Tingjun Zhang

Carbon and Nitrogen Properties of Permafrost over the Eboling Mountain in the Upper Reach of Heihe River Basin, Northwestern China

Abstract The sensitivity of soil carbon and nitrogen to warming is a major uncertainty in projections of climate. However, previous studies about soil organic carbon (SOC) stocks and potential emission predominantly concentrated on the shallow soil layer in high latitude ecosystems. In this study, we analyzed the SOC, total nitrogen (TN) and soil inorganic carbon (SIC) stocks, C/N ratios, and stable carbon isotope (&dgr;13C) in the active layer and permafrost layer on the Eboling Mountain in the upper reach of Heihe River basin, northwestern China. Our results showed that the average stocks of SOC, TN, and SIC in permafrost layer above soil parent materials (71.7 kg m-2, 8.0 kg m-2, 34.7 kg m-2) were much higher than those in the active layer (44.3 kg m-2, 5.3 kg m-2, 12.2 kg m-2). The &dgr;13C pattern in the soil profiles indicated that historical drainage conditions and pedogenesis were important factors in determining soil organic matter (SOM) stocks in this permafrost region. The &dgr;13C and C/N ratios of the transient layer and some layers of permafrost implied that the degradation of SOM was different. These results highlight that carbon and nitrogen in permafrost regions with Alpine Kobresia meadow could make significant contribution to Chinas terrestrial carbon cycle.

Changes of Soil Thermal Regimes in the Heihe River Basin Over Western China

Abstract Investigation of the changes in soil thermal regimes is essential to the understanding of ecohydrological processes, resource development, and climate change. We use soil temperatures from 12 meteorological stations of the China Meteorological Administration in the Heihe River Basin to estimate soil seasonal freeze depth, the onset and end dates of soil freeze, and the duration of soil freeze. Based on the characteristics of the soil temperature in the seasonal freezing layer, the freeze/thaw processes of this layer were divided into four stages: the winter freezing stage, spring thawing stage, summer warming stage, and autumn cooling stage. Spring, summer, autumn, and winter ground surface temperatures in the basin exhibit significant increasing trends in 1972–2006, of 0.65 °C decade-1, 0.73 °C decade-1, 0.48 °C decade-1, and 0.44 °C decade-1, respectively. Mean annual soil temperature at 0.0–0.20 m depths reveals an increasing trend of 0.58–0.63 °C decade-1 in 1972–2006. The onset date of soil freeze, the end date of soil freeze, and the duration of soil freeze in 1972–2006 exhibit a statistically significant trend of +2 days decade-1, -4 days decade-1, and -6 days decade-1, respectively. The maximum thickness of the seasonally frozen ground for 1960–2007 reveals a statistically significant trend of -4.0 cm decade-1 and a net change of -19.2 cm for the 48-year period. These are all related to the increase in spring, summer, autumn, and winter air temperature and the mean annual air temperature in the basin, a possible result of global warming.

Changes in Freezing-Thawing Index and Soil Freeze Depth Over the Heihe River Basin, Western China

ABSTRACT Freezing/thawing index is an important indicator of climate change, and can be used to estimate depths of the active layer and seasonally frozen ground (SFG). Using the mean monthly grid air temperature from 2000 to 2009 as well as daily air and ground surface temperatures from 12 meteorological stations across the Heihe River Basin, this study investigated spatial and temporal variability of the freezing/thawing index and seasonal soil freeze depth. The mean annual air temperature increased at a rate of 0.35 °C decade-1 from 1960 to 2013, or approximately 1.9 °C for the 54-year period. We found that the freezing index (FI) showed a decreasing trend over the study area, while the thawing index (TI) had an increasing trend. Changes in both FI and TI are consistent with an increasing mean annual air temperature. The TI and freezing n-factor (nf) decrease with elevation increase, while FI and thawing n-factor (nt) increase with elevation. Soil potential seasonal freezing depth was primarily between 1.5 and 2.5 m in permafrost regions. However, the soil maximum freezing depth is below 2.5 m in SFG region.

Response of Changes in Seasonal Soil Freeze/Thaw State to Climate Change from 1950 to 2010 across China

