Contribution of fine tree roots to the silicon cycle in a temperate forest ecosystem developed on three soil types
Authors: Turpault Marie-Pierre , Calvaruso Christophe , Kirchen Gil , Redon Paul-Olivier , Cochet Carine 
Published: April 2018

Abstract. The role of forest vegetation in the silicon (Si) cycle has been widely
examined. However, to date, little is known about the specific role of fine
roots. The main objective of our study was to assess the influence of fine
roots on the Si cycle in a temperate forest in north-eastern France. Silicon
pools and fluxes in vegetal solid and solution phases were quantified within
each ecosystem compartment, i.e. in the atmosphere, above-ground and below-ground
tree tissues, forest floor and different soil layers, on three plots, each
with different soil types, i.e. Dystric Cambisol (DC), Eutric Cambisol (EC) and Rendzic Leptosol (RL). 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 compositions and
management. Soil solutions were measured monthly for 4 years to study the
seasonal dynamics of Si fluxes. A budget of dissolved Si (DSi) 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. Due to the abundance of fine roots mainly in the superficial soil
layers, their high Si concentration (equivalent to that of leaves and 2
orders higher than that of coarse roots) and their rapid turnover rate
(approximately 1 year), the mean annual Si fluxes in fine roots in the
three plots were 68 and 110 kgha-1yr-1 for the RL and
the DC, respectively. The turnover rates of fine roots and leaves were approximately
71 and 28 % of the total Si taken up by trees each year,
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 DC compared to the
Si-poor RL. In terms of the DSi budget, DSi production was large in the
three plots in the forest floor (9.9 to 12.7 kgha-1yr-1), as
well as in the superficial soil layer (5.3 to
14.5 kgha-1yr-1), and decreased with soil depth. An
immobilization of DSi was even observed at 90 cm depth in plot DC
(−1.7 kgha-1yr-1). The amount of Si leached from the soil
profile was relatively low compared to the annual uptake by trees (13 %
in plot DC to 29 % in plot RL). The monthly measurements demonstrated
that the seasonal dynamics of the DSi budget were mainly linked to biological
activity. Notably, the peak of dissolved Si production in the superficial
soil layer occurred during winter and probably resulted from fine-root
decomposition. Our study reveals that biological processes, particularly
those involving fine roots, play a predominant role in the Si cycle in
temperate forest ecosystems, while the geochemical processes appear to be
limited.