Abstract:

【Objective】Soil organic carbon (SOC) sequestration is crucial for improving soil fertility and agricultural production sustainability. The soil inorganic carbon (SIC) is closely related to SOC with regarding with inter-transformation, which has also great effect on carbon sequestration. Crop straw return has been recognized as one of the most important organic amendment improving soil organic carbon sequestration in farmland. Meanwhile, the addition of lime also contributes greatly to increasing SIC, thereby affecting the SOC sequestration. However, the mechanism of simultaneous incorporation of crop straw and lime affecting on the CO₂ emission, SOC and SIC dynamics are not well understood, and how the SOC sequestration responds to the initial level of SOC is not clear, particularly after straw return. 【Method】 The incubation experiment and stable isotope technique (¹³C) were used in the study. The two tested soils were collected from a field with continuously cropping of winter wheat for 16 years, which was subjected to differential crop residue and nitrogen managements over long-term, including (1) S₀N₀ soil (no straw return+ nitrogen fertilizer application: 0); (2) S₁N₁ soil (high amount of straw return+ nitrogen fertilizer application: 240 kg·hm^-2). And then the two soils were both incubated with or without addition of straw and lime for 120 days under 25℃. 【Result】 The study showed that the soil cumulative CO₂ emission was observed 42.9% higher in S₁N₁ soils than that in S₀N₀ soils, when without straw and lime addition. In both soils, the straw addition alone increased the soil cumulative CO₂ emission by averages of 81.6% and 70.4%, respectively, compared with straw absence. Meanwhile, the increase of the cumulative CO₂ emission in S₀N₀ soils was higher than in S₁N₁ soils. This showed that straw addition had a greater impact on the native OC mineralization in soil with low initial SOC. Lime addition decreased soil cumulative CO₂ emission in both soils whether straw addition or not. Straw addition increased the SOC by 2.95 g·kg^-1 and 3.19 g·kg^-1 in S₀N₀ soils and S₁N₁ soils, respectively, while reduced the SOC by 1.36 g·kg^-1 in S₁N₁ soils and did not affect the SOC in S₀N₀ soils when combining with lime addition. Using¹³C stable isotope technology, the newly formed organic carbon (OC) in the two soils both significantly increased after straw addition, therein which increased 25.8% in S₀N₀ soils when compared with S₁N₁ soils. However, the conjoint addition of lime and straw did not modify the newly formed SOC when compared with the addition of straw alone, which showed that the lime addition had no effect on the decomposition process of straw in the soil. Over all, straw addition alone increased the SOC net sequestration by 3 066.3 mg?kg^-1and 2 480.53 mg?kg^-1 in S₀N₀ soils and S₁N₁ soils, respectively. The conjoint addition of lime and straw had no significant effect on the SOC net sequestration in S₀N₀ soils, but there was a decreasing trend on the SOC net sequestration in S₁N₁ soils. Lime addition reduced the cumulative CO₂ emission by 469 mg?kg^-1 and 529 mg?kg^-1 in S₀N₀ soils and S₁N₁ soils, respectively, which approximately equaled to the increases in SIC (by 443 mg?kg^-1 and 566 mg?kg^-1, respectively).【Conclusion】 In conclusion, the soil with low initial SOC content had higher potential in SOC sequestration. Lime addition might be an effective method to affect soil carbon sequestration and reduce soil CO₂ emission through chemical reactions.

Key words: soil carbon, soil respiration, straw return, lime, isotope technology