Drivers of volatile reactive soil emissions in the boreal forest

To study the drivers of volatile organic compound (VOC) emissions from boreal forest soil emissions, a study was conducted at two long-term and well-documented experiments in Finland. The first experiment, located in Karkkila (60.577°N, 24.261°E), is a spruce dominated stand with a nitrogen (N) fertilization experiment. The second experiment is in Taivalkoski (65.316°N, 28.161°E), where a tree species experiment is taking place with individual plots on which either silver birch, Scots pine, or Norway spruce were grown on originally similar soil.

Between May and October 2023, we collected soil VOC emissions on multi-bed adsorbent tubes at three locations in each plot (control and N-fertilized in Karkkila, and different tree species in Taivalkoski) about once a month, using an enclosure put on metallic frames placed at each sampling location at least one month before the first sampling. The collected emission samples were analysed with a thermal desorption gas chromatograph coupled to mass spectrometry (TD-GC-MS). The conditions both inside and outside the enclosure were recorded during sampling. Furthermore, the soil moisture was measured, and vegetation was visually assessed after each sampling. In addition, 15 to 20 soil cores were taken by a soil auger around 1-2m of each VOC sampling location during the early growing season, and the organic layer separated. Organic layer samples were combined to make one composite sample for each sampling location. Pretreatments and analysis were performed as described in Soronen et al. (2024) to derive soil parameters, including microbial properties. We also used a microdialysis sampling technique to determine induced diffusive fluxes of plant-available N compounds in the organic layer (Soronen et al., 2024). Fluxes were measured twice during the growing season at each site (early and late season).

For a given plot, emissions collected on the same day show similar composition with varying quantities. However, seasonal variations influenced the emissions’ composition, reflecting various direct and indirect underlying processes. Monoterpenes usually dominated the emissions, but chloroform was also emitted, especially from the N-fertilized plot. The relationships between stand properties, soil properties, environmental conditions, and VOC emissions were explored using multiple linear regression. We found that individual compounds are affected differently, emphasizing the importance of speciation. In general, the dominant tree species, moss cover, and shrub cover appear to be the vegetation factors most influencing VOC emissions. From soil properties, increasing dissolved organic carbon, organic matter, and the microbial biomass C-to-N ratio increased BVOC emissions.

Rising global temperatures lead to increased biomass in boreal areas (including litter and organic soil matter), an extended growing season, and heat stress. These alterations can affect vegetation, soil microbes, and forest floor plants, potentially causing changes in VOC emissions. The analysis presented here could be used in models of biogenic VOC emissions from boreal forest soils to investigate future scenarios.

Reference:

Soronen, P., Henttonen, H.M. and Smolander, A. (2024). Grey alder at the regeneration stage: Long-term effects on soil nitrogen and carbon pools and Norway spruce growth. Forest Ecology and Management, 554, 121686. doi:10.1016/j.foreco.2023.121686.