Variations in soil N cycling and trace gas emissions in wet tropical forests

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.

Appendix: N2O source study equations

• 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.

• 2. We determined the amount of ¹⁵N₂O-N accumulated in the chamber headspace (¹⁵N₂O_h 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 N₂O_ap 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%.

• 3. Soil ¹⁵N enrichment (N_ap 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 N_native is the concentration of NH ⁺₄ or NO ⁻₃ of the soil prior to labeling (measured from control plots) and N_added 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.

• 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 N_ap 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.

• 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 ¹⁵N₂O_h 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.

• 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)