Soil organic carbon stocks in China and changes from 1980s to 2000s

Abstract

The estimation of the size and changes of soil organic carbon (SOC) stocks is of great importance for decision makers to adopt proper measures to protect soils and to develop strategies for mitigation of greenhouse gases. In this paper, soil data from the Second State Soil Survey of China (SSSSC) conducted in the early 1980s and data published in the last 5 years were used to estimate the size of SOC stocks over the whole profile and their changes in China in last 20 years. Soils were identified as paddy, upland, forest, grassland or waste-land soils and an improved soil bulk density estimation method was used to estimate missing bulk density data. In the early 1980s, total SOC stocks were estimated at 89.61 Pg (1 Pg 5 10 3 Tg 5 10 15 g) in China's 870.94 Mha terrestrial areas covered by 2473 soil series. In the paddy, upland, forest and grassland soils the respective total SOC stocks were 2.91 Pg on 29.87 Mha, 10.07 Pg on 125.89 Mha, 34.23 Pg on 249.32 Mha and 37.71 Pg on 278.51 Mha, respectively. The SOC density of the surface layer ranged from 3.5 Mg ha À1 in Gray Desery grassland soils to 252.6 Mg ha À1 in Mountain Meadow forest soils. The average area-weighted total SOC density in paddy soils (97.6 Mg ha À1 ) was higher than that in upland soils (80 Mg ha À1 ). Soils under forest (137.3 Mg ha À1 ) had a similar average area-weighted total SOC density as those under grassland (135.4 Mg ha À1 ). The annual estimated SOC accumulation rates in farmland and forest soils in the last 20 years were 23.61 and 11.72 Tg, respectively, leading to increases of 0.472 and 0.234 Pg SOC in farmland and forest areas, respectively. In contrast, SOC under grassland declined by 3.56 Pg due to the grassland degradation over this period. The resulting estimated net SOC loss in China's soils over the last 20 years was 2.86 Pg. The documented SOC accumulation in farmland and forest soils could thus not compensate for the loss of SOC in grassland soils in the last 20 years. There were, however, large regional differences: Soils in China's South and Eastern parts acted mainly as C sinks, increasing their average topsoil SOC by 132 and 145 Tg, respectively. In contrast, in the Northwest, Northeast, Inner Mongolia and Tibet significant losses of 1.38, 0.21, 0.49 and 1.01 Pg of SOC, respectively, were estimated over the last 20 years. These results highlight the importance to take measures to protect grassland and to improve management practices to increase C sequestration in farmland and forest soils.

Organic and inorganic carbon in the topsoil of the Mongolian and Tibetan grasslands: pattern, control and implications

“…Yang et al (2010a) reported an average inorganic carbon density of 5.7 kg m −2 in the top 30 cm soil of the Tibetan grasslands, equal to 19.0 kg m −3 in 0-30 cm soil depth. Even considering different focused soil depth, their result is still much higher than the results of Mi et al (2008) average value of 38.5 g kg −1 in Chinese grasslands reported by Xie et al (2007). Differences in soil formation and climatic conditions between Inner Mongolian grasslands and the Tibetan grassland may contribute to the above-described patterns.…”

“…The pattern of SOC could be attributed to climatic differences between these two regions. Low temperature leads to slower decomposition rates (Wu et al, 2003;Xie et al, 2007;Kirschbaum, 1995), and high precipitation causes high vegetation productivity (Jobbagy and Jackson, 2000;Wynn et al, 2006;Callesen et al, 2003). In addition, high moisture, and especially temporal water saturation in the Tibetan grasslands due to the frozen ground, also leads to slower decomposition rates (Baumann et al, 2009).…”

“…SOC stocks are reported to range from 37.7 Pg (Xie et al, 2007) to 41.0 Pg (Ni, 2002) in Chinese grasslands, and about 16.7 Pg in northern China's grasslands (Yang et al, 2010b). On the scale of the whole of China, SOC densities are significantly influenced by temperature (Xie et al, 2007), while soil moisture and texture control SOC density in the Tibetan grasslands (Yang et al, 2008;Baumann et al, 2009). For SIC, total stocks of the top 1 m are estimated to vary from 53.3 Pg to 77.9 Pg Mi et al, 2008;Li et al, 2007) in China.…”

“…In recent years, many studies have been conducted to investigate the pattern, controls and dynamic of SOC (e.g. Ni, 2002;Yang et al, 2010b;Wu et al, 2003;Yu et al, 2007;Xie et al, 2007;Baumann et al, 2009) as well as SIC (e.g. Yang et al, 2010a;Mi et al, 2008;Feng et al, 2002) in Chinese grasslands.…”

Ecosystem Carbon Stock Loss after Land Use Change in Subtropical Forests in China

“…Secondly, the digested solution samples were titrated with standardized 0.2 mol¨L´1 FeSO 4 solution mixed with 15 mL concentrated H 2 SO 4 per litre to prevent oxidization [32,33]. After the analysis, the soil organic C stock (Mg¨ha´1) for each layer was calculated using the following formula [34]:…”