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=103 Tg=1015 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−1 in Gray Desery grassland soils to 252.6 Mg ha−1 in Mountain Meadow forest soils. The average area-weighted total SOC density in paddy soils (97.6 Mg ha−1) was higher than that in upland soils (80 Mg ha−1). Soils under forest (137.3 Mg ha−1) had a similar average area-weighted total SOC density as those under grassland (135.4 Mg ha−1). 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.

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

Soil carbon sequestration is being considered as a potential pathway to mitigate climate change. Cropland soils could provide a sink for carbon that can be modified by farming practices; however, they can also act as a source of greenhouse gases (GHG), including not only nitrous oxide (N2 O) and methane (CH4 ), but also the upstream carbon dioxide (CO2 ) emissions associated with agronomic management. These latter emissions are also sometimes termed "hidden" or "embedded" CO2 . In this paper, we estimated the net GHG balance for Chinese cropping systems by considering the balance of soil carbon sequestration, N2 O and CH4 emissions, and the upstream CO2 emissions of agronomic management from a life cycle perspective during 2000-2017. Results showed that although soil organic carbon (SOC) increased by 23.2 ± 8.6 Tg C per year, the soil N2 O and CH4 emissions plus upstream CO2 emissions arising from agronomic management added 269.5 ± 21.1 Tg C-eq per year to the atmosphere. These findings demonstrate that Chinese cropping systems are a net source of GHG emissions and that total GHG emissions are about 12 times larger than carbon uptake by soil sequestration. There were large variations between different cropping systems in the net GHG balance ranging from 328 to 7,567 kg C-eq ha-1  year-1 , but all systems act as a net GHG source to the atmosphere. The main sources of total GHG emissions are nitrogen fertilization (emissions during production and application), power use for irrigation, and soil N2 O and CH4 emissions. Optimizing agronomic management practices, especially fertilization, irrigation, plastic mulching, and crop residues to reduce total GHG emissions from the whole chain is urgently required in order to develop a low-carbon future for Chinese crop production.

/pdf/chinese-cropping-systems-are-a-net-source-of-greenhouse-ybamfzbly5.pdf

Chinese cropping systems are a net source of greenhouse gases despite soil carbon sequestration

The soil organic carbon (SOC) and total nitrogen (TN) of topsoil could be greatly affected by the conversion of grassland to cropland (CGC) due to vegetation harvesting and soil erosion. However, few studies have evaluated the effects of these changes in the subsoil and distinguished its main controls at the regional scale. This paper investigated the total changes in the SOC stock (SOCS) and TN stock (TNS) of soils from 0 to 30 cm and 0–100 cm in depth over the past three decades in the Songnen Plain, China, a typical Mollisol region. The CGC resulted in a moderate loss of topsoil C and N but an increase in subsoil C and N, and the soil mapping results showed that approximately 7.47 × 1011 kg C and 1.51 × 1011 kg N were accumulated during the past three decades. Prediction uncertainty was assessed by 100 model runs with random jackknife partitions, and high uncertainty was found in the areas with rapidly changing SOCS and TNS levels. Our findings indicated that subsoil should be considered in the estimation of the SOCS and TNS at the regional scale. The SOCS and TNS of the CGC areas were anticipated to increase after long-term cultivation. In view of ongoing cropland expansion, up-to-date land use and soil type data are vital for selecting monitoring sites and understanding long-term soil evolution at the regional scale.

The influence of the conversion of grassland to cropland on changes in soil organic carbon and total nitrogen stocks in the Songnen Plain of Northeast China

Effect of Long-Term Application of Chemical Fertilizers on Microbial Biomass and Functional Diversity of a Black Soil

Soil salinity and sodicity is considered one of the most import impediments to agricultural development in Northeast China The contents of TP and TK decrease with soil depth and high coefficients of variation were found in TOC AN and AP Mean EC in the 0-50 cm soil layers ranged from 0 61 to 0 89 dS m(-1) and the average soluble ion concentrations in the topsoil (0-10 cm) were approximately 11 38 mmol L-1 for Na+ 1 21 mmol L-1 for Ca2+ and 040 mmol L-1 for Mg2+ High SAR existed in the layers 10-50 cm indicating the studied soil was bearing low salinity in the top layer and high sodic layer in the subsurface The soil presented strong alkali reactions all through the profile with pH over 9 5 To improve and utilize saline sodic soil rationally several strategies were put forward based on long term field studies and demonstration works The results implied that ameliorating with sand applying farm yard manure regen erating salt tolerant grasses and leaching with groundwater and growing rice were effective measures for improving physical and chemical qualities of saline sodic soil

http://ir.yic.ac.cn/bitstream/133337/4397/1/Biogeochemical%20Characterizations%20and%20Reclamation%20Strategies%20of%20Saline%20Sodic%20Soil%20in%20Northeastern%20China.pdf

Biogeochemical Characterizations and Reclamation Strategies of Saline Sodic Soil in Northeastern China

The study results of the content variation of organic carbon and its component in black soil during different tillage period show that both the content of TOC and its component decreasing and the soil Kos value increasing in ploughing layer and ploughing bottom layer are rapid at first 50 years cultivation. After 130 years tillage, the content of organic carbon and its component are relative stable on the whole. Total organism carbon content in ploughing layer of black soil lost 1.76%~24.81% after 200 years tillage. Among organic carbon component, the FA-C is affected relative little by tillage time, while Hu-C is affected seriously.

Organic Carbon Variation Law of Black Soil During Different Tillage Period

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Organic Carbon Variation Law of Black Soil During Different Tillage Period