Biogenic and non-biogenic Si pools in terrestrial ecosystems: results from a novel analysis method.

Date: 9 March 2015

Venue: UAntwerp - Campus Groenenborger - Lokaal V0.08 - Groenenborgerlaan 171 - 2020 Antwerp

Time: 3:00 PM

Organization / co-organization: Department of Biology

PhD candidate: Ana Lucia BARAO

Principal investigator: Patrick Meire & Eric Struyf

Short description: PhD defense Ana Lucia BARAO - Department of Biology



Abstract

Silicon (Si) is a chemical element highly abundant in the Earth crust. The weathering of silicate minerals over millions of years provides dissolved Si (DSi) that once reaching the oceans is used for the frustules of the biogenic organisms - diatoms (BSi). Terrestrial systems, where this DSi can be taken up by plants and be deposited in plant cells in a biogenic form (BSi), is a buffer system triggering the availability of DSi downstream and affecting the soil dynamics. However, soil mineral heritage and pedogenic processes ensure that a myriad of different non-biogenic Si fractions (non-BSi) are present with potentially influence for the Si cycle in the short term.  The majority of methods widely used nowadays to measure BSi were initially developed for aquatic systems where diatom concentration is higher than the non-BSi contribution and as so, a proper separation is not a current practise.

In this thesis, we used a laboratorial and analysis method that separates BSi from non-BSi fractions and we developed it for terrestrial systems. We applied this new approach on: 1) a land to ocean continuum of samples, to assess where the separation of BSi is more important and the implications for studies targeting the BSi; 2) land use gradients, to understand the human influence on non-BSi pools and 3) tephra deposits and lake sediments subjected to volcanic interference, to provide a solution for the separation especially in studies where BSi is used as a proxy for primary production but the contribution of reactive volcanic Si is significant.

The results show that deeper soil horizons, river and coastal oceanic sediments are more likely to contain higher amounts of non-BSi fractions. Moreover, in soils, clay minerals with different weathering degrees may even overprint the BSi influence. Cultivation of soils also alters the distribution of Si pools, depleting BSi through crop harvest. However, non-BSi pools may also compensate this effect, although also intrinsically dependent on the climate, and soil parent material and weathering degree. Volcanic interference showed to be critical when measuring BSi with l direct and indirect influences on studies where BSi is a proxy for biological effect.