Marius Tuyishime

Phosphorus in managed forest soils: effect of whole-tree harvesting, ash fertilization and climate change

Background

Forests cover more than half (57%) of the total area in Sweden. Even though phosphorus (P) is a critical element for forest ecosystem functions, the chemistry of P is not well known. One reason is that nitrogen (N) is most often found to be the most important limiting element, and thus the P cycle has been largely overlooked. Today’s N deposition with forest biomass harvest will lead to a successive depletion of bioavailable P. Global warming also has an effect on apatite weathering which is considered as the major host phase for P in Swedish forest subsoils, thus increasing P leaching. Therefore, research on P in forest soils deserves more attention. From environmental perspectives, there is a need to understand the P chemistry in Swedish forest soils that have been lacking to date. This is important to predict P availability and dynamics in response to the present forest management practices as well as climate change.

Aim

The purpose of the PhD project is to generate a comprehensive knowledge on P status in the forest soil and assess how whole-tree harvesting, ash fertilization and climate change affect P speciation and solubility. Previous research has shown that organic P predominates P speciation in the O horizon while P adsorbed/bound to Al/Fe minerals and apatite bound P dominates in the B and C horizons respectively. We will test this hypothesis on different Swedish forest soils covering a variation in soil properties and climate. The data on P speciation in managed forest will be important to assess the reactivity of apatite and to improve the modelling of P balance and dynamics in response to forest management and climate change.

Methodology

Different forest soil profiles are used to encompass a variation in soil properties and climate of Sweden. Experimental methods of this doctoral project include X-ray Absorption Near-Edge Structure (XANES) spectroscopy at the K-edge of P, batch experiments, and chemical soil extractions. For P prediction, we will run simulations using existing modelling tool HD-MINTEQ (Löfgren et al. 2017; Gustafsson et al. 2018) which unites the chemical equilibrium code Visual MINTEQ (Gustafsson et al. 2018). We will incorporate P chemical speciation information as obtained from this project as well as other relevant processes such as plant uptake, atmospheric deposition and mineralization from existing literatures.