Effects of Biochar Addition on Topsoil Carbon–Nitrogen Cycling and CO2 Emissions in Reduced-Nitrogen, Film-Mulched Drip-Irrigated Silage Maize Systems

This study conducted a systematic evaluation over two years (2023–2024) through field experiments to assess the regulatory effects of biochar on soil properties, carbon and nitrogen cycling, and CO2 emissions under mulched drip irrigation with varying nitrogen application levels. The core findings indicate that the effects of biochar are strongly dependent on the nitrogen levels. Under reduced nitrogen conditions, biochar demonstrated a synergistic benefit: with a 15% nitrogen reduction (N2-BC), it significantly enhanced soil water retention (increasing moisture by 68.6% at the tasseling stage); with a 30% nitrogen reduction (N1-BC), it improved soil structure (bulk density decreased by 2.1%, porosity increased by 4.3%). Additionally, biochar differentially activates soil carbon and nitrogen pools: under the 30% nitrogen reduction treatment (N1-BC), soil organic carbon increased to 8.34 g kg−1 during the jointing stage, while dissolved organic carbon reached 0.536 g kg−1 at tasseling, and total nitrogen content rose significantly. Notably, the regulatory effect of biochar on CO2 emissions shifted toward marked suppression as nitrogen input decreased (N1-BC), achieving a net cumulative reduction of 21.4% under deep nitrogen reduction treatment. Correlation analysis further integrated these processes, demonstrating that improvements in the soil physical structure are closely linked to enhanced carbon and nitrogen cycling. This study clarifies that in reduced-N systems, the application of biochar can synergistically achieve “carbon sequestration–nitrogen conservation–emission reduction,” providing a basis for developing green, low-C farmland production models.