (cy17:0/16:1ω7c and cy19:0/18:1ω7c) and monoenoic-to-saturated PLFAs (mono/sat), as a combined effect of temperature and temperature-induced substrate constraints. The microbial PLFA decay patterns and ratios suggest that SOM quality intimately controls microbial responses to global warming.