Abstract
We used a previously described precipitation gradient in a tropical montane ecosystem of Hawai’i to evaluate how changes in mean annual precipitation (MAP) affect the processes resulting in the loss of N via trace gases. We evaluated three Hawaiian forests ranging from 2200 to 4050 mm year−1 MAP with constant temperature, parent material, ecosystem age, and vegetation. In situ fluxes of N2O and NO, soil inorganic nitrogen pools (NH +4 and NO −3 ), net nitrification, and net mineralization were quantified four times over 2 years. In addition, we performed 15N-labeling experiments to partition sources of N2O between nitrification and denitrification, along with assays of nitrification potential and denitrification enzyme activity (DEA). Mean NO and N2O emissions were highest at the mesic end of the gradient (8.7±4.6 and 1.1±0.3 ng N cm−2 h−1, respectively) and total oxidized N emitted decreased with increased MAP. At the wettest site, mean trace gas fluxes were at or below detection limit (≤0.2 ng N cm−2 h−1). Isotopic labeling showed that with increasing MAP, the source of N2O changed from predominately nitrification to predominately denitrification. There was an increase in extractible NH +4 and decline in NO −3 , while mean net mineralization and nitrification did not change from the mesic to intermediate sites but decreased dramatically at the wettest site. Nitrification potential and DEA were highest at the mesic site and lowest at the wet site. MAP exerts strong control N cycling processes and the magnitude and source of N trace gas flux from soil through soil redox conditions and the supply of electron donors and acceptors.
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Acknowledgments
This work was funded by a grant from the Andrew W. Mellon foundation to P. Matson. While gratefully acknowledging our funder, we thank E.A.G. Schuur for establishing these sites as well as B. Hobdy (Hawai’i DLNR) and M. Vaught (EMI Co.) for access to the Makawao and Ko’olau Forest Reserves. H. Farrington, E. Hinckley, M. Beman, Z. Moore, C. Nielsen, C. Snyder, J. Moen, P. Singleton, H. Kaiser, D. Herman and P. Brooks provided field and laboratory assistance. P. Vitousek, B. Houlton, L. Hedin, M. Vile, J. von Fischer, S. Alin, and H. Farrington inspired helpful discussions and provided useful comments. Experiments comply with all current laws of the United States of America.
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Appendix: N2O source study equations
Appendix: N2O source study equations
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1.
Atom percent 15N2O, 15NH +4 , and 15NO −3 of soil and gas samples were calculated from the 46/45 (N2O), 29/28 and 30/28 (NH +4 , NO −3 ) mass ratios. Data were blank corrected and corrected for analytical drift between runs.
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2.
We determined the amount of 15N2O-N accumulated in the chamber headspace (15N2Oh in μmol) during incubation by the following equation:
$$ ^{{{\text{15}}}} {\text{N}}_{{\text{2}}} {\text{O}}_{{\text{h}}} = S_{1} {\left( {\frac{{{\text{N}}_{{\text{2}}} {\text{O}}_{{{\text{ap}}}} }} {{100}}} \right)} - S_{0} (0.003663) $$(1)where N2Oap is the measured atom percent 15N2O-N at the end of the incubation, S 1 is the highest observed headspace N2O concentration, and S 0 is the lowest. N2O concentration was converted to μmol using the Ideal Gas Law. 15N natural abundance is assumed to be 0.3663%.
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3.
Soil 15N enrichment (Nap in atom percent) after labeling with 15NH +4 or 15NO −3 was estimated using a standard mixing equation (from Panek et al. 2000):
$$ {\text{N}}_{{{\text{ap}}}} = \frac{{{\text{N}}_{{{\text{native}}}} \times 0.3663 + {\text{N}}_{{{\text{added}}}} \times 99.0}} {{{\text{N}}_{{{\text{native}}}} + {\text{N}}_{{{\text{added}}}} }} $$(2)where Nnative is the concentration of NH +4 or NO −3 of the soil prior to labeling (measured from control plots) and Nadded is the concentration of NH +4 or NO −3 added to the plots through injections. Soil 15N natural abundance is assumed to be 0.3663% and label enrichment was 99.0 atom percent.
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4.
