Impact of cropland use transformation on ecosystem carbon sinks in a typical agroforestry mixed region: An analysis from explicit and implicit perspectives

Abstract

Cropland use transformation (CUT) not only increases grain production but also considerably impacts regional ecosystem carbon sinks. However, comprehensive studies exploring connections between CUT and ecosystem carbon sinks are limited. We constructed an indicator system and transformation framework for CUT by considering explicit and implicit dimensions. Moreover, we analyzed changes in the CUT in the middle and lower reaches of the Yangtze River Basin (MLYB), as well as the trends in net ecosystem productivity (NEP). Structural equation modeling was used to explore the direct and indirect impacts of CUT on NEP. The MLYB lost approximately 0.82–0.86 × 10³ km² of cropland annually, with the cropland being primarily transformed into construction and forest land. The proportion of the three major grain crops showed fluctuating increases. The average NEP in the MLYB from 1999 to 2019 was 549.36 gC/m²/yr, while the capacity of ecosystem carbon sinks gradually weakened, with approximately 29.96 % of the area showing a highly significant degradation trend, and 19.42 % showing a significant degradation trend. In terms of explicit CUT indicators, the total effect of the cropland area on NEP was negative, with a direct effect of −0.21, an indirect effect of 0.10, and a total effect of −0.11. The overall effect of the cropping structure on NEP was positive, with a direct effect of 0.21, an indirect effect of 0.02, and a total effect of 0.23. Regarding implicit CUT indicators, the input of agricultural production factors directly inhibited ecosystem carbon sinks, with an effect value of −0.62. Cropland output directly promoted NEP, with an effect value of 0.65. This study explains the relationship between CUT and ecosystem carbon sinks, providing a scientific basis for advancing green agriculture in China.