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Shifts in an ecosystem’s state can alter biogeochemical cycling and the extent of nutrient conservation within a terrestrial landscape on multiple time scales. Transient biogeochemical changes may follow disturbance and succession, although persistent long-term differences may exist under different climates and vegetation types. We evaluate the potential for such biogeochemical changes in the context of long-term ecological history by measuring the nitrogen isotope composition of organic matter in a lake sediment core. We targeted Little Windy Hill Pond (LWH) in the Medicine Bow Mountains, Wyoming because reconstructions of the lake level, fire, and vegetation histories from the lacustrine sediments indicated a century-scale transformation from an arid, shrub-dominated landscape to a sub-alpine, tree-dominated ecosystem with extensive woody cover and large, live biomass pools. We demonstrate that the afforestation at the beginning of the Holocene transformed the Artemisia-dominated ecosystem, which had persisted for millennia during the Pleistocene. The changes affected nitrogen cycling dynamics, especially through intensified nutrient conservation when live biomass pools increased with greater woody cover. The LWH sediments record a baseline delta N-15 shift from 2.2-3.0 to 0.3-2.0aEuro degrees as less N-15-enriched organic matter accumulated in the lake. We also observed a transient pattern of maximum nutrient conservation and minimum delta N-15 values as terrestrial biomass increased during the aggradation (similar to 175 years) and transition phases of ecological succession. Our nitrogen isotope results support theoretical expectations of long-term biogeochemical dynamics as nutrient conservation increases during afforestation.