Abstract: Objective Farmland shelterbelts in arid and semi-arid regions have experienced varying degrees of degradation during long-term management, which has become an important constraint on the maintenance of their ecological functions. This study aimed to clarify the nutrient variation characteristics of the plant-soil-microbial system in degraded farmland shelterbelts and to determine whether different tree species exhibit distinct nutrient response pathways under broadly similar regional environmental conditions. Methods Three typical poplar farmland shelterbelts on the northeastern margin of the Ulan Buh Desert, dominated by Populus alba, Populus nigra, and Populus popularis, were selected as the study objects. Sample plots representing different degradation levels were established for each shelterbelt type. Carbon (C), nitrogen (N), and phosphorus (P) contents and their stoichiometric ratios were measured in plant leaves, soils, and soil microbial biomass. On this basis, nutrient variation characteristics of the plant-soil-microbial system were analyzed across degradation levels, and the coordinated responses among plants, soils, and microorganisms were further examined. Results The C, N, and P contents and their stoichiometric ratios in the plant-soil-microbial systems of the three shelterbelt types all changed, to different extents, under different degradation levels. Leaf N:P generally showed a declining trend, indicating a gradual intensification of nitrogen limitation in the shelterbelt systems. Clear interspecific differences were observed in nutrient response pathways. In Populus alba shelterbelts, degradation was mainly characterized by decreases in soil organic carbon (SOC) and total nitrogen (TN), accompanied by an increase in soil microbial biomass. In Populus nigra shelterbelts, the dominant response was a decrease in leaf N concentration, whereas soil and microbial indicators remained relatively stable. In Populus popularis shelterbelts, the most pronounced responses were reflected in an increase in leaf C:N and a decrease in soil N:P. Overall, nutrient allocation patterns in the shelterbelt systems were markedly altered under different degradation levels, and significant correlations were detected among C, N, and P contents and their ratios in plants, soils, and microorganisms. Conclusions Different degradation levels significantly affected nutrient variation in farmland shelterbelts on the northeastern margin of the Ulan Buh Desert. Although the three shelterbelt types were distributed under similar regional environmental conditions, they still exhibited distinct nutrient response pathways, suggesting that species-specific traits play an important role in regulating nutrient dynamics during shelterbelt degradation. At the system level, the general response was a progressive enhancement of nitrogen limitation and a marked adjustment of nutrient allocation patterns within the plant-soil-microbial continuum.