The effect of single tree species on soil microbial activities related to C and N cycling in the Siberian artificial afforestation experiment

The effects of grassland conversion to forest vegetation and of individual tree species on microbial activity in Siberia are largely unstudied. Here, we examined the effects of the six most commonly dominant tree species in Siberian forests (Scots pine, spruce, Arolla pine, larch, aspen and birch) on soil C and N mineralization, N₂O-reduction and N₂O production during denitrification 30 years after planting. We also documented the effect of grassland conversion to different tree species on microbial activities at different soil depths and their relationships to soil chemical properties. The effects of tree species and grassland conversion were more pronounced on N than on C transformations. Tree species and grassland conversion did significantly alter substrate-induced respiration (SIR) and basal respiration, but the differences were not as large as those observed for N transformations. Variances in SIR and basal respiration within species were markedly lower than those in N transformations. Net N mineralization, net nitrification, and denitrification potential were highest under Arolla pine and larch, intermediate under deciduous aspen and birch, and lowest beneath spruce and Scots pine. Tree species caused similar effects on denitrification potential, net N mineralization, and net nitrification, but effects on N₂O reduction rate were idiosyncratic, indicating a decoupling of N₂O production and reduction. We predict that deciduous species should produce more N₂O in the field than conifers, and that Siberian forests will produce more N₂O if global climate change alters tree species composition. Basal respiration and SIR showed inverse responses to tree species: when basal respiration increased in response to a given tree species, SIR declined. SIR may have been controlled by NH₄ ⁺ availability and related therefore to N mineralization, which was negatively affected by grassland conversion. Basal respiration appeared to be less limited by NH₄ ⁺ and controlled mostly by readily available organic C (DOC), which was higher in concentration under forests than in grassland and therefore basal respiration was higher in forested soils. We conclude that in the Siberian artificial afforestation experiment, soil C mineralization was not limited by N.     

Denitrification Potential and CO2 Emission in the Northern Forest Soils of the Yenisei Meridian (the Siberian IGBP Transect)

To estimate the probable contribution of northern forest soils to the global budget of greenhouse microgases, the cryogenic soils along the Yenisei meridian have been studied with respect to their potential denitrification and carbon mineralization activities. It is shown that the forest soils of the boreal zone have a high denitrification potential and, under conditions of a high nitrate nitrogen content, may be a source of nitrous oxide emission. A significant correlation is observed between N₂O and CO₂ emissions (r = 0.85, p < 0.001).     

Tree species mediated soil chemical changes in a Siberian artificial afforestation experiment

Natural and human-induced changes in the composition of boreal forests will likely alter soil properties, but predicting these effects requires a better understanding of how individual forest species alter soils. We show that 30 years of experimental afforestation in Siberia caused species-specific changes in soil chemical properties, including pH, DOC, DON, Na⁺, NH₄ ⁺, total C, C/N, Mn²⁺, and SO₄ ^2-. Some of these properties –- pH, total C, C/N, DOC, DON, Na⁺ –- also differed by soil depth, but we found no strong evidence for species-dependent effects on vertical differentiation of soil properties (i.e., no species × depth interaction). A number of soil properties –- NO₃ ^–, N, Al³⁺, Ca²⁺, Fe³⁺, K⁺, Mg²⁺ and Cl^– –- responded to neither species nor depth. The six studied species may be clustered into three groups based on their effects on the soil properties. Scots pine and spruce had the lowest pH, highest C/N ratio and intermediate C content in soil. The other two coniferous species, Arolla pine and larch, had the highest soil C contents, highest pH values, and intermediate C/N ratios. Finally, the two deciduous hardwood species, aspen and birch, had the lowest C/N ratio, intermediate pH values, and lowest C content. These tree-mediated soil chemical changes are important for their likely effects on soil microbiological activities, including C and N mineralization and the production and consumption of greenhouse gases.     

Effect of Siberian tree species on N2O production and consumption

The effect of six Siberian tree species on two stages of denitrification—N₂O production and consumption—was studied. Broadleaf species (aspen and birch) proved to have lower rates of N₂O consumption compared to coniferous species. The factors influencing production and consumption of N₂O were also evaluated. The replacement of coniferous forests with broadleaf trees will double the N₂O/N₂ ratio in the denitrification end-products. Doubled N₂O emission from Siberian forest soils to the atmosphere can be expected due to changes in tree species composition of forest ecosystems even without considering changes in water and temperature regimes in soil.