Variations in seasonal soil freeze/thaw state are important indicators of climate change and influence ground temperature, hydrological processes, surface energy, and the moisture balance. Previous studies mainly focused on the active layer and permafrost, while seasonally frozen ground research in nonpermafrost regions has received less attention. In this study, we investigate the response of changes in seasonal soil freeze/thaw state to changes in air temperatures by combining observations from more than 800 stations with gridded mean monthly air temperature data across China. The results show that mean annual air temperature (MAAT) increased statistically significantly by 0.29 ± 0.03°C/decade from 1967 to 2013, with greater warming on the Qinghai-Tibetan Plateau. There is a statistically significant decrease in the freeze/thaw cycle (FTC) at 0.39 ± 0.05 cycles/decade. In addition, there are strong negative correlations between FTC and MAAT. Estimating the soil freeze/thaw state classification based on the number of days in the month, we find that changes of mean annual area extent of seasonal soil freeze/thaw state decreased significantly for completely frozen (CF) ground, while the area extent of partially frozen (PF) and unfrozen (UF) ground both increased. Changes in mean monthly area extent of seasonal soil freeze/thaw state indicate that the extent of CF and UF area was decreasing and increasing, respectively. But for the extent of PF areas, both increasing and decreasing trends were observed. Quantifying the spatial pattern of the seasonal soil freeze/thaw, we find that CF and PF areas are located in northern China and the Tibetan Plateau from December to March, and UF areas are located in southern China. The variations of mean annual area extent departure of soil freeze/thaw states are consistent with MAAT changes in different land cover types across China.

Spatial variability of active layer thickness detected by ground-penetrating radar in the Qilian Mountains, Western China

The active layer plays a key role in geomorphic, hydrologic, and biogeochemical processes in permafrost regions. We conducted a systematic investigation of active layer thickness (ALT) in northeastern Qinghai-Tibetan Plateau by using ground-penetrating radar (GPR) with 100 and 200 MHz antennas. We used mechanical probing, pit, and soil temperature profiles for evaluating ALT derived from GPR. The results showed that GPR is competent for detecting ALT, and the error was +/- 0.08 m at common midpoint co-located sites. Considerable spatial variability of ALT owing to variation in elevation, peat thickness, and slope aspect was found. The mean ALT was 1.32 +/- 0.29 m with a range from 0.81 to 2.1 m in Eboling Mountain. In Yeniu Gou, mean ALT was 2.72 +/- 0.88 m and varied from 1.07m on the north-facing slope to 4.86 m around the area near the lower boundary of permafrost. ALT in peat decreased with increasing elevation at rates of -1.31 m/km (Eboling Mountain) and -2.1 m/km (Yeniu Gou), and in mineral soil in Yeniu Gou, the rate changed to -4.18 m/km. At the same elevation, ALT on the south-facing slope was about 0.8m thicker than that on the north-facing slopes, while the difference was only 0.18 m in peat-covered area. Within a 100 m(2) area with a local elevation difference of 0.8 m, ALT varied from 0.68 m to 1.25 m. Both field monitoring and modeling studies on spatial ALT variations require rethinking of the current strategy and comprehensive design.

Spatiotemporal Changes in Active Layer Thickness under Contemporary and Projected Climate in the Northern Hemisphere

AbstractVariability of active layer thickness (ALT) in permafrost regions is critical for assessments of climate change, water resources, and engineering applications. Detailed knowledge of ALT variations is also important for studies on ecosystem, hydrological, and geomorphological processes in cold regions. The primary objective of this study is therefore to provide a comprehensive 1971–2000 climatology of ALT and its changes across the entire Northern Hemisphere from 1850 through 2100. To accomplish this, in situ observations, the Stefan solution based on a thawing index, and the edaphic factor (E factor) are employed to calculate ALT. The thawing index is derived from (i) the multimodel ensemble mean of 16 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) over 1850–2005, (ii) three representative concentration pathways (RCP2.6, RCP4.5, and RCP8.5) for 2006–2100, and (iii) Climatic Research Unit (CRU) gridded observations for 1901–2014. The results show significant spatial variab...

Climatology of the Timing and Duration of the Near-Surface Soil Freeze-Thaw Status Across China

ABSTRACT The near-surface soil is an important interface in ground—atmosphere interactions. The near-surface soil freeze-thaw status is critical for energy, moisture, and carbon exchange between the ground and the atmosphere, plant growth, and the ecosystem as a whole. The main objective of this study is to investigate climatology of the timing and duration of the near-surface soil freeze-thaw status using data from 636 meteorological stations across China for the baseline period from July 1971 through June 2001. The long-term average first date of the near-surface soil freeze is 14 September (30 July–30 October), the last date is 15 May (8 April–21 June), the duration is 245 ± 85 days, and the actual number of the near-surface soil freeze days is 202 ± 90 days over China as a whole. On the Qinghai—Tibetan Plateau, the near-surface soil freeze can occur essentially in any month of a year. The spatial variations of the near-surface soil freeze-thaw status are strongly controlled by latitude in east China, and by elevation in west China. The long-term average 220-day and 260-day contours of the near-surface soil freeze coincide approximately with the southern boundary of high—latitude permafrost regions in northeastern China and the lower boundary of high—altitude permafrost regions in west China, respectively. The number of days and duration of the near-surface soil freeze decreased with increasing long-term mean annual air temperature (MAAT). Variation of the actual number of the near-surface soil freeze days presents nonlinear linkage to the length of the near-surface soil freeze duration and also to the MAAT climatology. The timing and duration of the near-surface soil freeze-thaw status are strongly nonlinearly related to air freezing index, but are nearly linearly related to air thawing index.