Abstract
Warming will likely stimulate Arctic primary production, but also soil C and N mineralization, and it remains uncertain whether the Arctic will become a sink or a source for CO2. Increasing insect herbivory may also dampen the positive response of plant production and soil C input to warming. We conducted an open-air warming experiment with Subarctic field layer vegetation in North Finland to explore the effects of warming (+3°C) and reduced insect herbivory (67% reduction in leaf damage using an insecticide) on soil C and N dynamics. We found that plant root growth, soil C and N concentrations, microbial biomass C, microbial activity, and soil NH4+availability were increased by both warming and reduced herbivory when applied alone, but not when combined. Soil NO3–availability increased by warming only and in-situ soil respiration by reduced herbivory only. Our results suggest that increasing C input from vegetation under climate warming increases soil C concentration, but also stimulates soil C turnover. On the other hand, it appears that insect herbivores can significantly reduce plant growth. If their abundance increases with warming as predicted, they may curtail the positive effect of warming on soil C concentration. Moreover, our results suggest that temperature and herbivory effects on root growth and soil variables interact strongly, which probably arises from a combination of N demand increasing under lower herbivory and soil mineral N supply increasing under higher temperature. This may further complicate the effects of rising temperatures on Subarctic soil C dynamics.
Highlights
The Arctic is predicted to experience rising temperatures of 4–7◦C by the end of this century (IPCC, 2014)
The fate of the C sink in the Arctic will depend on how the balance between C accrual through plant production and C loss through soil organic matter (SOM) decomposition will change under a warming climate
Using an open-air warming experiment established in a mountain birch forest in northern Finland (Silfver et al, 2020), we investigated the responses of field layer vascular plant shoot and root growth, soil microbial biomass and activity, and soil C and N concentrations and mineralization to warming and reduced insect herbivory
Summary
The Arctic is predicted to experience rising temperatures of 4–7◦C by the end of this century (IPCC, 2014). This may increase plant production (Pouliot et al, 2009; Beck and Goetz, 2011; Natali et al, 2011; Elmendorf et al, 2012) and advance the Subarctic tree line (Harsch et al, 2009) as these are generally assumed to be suppressed by low temperatures (Körner and Paulsen, 2004). The fate of the C sink in the Arctic will depend on how the balance between C accrual through plant production and C loss through soil organic matter (SOM) decomposition will change under a warming climate. Some studies suggest that the Arctic will become a stronger C sink (Sistla et al, 2013; Silfver et al, 2020) while others suggest it will become a C source (Oechel et al, 1993; Lundin et al, 2016)
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