1. Evaluating climate effects on plant-soil interactions in terrestrial ecosystems remains challenging due to the fact that floristic composition co-varies with climate, particularly along rainfall gradients. It is difficult to separate effects of precipitation per se from those mediated indirectly through changes in species composition. As such, afforestation (the intentional planting of woody species) in terrestrial ecosystems provides an ecological opportunity to assess the relative importance of climate and vegetation controls on ecosystem processes. 2. We investigated the impacts of 35 years of afforestation on ecosystem N dynamics, in ecosystems ranging from arid shrub-steppe to closed-canopy forest in Patagonia, Argentina. Site of natural vegetation and adjacent sites planted with a single exotic species, Pinus ponderosa, were identified in five precipitation regimes along a continuous gradient of 250 to 2200 mm mean annual precipitation (MAP). We evaluated C and N parameters of vegetation and soil, as well as natural abundance of 13C and 15N, in leaves, roots, ectomycorrhizae (EcM), and soils. 3. In natural vegetation, most leaf traits (%N, C:N ratios, leaf mass per area, δ15N values) demonstrated strong significant relationships with MAP, while these relationships were nearly absent in afforested sites. In addition, the EcM of native southern beech and pine trees were significantly enriched in 15N relative to leaves at all sites where they were present. While soil C and N pools in both vegetation types increased with MAP, overall pool sizes were significantly reduced in afforested sites. 4. Synthesis Observed relationships between leaf traits and precipitation in natural vegetation may be driven largely by shifts in species composition and plant-soil interactions, rather than direct effects of precipitation. Our results suggest that a change in species composition of the dominant vegetation is sufficient to alter C and N cycling independently of climate constraints: pine afforestation homogenized N dynamics across sites spanning an order of magnitude of MAP. These results highlight the important control of ectomycorrhizal associations in affecting C and N dynamics. Additionally, it serves to demonstrate that altering natural species composition alone is sufficient to cause large, detectable impacts on N turnover independently of direct climate effects.