The soil priming effect: Consistent across ecosystems, elusive mechanisms

• Published in: Soil biology & biochemistry 2020-01, Vol.140, p.107617, Article 107617
• Main Authors: Liu, Xiao-Jun Allen, Finley, Brianna K., Mau, Rebecca L., Schwartz, Egbert, Dijkstra, Paul, Bowker, Matthew A., Hungate, Bruce A.
• Format: Article
• Language: English
• Subjects: Carbon use efficiency; Microbial growth and respiration; Nutrient limitation; Soil carbon sequestration; Soil organic matter decomposition; Stable isotope tracer
• ISSN: 0038-0717; 1879-3428
• DOI: 10.1016/j.soilbio.2019.107617

Organic matter input to soils can accelerate the decomposition of native soil carbon (C), a process called the priming effect. Priming is ubiquitous and exhibits some consistent patterns, but a general explanation remains elusive, in part because of variation in the response across different ecosystems, and because of a diversity of proposed mechanisms, including microbial activation, stoichiometry, and community shifts. Here, we conducted five-week incubations of four soils (grassland, piñon-juniper, ponderosa pine, mixed conifer), varying the amount of substrate added (as 13C-glucose, either 350 or 1000 μg C g−1 week−1) and either with no added nitrogen (N), or with sufficient N (as NH4NO3) to bring the C-to-N ratio of the added substrate to 10. Using four different ecosystems enabled testing the generality of mechanisms underlying the priming effect. The responses of priming to the amount and C-to-N ratio of the added substrate were consistent across ecosystems: priming increased with the rate of substrate addition and declined when the C-to-N ratio of the substrate was reduced. However, structural equation models failed to confirm intermediate responses postulated to mediate the priming effect, including responses postulated to be mediated by stoichiometry and microbial activation. Specifically, priming was not clearly associated with changes in microbial biomass or turnover, nor with extracellular enzyme activities or the microbial C-to-N ratio. The strongest explanatory pathways in the structural equation models were the substrate, soil, and C-to-N ratio treatments themselves, with no intermediates, suggesting that either these measurements lacked sufficient sensitivity to reveal causal relationships, or the actual drivers for priming were not included in the ancillary measurements. While we observed consistent changes in priming caused by the amount and C-to-N ratio of the added substrate across a wide array of soils, our findings did not clearly conform to common models offered for the priming effect. Because priming is a residual flux involving diverse substrates of varying chemical composition, a simple and generalizable explanation of the phenomenon may be elusive. •Priming occurs when input of new soil carbon stimulates decomposition of old soil carbon.•Across soils, adding more carbon increases priming, and adding nitrogen suppresses it.•Proposed mechanisms invoke shifts in microbial physiology and nutrient demand.•Tests using structural equat