Epsilon Open Archive

Abstract

In boreal and temperate forests, long-term elevated nitrogen (N) load may eventually saturate forest ecosystems with N, i.e. total N ecosystem input exceed ecosystem sinks for N, and N losses via soil water transport may then increase and negatively impact environmental quality.

This thesis is based upon four studies (reported in papers I-IV), and the overall aims were to assess and analyse effects on soil water N in coniferous forests of two types of anthropogenic disturbance: “chemical disturbance” (long-term experimental N addition and N deposition), and “physical disturbance” (clear-cutting and subsequent soil scarification). Effects of these disturbances were addressed in both field experiments and process-based ecosystem modelling. In the field experiments, soil water N was collected from both organic (O) horizons and mineral soil, at 0.5 m depth, during several growing seasons to assess temporal variation in the N concentration (Paper I). In addition, microbial variables in soil samples of the O-horizon were analysed in the laboratory to assess responses of the soil microbial community to long-term N addition in forest experiments and along a N deposition gradient (Papers II and IV). In the modelling, a process-based ecosystem carbon and N model (CoupModel) was calibrated to measurements obtained during the regeneration phase of a Scots pine (Pinus sylvestris L.) forest in an N fertilization experiment where soil scarification was applied (Paper III).

The results showed that long-term N addition to a boreal Norway spruce (Picea abies (L.) Karst) forest can alter the quantity and seasonal dynamics of dissolved organic nitrogen (DON) concentrations in soil water collected from the O-horizon. However, DON concentrations were low in soil water collected from mineral soil under all N treatments and probably only contributed to small net N losses in this forest. Although microbial variables of the O-horizon were affected by N loading they were similar under N loading that resulted in the leaching of small amounts of nitrate (<2 kg ha⁻¹ year⁻¹ of NO₃-N) and those that resulted in the leaching of large amounts (>15 kg ha⁻¹ year⁻¹ of NO₃-N). Further, soil scarification increased soil water N leaching from a Scots pine forest, as calculated with the CoupModel, during the regeneration phase, particularly in previously N-fertilized pine stands.