Si cycling in a forest biogeosystem – the importance of transient state biogenic Si pools

J. Breuer,D. Barkusky,M. Sommer,D. Kaczorek,D. Puppe,M. Wanner,A. Höhn,K. Meier,J. Busse,Z. Zagorski,H. Jochheim

doi:10.5194/bg-10-4991-2013

Abstract

Abstract. The relevance of biological Si cycling for dissolved silica (DSi) export from terrestrial biogeosystems is still in debate. Even in systems showing a high content of weatherable minerals, like Cambisols on volcanic tuff, biogenic Si (BSi) might contribute > 50% to DSi (Gerard et al., 2008). However, the number of biogeosystem studies is rather limited for generalized conclusions. To cover one end of controlling factors on DSi, i.e., weatherable minerals content, we studied a forested site with absolute quartz dominance (> 95%). Here we hypothesise minimal effects of chemical weathering of silicates on DSi. During a four year observation period (05/2007–04/2011), we quantified (i) internal and external Si fluxes of a temperate-humid biogeosystem (beech, 120 yr) by BIOME-BGC (version ZALF), (ii) related Si budgets, and (iii) Si pools in soil and beech, chemically as well as by SEM-EDX. For the first time two compartments of biogenic Si in soils were analysed, i.e., phytogenic and zoogenic Si pool (testate amoebae). We quantified an average Si plant uptake of 35 kg Si ha−1 yr−1 – most of which is recycled to the soil by litterfall – and calculated an annual biosilicification from idiosomic testate amoebae of 17 kg Si ha−1. The comparatively high DSi concentrations (6 mg L−1) and DSi exports (12 kg Si ha−1 yr−1) could not be explained by chemical weathering of feldspars or quartz dissolution. Instead, dissolution of a relictic, phytogenic Si pool seems to be the main process for the DSi observed. We identified canopy closure accompanied by a disappearance of grasses as well as the selective extraction of pine trees 30 yr ago as the most probable control for the phenomena observed. From our results we concluded the biogeosystem to be in a transient state in terms of Si cycling.

Highlights

In recent years our understanding of Si cycling in different terrestrial biogeosystems has been improved substantially
Throughfall and stemflow can be neglected in terms of Si fluxes
The small annual increase in Si stored in the vegetation is lower compared to reported data (European beech = 3.5 kg Si ha−1 yr−1, Cornelis et al, 2010a) which might be explained by the lower forest growth of our mature beech stand (120 yr)

Summary

Introduction

In recent years our understanding of Si cycling in different terrestrial biogeosystems has been improved substantially. Research on steppe (Blecker et al, 2006; Borrelli et al, 2010, White et al, 2012), savannah (Melzer et al, 2010, 2012; Alexandre et al, 2011), forest (Gerard et al, 2008; Cornelis et al, 2010a, 2011a) and wetland biogeosystems (Struyf and Conley, 2009; Struyf et al, 2009) enhanced our understanding of the “plant factor”, i.e., the importance of the biological Si cycling in terrestrial biogeosystems which had been studied in tropical and temperate climates so far (Sommer et al, 2006). The results of these studies challenge the steady state assumption implied in most studies on Si budgets

Methods

Results

Discussion

Conclusion