Assessing silicon’s role in leaf-litter decomposition, carbon and nitrogen cycling across microclimates in temperate beech forests

Leaf-litter decomposition is a key driver of carbon (C) and nutrient cycling in terrestrial ecosystems, governed by climate and the litter chemical composition. Silicon (Si), a ubiquitous element in terrestrial ecosystems that has various beneficial effects on plants, is an integral component of leaf-litter. However, the relationship between leaf-litter decomposition and Si content remains poorly constrained, particularly in temperate beech forests where Si uptake predominantly occurs through passive mechanisms. Here, we investigated the relationships between total beech-leaf Si content, mass loss, decomposition rate (k), and contents of C and nitrogen (N) of beech (Fagus sylvatica) leaf-litter under five temperate beech-forest stands with differing microclimatic conditions, in two sandstone regions of Lower Saxony, central Germany.

We incubated 441 leaf-litter bags that were sampled bi-monthly over two years to capture fine-scale temporal decomposition dynamics. Each site was equipped with soil temperature and moisture loggers, allowing differentiation of the sites into three microclimatic classes: (i) warm-dry (14.5°C mean topsoil (0-6 cm) temperature, 21% mean soil moisture), (ii) intermediate (13.2°C, 31.4%), (iii) cool-wet (9.7 °C, 38.8%).

The median total Si content of beech leaves across all sites was 1.1% dw^-1, comparable to the 1.0% dw^-1 observed in unincubated leaf-litter samples. Decomposition rate (k) was positively related to Si content under intermediate (R² = 0.14, p < 0.05) and warm-dry (R² = 0.2, p < 0.05) conditions, whereas no such relationship was observed under cool-wet conditions. Median k values were noticeably higher under both cool-wet and intermediate conditions (0.31 g yr^-1) compared to warm-dry conditions (0.18 g y^-1). There was no relationship between Si and C content, but N content exhibited a weak but positive correlation with Si under all climate conditions, with the strongest relationship observed under warm-dry conditions (R²= 0.21, p < 0.05). Over the two-year study, C content decreased from an initial 49% to 41% under intermediate conditions, representing only 16.8% decrease, while N content increased from 0.9% to 1.42% under the same conditions. During mass loss for the same period, Si and N contents increased while C content decreased across all sites. These findings reflected immobilization of N by microbes, but a release of C with mass loss.

The stability of Si content over time, along with the positive relationship to mass loss, suggests that the total Si pool of beech leaves predominantly comprises structured opal compounds that resist mineralization under temperate forest conditions. While this has theoretical implications for linking the Si cycle to C sequestration, the weak relationship observed here, coupled with the decreased C content, suggest further investigation. We conclude that Si influences litter decomposition in a context-dependent manner, with stronger effects under drier and warmer conditions, where soil moisture limitations may intensify its role in C and nutrient cycling.