Hongbing Fang

Environmental controls on soil organic carbon and nitrogen stocks in the high-altitude arid western Qinghai-Tibetan Plateau permafrost region

While permafrost in the circum-Artic has great influence on soil organic carbon (SOC) and total nitrogen (TN) stocks, this might not be the case in low-latitude arid permafrost regions. We test this hypothesis in the western Qinghai-Tibetan Plateau (QTP) permafrost region. Fifty-nine soil profiles were analyzed to examine the SOC and TN distribution and the controlling factors in western QTP, which is a desert steppe ecoregion. Mean stocks of SOC (5.29 kg m−2) and TN (0.56 kg m−2) for the top 200 cm in this area were lower than those of the east QTP and circum-Arctic regions. The SOC and TN stocks under vegetative cover with permafrost conditions were significantly higher than those of desert conditions. The SOC and TN stocks for the layers of different depths were related to the content of clay, silt, and moisture. Although the active layer thickness (ALT) had a significant negative correlation to soil moisture, the ALT explained little or no variance in the SOC and TN stocks. The results showed that in the vast permafrost regions of the western QTP, the SOC and TN stocks are very low, and the main controlling factors for the SOC and TN are soil texture, moisture, and vegetation type. The SOC pool in this area may not be as vulnerable to degradation associated with climate warming and thus not emit greenhouse gases at the same rate as other permafrost regions. The different response of the SOC in this region should be considered in carbon cycling models.

Vertical patterns and controls of soil nutrients in alpine grassland: Implications for nutrient uptake

Vertical patterns and determinants of soil nutrients are critical to understand nutrient cycling in high-altitude ecosystems; however, they remain poorly understood in the alpine grassland due to lack of systematic field observations. In this study, we examined vertical distributions of soil nutrients and their influencing factors within the upper 1m of soil, using data of 68 soil profiles surveyed in the alpine grassland of the eastern Qinghai-Tibet Plateau. Soil organic carbon (SOC) and total nitrogen (TN) stocks decreased with depth in both alpine meadow (AM) and alpine steppe (AS), but remain constant along the soil profile in alpine swamp meadow (ASM). Total phosphorus, Ca2+, and Mg2+ stocks slightly increased with depth in ASM. K+ stock decreased with depth, while Na+ stock increased slightly with depth among different vegetation types; however, SO42- and Cl- stocks remained relatively uniform throughout different depth intervals in the alpine grassland. Except for SOC and TN, soil nutrient stocks in the top 20cm soils were significantly lower in ASM compared to those in AM and AS. Correlation analyses showed that SOC and TN stocks in the alpine grassland positively correlated with vegetation coverage, soil moisture, clay content, and silt content, while they negatively related to sand content and soil pH. However, base cation stocks revealed contrary relationships with those environmental variables compared to SOC and TN stocks. These correlations varied between vegetation types. In addition, no significant relationship was detected between topographic factors and soil nutrients. Our findings suggest that plant cycling and soil moisture primarily control vertical distributions of soil nutrients (e.g. K) in the alpine grassland and highlight that vegetation types in high-altitude permafrost regions significantly affect soil nutrients.

Soil moisture and texture primarily control the soil nutrient stoichiometry across the Tibetan grassland

Soil nutrient stoichiometry and its environmental controllers play vital roles in understanding soil-plant interaction and nutrient cycling under a changing environment, while they remain poorly understood in alpine grassland due to lack of systematic field investigations. We examined the patterns and controls of soil nutrients stoichiometry for the top 10cm soils across the Tibetan ecosystems. Soil nutrient stoichiometry varied substantially among vegetation types. Alpine swamp meadow had larger topsoil C:N, C:P, N:P, and C:K ratios compared to the alpine meadow, alpine steppe, and alpine desert. In addition, the presence or absence of permafrost did not significantly impact soil nutrient stoichiometry in Tibetan grassland. Moreover, clay and silt contents explained approximately 32.5% of the total variation in soil C:N ratio. Climate, topography, soil properties, and vegetation combined to explain 10.3-13.2% for the stoichiometry of soil C:P, N:P, and C:K. Furthermore, soil C and N were weakly related to P and K in alpine grassland. These results indicated that the nutrient limitation in alpine ecosystem might shifts from N-limited to P-limited or K-limited due to the increase of N deposition and decrease of soil P and K contents under the changing climate conditions and weathering stages. Finally, we suggested that soil moisture and mud content could be good predictors of topsoil nutrient stoichiometry in Tibetan grassland.

Distribution of Soils and Landform Relationships in Permafrost Regions of the Western Qinghai-Xizang (Tibetan) Plateau, China

Abstract We evaluated software predictions involving soil distributions across landscape positions using selected soil morphology and terrain features in the permafrost regions of the Qinghai-Xizang (Tibetan) Plateau. The relationships presented are based on the Soil Taxonomy System and the 2010 soil suborders coupled with defined environmental factors and predict the spatial distribution of the soil suborders, using the See 5.0 decision tree software integrated with the soil-land inference model. Ten environmental factors closely related to permafrost-affected soil formation were selected as variables for the model: soil parent material, land surface temperatures, elevation, slope gradient, slope aspect, planform and profile curvatures, wetness index, and Normalized Difference Vegetation Index. A total of 62 soil profiles from 2010 were used for model building and validation. The results showed that the soils in the study area were primarily Turbels, Cryids, and Psamments, which cover 47.9%, 22.3%, and 11.7%, respectively, of the total area. Altitude had the closest relationship with soil type; in addition, permafrost played an important role in the soil-forming processes, which led to higher water content in the higher altitudes than in the lower altitudes in the arid areas. The thickness of the active layer in the study area was usually more than 2 m; however, additional consideration of the thickness of the active layer within the Soil Taxonomy System is required.

Soil organic carbon and total nitrogen pools in permafrost zones of the Qinghai-Tibetan Plateau

There are several publications related to the soil organic carbon (SOC) on the Qinghai-Tibetan Plateau (QTP). However, most of these reports were from different parts of the plateau with various sampling depth. Here, we present the results from a systematic sampling and analysis of 200 soil pits. Most of the pits were deeper than 2 m from an east-west transect across the plateau. The SOC and total nitrogen (TN) pools of the 148 × 104 km2, the area of the permafrost zone, for the upper 2 m soils calculated from the vegetation map were estimated to be 17.07 Pg (interquartile range: 11.34–25.33 Pg) and 1.72 Pg (interquartile range: 1.08–2.06 Pg), respectively. We also predicted the distribution of land cover types in 2050 and 2070 using decision tree rules and climate scenarios, and then predicted SOC and TN pools of this region. The results suggested that the SOC and TN pools will decrease in the future. The results not only contribute to the carbon and nitrogen storage and stocks in the permafrost regions as a whole but most importantly, to our knowledge of the possible changes of C and N storage on the QTP in the future.