The forest soil organic carbon pool is the primary carbon sink of terrestrial ecosystems, and soil humus, as the main component of organic carbon, is an important contributor to the surface soil organic carbon (SOC) pool and plays an irreplaceable role in carbon sequestration in forest ecosystems(Cicuzza and others 2015).Changes in its carbon stocks and composition may alter the concentration of CO2 in the atmosphere, thereby affecting global climate change (Faggian and others 2012).
Humus comprises 65–75% of organic matter in the soil. Humus can be divided into humic acid carbon (HAC), fulvic acid carbon (FAC) and humin carbon (HMC) based on its solubility. FAC has a lower molecular weight than HAC and is more fluid. Humus plays a role in water retention, fertilizer retention and buffering, which can promote the formation of aggregates and improve soil physical properties (Wei and others 2020). Moreover, humus also significantly contributes to soil detoxification (Lasota and others 2020; Pandelova and others 2018). Most importantly, humus is an organic polymer with complex biochemical characteristics, which can effectively resist microbial decomposition, and the content of its carbon component is a true reflection of the ability of soil to sequester carbon (Andreetta and others; Cheng and others 2015). Therefore, in the context of global warming, studying the storage and distribution of the humus carbon component of forest soils can provide a scientific basis for addressing global warming (Han and others 2019).
As the main component of organic carbon, the mechanism of formation of humus is a highly complex process that is affected by many factors, including the content of humus matrix and the time of transformation of plant active residues (Labaz and others 2014; Prescott, 2010). Other environmental factors, such as climate, topography, altitude, soil type, and land use pattern,(Hellwig and others 2019; Liaudanskiene and others 2021; Startsev and others 2020) also affect the humification process by affecting the input and decomposition of humification substrates (Buresova and others 2021). Thus, these factors affect carbon sequestration by soil humus.
Latitude and altitude determine the productivity, stability and functional characteristics of forest ecosystems by influencing precipitation, temperature and light intensity, but the correlation between latitude and soil humus is not the only general and universal characteristics (Xu and others 2018a). Latitude affects the accumulation of organic carbon, and the content of SOC and humus components increase in parallel with latitude from tropical to temperate forest regions. Climate, soil texture and microorganisms comprised 90% of the change in humus carbon components, and the effect of soil microorganisms on FAC was greater than that on HAC. Compared with other influencing factors, climate factors play a leading role in the latitude pattern distribution of HAC and FAC as determined by the plant species, production methods and the process of decomposition of litter ( Alvarez and Lavado, 1998 ; Ponge and others 2011). The altitude plays an important role in soil humification because it can affect the soil humification process by changing environmental factors, such as, temperature and water content, and thus, affect the characteristics of the distribution of carbon storage of soil components (Shedayi and others 2016). (Labaz and others 2014) found that the carbon content of soil humus in mountainous areas in temperate climates increases with the increase in altitude, and the form of soil humus changes simultaneously, primarily owing to the combined effect of the increase in soil acidity and the decrease in temperature with the increase in altitude (Badía-Villas and Girona-García, 2018; Zhang and others 2021). However, (Bojko and others 2017) concluded that the soil humus components responded differently at different altitudes, and the FAC content increased significantly in areas above 1,000 m a.s.l., while the content of stable component HMC decreased significantly. In areas below 1,000 meters a.s.l., the most stable humus may be reduced. In summary, there is no clear understanding of the composition and changes of soil humus under the influence of different latitudes and altitudes.
The low latitude plateau area is primarily located at the southeast edge of the Qinghai Tibet Plateau in China. The terrain in this area is complex, high in the north and low in the south, and the altitude differs by 6,664 m. It is primarily characterized by a combination of high-latitude and high-altitude and a combination of low-latitude and low-altitude, which has typical mountain geomorphic characteristics. The unique geographical location of the low-latitude plateau makes it a typical area with complex terrain and comprehensive climate diversity (Wu and others 2019), and it is considered to be one of the areas with the highest soil organic carbon reserves in China (Duan and others 2014). However, in recent years, mountain soils have become a potentially vulnerable part of the global carbon cycle. Therefore, evaluation of the storage of SOC and the study of influencing factors in this area have attracted extensive attention. However, it is not clear how latitude and altitude affect the characteristics of distribution of soil humus components in this area. Therefore, a study on the characteristics and influencing factors of organic carbon and humus components in this area is highly significant to understand the assessment of soil carbon storage in mountainous areas and its effects on managing global climate change.
The main purpose of this study was to (1) investigate the spatial distribution of humus components in forest soils affected by different latitudes and investigates the subsurface soil in low-latitude and high-altitude areas, and (2) explore the influencing factors of SOC and humus composition in this area. The main question of this study was to identify how the latitude, altitude and related environmental factors affect variation in the SOC and its humus components. Thus, we hypothesized that (1) the SOC and humus components vary in different latitudes and altitudes, and (2) the SOC and humus components would have a stronger association with latitude than altitude.