Consequences of timber harvesting techniques on soil nitrous oxide fluxes of temperate forest soils susceptible to compaction

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200212152

Under natural conditions temperate forests are a sink for CO2 and methane while soil emissions of nitrous oxide (N2O) are low. Rewetting events and freeze-thaw cycles are known triggers for N2O emission. However, ground-based harvesting systems disturb soil conditions and can significantly increase soil N2O emissions. Logging causes severe soil compaction, soil displacement, and incorporation of organic matter (OM). These changes in soil optimize denitrification conditions. Incorporated OM in the soil matrix delivers carbon and nitrogen compounds, while oxygen gets limited due to reduced soil porosity and gas diffusivity. Soil disturbance is particularly strong and long-lasting in compaction-prone silty and loamy soils. In an empirical study, we compare the effects of harvester-forwarder with and without tracks (HF/HFt) and cable-yarding with motor-manual-felling (CMM) on the microbial community (MBC), nitrogen balance, and N2O fluxes. N2O soil flux rates are measured with a trace gas analyzer (Li-Cor), either manually at the recently thinned stands, or continuously at plots that were thinned in 2016. Changes in the MBC were determined by phospholipid fatty acids and microbial biomass analysis. Supplementary soil analyses provide estimates for nitrogen availability and turnover. Results show a significant impact the applied harvesting techniques on soil N2O fluxes of both skid trails and CCM corridors, compared to the control plots at the thinned stand. Our findings indicate that OM translocation and aggregation play a key role in the changes of soil N2O fluxes. In an irrigation experiment, CCM sections with high amounts of particular OM showed greater N2O fluxes than HFt skid trails. Areas with loose topsoil structure showed a tendency for elevated N2O emissions after rewetting. In a transect across a skid trail measured one day after precipitation, N2O fluxes were 2-fold higher in the middle mound compared to the ruts. Our findings underline that soil compaction is a central concern, but it is crucial to consider logging induced soil displacement and OM incorporation in research for sustainable management methods.
Keywords: Temperate Forests, Harvesting Techniques, N2O Fluxes, Microbial Community, Soil Disturbance