Linkages of Nitrogen and Phosphorus Availability to Ecosystem Processes and Succession in Forests of Northern Sweden and New Zealand

Initially after catastrophic disturbance ecosystems undergo a build-up phase but in the long term absence of catastrophic disturbance ecosystems can enter a phase of decline called ecosystem retrogression. It is characterised by reduced productivity, decomposition rates and rates of nutrient cycling as a consequence of nutrient limitation. Over the course of long-term soil development, phosphorus (P) availability decreases due to leaching and chemical immobilisation, leading to increased soil N: P ratios during retrogression. In contrast, nitrogen (N) is continuously supplied to natural ecosystems through biological N2 fixation, though late successional plant species can have a negative effect on N availability during retrogression, reinforcing N limitation. I studied variation in supply and availability of N and P throughout a 5000 year retrogressive chronosequence in which the soil N: P ratio increases, and investigated how it affected the nutrient status of soil microbes and plants. I found that total N increases considerably during retrogression as a consequence of increased N2 fixation. Total P did not change across the chronosequence but labile mineral P declined in the retrogressive stage and this was connected to decreased soil microbial activity. Plant nutrient status showed diverging trends between trees and dwarf shrubs across the chronosequence, indicating increasing resource partitioning as retrogression proceeds. Variation in leaf traits was due to shifting species composition rather than within species changes. I also studied factors influencing the availability of N and P during the build-up phase of succession as affected by an important extrinsic driver, i.e., herbivory. Herbivory of a N2 fixing shrub had considerable negative effects on the nutrient status of not only its own growth and nutrient status, but also that of neighbouring non-fixing plants and soil processes. These studies in combination have demonstrated how changing availability of nutrients during succession, both in the build-up phase and the retrogressive phase, can be driven by biological processes such as N2 fixation and species effects on carbon (C) quality and, how this in turn leads to differences in the relative success of coexisting plant species in the community. As such, these results demonstrate the dynamic and variable nature of nutrient limitation and the processes leading to it.