Abstract
The grassland soils of the Qinghai-Tibet Plateau (QTP) store a large amount of organic carbon because of the cold, humid climate, and topsoil organic carbon is quite sensitive to global climate changes. However, the spatio-temporal dynamics and factors that influence the soil organic carbon (SOC) on the QTP’s grassland are not understood well. Moreover, there are few comparative analyses of different approaches to estimate the QTP’ SOC. In this study, we estimated the storage and patterns of SOC density (SOCD) using several methods, including MODIS (moderate-resolution imaging spectroradiometer) retrieval, field data and previous empirical models (Models1-4, and soil organic matter (SOM)). And their relations with aboveground biomass, soil moisture, temperature, elevation, and soil conductivity were further explored. The results showed that SOC showed a similar variation trend in the different models, in which it decreased with increasing bulk density (BD) in the topsoil at 30 cm. For meadow and steppe grasslands, Models 1, 2, and 4 showed similar estimated values of SOCD, while Model3 had a lower value than them. SOC storage in the BD 3 and SOM methods had abnormal values, while the MODIS-NDVI, BD 1, 2, and 4 methods had similar SOC stock values for meadow and steppe grassland. Moreover, meadow grassland had a higher SOC storage than did steppe grassland, with means values of 397.9×1010 kg and 242.2×1010 kg, respectively. SOCD’s spatial distribution using MODIS-NDVI method differed clearly from the empirical models, with a significant tendency for spatial variation that increased from the northwestern to southeastern regions on the QTP. Therefore, based on the values estimated and spatial variation features, the MODIS-NDVI method may be a more feasible and valid model to estimate SOC. Moreover, the mean annual SOCD values during 2000–2015 showed an increasing trend, with a higher mean value in meadow and a lower mean value in steppe. Further, SOCD was correlated significantly and positively with aboveground biomass and soil moisture, and negatively correlated with elevation and soil conductivity. Increasing temperature had negative effects on SOCD, which was consistent with the global trend. These results indicated that topsoil moisture plays a key role in SOCD spatial patterns. Our results provide valuable support for the long-term estimation of SOCD in future research on the QTP.
Highlights
Grasslands cover 32% of the earth’s vegetated area and account for 40% of the total area in China [1, 2]
Model 3 differed clearly and unreasonably from other three models when bulk density (BD) was less than 1.1 g cm-3, while it had a similar decreasing tendency with the other three models when BD was more than 1.1 g cm-3
SOC density (SOCD) showed no significant differences between different grassland types, with mean values of 5.39 kg cm-2 in the meadow grassland and 5.29 kg cm-2 in the steppe grassland, respectively
Summary
Grasslands cover 32% of the earth’s vegetated area and account for 40% of the total area in China [1, 2] They are important in global carbon cycle and carbon sequestration in both soil and vegetation [3]. As a sensitive area with respect to possible effects of global climate change, the rising atmospheric CO2 levels attributable to anthropogenic activities that have been occurring since the 1750s, increasing attention has been paid to the QTP grasslands’ SOC [3, 9]. The QTP may be an ideal observation area to monitor SOC stock changes and assess environmental threats caused by human activities [10]. The spatio-temporal patterns of SOC on the QTP remain uncertain, as data from repeated inventories and sufficient field observations still are lacking
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