The Abundance of Fungi, Bacteria and Denitrification Genes during Insect Outbreaks in Scots Pine Forests

Abstract

Abstract:Outbreaks of defoliating insects may affect microbial populations in forests and thereby mass balances and ecosystem functioning. Here, we investigated the microbial dynamics in Scots pine (Pinus sylvestris L.) forests during outbreaks of the nun moth (Lymantria monacha L.) and the pine-tree lappet (Dendrolimus pini L.). We used real-time PCR (polymerase chain reaction) to quantify genes that characterize bacterial and fungal abundance and the denitrification processes (nirK, nirS, nosZ clades I and II) in different forest compartments and we analyzed the C and N content of pine needles, insect feces, larvae, vegetation layers, organic layers, and mineral soil horizons. The infestation of the nun moth increased the bacterial abundance on pine needles, in the vegetation layer, and in the upper organic layer, while fungal populations were increased in the vegetation layer and upper organic layer during both outbreaks. In soil, the abundance of nirK increased after insect defoliation, while the C/N ratios decreased. nosZ clades I and II showed variable responses in different soil layers and to different defoliating insects. Our results illustrate changes in the microbial populations in pine forests that were infested by defoliating insects and changes in the chemical soil properties that foster these populations, indicating a genetic potential for increased soil N 2 O emissions during the defoliation peak of insect outbreak events.

Conversion of monoculture cropland and open grassland to agroforestry alters the abundance of soil bacteria, fungi and soil-N-cycling genes

“…Standard curves for qPCRs were generated in two replicates using 1:3 serial dilutions in 0.5X TE buffer. Standards of Bacterial 16S rRNA, fungal 18S rRNA, nirK , nirS , nosZ clade I and nosZ clade II genes were obtained as described previously [46]. AOA amoA gene was obtained from an environmental clone, cloned in plasmid pGEM-T (Promega, Mannheim, Germany) and multiplied in Escherichia coli JM109.…”

The Effect of Insect Defoliations and Seed Production on the Dynamics of Radial Growth Synchrony among Scots Pine Pinus sylvestris L. Provenances

“…The defoliation caused by insect outbreaks has reduced the carbon sequestration in cell walls [33], specific leaf area, and water-use efficiency in trees [34], but has also indirectly affected the physiology of trees by disturbing the nitrogen cycling as well as microbial populations in the forest soil [35,36]. Under water shortage, an increasing vulnerability of trees to insect outbreaks has been observed [37][38][39][40].…”

Increased Forest Soil CO2 and N2O Emissions During Insect Infestation

“…The increased soil C/N ratios of the feces treatment in our incubation experiment may therefore be an indicator of microbial immobilization, and this is supported by the relatively slow decrease of the gas emission rates following peak emissions. In contrast, C/N ratios under field conditions are often observed to decrease during insect outbreaks [20,23,24,40], even on our sampling sites [41].…”

“…Further, N 2 O emissions from forest soils are spatial and temporal highly variable ("hot spots" and "hot moments" of N 2 O emissions [48]), which makes measurement and comparability across sites difficult [45]. However, on our infested study site, the abundance of NO 2 reducers (nirK genes) in the soil was also found to be increased [41], indicating a genetic potential for accelerated N 2 O emissions. To our knowledge, we show for the first time that both CO 2 and N 2 O emissions can be triggered simultaneously by organic inputs deriving from pest insects.…”