The transformation and sequestration of soil organic carbon (SOC) has long been a focus of carbon cycle research (Buckeridge et al. 2022, Jia et al. 2021, Lehmann and Kleber 2015), particularly in forests, which account for 50% of the global carbon storage (Pan et al. 2011). SOC can be divided into plant and microbial-derived carbon to better understand SOC formation pathways and stability (Angst et al. 2021). The input from plant materials has traditionally been considered the primary source of SOC because plant lignin and cellulose have higher chemical stability and are hard to decompose (Talbot and Treseder 2012). However, further research has shown that lignin was less stable than previously thought (Hall et al. 2020, He et al. 2019), sometimes decomposing more rapidly than the turnover rate of SOC (Schmidt et al. 2011). Besides, emerging studies have demonstrated that microorganisms play an important role in SOC turnover (Kallenbach et al. 2016, Liang et al. 2017, Simpson et al. 2007). Microorganisms generate many cell residues through continuous iterative processes such as cell growth, biomass increment, and death accumulation. The cell residues are easy to bind with soil minerals, generating highly stable microbial residues in the soil and thus significantly contributing to the SOC pool (Lavallee et al. 2020, Zhang et al. 2023). Therefore, the preservation and turnover of SOC relied on the carbon derived from plants and microorganisms (Xia et al. 2019, Zhu et al. 2020). Although many studies have found varying results on their relative contributions to SOC for different ecosystems (Hu et al. 2022, Wang et al. 2022b, Yang et al. 2022), their relative contributions in forest ecosystems are still unclear, especially across broad environmental gradients.
Biomarkers have been widely adopted to represent carbon sources for SOC pools (Buckeridge et al. 2022, Chen et al. 2020, Ma et al. 2018). Studies in farmland (Li et al. 2020), permafrost (Zhou et al. 2023), and forest (Luo et al. 2022, Wang et al. 2022a) have used amino sugar and lignin phenol to indicate microbial and plant-derived carbon. Lignin is primarily present in vascular plants and thus can be used as a biomarker to indicate the source of plants (Ma et al. 2018). Lignin phenols include vanillin (V), Syringyl (S), and cinnamyl (C) compounds (Hedges and Ertel 1982). Amino sugars are the basic unit of cell wall peptidoglycan and chitin (Amelung et al. 2001, Stevenson 1983). Many studies have used amino sugars as markers of microbial residue (Liang et al. 2019, Ma et al. 2018). Of the four detectable amino sugars, Muramic acid (MurN) is wholly derived from bacterial peptidoglycans, and Glucosamine (GluN) is mainly derived from fungi and bacteria (Joergensen 2018). Therefore, GluN/MurN is commonly used to evaluate fungal and bacterial residues in soil relative contribution to SOC transformation (Glaser et al. 2004).
The changes in concentration of amino sugars and lignin phenols were affected by abiotic and biotic factors. However, we need to learn more about how the effects may vary for different forest ecosystems in a broad environmental gradient. The latitude gradient contains substantial variations in biotic and abiotic factors, which could significantly affect litter decomposition and soil carbon dynamics. For example, in low-latitude tropical forests, microbial turnover is fast due to high temperature and precipitation, causing a lower SOC content (Ma et al. 2020, 2021). However, in high-latitude boreal forests, microorganisms decomposed slowly, and the organic carbon turnover time was longer than in low-latitude forests (Chen et al. 2020). According to IPCC projections, by the end of the century, climate change will result in a global temperature increase of 1.1–2.6°C, and more significant warming of ecosystems in high latitudes (IPCC 2014). Therefore, studying the latitudinal variation of plant and microbial-derived carbon and its controlling factors is vital to reveal the interaction between climate change and SOC dynamics. There is a recent attempt to study the changes in microbial-derived soil carbon along the latitude gradient (Chen et al. 2020). However, the distribution and preservation of plant-derived C along the latitude gradient remains elusive.
Various abiotic and biotic factors can regulate the retention of plant and microbial residue in soil. For instance, soil texture is essential to microbial residue accumulation (Zhang et al. 2023). Silt and clay components contain more oxidized and decomposed lignin phenols (Clemente and Simpson 2013, Tamura et al. 2017). Soil texture may also interact with climate to affect soil carbon accumulation. In coarser soils, microbial residue was influenced by the degree of drought (Ma et al. 2018). Fungi are decomposers of lignin (Jex et al. 2014, Thevenot et al. 2010). Therefore, changes in the ratio of fungi to bacteria in the microbial community can also affect the accumulation of lignin. Given the wide gradient of climate, soil physicochemical properties, and vegetation and their interactions along the latitude, it is crucial to research the geographical patterns of plant and microbial-derived C accumulation.
Previous studies on how environmental gradient and soil carbon accumulation related to SOC composition mainly concentrated on the topsoil (Chen et al. 2020, Mou et al. 2021, Yang et al. 2020). The subsoil is rarely studied. Subsoil carbon plays a crucial role in the terrestrial carbon cycle, but its understanding remains limited (Rumpel and Kögel-Knabner 2011). Subsoil (greater than 30 cm) buried a large amount of relatively stable carbon, stored greater than 50% of the SOC in terrestrial ecosystems (Balesdent et al. 2018). Hence, understanding the preservation and turnover mechanisms of SOC in the subsoil is essential for accurately predicting soil carbon dynamics and its responsiveness to climate change. One study on alpine ecosystems revealed that the preservation of topsoil and subsoil organic carbon may be regulated by different factors (He et al. 2022). However, it remains to be evaluated whether these findings are applicable to other forest ecosystems. Some studies have also investigated the vertical distribution of amino sugars through data collection. (Ni et al. 2020, 2021). But systematic microbial and plant-derived carbon observations at different soil depths still need to be included.
Here, we explored the pattern and controlling factors of soil amino sugar and lignin phenol in forest ecosystems along the latitudinal gradient. Amino sugar and lignin phenol were chosen as the biomarker of microbial-derived and plant-derived carbon(C), respectively. The topsoil (0–10 cm) and the subsoil (30–50 cm) of six major forest types in China along the latitudinal gradient were compared. Meanwhile, the factors controlling the distribution and conservation of amino sugars and lignin phenols were analyzed with climate, soil, and vegetation characteristics. We hypothesized that: (i) The accumulations of SOC from plants and microorganisms would vary with latitude. At low latitudes with high temperatures and abundant precipitation, fast litter decomposition and turnover of microbial community cause a relatively higher contribution of amino sugars and a lower contribution of lignin phenols in soils.
In contrast, at high latitudes where low temperatures and significant SOC accumulation prevail, the slower litter decomposition and microbial turnover (Fanin and Bertrand 2016), which causes a relatively higher contribution of lignin phenols and a lower contribution of amino sugars in soils. (ii) Topsoil and subsoil would have different amounts of amino sugars and lignin phenols. Topsoil would have a higher proportion of plant-derived SOC due to more plant material input. In comparison, subsoil would have a higher proportion of microbial-derived carbon due to enhanced decomposition of soil organic materials.