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The "priming effect": bridging the gap between terrestrial and aquatic ecology

Journal "Ecology" ESA - Ecological Society of America
Bertrand Guenet, Michael Danger, Luc Abbadie, Gérard Lacroix

The dynamics of organic carbon has been one of the major research topics of soil science for decades.

Organic carbon is indeed a major determinant of soil fertility and is a sink or a source of atmospheric carbon dioxide. The level of carbon in soil depends crucially on two antagonistic processes: the incorporation into the soil of compounds from the photosynthetic activity of plants, and mineralization of these compounds by soil organisms, mostly microorganisms.

The concept of Jenkinson validated

Both processes have long been considered and modeled as processes independent of one another. However, frequent experimental results show increasingly that it is not. The concept of "priming effect" proposed by Jenkinson in 1966 is validated: the incorporation of fresh plant residues, high content of organic compounds of small molecular weight, often leads to activation of soil microorganisms that degrade organic compounds ex-stabilized by their more complex molecular structure. In other words, the incorporation of fresh organic matter in soil, although it tends to increase the content of soil organic carbon, can also lead to the destocking of existing carbon at the point of cancelling the intake or causing a negative balance.

This priming effect occurs when microorganisms have simultaneous access to two sources of organic compounds: relatively simple compounds (sugars or sugar polymers), often poor in nutrients (nitrogen, phosphorus, etc..) and more difficult compounds to degrade, but rich in nutrients. This co-occurrence of two types of organic compounds does not occur in the soil: it also occurs in aquatic environments where rather recalcitrant particles of terrestrial or aquatic origin may come into contact with dissolved organic molecules of plankton origin. This could explain why a significant proportion of organic carbon eroded over the continents is not found in the oceans.

A role in the biosphere-atmosphere interaction

The authors of the paper propose to consider the priming effect as a general process that occurs in all types of continental and oceanic environments. The priming effect would be a major mechanism for regulating the carbon cycle and may play a role in the interactions between the biosphere and atmosphere, or in the functioning of ecosystems. One may therefore conclude that the excretion of simple organic compounds by phytoplankton stimulates by priming effect the production of nutrients from recalcitrant organic matter which is likely to stimulate algal growth. We can also assume that the increase in the incorporation of carbon in the soil (root exudates, leaf litter and dead roots) due to the increased content of CO2 in the atmosphere could negatively impact the stock of carbon in soil and positively impact CO2 emissions from soils.

A key phenomenon of the carbon cycle

This article illustrates the desirability of opening up the disciplines and subdisciplines. It is likely that the priming effect, virtually ignored by the scientific communities studying oceanic or freshwater, is a key phenomenon of the carbon cycle, and secondarily that of many mineral elements such as nitrogen or phosphorus. Its inclusion in all environments seems essential for better forecasting of changes in global biogeochemical cycles and premises under the constraint of global change.

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