Restructuring Nematode Food Web Energetics Drives Microbial Necromass Formation and Soil Carbon Sequestration Under Organic Amendments in Agricultural Soils

While organic amendments are extensively applied to increase soil organic carbon (SOC), the mechanisms by which they restructure soil food web energetics to regulate microbial necromass formation and carbon sequestration remain unclear. We conducted a 105-day incubation experiment to investigate how three organic amendments (straw, biochar, liquid organic fertilizer), differing in C:N ratio and pH, and applied at three rates, affect nematode food web energetics and carbon sequestration in black soils of Northeast China. Results showed that amendment properties and application rates jointly shaped nematode energetic structure, differentially regulating total energy flux (Et) and flow uniformity (U). Straw (moderate C:N, near-neutral pH) strongly increased both Et (up to 587% above control) and U (peak at 1.68) by fostering fungivore-dominated energy channels. Despite its high C:N ratio, biochar’s alkaline pH favored bacterial-dominated channels, improving flow uniformity (U up to 1.61) without increasing Et. Liquid fertilizer (low C:N, high pH) produced rate-dependent outcomes: the low rate increased Et by 210% above control via enhanced bacterial channels, whereas the high rate suppressed Et but elevated U by reducing energy allocated to plant parasites. Critically, energy flow uniformity correlated positively with fungal necromass carbon, which drove SOC accumulation, whereas total flux was strongly linked to bacterial necromass carbon, suppressing SOC. All amendments finally enhanced omnivore-predator energy flux, boosting top-down regulation and uniform energy distribution. However, mineral fertilizer truncated the food web and suppressed flow uniformity, failing to increase SOC. Therefore, organic amendments enhance SOC by restructuring nematode-mediated energy flow by reinforcing fungal-dominated channels and/or increasing flow uniformity, thereby promoting fungal necromass accumulation and providing a mechanistic basis for targeted management of degraded black soils.