Biochar amendment into soils has been suggested as an effective means to abate global warming by storing the carbon rich but decomposition resistant biomass materials in soils, while simultaneously bettering soil properties and increasing plant biomass yields (Christian, 2001; Farji-Brener and Ghermandi, 2000). Biochar technology could potentially reduce about 1.8 Pg CO₂-C yearly, equal to 12% of current anthropogenic CO₂-C emissions (Christian, 2001). It was estimated that biochar application would reduce 3–4 folds of current carbon loss from the soil pool (Zhao et al., 2015). Biochar-amended soil could sequester C over a long time by physical stabilization (Novak et al., 2009; Yin et al., 2014). The mean residence time of biochar is estimated to about 2000 years, and the half-life is about 1400 years. Biochar application also has positive effects up to 30% on biomass production with the large-volume application of BC (30~60t/ha), and this would yield more plant-derived biomass input into soils (Yin et al., 2014). However, carbon mineralization usually was stimulated or suppressed by biochar through positive or negative priming effects and varied significantly and separately (Prommer et al., 2014; Sui et al., 2016). This might offset the benefit of increasing SOC by biochar addition in short-term time. It was believed that credible data are needed from varied field experiments and soil types while using biochar as a soil amendment may be a potentially useful option to mitigate climate change.

Saline-sodic soils cover 3.1% of the global land area, and world soils that are currently saline have lost an average of 3.47 t SOC/ha since they become saline (Yang et al., 2018). Improvement of saline-sodic soil is increasingly undertaken as a means of reclaiming otherwise unproductive agricultural land, and biochar application is a potential choice for fertilizing saline-sodic soils for replantation (Munda et al., 2018). Saline-sodic soils are flocculated with high soluble salts and exchangeable Na⁺ and would become disperse when pH is higher than 8.5. Organic matter contents in saline-sodic soils are usually low due to poor plant growth, which is restricted by poor aeration, compaction, and lower nutrient bioavailability (Sun et al., 2016). Since biochar materials are charcoal-like, high porous, fine-grained, and have a large surface area, its incorporation application to soils has attracted considerable attentions as an effective and economic soil amendment for improving soil physicochemical properties, enhancing plant biomass and increasing soil organic carbon pool. Biochar has been successfully used to reduce the nutrient deficiency and salt stress (Sun et al., 2016), reclaim degraded soils (Sun et al., 2016), facilitate plant growth (Brodowski et al., 2005), and suppress SOC mineralization (Lin et al., 2015). However, the effects of biochar addition on SOC pool were usually with inconsistent results depending on the nature of soil and biochar, soil types, and incubation time. Besides as a direct carbon source that can increase SOC pool, biochar addition would act as nutrient sources such as nitrogen and phosphorus, which facilitate plant growth and increase the SOC pool indirectly. The large adsorption capacity of biochar could also preserve organic carbon in the pores that prevent them from decomposition by microorganisms. However, most studies about biochar amendment were carried out on nonsalt-affected soils, and knowledge about the effects of biochar application on carbon dynamics in saline-sodic soils is still scant and not well understood, which need further evaluation (Sollins et al., 1996).

As one of the largest saline-sodic soil areas in the world, the salt-affected areas were estimated to 3.84×10⁶ ha in the Songnen Plain of northeast China. Saline-sodic soils here have high montmorillonite clay and sodium bicarbonate and very low SOC content, and carbon sequestration rates were <60 gC/m²/yr in the Momoge wetland site (Sollins et al., 1996). How to improve soil physicochemical properties, fertilizer saline-sodic soil for more biomass yield, and to increase soil carbon pool for mitigating rising atmospheric CO₂ is one the most important environmental spot here. The objectives of the present work are: (1) to compare impacts of biochar addition on soil organic carbon pools based on incubation experiments in the field, and (2) to reveal potential factors that govern SOC contents before and after biochar addition. Based on these two aims, it is hoped to manageably provide carbon farming solutions to the global climate and satisfying food demand using biochar technology.