Threshold response of the agricultural modernization to the open crop straw burning CO2 emission in China's nine major agricultural zones

Achieving carbon emission reduction in agriculture has become a key issue in promoting agricultural modernization. However, the continued open crop straw burning (OCSB) in China has exerted enormous pressure on agricultural modernization. Related research mostly focused on the synergic relationship between CO2 emission and agricultural modernization level (AML), ignoring the nonlinear response of agriculture emission to AML and regional differences, especially OCSB CO2 emission. Thus, based on the fire point data and crop straw yield, the OCSB CO2 emission of prefecture-level cities in China was calculated, and the AML was measured by constructing an evaluation index system of 23 indicators. With the constraint lines and restricted cubic splines (RCSs), their underexplored threshold relationships in Chinese nine agricultural zones were revealed, and targeted control areas were proposed. Results showed that during 2013-2021, Chinese OCSB CO2 emission showed an overall downward trend with a total 18,823 Mt reduction, while average AML steadily improved by 0.026. Significant negative U-shaped relationship was found between OCSB CO2 emission and AML, while OCSB CO2 emission attained the potential extreme values when AML reached 0.349. Meanwhile, AML thresholds varied significantly across the agricultural zones as OCSB CO2 emission from different crops reached the potential extreme values. Accordingly, cities were spatially divided into three types of control areas: priority prevention area, centralized remediation area, and strictly controlled area. We quantified OCSB CO2 emission and AML of prefecture-level cities and identified potential high OCSB CO2 emission areas, which can provide data support and differentiated management countermeasures for low-carbon agricultural development in various agricultural zones of China.