Microbial decomposition of soil organic matter is mediated by quality and quantity of crop residues: mechanisms and thresholds
Crop residue quality and quantity have contrasting effects on soil organic matter (SOM) decomposition, but the mechanisms explaining such priming effect (PE) are still elusive. To reveal the role of residue quality and quantity in SOM priming, we applied two rates (5.4–10.8 g kg−1) of 13C-labeled wheat residues (separately: leaves, stems, roots) to soil and incubated for 120 days. To distinguish PE mechanisms, labeled C was traced in CO2 efflux and in microbial biomass and enzyme activities (involved in C, N, and P cycles) were measured during the incubation period. Regardless of residue type, PE intensity declined with increasing C additions. Roots were least mineralized but caused up to 60% higher PE compared to leaves or stems. During intensive residue mineralization (first 2–3 weeks), the low or negative PE resulted from pool substitution. Thereafter (15–60 days), a large decline in microbial biomass along with increased enzyme activity suggested that microbial necromass served as SOM primer. Finally, incorporation of SOM-derived C into remaining microbial biomass corresponded to increased enzyme activity, which is indicative of SOM cometabolism. Both PE and enzyme activities were primarily correlated with residue-metabolizing soil microorganisms. A unifying model demonstrated that PE was a function of residue mineralization, with thresholds for strong PE increase of up to 20% root, 44% stem, and 51% leaf mineralization. Thus, root mineralization has the lowest threshold for a strong PE increase. Our study emphasizes the role of residue-feeding microorganisms as active players in the PE, which are mediated by quality and quantity of crop residue additions.
Keywords13C-labeled crop residues Enzyme activities Litter quality Microbial necromass Priming effect Soil organic matter
We acknowledge the financial support provided by the German Academic Exchange Service (DAAD) to MS and Alexander von Humboldt Foundation (AvH) to MSU. EB’s participation was supported by the Russian Science Foundation (project N 14-14-00625). We also acknowledge the technical support of Klaus Schützenmeister in isotope labeling of the plant material. We are thankful to Karin Schmidt and Anita Kriegel for laboratory assistance. The isotopic analyses were performed at the Kompetenzzentrum Stabile Isotope (KOSI), Goettingen. This study was funded by the Deutsche Forschungsgemeinschaft (DFG, projects HE 6726/6 and KU 1184/29).
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