Atmospheric nitrogen (N) deposition caused by human activities has increased approximately three − fold over the past 150 years worldwide (Clark and Tilman 2008; Reay et al., 2008). The situation is severe, in particular in tropical and subtropical areas (Galloway et al., 2008; Richter et al., 2005). In the past three decades, due to the combined effects of economic structure adjustment and various environmental control measures, atmospheric N deposition in China has shifted from a rapid growth to a stable state, but nitrate − N deposition continues to increase (Yu et al., 2019), with the highest levels in subtropical regions (Liu et al., 2020). Long − term N deposition has considerable adverse ecological consequences, such as nutrient imbalances (Thayer et al., 2008; Zhang et al., 2017; Mo et al., 2021), soil acidification (Wright et al., 2001; Yang et al., 2012), ion toxicity (Bowman et al., 2008; Tian et al., 2015), and leaching and of soil cations causing cation deficiency (Lu et al., 2018; Du et al., 2020) in forests, further affecting the structure and functioning of forest ecosystems (Tian et al., 2019). Notably, those processes may alter other biogeochemical cycles, such as silicon (Si) cycling (Zhao et al., 2016; Minden et al., 2021). However, the effects of long − term atmospheric N deposition on Si cycling in forest ecosystems are unknown.
Silicon is the second-most-abundant element in soil, accounting for approximately 29% of the soil mass in the Earth’s crust (Wedepohl et al., 1995). There are many forms of Si in soils, such as plant − available Si (dissolved monosilicic acid that can be taken up by plants), adsorbed Si (Si adsorbed on soil particles such as Fe/Mn oxides), non − crystalline Si (including biogenic and non − biogenic amorphous forms) and crystalline Si (Sauer et al., 2006; Cornelis et al., 2010). Uptake of plant − available Si leads to amorhpous silica deposits in plant tissues, as phytoliths (from Greek, meaning plant stone) (Epstein et al., 2009; Debona et al., 2017; Nawaz et al., 2019). Despite being a non − essential nutrient for plants, Si alleviates biotic (e.g., due to herbivores and pathogens) and abiotic stresses (e.g., metal toxicity, water shortage, nutrient limitation) (Debona et al., 2017; Coskun et al., 2019; Bokor et al., 2021). Silicon can also play a biomechanial role in some species, thereby influencing light capture and photosynthesis (de Tombeur et al., 2023). During litter decomposition, phytoliths are eventually returned to the soil, where they dissolve faster than most non − biogenic Si forms (Fraysse et al., 2009, Cornélis and Delvaux 2016). Investigating Si dynamics in plant − soil systems is necessary to better understand both ecological mechanisms (i.e. plant − herbivore interactions) and biogeochemical cycles given the significant interactions between the Si and carbon cycles at different scales (Epstein et al., 2009; Song et al., 2013; Song et al., 2016).
The dynamics of Si in soil − plant systems is affected by both edaphic properties and plant activities (Cornelis et al., 2016; Meunier et al., 2018; de Tombeur et al., 2020a; Puppe et al., 2020). The soil parent material, the degree of soil weathering and the subsequent soil physicochemical properties all affects Si mobility in soil − plant systems (Cornélis and Delvaux 2016; de Tombeur et al., 2020b). Furthermore, plants strongly affect Si biogeochemistry by increasing the rates of silicate weathering, and through the formation of a reactive biogenic Si pool (Alexandre et al., 1997; Fraysse et al., 2009). Recently, a number of interactions between the major nutrients (N and P) and the Si cycle have been suggested. In particular, plant Si uptake increases in situation of N or P stress (Quigley et al., 2020; Minden et al., 2021; de Tombeur et al., 2021a, 2022), and rhizosphere processes may be involved. For instance, in P − poor landscapes, many species release significant amounts of carboxylates from their roots to mobilize poorly − available P forms (Lambers et al., 2015). A recent study suggests that carboxylates can also affect Si mobility in the rhizosphere, and its accumulation in plants (de Tombeur et al., 2021b; Lambers, 2022). This release of carboxylates in soils may be enhanced by P limitation, induced by N addition (Lambers et al., 2012; Tian et al., 2020). As such, rhizosphere processes, possibly mediated by nutrient availability, might affect Si biogeochemistry through increased silicate weathering. Beyond root exudates, N addition may affect Si availability through competition for binding sites at mineral surfaces (Reithmaier et al., 2017; Schaller et al., 2019). These results show significant interactions between N, P and Si dynamics in soil − plant systems, for which the directions and underlying mechanisms remain unclear.
In line with recent studies showing an increase in leaf phytolith concentration under N and/or P limitation (Quigley et al., 2020; Minden et al., 2021; de Tombeur et al., 2021a, 2022; Lu et al., 2022), a field experiment conducted in a temperate region of China has recently shown that N addition significantly decreases phytolith concentrations in grasses (Zhao et al., 2016). This might be explained by some dilution effects (same Si uptake in more biomass) or by plants investing more in Si − based defenses under nutrient − limited conditions (de Tombeur et al., 2022). However, whether this result can be extended to tropical and subtropical regions is unknown. Compared with N − limited temperate regions, plants in tropical and subtropical regions are typically P limited (Cleveland and Townsend 2006; Fan et al., 2019; Du et al., 2020). Thus, the responses of plant species in tropical and subtropical regions to N addition may differ from those in temperate region, but studies on Si cycling in P − limited tropical and subtropical forests responding to N deposition are lacking.
Current field N-addition experiments are mainly carried out as understory N addition (UN) (de Vries et al., 2006; Tian et al., 2018), which may overlook various canopy processes, such as N retention, interception, absorption, and transformation by the forest canopy (Houle et al., 2015; Liu et al., 2020). The quality and quantity of N reaching the soil, plant intrinsic water − use efficiency (Hu et al., 2019), maximum photosynthetic rate (Zhang et al., 2023)d − based leaf defenses (Tang et al., 2021) are significantly different between canopy N addition (CN) and UN. These discrepancies suggest that traditional UN experiments may not fully reflect the effects of atmospheric N deposition on the functions and processes of forest ecosystems (Tian et al., 2018; Hu et al., 2019). Therefore, CN experiments can be utilized together with UN experiments (Zhang et al., 2015), thus more accurately and realistically reflecting the process of Si biocycling under atmospheric N deposition.
In this study, we explored the responses of plant − soil Si dynamics to long − term N deposition based on a nine − year experiment with both CN and UN treatments in an evergreen broadleaved forest of subtropical China (Zhang et al., 2015). Castanea henryi, the dominant tree species in this forest (Xie et al., 2022), as well as the topsoil (0 − 10 cm depth), were sampled for soil Si pools and phytolith concentrations. We tested three hypotheses: 1) N addition will simulate the root release of organic acids due to increased P limitation, thus enhancing plant − available Si concentrations in soils through silicate dissolution, 2) tree foliar phytolith concentrations will be enhanced by long − term N addition because of increased plant − available Si concentrations, and 3) a high rate of N addition has greater effects on Si cycling than a low rate of N addition. Results are expected to strengthen our understanding of how atmospheric N deposition affects soil − plant Si dynamics.