Impact of nitrogen deposition on carbon stocks in coniferous forest soils

Nutrient limitations constrain the capacity for northern coniferous forests to capture and store carbon (C). Nitrogen (N) deposition may enhance tree productivity in these forests, but it remains uncertain how the storage of C in soils is responding to changes in N availability, and how limitations by other elements, such as phosphorus (P), develop in response to N enrichment. In this thesis, I present results from five studies on C cycling in long-term N addition experiments in a Scots pine forest (study I-III) and a Norway spruce forest (study IV-V) in northern Sweden. In these forests, N has been added at low (≤12.5 kg N ha-1 yr-1 ) and high (50 kg N ha-1 yr-1 ) rates for up to 20 years to simulate N deposition. In the Scots pine forest, I showed that the highest N addition rate increased soil C stocks in the organic layer by 22.3 kg C kg-1 N added (study I), which supports the view that N enrichment can increase the C sink of northern coniferous forests. In study I and II I showed that N additions increased the input of C by both aboveground litter and fineroot litter, which coupled to a decrease in autotrophic soil respiration contributes to soil C accumulation. Study II and III further showed that the increase in fine-root production occurred without a concomitant increase in tree-root associated microbiota, indicating that N enabled higher retention of C in fine-root biomass production. In the Norway spruce forest, study IV showed that N decreased the activity of enzymes in the soil involved in the degradation of recalcitrant organic matter, and organic N uptake, and increased the activity of enzymes involved in the degradation of simple carbohydrates, whereas enzyme activity targeting P was unaffected. Study V further showed that N had no effect on the mobilization of P in soil organic matter or on the fungal exploration of P rich apatite minerals in the soil. In both forests, N changed the composition of the microbial community, which is likely to impact soil respiration and decomposition. However, more studies are needed on the links between N availability and soil microbial activity. In conclusion, the result presented in this thesis support the view that N deposition is enhancing the C sink of N poor boreal forests, however at a rate that is too low to make a major contribution to the global land C sink.