Abstract
Abstract. Local and regional modelling of NH3 surface exchange is required to quantify nitrogen deposition to, and emissions from, the biosphere. However, measurements and model parameterisations for many remote ecosystems – such as tropical rainforest – remain sparse. Using 1 month of hourly measurements of NH3 fluxes and meteorological parameters over a remote Amazon rainforest site (Amazon Tall Tower Observatory, ATTO), six model parameterisations based on a bidirectional, single-layer canopy compensation point resistance model were developed to simulate observations of NH3 surface exchange. Canopy resistance was linked to either relative humidity at the canopy level (RHz0′), vapour pressure deficit, or a parameter value based on leaf wetness measurements. The ratio of apoplastic NH4+ to H+ concentration, Γs, during this campaign was inferred to be 38.5 ± 15.8. The parameterisation that reproduced the observed net exchange of NH3 most accurately was the model that used a cuticular resistance (Rw) parameterisation based on leaf wetness measurements and a value of Γs=50 (Pearson correlation r=0.71). Conversely, the model that performed the worst at replicating measured NH3 fluxes used an Rw value modelled using RHz0′ and the inferred value of Γs=38.5 (r=0.45). The results indicate that a single-layer canopy compensation point model is appropriate for simulating NH3 fluxes from tropical rainforest during the Amazonian dry season and confirmed that a direct measurement of (a non-binary) leaf wetness parameter improves the ability to estimate Rw. Current inferential methods for determining Γs were noted as having difficulties in the humid conditions present at a rainforest site.
Highlights
The global cycling of nitrogen is of critical importance to Earth’s biogeochemistry
Using the static canopy compensation point model of NH3 surface exchange, in combination with new NH3 flux and meteorological data measured at a remote, tropical rainforest site, this study aims to present a series of local model formulations for χs and Rw which adequately simulate the bidirectional fluxes of NH3 observed by Ramsay et al (2020), with a focus on the most suitable control metric for Rw
Application of a capacitance model that incorporates the process of cuticular desorption did not lead to improved model results, suggesting that the emission periods were under stomatal control
Summary
The global cycling of nitrogen is of critical importance to Earth’s biogeochemistry. One of the major contributors to the global atmospheric reactive nitrogen (Nr) budget is ammonia (NH3), which is primarily generated from anthropogenic sources (Galloway et al, 2003). Fowler et al (2013) detail how increased deposition of N can lead to increased vegetation growth rates in forests, leading to potentially greater carbon sequestration rates. This potential positive impact, is offset by the effect of N saturation on forests as detailed by Nadelhoffer (2008). The combined impact of disturbance to forest soil microbial systems involved in the nitrification–denitrification cycle (Fowler et al, 2009), and damage to vegetation (Krupa, 2003) leads to a sharp decrease in net primary productivity
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