Nitrogen retention across a gradient of 15N additions to an unpolluted temperate forest soil in Chile

Accelerated nitrogen (N) inputs can drive nonlinear changes in N cycling, retention, and loss in forest ecosystems. Nitrogen processing in soils is critical to understanding these changes, since soils typically are the largest N sink in forests. To elucidate soil mechanisms that underlie shifts in N cycling across a wide gradient of N supply, we added ¹⁵NH₄¹⁵NO₃ at nine treatment levels ranging in geometric sequence from 0.2 kg to 640 kg N·ha⁻¹·yr⁻¹ to an unpolluted old-growth temperate forest in southern Chile. We recovered roughly half of ¹⁵N tracers in 0–25 cm of soil, primarily in the surface 10 cm. Low to moderate rates of N supply failed to stimulate N leaching, which suggests that most unrecovered ¹⁵N was transferred from soils to unmeasured sinks above ground. However, soil solution losses of nitrate increased sharply at inputs >160 kg N·ha⁻¹·yr⁻¹, corresponding to a threshold of elevated soil N availability and declining ¹⁵N retention in soil. Soil organic matter (