Comparison of Soil Organic Matter Transformation Processes in Different Alpine Ecosystems in the Qinghai‐Tibet Plateau

Abstract

Soils in the Qinghai‐Tibet Plateau are young in terms of development and form a thin, skeletal soil layer because the gravel parent materials are glaciofluvial deposit, eluvium, and fluvial sediment, which are extremely sensitive to global climate change. The lack of understanding of soil organic matter (SOM) transformation processes in this region hinders the prediction of SOM stocks under future climate conditions. In this study, SOM transformation processes were investigated by density groupings and pyrolysis‐gas chromatography/tandem‐mass spectrometry (Py‐GC–MS/MS) in five alpine ecosystems: alpine desert, alpine grassland, alpine meadow, alpine wetland, and alpine forest. The roles of microbial community in different density fractions were elucidated. Soil samples were separated into three fractions with NaI solution: labile fraction (FL; ρ ≤ 1.6 g/cm3), moderate fraction (FM; 1.6 < ρ < 2.25 g/cm3), and recalcitrant fraction (FR; ρ ≥ 2.25 g/cm3). The following results were obtained. (1) The ratios of different fractions (FM/FL, FR/FM, and FR/FL) indicated the transformation processes of SOM, and ratios less than 1 represented degradation. The transformation processes were similar in alpine grassland and meadow and were similar in alpine desert, wetland, and forest but differed from each other. (2) Fungi preferred to degrade plant detritus, which mainly affected FL, whereas bacteria chiefly affected FM. (3) Precipitation altered vegetation type and soil pH, thus affecting communities and microorganism activities and resulting in the above differences. These findings highlight the importance of understanding the limited transformation processes of SOM in ecosystems to predict the impact of climate change on SOM preservation.

Multilevel Nitrogen Additions Alter Chemical Composition and Turnover of the Labile Fraction Soil Organic Matter via Effects on Vegetation and Microorganisms

“…According to our previous research results, the properties of SOM with density ≤2.25 g cm −3 and with density >2.25 g cm −3 were significantly different in this studied alpine grassland, and SOM with density ≤2.25 g cm −3 was the main carbon source used by soil microorganisms, which was more susceptible to external climate change (Chen et al, 2019). Therefore, in this study, soil samples were divided into two fractions: labile fraction SOM (LF‐SOM, ρ ≤ 2.25 g cm −3 ) and refractory fraction SOM (RF‐SOM, ρ > 2.25 g cm −3 ).…”

“…However, with the increase in N additions, LF‐SOM decreased significantly (Figure 1b), and there was a significant negative correlation between LF‐SOM and AGB (Table 1), indicating that N addition promoted the degradation of LF‐SOM in alpine grassland of the QTP. As the most active fraction with the fastest turnover rate, LF‐SOM is closely related to soil microorganisms and shows the greatest sensitivity to soil physical or chemical disturbances (Chen et al, 2019). Therefore, LF‐SOM plays a very important role in carbon cycling (Khanna et al, 2001) and is often used to indicate the impact of climate change on SOM degradation.…”

“…In addition, the activities of enzymes (cellulase and amylase) that degrade plant‐derived compounds are mainly limited by N. The input of external N relieved the N limitation in this area and enhanced the activities of cellulase and amylase, thus promoting plant‐derived compound degradation (Zhao et al, 2017). Microbial‐derived compounds in the soil generally interact with soil minerals to enhance their stability to resist microbial degradation (Chen et al, 2019), and aromatic compounds in soil have strong chemical stability due to their unique chemical structures. Therefore, the responses of microbial‐derived and aromatic compounds to N addition were relatively insensitive compared with those of plant‐derived compounds.…”

“…The chromatographic column was HP‐5 (100 m × 0.32 mm × 0.25 μm), and the mass spectrometry conditions were as follows: electron ionization (EI) of 70 eV, ion source temperature of 230 °C, quadrupole temperature of 150 °C, and a full scan scanning range of 50–550 amu. The temperature program of the PY‐3030D and the column was based on a previous study (Chen et al, 2019).…”

Composition and Diversity of Soil Microbial Community Associated With Land Use Types in the Agro–Pastoral Area in the Upper Yellow River Basin

“…The soil microorganism plays an important role in soil biochemistry, which is involved in the decomposition of soil organic matter, humus formation, and nutrient cycling. At the same time, soil microorganism is critical to maintaining ecosystem function as the connection between soil and plants ( Chen et al, 2019 ). Therefore, the microbial diversity is widely used to evaluate soil fertility and land quality ( Lamb et al, 2011 ; Barberan et al, 2012 ; Fierer et al, 2012 ; Berthrong et al, 2013 ; Lauber et al, 2013 ).…”