Soil organic carbon contents, aggregate stability, and humic acid composition in different alpine grasslands in Qinghai-Tibet Plateau

Abstract

Alpine grassland soils on Qinghai-Tibet Plateau store approximately 33.5 Pg of organic carbon (C) at 0-0.75 m depth and play an important role in the global carbon cycle. We investigated soil organic C (SOC), water-soluble organic C (WSOC), easily oxidizable organic C (EOC), humic C fractions, aggregate-associated C, aggregate stability, and humic acid (HA) composition along an east-west transect across Qinghai-Tibet Plateau, and explored their spatial patterns and controlling factors. The contents of SOC, WSOC, EOC, humic C fractions and aggregate-associated C, the proportions of macroaggregates (2-0.25) and micro-aggregates (0.25-0.053 mm), and the aggregate stability indices all increased in the order alpine desert < alpine steppe < alpine meadow. The alkyl C, O-alkyl C, and aliphatic C/aromatic C ratio of HA increased as alpine desert < alpine meadow < alpine steppe, and the trends were reverse for the aromatic C and HB/HI ratio. Mean annual precipitation and aboveground biomass were significantly correlated with the contents of SOC and its fractions, the proportions of macro-and microaggregates, and the aggregate stability indices along this transect. Among all these C fractions, SOC content and aggregate stability were more closely associated with humic C and silt and clay sized C in comparison with WSOC, EOC, and macro-and microaggregate C. The results suggested that alpine meadow soils containing higher SOC exhibited high soil aggregation and aggregate stability. Mean annual precipitation should be the main climate factor controlling the spatial patterns of SOC, soil aggregation, and aggregate stability in this region. The resistant and stable C fractions rather than labile C fractions are the major determinant of SOC stocks and aggregate stability. Keywords

The spatio-temporal patterns of the topsoil organic carbon density and its influencing factors based on different estimation models in the grassland of Qinghai-Tibet Plateau

“…However, for different grassland types, the different methods differed significantly, with a larger value in the meadow than steppe grassland. The results of this experiment were similar to those of previous studies [13, 46, 47]. Ding et al (2017) concluded that the alpine meadow’ mean SOCD was higher than that of the alpine steppe in 2000 (2.71 and 6.96 kg cm -2 , respectively) and 2010 (2.95 and 7.37 kg cm -2 , respectively) [47].…”

“…Ding et al (2017) concluded that the alpine meadow’ mean SOCD was higher than that of the alpine steppe in 2000 (2.71 and 6.96 kg cm -2 , respectively) and 2010 (2.95 and 7.37 kg cm -2 , respectively) [47]. The soil carbon content of different grassland types on the QTP had obvious zonal differences that were consistent with the distribution of soil and grassland types [46]. The reason for this may be that higher precipitation and lower temperatures influence meadow grassland to a greater extent than steppe grassland, and the decomposition process of SOM is restricted, which results in higher accumulation of SOC.…”

Humus composition and humification degree of humic acids of alpine meadow soils in the northeastern part of the Qinghai–Tibet Plateau

“…(Gao et al 2007;Wang et al 2002). Alpine grasslands, covering approximately 35% of the Qinghai-Tibet Plateau area, are the major pastureland of the region (Zheng et al 2000), and approximately 33.5 Pg of organic C is stocked in the 0-75 cm layer of soil (Cao et al 2016). Therefore, many studies have estimated the SOM stocks in this region, indicating a considerable accumulation of SOM (Tan et al 2010;Yang et al 2008;Zhang et al 2007).…”

“…SOM is heterogeneous and consists of various fractions with different stability, resolution, and rotation rates. Two dominant fractions of SOM are the labile soil organic C (SOC) fractions and the chemically recalcitrant SOC fractions, which are potential indicators of global climate change (Steinberg 2003;Cao et al 2016;Hu et al 2017;Wang et al 2016). The latter (e.g., humic substances) is traditionally separated into humic acid (HA), fulvic acid (FA), and humins (HN) based on the solubility characteristics of each fraction, and the humus composition is an essential characteristic of humic substances in SOM.…”

Profile distributions of soil organic carbon fractions in a permafrost region of the Qinghai–Tibet Plateau

“…[30][31][32][33][34] Permafrost distribution and other features are closely related to vegetation types, and their changes can potentially affect SOC dynamics. 16,31,35,36 Some previous studies have provided valuable information on the SOC pools and fraction dynamics under different vegetation types on the QTP (e.g., 16,25,27,[36][37][38] ). For example, Wu et al 16 suggested that the permafrost and land cover types were the main controlling factors for the LFOC content and stock, and that permafrost degradation would inevitably lead to a decrease in the LFOC, thus accelerating the decomposition of SOC.…”