Impact of paddy-to-vegetable land conversion on soil heterotrophic respiration and its driving factors

In recent years, some rice paddies in southern China have been converted to vegetable cultivation, which may lead to a loss of soil carbon stocks in these fields. However, the dynamics of soil heterotrophic respiration and its main driving factors during the initial stage of this conversion remain unclear. In this study, three treatments, including rice paddies, conventional vegetable fields, and greenhouse vegetable fields, were established in a mid-subtropical region. A one-year in situ observation was conducted to measure soil heterotrophic respiration and related environmental factors, aiming to explore the effects of rice-to-vegetable conversion on soil heterotrophic respiration and its driving mechanisms. The results showed that converting rice paddies to vegetable fields significantly increased both the rate and annual cumulative amount of soil heterotrophic respiration. The annual cumulative amounts in conventional and greenhouse vegetable fields increased by 82% and 80% compared to rice paddies, respectively, but the difference between them was not significant. Soil heterotrophic respiration rates exhibited a significant exponential relationship with soil temperature (T_s). The Q₁₀ value of conventional vegetable fields (1.67) decreased by 8% compared to rice paddies (1.82), whereas the Q₁₀ value of greenhouse vegetable fields (1.86) increased by 2%. The conversion from rice to vegetable fields significantly decreased soil water content (SWC) and increased the contents of nitrate nitrogen (NO₃⁻-N), ammonium nitrogen (NH₄⁺-N), and dissolved organic carbon (DOC), thereby promoting soil heterotrophic respiration. Partial least squares path modeling indicated that soil heterotrophic respiration was significantly correlated with soil temperature (T_s) under all land-use types. Specifically, soil heterotrophic respiration in rice paddies was directly regulated by moisture, while after conversion to vegetable fields, soil respiration in conventional vegetable fields was mainly related to NO₃⁻-N content, and in greenhouse fields, it was mainly influenced by DOC content. This study reveals that soil temperature, water content, and carbon-nitrogen availability are key drivers of soil carbon emissions during the conversion of rice paddies to vegetable fields, providing empirical data for understanding soil carbon cycling under land-use change.