The impact of trees on soil organic carbon dynamics in the Subarctic - Priming effects and microbial N mining

Climate warming and increased nitrogen deposition are inducing an advance of the Subarctic treeline. Yet, also outbreaks of foliage-feeding geometrid moths became more frequent over the past decades, often resulting in forest dieback. This calls for a better understanding of the role of living, dead, or absent trees in soil organic carbon (SOC) cycling.

It has been shown in several studies that living trees may deplete SOC stocks in comparison with treeless tundra despite C input via litter and roots. This may be explained by a phenomenon termed “priming effect”: labile C supply from root exudates or fresh litter stimulates microbial decomposition of soil organic matter (SOM). It is assumed that microbes increasingly decompose SOM in order to acquire N from it (“microbial N-mining”), which may be relevant especially in N poor soils like in the Subarctic. Reversing this finding, it may be speculated that forest dieback caused by moth outbreaks induces an increase of SOC stocks when root exudation and plant N demand decrease and decomposing wood is returned to the soil.

I will present results from two studies related to the effect of trees and nitrogen availability in the Subarctic. The first is a field study, in which we investigated SOC stocks and biochemical soil properties under living trees, treeless tundra, and under trees that died 12 and 55 years ago during moth outbreaks. The second study is a greenhouse experiment, in which we investigated SOC turnover in the presence and absence of a tree with or without nitrogen addition.

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