Abstract
Forest soils are major sinks of terrestrial carbon, but this function may be threatened by mass outbreak events of forest pests. Here, we measured soil CO2-C and N2O-N fluxes from a Scots pine (Pinus sylvestris L.) forest that was heavily infested by the nun moth (Lymantria monacha L.) and an adjacent noninfested (control) forest site during one year. In the infested forest, net emissions of CO2-C were higher during main defoliation, summer and autumn, while indications of increased N2O-N emissions were found at one sampling date. On basis of this, a microcosm incubation experiment with different organic matter treatments was conducted. Soil treatments with needle litter, insect feces plus needle litter, and insect feces showed 3.7-, 10.6-, and 13.5-fold higher CO2-C emissions while N2O-N of the insect feces plus needle litter, and insect feces treatment was 8.9-, and 10.4-fold higher compared with soil treatments without added organic matter (control). Hence, the defoliation in combination with high inputs of organic matter during insect outbreaks distinctly accelerate decomposition processes in pine forest soils, which in turn alters forests nutrient cycling and the functioning of forests as carbon sinks.
Highlights
The worldwide forest area affected annually by insect outbreaks amounts about 36.5 million hectares [1], thereby representing a thread to the function of forests as a carbon sink [2,3,4,5].Insect defoliation leads to decreased canopy biomass, tree growth, and inhibits the production new foliage, but can be accompanied by changes in the N nutritional status of infested trees [6,7]
Scots pine forests infested with the nun moth showed increased soil emissions of CO2 -C during several sampling dates and indications of increased N2 O-N emissions at one date, which both may be related to the altered quality and quantity of organic inputs during the pest outbreak
We show for the first time that both CO2 and N2 O emissions can be triggered simultaneously by organic inputs deriving from pest insects
Summary
The worldwide forest area affected annually by insect outbreaks amounts about 36.5 million hectares [1], thereby representing a thread to the function of forests as a carbon sink [2,3,4,5].Insect defoliation leads to decreased canopy biomass, tree growth, and inhibits the production new foliage, but can be accompanied by changes in the N nutritional status of infested trees (e.g., decreased net N uptake, N accumulation in fine roots and needles) [6,7]. Defoliation of forest areas during forest pest outbreaks distinctly increase organic input into the soil in the form of insect feces, cadavers, litter, and other plant material [7,8,9]. Such changes of litter quality and quantity affect the soil organic matter composition [10], and alter nutrient cycling [11,12,13]. Ponderosa pine (Pinus ponderosa Laws.) stands infested by different bark beetle species (Scolytinae) [21] as well as temperate mixed forests infested by the emerald ash borer (Agrilus planipennis F.) [22] showed no effect on soil CO2 emissions.
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