Complex controls on nitrous oxide flux across a long 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 long elevation gradient (600–3700 m a.s.l.) in the southern Peruvian Andes. Nitrous oxide flux and environmental variables were quantified in four major habitat types (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. Ecosystems in this region were net atmospheric sources of N2O, emitting 0.27 ± 0.07 mg N-N2O m−2 d−1. Nitrous oxide flux was inversely related to elevation; N2O flux was greatest in premontane forest (0.75 ± 0.18 mg N-N2O m−2 d−1), followed by lower montane forest (0.46 ± 0.24 mg N-N2O m−2 d−1), montane grasslands (0.07 ± 0.08 mg N-N2O m−2 d−1), and upper montane forest (0.04 ± 0.07 mg N-N2O m−2 d−1). 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 bimodal and non-linear, diverging from theoretical predictions of how WFPS relates to N2O flux. Nitrous oxide flux was greatest at 90 and 50 % WFPS, and lowest at 70 and 30 % WFPS. This bimodal distribution of N2O flux suggests a complex relationship between WFPS, environmental variables, and nitrate-reducing processes. Changes in labile organic matter inputs, through the manipulation of leaf litter-fall, did not alter N2O flux, suggesting that litter inputs have a negligible impact on N2O flux. Nitrate addition experiments demonstrated that variations in NO3− availability 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). Nitrous oxide flux did not respond to short-term changes in NO3− concentration.