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NITROGEN RETENTION ACROSS A GRADIENT OF 15 N ADDITIONS TO AN UNPOLLUTED TEMPERATE FOREST SOIL IN CHILE
Author(s) -
Perakis Steven S.,
Compton Jana E.,
Hedin Lars O.
Publication year - 2005
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1890/04-0415
Subject(s) - nitrification , soil water , temperate forest , cycling , environmental science , nitrogen cycle , leaching (pedology) , organic matter , ecosystem , nitrogen , temperate climate , temperate rainforest , soil organic matter , sink (geography) , ammonium , ecology , nitrate , environmental chemistry , agronomy , chemistry , soil science , biology , forestry , geography , organic chemistry , cartography
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 15 NH 4 15 NO 3 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 15 N 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 15 N 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 15 N retention in soil. Soil organic matter (<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 15 N 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. 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 15 N 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.

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