Nitrogen Addition Decreased Respiration and Heterotrophic Respiration but Increased Autotrophic Respiration in a Cabbage ( Brassica pekinensis Rupr) Experiment in the Northeast Plains

Farmland soil respiration ( Rs ) significantly impacts the global carbon (C) cycle. Although nitrogen (N) can promote crop growth and increase yields, its relationship with Rs and its constituents, including autotrophic respiration ( Ra ) and heterotrophic respiration ( Rh ), remains unclear. Therefore, a field study was carried out in a cabbage ( Brassica pekinensis Rupr) system to probe the impact of N addition on Rs , Ra , and Rh . Five levels of N addition, including 0 kg N hm −2 ·yr −1 (N0), 50 kg N hm −2 ·yr −1 (N50), 100 kg N hm −2 ·yr −1 (N100), 150 kg N hm −2 ·yr −1 (N150), and 200 kg N hm −2 ·yr −1 (N200), started in March 2022. The Rs ( Ra and Rh ) and soil samples were measured and collected twice a month. The findings revealed the following: (1) N fertilizer enhanced Ra while reducing Rs and Rh ; (2) soil temperature (ST), belowground net primary productivity (BNPP), soil inorganic N (SIN), and soil total C/total N (C/N) were the significant elements influencing Ra , and microbial biomass carbon (MBC), SIN, and microbial diversity (MD) were the primary factors influencing Rh ; (3) partial least squares-path models (PLS-PM) showed that ST and SIN directly impacted Rh , while ST and BNPP tangentially influenced Ra ; (4) 150 kg N hm −2 ·yr −1 was the ideal N addition rate for the cabbage in the region. In summary, the reactions of Ra and Rh to N fertilizer in the Northeast Plains are distinct. To comprehend the underlying processes of Rs , Ra , and Rh , further long-term trials involving various amounts of N addition are required, particularly concerning worsening N deposition.