Phospholipid 13C stable isotopic probing during decomposition of wheat residues

Abstract

Disentangling the kinetics of the soil microbial community succession, which is simultaneously driven by newly added plant materials and extant soil organic matter (SOM), can enrich our knowledge on microbial carbon (C) utilization patterns under residue amendment. This understanding might be useful to predict the rapid responses of specific microbial functional groups and develop strategies for balancing the terrestrial C budget. Therefore, our objective was to characterize and estimate the parameters of the microbial community dynamics profiled by phospholipid fatty acids (PLFA) from 13C-labeled wheat residues and SOM. We conducted a 21-day microcosm study using two different arable systems (conventional tillage, CT; no-till, NT) amended with three types of 13C-labeled wheat residues (grains, leaves and roots). The abundances and isotopic fractions of 13CO2 flux and 13C-labeled PLFA were measured via gas trace isotope ratio mass spectrometry (IRMS) and gas chromatography-combustion-isotope ratio mass spectrometry (GC-c-IRMS), respectively. A double exponential model was used to describe the synthesis-degradation kinetics of PLFA from different microbial origins. We found that the PLFA formation generally reaches its maximal abundance within 7 days (except for PLFA from actinomycetes). The SOM- and wheat residue-derived C fluxes, as well as their PLFA profiles, were inconsistently impacted by the residue quality or the tillage regime over the incubation period. Specifically, the abundances of residue-derived CO2 and PLFAs significantly decreased in the following order: grains>leaves>roots. However, those abundances derived from SOM were the lowest with the leaf residue treatments. Residue-derived PLFA patterns were highly influenced by fungi and G− bacteria, while G+ bacterial and actinomycete PLFAs were preferentially linked to extant SOM mineralization. Compared to the residue-derived counterparts, the SOM-derived microbes were characterized by higher G+/G− bacteria and cy17:0/C16:1ω7c ratios, as well as lower fungi/bacteria PLFA ratios. Such distinction between residue and SOM was also evidenced by the contrasting tillage effects on C mineralization and the ratios of cy17:0/C16:1ω7c and fungal/G− bacterial PLFA. Our study provides evidence with important implications for adapting the microbial-mediated processes of soil C management through residue quality control.