PhD defence: Geshere Abdisa Gurmesa

Geshere Abdisa Gurmesa defends his PhD thesis: 

Topic: Fate of deposited nitrogen in tropical forests in southern China

Supervisor
Professor Per Gundersen, IGN 

Assessment Committee
Professor Lars Vesterdal, IGN (chairman)

Assistant Professor Albert Tietema, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam 

Senior Scientist Marife D. Corre, Soil Science of Tropical and Subtropical Ecosystems, Georg-August University of Goettingen

After the PhD defense there will be a reception at Rolighedsvej 23, in the canteen (common area) on ground floor in the new building.

Summary
Increased atmospheric nitrogen (N) deposition may have several negative effects in tropical forests which are generally regarded as naturally N-rich ecosystems. The effects largely depend on the N retention efficiency of these forests. However, the fate of deposited N in tropical forest ecosystems and its retention mechanisms remain elusive. This PhD thesis used stable N isotope methods to uncover two aspects of N cycling in tropical forests: i) the plant species traits of N uptake and isotope cycling patterns in the ecosystem at ambient and increased N-input (+50 kg N ha-1yr-1) using natural 15N abundance (δ15N); ii) the fate of deposition N at ambient and increased N-input by adding 15N tracer over one year at the ecosystem scale. The study took place in the Dinghushan Biosphere Reserve in southern China, including a N-rich old-growth forest and a previously disturbed pine plantation.

Total ambient N deposition in the study period was 51 kg N ha-1yr-1 and it was dominated by NH4-N from agricultural emissions. Leaching loss (dominated by NO3-N) was similar to the input amount in the old-growth forest, but less in the pine plantation. Plants and soils were more 15N-depleted than in other tropical forests due to an imprint from the high and 15N-depleted N deposition. Despite this strong imprint from deposition, tree species differences in N uptake and cycling was inferred from the isotopic signatures. Although the forests were inherently N-rich (and more so in the old-growth forest) a major uptake and incorporation of deposition N was indicated. 

The fates of deposition N in the old-growth forest were quantified as percent of added 15N recovered in plant and soil pools four months after the last addition and by monitoring leaching loss of 15N in soil water. The results showed that in the short-term deposited N is effectively retained in plant and soil pools resembling the retentions observed for temperate forests, and that increased N input decreased the N retention efficiency, but increased N leaching. The old-growth forest retained more N in both plants and soils than the pine forests although the difference was not significant. Overall, the results indicate that short-term retention mechanisms of deposited N (plant N uptake and soil N retention) are surprisingly similar in N-rich and N-limited forests and across climatic gradients despite major differences in the N-budgets. Thus the differences between N-rich and N-limited forests must occur in the turnover and N cycling rates that remain to be evaluated in long-term studies of the fate of deposition N in these tropical forests.

The thesis is available from the PhD administration office 04.1.417