Artificial Light at Night Drives Asymmetric Biological Allocation of Soil Carbon and Nitrogen by Fragmenting Micro‐Food Webs

Artificial light at night (ALAN) is an increasingly pervasive global change driver, yet its effects on the interconnected biogeochemical cycling of soil carbon (C) and nitrogen (N) remain poorly understood. Here, we conducted an in situ dual‐isotope ( 13 C‐CO 2 pulse labelling and 15 N addition) experiment combined with multitrophic microbial network analysis in a semi‐arid meadow steppe to explore the impact of ALAN on belowground C and N cycling and the underlying mechanisms. We found that ALAN significantly accelerated the turnover of newly fixed plant C, a process primarily fueled by the increased root‐derived C input from ALAN‐stimulated plant biomass (the bottom‐up C‐pump). Concurrently, while this plant‐driven C surplus primarily fueled elevated soil respiration, ALAN substantially enhanced microbial 15 N retention, resulting in a pronounced asymmetric biological allocation of assimilated C and N within the biotic pools. This stoichiometric divergence was underpinned by a physiological shift toward a conservative microbial N retention strategy, whereby soil microorganisms increased N retention efficiency to counterbalance the stoichiometric C surpluses induced by enhanced plant inputs. Crucially, we found that this enhanced biological N retention was predicted not by microbial taxonomic diversity but was closely associated with a directional trend toward reduced micro‐food web structural connectivity. This fragmented network architecture potentially altered top‐down trophic controls, which compounded the plant‐driven stoichiometric imbalance by limiting predator‐mediated N mineralization and thereby promoting relative N retention within the microbial biomass. Our findings highlight light pollution as an underappreciated global change driver that specifically accelerates plant‐mediated bottom‐up C loss while simultaneously promoting microbial N retention via fragmented micro‐food webs in increasingly illuminated ecosystems.