Review of mechanisms and quantification of priming effects

Priming effects are strong short-term changes in the turnover of soil organ ic matter caused by comparatively moderate treatments of the soil. In the c ourse of priming effects large amounts of C, N and other nutrients can be r eleased or immobilized in soil in a very short time. These effects have bee n measured in many field and laboratory experiments; however, only a few of the studies were aimed at an extended investigation of the mechanisms of s uch phenomena. The aim of this overview is to reveal possible causes and pr ocesses leading to priming actions using the references on agricultural eco systems and model experiments. Multiple mechanisms and sources of released C and N are presented and summarized in Tables for positive and negative re al and apparent priming effects induced after the addition of different org anic and mineral substances to the soil. Soil microbial biomass plays the k ey role in the processes leading to the real priming effects. The most impo rtant mechanisms for the real priming effects are the acceleration or retar dation of soil organic matter turnover due to increased activity or amount of microbial biomass. Isotopic exchange, pool substitution, and different u ncontrolled losses of mineralized N from the soil are responsible for the a pparent N priming effects. Other multiple mechanisms (predation, competitio n for nutrients between roots and microorganisms, preferred uptake, inhibit ion, etc.) in response to addition of different substances are also discuss ed. These mechanisms can be distinguished from each other by the simultaneo us monitoring df C and N release dynamics; its comparison with the course o f microbial activity; and by the labelling of different pools with C-14 or C-13 and N-15. Quantitative methods for describing priming effects and thei r dynamics using C-14 and N-15 isotopes, as well as for non-isotopic studie s are proposed. (C) 2000 Elsevier Science Ltd. All rights reserved.