Nitrogen inputs and losses in response to chronic CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; exposure in a subtropical oak woodland

B A Hungate,D W Johnson,B D Duval,P Stiling,W Cheng,P Dijkstra,A Hartley,M E Ketterer,J Millman,D B Stover

doi:10.5194/bg-11-3323-2014

Abstract

Abstract. Rising atmospheric CO2 concentrations may alter the nitrogen (N) content of ecosystems by changing N inputs and N losses, but responses vary in field experiments, possibly because multiple mechanisms are at play. We measured N fixation and N losses in a subtropical oak woodland exposed to 11 years of elevated atmospheric CO2 concentrations. We also explored the role of herbivory, carbon limitation, and competition for light or nutrients in shaping the response of N fixation to elevated CO2. Elevated CO2 did not significantly alter gaseous N losses, but lower recovery and deeper distribution in the soil of a long-term 15N tracer indicated that elevated CO2 increased leaching losses. Elevated CO2 had no effect on nonsymbiotic N fixation, and had a transient effect on symbiotic N fixation by the dominant legume. Elevated CO2 tended to reduce soil and plant concentrations of iron, molybdenum, phosphorus, and vanadium, nutrients essential for N fixation. Competition for nutrients and herbivory likely contributed to the declining response of N fixation to elevated CO2. These results indicate that positive responses of N fixation to elevated CO2 may be transient and that chronic exposure to elevated CO2 can increase N leaching. Models that assume increased fixation or reduced N losses with elevated CO2 may overestimate future N accumulation in the biosphere.

Highlights

Nitrogen (N) is the element most frequently limiting to plant growth (LeBauer and Treseder, 2008)
We synthesize the effects of 11 years of chronic exposure to increased CO2 concentrations on N inputs and losses from a subtropical oak woodland
Our results indicate that processes promoting N loss were more responsive to elevated CO2 than were processes promoting N accumulation

Summary

Introduction

Nitrogen (N) is the element most frequently limiting to plant growth (LeBauer and Treseder, 2008). Nitrogen inputs and losses from terrestrial ecosystems determine ecosystem N pool size, and in turn influence the potential for carbon (C) uptake when plant growth is N limited. Carbon uptake and storage are sensitive to the balance of N inputs and losses (Pepper et al, 2007; Gerber et al, 2010; Esser et al, 2011). Nitrogen fixation is the major biological pathway through which the biosphere accumulates N. Nitrogen fixation has a high demand for reducing power to break the triple covalent bond shared by the two atoms in the N2 molecule (Benemann and Valentine, 1972). Symbioses between bacteria capable of N fixation and photosynthetic organisms have evolved

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Conclusion