Seasonal freeze-thaw characteristics of soil carbon pools under different vegetation restoration types on the Longzhong Loess Plateau

Abstract

Soil carbon pools are important for maintaining the stability of the carbon cycle in terrestrial ecosystems and regulating climate change. However, it is not clear how soil carbon pools change under different vegetation restoration types at high altitudes during frequent seasonal freeze-thaws (FTs). Therefore, we studied the seasonal FT variability (before freezing, early stages of freezing, stable freeze stage, thawing stage) of soil organic carbon (SOC), microbial biomass carbon (MBC), dissolved organic carbon (DOC), and easily oxidized organic carbon (EOC) under three vegetation restoration types (Grassland, GL; Caragana korshinskii, CK; Xanthoceras sorbifolia, XS) on the Longzhong Loess Plateau region. We found that during the seasonal FT, the 0–40 cm SOC, MBC, DOC, and EOC contents were higher in XS vegetation than in GL and CK vegetation, but the sensitivity index of SOC was lower in XS vegetation (sensitivity index = 2.79 to 9.91). In the 0–40 cm soil layer, the seasonal FT process reduced the MBC content and increased the DOC content in the three vegetation soils. Meanwhile, DOC and EOC contents accumulated obviously in the stable freezing period and decreased significantly in the thawing period. We also found that the SOC, MBC, DOC, and EOC contents were higher in the surface soils (0–10 cm) than in the underlying soils (10–20 and 20–40 cm), while the sensitivity of the soil carbon pool fractions to seasonal FT processes differed considerably between soil depths. Redundancy analysis (RDA) showed that soil total nitrogen, temperature, total phosphorus, and soil water content were important environmental factors influencing soil carbon pool fractions during seasonal FT. This study suggested that in the Longzhong Loess Plateau region, soil MBC and DOC were more susceptible to seasonal FT phenomena and that the soil system of the Xanthoceras sorbifolia vegetation had a stronger soil C sequestration function during the seasonal FT process.

Distribution Characteristics of Carbon Density in Plant–Soil System of Temperate Steppe and Temperate Desert in the Longzhong Loess Plateau

“…Both values were lower than the soil organic carbon density content (between 8500 and 15,100 g•m −2 ) in China [43], presenting a lower level in the whole country. This was mainly due to the fact that the Longzhong Loess Plateau area is constrained by geographic conditions such as aridity, windy conditions, and soil erosion [19], which results in low and sparse vegetation growth and low productivity, thus making soil carbon sources limited. In addition, the extreme climatic conditions (cold winters and hot summers) constrains the rate and amount of biological conversion of organic carbon by soil microorganisms, resulting in a slow rate of accumulation of soil organic carbon and consequently a low soil organic carbon density content.…”

“…The main grassland types are temperate steppe and temperate desert, and their plant-soil system carbon stocks contribute significantly to the carbon balance of grasslands in China, which is important for maintaining the ecological environment of the grasslands in the Loess Plateau in Longzhong and coping with global warming [18]. However, the impacts of global climate change and human activities, coupled with the fragile ecological environment of grasslands in the Longzhong Loess Plateau, has resulted in intense grassland soil erosion, serious loss of organic carbon from grasslands, and serious threats to the ecological security of grasslands [19]. Therefore, there is an urgent need for in-depth research on the characteristics of the changes in carbon density of grassland ecosystems in the Longzhong Loess Plateau.…”

Soil Carbon and Nitrogen Pools and Their Storage Characteristics under Different Vegetation Restoration Types on the Loess Plateau of Longzhong, China

“…The Longzhong Loess Plateau is frequently regarded as one of the Earth's most vulnerable ecosystems [18]. Since 1990, the "Return of Farmland to Forest and Grassland Program" has been implemented in the region to restore its ecology [18,19].…”

“…The Longzhong Loess Plateau is frequently regarded as one of the Earth's most vulnerable ecosystems [18]. Since 1990, the "Return of Farmland to Forest and Grassland Program" has been implemented in the region to restore its ecology [18,19]. Researchers have shown that converting farmland to forest and grassland can improve the C and N sinks and C emission reduction capacity of ecosystems and mitigate regional climate change [20].…”

“…has reduced carbon dioxide (CO 2 ) and nitrous oxide (N 2 O) emissions in the soil of the Loess Plateau [23] and the Yangtze River Delta [24] regions, respectively. In the Loess Plateau region, large areas of sloping land have been abandoned and revegetated by allowing it to recover naturally to grassland or by converting it to trees and shrublands [18,25]. Several studies have demonstrated that different vegetation restoration types can have effects on nutrient supply [26] and microbial and enzymatic activities [16,27] in the area.…”