100 Years of Soil Organic Matter Priming Research

The priming effect (PE)-the change of soil organic matter (SOM) decomposition after the input of fresh organic matter or fertilizers-was discovered 100 years ago, yet major uncertainties persist regarding why PE varies in direction and magnitude, how microbial energy and nutrient constraints shape these responses, and under what conditions priming results in net soil C gain or loss. Here, we review the mechanisms, processes, and consequences of PE that are driven by sophisticated experiments, brilliant ideas, and methodological advances. We intensively discussed 13 PE mechanisms responsible for the acceleration and reduction of SOM decomposition and highlight an emerging paradigm in which microbial energy limitation regulates enzyme investment and C and energy balance over the course of PE. 97% of studies measured positive PE, meaning that the addition of available compounds accelerates SOM turnover. The few PE studies assessing the C balance (the difference between added C amount and the sum amount of primed C from SOM decomposition and C loss from added organic C decomposition) in soil after 1 year showed that about 40% of the added C is incorporated into microbial biomass and SOM, indicating that the C balance is positive despite positive PE. We assessed that microorganisms use approximately 13% of their energy during priming to produce nine enzymes involved in C, nitrogen, and phosphorus mobilization, but the actual proportion is much higher because not all enzyme types are included in the estimation. We identify major knowledge gaps related to the controls of PE direction and intensity, context-dependent microbial strategies, and the scarcity of long-term C balance assessments. Finally, we discussed the environmental implications of PE (feedback to climate change, pollutant removal, and effects of terrestrial erosion on C dynamics in aquatic ecosystems) and suggested various open questions related to PE sources, main drivers, and mechanisms.