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

Steven S. Perakis, Jana E. Compton, Lars O. Hedin

Research output: Contribution to journalArticlepeer-review

60 Scopus citations

Abstract

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 15NH4 15NO3 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 (<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 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. 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 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.

Original languageEnglish (US)
Pages (from-to)96-105
Number of pages10
JournalEcology
Volume86
Issue number1
DOIs
StatePublished - Jan 2005

All Science Journal Classification (ASJC) codes

  • Ecology, Evolution, Behavior and Systematics

Keywords

  • Ammonium nitrate
  • Dissolved organic nitrogen
  • N stable isotope
  • Nitrification
  • Nitrogen fertilizer
  • Nitrogen saturation
  • Nutrient budget
  • Old-growth temperate forest
  • Roots
  • Soil organic matter

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