Complex controls on nitrous oxide flux across a large-elevation gradient in the tropical Peruvian Andes

Abstract. Current bottom–up process models suggest that montane tropical ecosystems are weak atmospheric sources of N₂O, although recent empirical studies from the southern Peruvian Andes have challenged this idea. Here we report N₂O flux from combined field and laboratory experiments that investigated the process-based controls on N₂O flux from montane ecosystems across a large-elevation gradient (600–3700 m a.s.l.) in the southern Peruvian Andes. Nitrous oxide flux and environmental variables were quantified in four major habitats (premontane forest, lower montane forest, upper montane forest and montane grassland) at monthly intervals over a 30-month period from January 2011 to June 2013. The role of soil moisture content in regulating N₂O flux was investigated through a manipulative, laboratory-based ¹⁵N-tracer experiment. The role of substrate availability (labile organic matter, NO₃⁻) in regulating N₂O flux was examined through a field-based litter-fall manipulation experiment and a laboratory-based ¹⁵N–NO₃⁻ addition study, respectively. Ecosystems in this region were net atmospheric sources of N₂O, with an unweighted mean flux of 0.27 ± 0.07 mg N–N₂O m⁻² d⁻¹. Weighted extrapolations, which accounted for differences in land surface area among habitats and variations in flux between seasons, predicted a mean annual flux of 1.27 ± 0.33 kg N₂O–N ha⁻¹ yr⁻¹. Nitrous oxide flux was greatest from premontane forest, with an unweighted mean flux of 0.75 ± 0.18 mg N–N₂O m⁻² d⁻¹, translating to a weighted annual flux of 0.66 ± 0.16 kg N₂O–N ha⁻¹ yr⁻¹. In contrast, N₂O flux was significantly lower in other habitats. The unweighted mean fluxes for lower montane forest, montane grasslands, and upper montane forest were 0.46 ± 0.24 mg N–N₂O m⁻² d⁻¹, 0.07 ± 0.08 mg N–N₂O m⁻² d⁻¹, and 0.04 ± 0.07 mg N–N₂O m⁻² d⁻¹, respectively. This corresponds to weighted annual fluxes of 0.52 ± 0.27 kg N₂O–N ha⁻¹ yr⁻¹, 0.05 ± 0.06 kg N₂O–N ha⁻¹ yr⁻¹, and 0.04 ± 0.07 kg N₂O–N ha⁻¹ yr⁻¹, respectively. Nitrous oxide flux showed weak seasonal variation across the region; only lower montane forest showed significantly higher N₂O flux during the dry season compared to wet season. Manipulation of soil moisture content in the laboratory indicated that N₂O flux was significantly influenced by changes in water-filled pore space (WFPS). The relationship between N₂O flux and WFPS was complex and non-linear, diverging from theoretical predictions of how WFPS relates to N₂O flux. Nitrification made a negligible contribution to N₂O flux, irrespective of soil moisture content, indicating that nitrate reduction was the dominant source of N₂O. Analysis of the pooled data indicated that N₂O flux was greatest at 90 and 50 % WFPS, and lowest at 70 and 30 % WFPS. This trend in N₂O flux suggests a complex relationship between WFPS and nitrate-reducing processes (i.e. denitrification, dissimilatory nitrate reduction to ammonium). Changes in labile organic matter inputs, through the manipulation of leaf litter-fall, did not alter N₂O flux. Comprehensive analysis of field and laboratory data demonstrated that variations in NO₃⁻ availability strongly constrained N₂O flux. Habitat – a proxy for NO₃⁻ availability under field conditions – was the best predictor for N₂O flux, with N-rich habitats (premontane forest, lower montane forest) showing significantly higher N₂O flux than N-poor habitats (upper montane forest, montane grassland). Yet, N₂O flux did not respond to short-term changes in NO₃⁻ concentration.