Review Review of mechanisms and quantification of priming eÄects

Priming eAects are strong short-term changes in the turnover of soil organic matter caused by comparatively moderate treatments of the soil. In the course of priming eAects large amounts of C, N and other nutrients can be released or immobilized in soil in a very short time. These eAects have been measured in many field and laboratory experiments; however, only a few of the studies were aimed at an extended investigation of the mechanisms of such phenomena. The aim of this overview is to reveal possible causes and processes leading to priming actions using the references on agricultural ecosystems and model experiments. Multiple mechanisms and sources of released C and N are presented and summarized in Tables for positive and negative real and apparent priming eAects induced after the addition of diAerent organic and mineral substances to the soil. Soil microbial biomass plays the key role in the processes leading to the real priming eAects. The most important mechanisms for the real priming eAects are the acceleration or retardation of soil organic matter turnover due to increased activity or amount of microbial biomass. Isotopic exchange, pool substitution, and diAerent uncontrolled losses of mineralized N from the soil are responsible for the apparent N priming eAects. Other multiple mechanisms (predation, competition for nutrients between roots and microorganisms, preferred uptake, inhibition, etc.) in response to addition of diAerent substances are also discussed. These mechanisms can be distinguished from each other by the simultaneous monitoring of C and N release dynamics; its comparison with the course of microbial activity; and by the labelling of diAerent pools with 14 Co r 13 C and 15 N. Quantitative methods for describing priming eAects and their dynamics using 14 C and 15 N isotopes, as well as for non-isotopic studies are