Silicon cycle in a temperate forest ecosystem: role of fine roots and litterfall recycling and influence of soil types

Abstract. The role of forest vegetation in the silicon (Si) cycle has been widely examined. However, to date, no study has investigated the specific role of fine roots. The main objectives of our study were to assess the influence of fine roots as well as the impact of soil properties on the Si cycle in a temperate forest in northeastern France. Silicon pools and fluxes in solid and solution phases were quantified within each ecosystem compartment, i.e., the atmosphere, aboveground and belowground tree tissues, forest floor, and different soil horizons, on three plots, each with different soil types, i.e., Dystric Cambisol (plot S1), Eutric Cambisol (plot S2), and Rendzic Leptosol (plot S3). In this study, we took advantage of a natural soil gradient, from shallow calcic soil to deep moderately acidic soil, with similar climates, atmospheric depositions, species composition and management. Soil solutions were measured monthly for four years to study the seasonal dynamics of Si fluxes. A budget of dissolved Si was also determined for the forest floor and soil layers. Our study highlighted the major role of fine roots in the Si cycle in forest ecosystems for all soil types. Because of the abundance of fine roots mainly in the superficial soil horizons, their high Si concentration (equivalent to that of leaves and two orders higher than that of coarse roots) and their rapid turnover rate (approximately one year), the mean annual Si fluxes in fine roots in the three plots ranged from 68 to 110 kg ha−1 y−1 for the Rendzic Leptosol and the Dystric Cambisol, respectively. The turnover of fine roots and leaves was approximately 71 % and 28 % of the total Si taken up by trees each year, respectively, demonstrating the importance of biological recycling in the Si cycle in forests. Less than 1 % of the Si taken up by trees each year accumulated in the perennial tissues. This study also demonstrated the influence of soil type on the concentration of Si in the annual tissues and therefore on the Si fluxes in forests. The concentrations of Si in leaves and fine roots were approximately 1.5–2.0 times higher in the Si-rich Dystric Cambisol compared to the Si poor Rendzic Leptosol. In terms of the dissolved Si budget, there were large amounts of dissolved Si in the three plots on the forest floor (9.9 to 12.7 kg ha−1 y−1) and in the superficial soil horizon (5.3 to 14.5 kg ha−1 y−1), and Si decreased with depth in plot S1 (1.7 kg ha−1 y−1). The amount of Si leached from the soil profile was relatively low compared to the annual uptake by trees (13 % in plot S1 to 29 % in plot S3). The monthly measurements demonstrated that the seasonal dynamics of the dissolved Si budget were mainly linked to biological activity. Notably, the peak of dissolved Si production in the superficial soil horizon was during the winter and probably resulted from fine root decomposition. Our study reveals that biological processes, particularly those of fine roots, play a predominant role in the Si cycle in temperate forest ecosystems, while the geochemical processes appear to be limited.