Divergent responses of organic matter composition to incubation temperature

•Soil with fresh or aged litter incubated for 14 days at 12 °C or 32 °C•Organic matter composition characterized with 13C NMR before and after incubation•Relative loss of lipids, gain of lignin at higher temp in aged litter only•O-alkyl losses in fresh litter but alkyl losses in aged litter.•CO2 release explained by both OM composition and microbial community structure

Increased or preferential decomposition of organic matter (OM) resulting from elevated global temperatures could have wide reaching impacts on ecosystem stability and function. The purpose of this experiment was to determine whether temperature and litter type interact to alter the chemical composition of OM decomposed during incubation. Either a higher-quality (fresh) eucalyptus leaf litter or a lower-quality (aged) eucalyptus leaf litter was mixed into a eucalyptus woodland soil. These soil and litter mixtures were incubated for 14 days at 12 °C or 32 °C. The OM composition of the starting and ending mixtures were characterized with solid-state 13C cross polarisation magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy. Despite significantly greater losses of CO2 at 32 °C relative to 12 °C in both litter mixtures, the OM composition of the fresh litter mixture was similar between the two temperatures, revealing compound classes were lost and gained in similar proportions. However, the OM composition of the aged litter mixture was significantly impacted by incubation temperature, with proportionately more lignin C and less lipid C at 32 °C relative to 12 °C. The differences in OM loss between the two litters in response to temperature (O-alkyl C in fresh litter; alkyl C in aged litter) reveal that there was not a consistent response of OM to elevated incubation temperature. Instead, the patterns of CO2 released during incubation were related to initial OM composition (r = 0.76) and microbial community structure (r = 0.74). In this experiment, the interacting effects of temperature, litter quality, and microbial community structure resulted in distinct decomposition trajectories for the two litter mixtures, and resulted (in the aged litter) in the enhanced loss of a compound considered to be relatively long-lived in soils. These results support the concept that in response to warming temperatures, the chemical composition of remaining OM will be impacted by both initial litter quality and microbial community structure.

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