Jana E. Compton L. O. Hedin Steven S. Perakis 2005 <p>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<span>&nbsp;</span>¹⁵NH₄¹⁵NO₃<span>&nbsp;</span>at nine treatment levels ranging in geometric sequence from 0.2 kg to 640 kg N·ha⁻¹·yr⁻¹<span>&nbsp;</span>to an unpolluted old-growth temperate forest in southern Chile. We recovered roughly half of<span>&nbsp;</span>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<span>&nbsp;</span>¹⁵N was transferred from soils to unmeasured sinks above ground. However, soil solution losses of nitrate increased sharply at inputs &gt;160 kg N·ha⁻¹·yr⁻¹, corresponding to a threshold of elevated soil N availability and declining<span>&nbsp;</span>¹⁵N retention in soil. Soil organic matter (&lt;5.6 mm) dominated tracer retention at low rates of N input, but coarse roots and particulate organic matter became increasingly important at higher N supply. Coarse roots and particulate organic matter together accounted for 38% of recovered<span>&nbsp;</span>¹⁵N in soils at the highest N inputs and may explain a substantial fraction of the “missing N” often reported in studies of fates of N inputs to forests.</p><p>Contrary to expectations, N additions did not stimulate gross N cycling, potential nitrification, or ammonium oxidizer populations. Our results indicate that the nonlinearity in N retention and loss resulted directly from excessive N supply relative to sinks, independent of plant–soil–microbial feedbacks. However, N additions did induce a sharp decrease in microbial biomass C:N that is predicted by N saturation theory, and which could increase long-term N storage in soil organic matter by lowering the critical C:N ratio for net N mineralization. All measured sinks accumulated<span>&nbsp;</span>¹⁵N tracers across the full gradient of N supply, suggesting that short-term nonlinearity in N retention resulted from saturation of uptake kinetics, not uptake capacity, in plant, soil, and microbial pools.</p> application/pdf 10.1890/04-0415 en Ecological Society of America Nitrogen retention across a gradient of 15N additions to an unpolluted temperate forest soil in Chile article