Mycelial pathway carbon input enhances nitrogen utilization in cotton more than the root pathway in symbiotic...

Plant roots and arbuscular mycorrhizal fungi (AMF) significantly impact soil carbon (C) and nitrogen (N) processes. However, it remains unclear whether there are differences in the priming effects of new C input from roots and extraradical mycelia under different N application rates in agricultural fields, and how these differences affect crop N uptake. Therefore, this study used upland cotton (Xinluzao No. 84) as the experimental material and employed a two-chamber partitioned growth system (length × width × height: 13 × 10 × 13 cm) to investigate the effects of different N application rates (1.5, 1 and 0 g per device) on soil microbial activity, the priming effects induced by C inputs from roots and mycelia, and the N uptake by roots and mycelia. This study found that, compared to no N fertilizer, root and mycelial C inputs under reduced N condition increased soil organic carbon (SOC) by 17.2 % and 73.7 %, respectively. Furthermore, the content of mycelial C input showed a significant negative correlation with the soil original C content (P < 0.05), whereas root C input exhibited a positive correlation (P < 0.01). Additionally, the study revealed a significant positive linear relationship between the mycelia to cotton N contribution and SOC content (P < 0.05), as well as significantly higher activities of soil extracellular enzymes (e.g., PPO and NAG) via the mycelial pathway compared to the root pathway under the same N rates. These results demonstrate that new C input from mycelia under reduced N condition induced a positive priming effect, thereby enhancing the decomposition and utilization of N-containing organic matter in agricultural soil. Concurrently, N reduction increased the uptake of soil NO₃^--N by the mycelia. Ultimately, the timely release of stored N by the mycelia, coupled with its functional shift from parasitism to symbiosis, significantly enhanced N delivery to the host plant. In contrast, as a component of soil new C input, root C input induced a negative priming effect, which contributed to soil C accumulation and helped maintain the balance of the soil C pool. These findings provide important insights into the mechanisms by which plant-AMF symbiosis regulates soil C- N process under different N conditions, and hold significant implications for improving N use efficiency and promoting sustainable agricultural development.