Københavns Universitet
Uafhængig af ledelsen


PhD defence: Bingru Wang

Ph.d.-forsvar — Bingru Wang defends her thesis, "Afforestation and tree species choice affect carbon, nitrogen, phosphorus dynamics in soil-microbe-root system"


Date & Time:

Auditorium Landskab, Rolighedsvej 23, 1958 Frederiksberg

Hosted by:
Section of Forest and Landscape Ecology


Bingru Wang defends hers thesis,

Afforestation and tree species choice affect carbon, nitrogen, phosphorus dynamics in soil-microbe-root system

Professor Inger Kappel Schmidt, IGN
Professor Lars Vesterdal, IGN

Assessment Committee:
Professor emeritus Heljä-Sisko Helmisaari, University of Helsinki
Professor Håkan Wallander, Lund University
Professor Per Gundersen (chair), IGN

Afforestation of former arable land and choice of tree species change nutrient cycling. Anthropogenic activities have increased the atmospheric deposition of nitrogen and agricultural soils have been enriched with nutrients due to fertilization, whereas forest management with intensive biomass harvest has removed nutrients. The attention on phosphorus (P) is increasing because P is scarce and an unrenewable resource. All factors are expected to change the amount and composition of nutrients in the soil. This PhD project aims at investigating how afforestation and tree species choice affect nitrogen (N) and P and their stoichiometry in the soil-microbes-root system, specifically, focusing on how afforestation chronosequences, tree species and mycorrhizal association affect N and P content and availability in soil and microbes. Further, the PhD investigates the effect of tree species on root dynamics.
I used unique long term field experiments and laboratory analysis focusing on soil and root sampling at a five-decades long afforestation site with three tree species chronosequences (Fagus sylvatica L., Quercus robur L., Picea abies (L.) Karst.), and a common garden trial consisting of six sites with six temperate European tree species (Fraxinus excelsior L., ., Acer pseudoplatanus L.,( (arbuscular mycorrhizal (AM) association), Fagus sylvatica L., Tilia cordata L., Quercus robur L., Picea abies (L.) Karst. (ectomycorrhizal (EcM) association)) in Denmark. In laboratory, I measured soil pH, carbon (C), N and P concentration in soil, microbes and roots, soil enzyme activities, organized data of fine root biomass, production and decomposition. Based on these, we found that total P in soil had a declining trend, while labile P decreased significantly along afforestation chronosequence. Microbial biomass N and P became increasingly constrained by nutrients along the afforestation age, particularly evident in deciduous trees like beech and oak. In the common garden study, beech and spruce stands had lower concentrations of microbial biomass C, N and P compared to other species. Spruce stands displayed lower soil pH, acid phosphatase activity, but higher levels of labile P compared to other species. Tree species associated with AM fungi showed higher β-glucosidase activity and lower C:N and C:P ratios in both soil and microbes, in contrast to species associated with EcM fungi. Root CNP stoichiometry showed significant variation across tree species, however, the variation cannot be summarized as mycorrhizal association effect. In the root study, we found greater root biomass and slower root decomposition rates of AM associated tree species, which may contribute to larger soil C stocks compared to EcM associated tree species. However, this effect could be offset by lower root productions rate.
This PhD study highlights the important role of mycorrhizal association in CNP stoichiometry of soil and microbes, fine root biomass and decomposition. Furthermore, this study emphases the significance to include soil-microbes-root as an organic complex regarding studies of nutrients and their dynamics.