Age-dependent biomass allocation indicates ecosystem responses to chronic nitrogen addition in artificial Poa pratensis grasslands

Nitrogen (N) addition strongly affects plant biomass allocation, yet most existing studies are based on short-term observations, leaving long-term changes in biomass allocation patterns and organ-level construction strategies across stand development poorly understood. Using artificial Poa pratensis grasslands, we conducted an eight-year field experiment with four N addition levels to quantify organ-level biomass allocation patterns across different growth ages. Our results showed that growth age, N addition, and their interaction significantly affected above- and belowground biomass as well as the root-to-shoot ratio (R/S). Aboveground biomass exhibited pronounced variation across growth ages, with noticeable shifts in biomass allocation occurring around growth ages 5-6, whereas belowground biomass responded more gradually. Nitrogen addition generally increased overall biomass, and N medium (45 kg·N·hm⁻² yr⁻¹) effectively promoted biomass accumulation across most growth ages. Relationships associated with biomass allocation also varied with growth age: in the 2-4 year interval, R/S was mainly associated with soil nutrient status and aboveground biomass; in the 5-6 year interval, R/S was more closely linked to soil moisture and belowground allocation; and in the 7-9 year interval, R/S was more strongly related to belowground biomass components. These findings demonstrate that R/S and organ-level biomass partitioning reflect growth-age-associated responses to nitrogen enrichment, underscoring the importance of considering variation among growth ages when interpreting long-term N effects in artificial alpine grasslands.