We calculated the amount of soil 15N (\(^{{15}} {\text{N}}_{{{\text{NH}}^{{\text{+}}}_{{\text{4}}} {\text{or NO}}^{-}_{{\text{3}}} }},\) in μmol) using estimated soil enrichment and measured soil inorganic N concentrations.
$$^{{\text{15}}} {\text{N}}_{{{\text{NH}}^{{\text{+}}}_{{\text{4}}} {\text{or NO}}^{-}_{{\text{3}}} }} = {\text{N}}_{{{\text{NH}}^{{\text{+}}}_{{\text{4}}} {\text{or NO}}^{-}_{{\text{3}}} }} \times \frac{{{\text{N}}_{{{\text{ap}}}} }}{{{\text{100}}}} \times {\text{BD}} \times V$$(3)where Nap is atom percent 15NH +4 or 15NO −3 from Eq. 2, \( {\text{N}}_{{{\text{NH}}^{{\text{+}}}_{{\text{4}}} {\text{or NO}}^{-}_{{\text{3}}} }} \) is the concentration of NH +4 or NO −3 at the end of the incubation (μmol N g−1 dry soil), BD (g dry soil cm−3), and V is the volume of the experimental plot to 10 cm depth (cm−3).
For the July 2002 experiment, we were able to measure soil 15N enrichment directly and compare with estimated values from Eq. 2. Measured were 75% (Site 1), 111% (Site 4), and 130% (Site 5) of estimated values and results presented in Fig. 3 were essentially unchanged with measured versus estimated enrichments. For consistency, we used estimated values for both experiments.
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5.
The amount of 15N2O-N in the headspace derived from either 15NH +4 or 15NO −3 was corrected for differences in soil pool enrichment among chambers and sites. It is assumed the atom percent 15N2O flux to the headspace is equal to that of the source pool (i.e. no fractionation).
$$ ^{{{\text{15}}}} {\text{N}}_{2} {\text{O}}_{{{\text{nitrification}}}} = \frac{{^{\text {15}}{{{{\text{N}}_{{\text{2}}} {\text{O}}_{{\text{h}}} }} }} }{{^{\text {15}}{\text{N}}_{{{\text{NH}}^{{\text{+}}}_{{\text{4}}} }} }}\quad {\left( {{\text{from}}\;^{{\text{15}}} {\text{NH}}^{{\text{+}}}_{{\text{4}}} \;{\text{labeled}}\;{\text{plots}}} \right)} $$(4)$$ ^{{{\text{15}}}} {\text{N}}_{2} {\text{O}}_{{{\text{denitrification}}}} = \frac{{^{{{\text{15}}}} {\text{N}}_{2} {\text{O}}_{{\text{h}}} }} {{^{{{\text{15}}}} {\text{N}}_{{{\text{NO}}^{-}_{{\text{3}}} }} }}\quad {\left( {{\text{from}}\;^{{15}} {\text{NO}}^{-}_{3} \;{\text{labeled}}\;{\text{plots}}} \right)} $$where 15N2Oh is the result from Eq. 1, and \(^{{{\text{15}}}} {\text{N}}_{{{\text{NH}}^{{\text{+}}}_{{\text{4}}} }} \) and \( ^{{15}} {\text{N}}_{{{\text{NO}}^{-}_{{\text{3}}} }} \) are results from Eq. 3 for the NH +4 or NO −3 pools, respectively.
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6.
The relative proportion of N2O derived from nitrification versus denitrification (R) was calculated on a block-by-block basis by comparing the 15N2O-N recovered from the 15NH +4 labeled plots to the 15NO −3 labeled plots. Ratios greater than one indicate nitrification as the predominant source of N2O, while ratios below one indicate denitrification.
$$ R = \frac{{^{{{\text{15}}}} {\text{N}}_{2} {\text{O}}_{{{\text{nitrification}}}} }} {{^{{{\text{15}}}} {\text{N}}_{2} {\text{O}}_{{{\text{denitrification}}}} }} $$(5)
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Holtgrieve , G.W., Jewett, P.K. & Matson, P.A. Variations in soil N cycling and trace gas emissions in wet tropical forests. Oecologia 146, 584–594 (2006). https://doi.org/10.1007/s00442-005-0222-1
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DOI: https://doi.org/10.1007/s00442-005-0222-1