Decoupling of priming and microbial N mining during a short-term soil incubation

• Published in: Soil biology & biochemistry Vol. 129; pp. 71 - 79
• Main Authors: Wild, Birgit, Li, Jian, Pihlblad, Johanna, Bengtson, Per, Rütting, Tobias
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.02.2019
• Subjects: C and N cycling; Carbon; Chitin; Decomposition; Depolymerization; Earth and Related Environmental Sciences; Enzymes; Extracellular enzymes; Fatty acids; Geochemistry; Geokemi; Geovetenskap och miljövetenskap; Lignin; Lignin depolymerization; Markvetenskap; Microorganisms; Natural Sciences; Naturvetenskap; Phospholipid fatty acids; Phospholipids; Priming effects; Protein; Protein depolymerization; Proteins; Soil respiration; Soil Science; Soils; SOM decomposition; Lignin; Decomposition; Chitin; Phospholipid fatty acids; Extracellular enzymes; Protein
• ISSN: 0038-0717; 1879-3428; 1879-3428
• DOI: 10.1016/j.soilbio.2018.11.014

Soil carbon (C) and nitrogen (N) availability depend on the breakdown of soil polymers such as lignin, chitin, and protein that represent the major fraction of soil C and N but are too large for immediate uptake by plants and microorganisms. Microorganisms may adjust the production of enzymes targeting different polymers to optimize the balance between C and N availability and demand, and for instance increase the depolymerization of N-rich compounds when C availability is high and N availability low (“microbial N mining”). Such a mechanism could mitigate plant N limitation but also lie behind a stimulation of soil respiration frequently observed in the vicinity of plant roots (“priming effect”). We here compared the effect of increased C and N availability on the depolymerization of native bulk soil organic matter (SOM), and of 13C-enriched lignin, chitin, and protein added to the same soil in two complementary ten day microcosm incubation experiments. A significant reduction of chitin depolymerization (described by the recovery of chitin-derived C in the sum of dissolved organic, microbial and respired C) upon N addition indicated that chitin was degraded to serve as a microbial N source under low-N conditions and replaced in the presence of an immediately available alternative. Protein and lignin depolymerization in contrast were not affected by N addition. Carbon addition enhanced microbial N demand and SOM decomposition rates, but significantly reduced lignin, chitin, and protein depolymerization. Our findings contrast the hypothesis of increased microbial N mining as a key driver behind the priming effect and rather suggest that C addition promoted the mobilization of other soil C pools that replaced lignin, chitin, and protein as microbial C sources, for instance by releasing soil compounds from mineral bonds. We conclude that SOM decomposition is interactively controlled by multiple mechanisms including the balance between C vs N availability. Disentangling these controls will be crucial for understanding C and N cycling on an ecosystem scale. •Low N and high C availability might boost soil N-polymer breakdown (“N mining”).•N mining might thus lie behind higher soil respiration near plant roots (“priming”).•We tested the effect of C and N input on the degradation of SOM and added polymers.•C input increased SOM degradation but reduced that of chitin, lignin and protein.•Our findings challenge the hypothesis of N mining as a key driver of priming.