Abstract
Atmospheric nitrogen deposition is the third largest cause of global biodiversity loss, with rates that have more than doubled over the past century. This is especially threatening for tropical regions where the deposition may soon exceed 25 kg of N ha-1 year-1, well above the threshold for physiological damage of 12-20 kg of N ha-1 year-1, depending on plant species and nitrogenous compound. It is thus urgent to monitor these regions where the most diverse biotas occur. However, most studies have been conducted in Europe, the USA and recently in China. This review presents the case for the potential use of biological organisms to monitor nitrogen deposition, with emphasis on tropical plants. We first present an overview of atmospheric chemistry and the nitrogen metabolism of potential biomonitors, followed by a framework for monitoring nitrogen deposition based on the simultaneous use of various functional groups. In particular, the tissue nitrogen content responds to the rate of deposition, especially for mosses, whose nitrogen content increases by 1‰ per kilogram of N ha-1 year-1. The isotopic signature, δ15N, is a useful indicator of the nitrogen source, as the slightly negative values (e.g. 5‰) of plants from natural environments can become very negative (-11.2‰) in sites with agricultural and husbandry activities, but very positive (13.3‰) in urban environments with high vehicular activity. Mosses are good biomonitors for wet deposition and atmospheric epiphytes for dry deposition. In turn, the nitrogen saturation of ecosystems can be monitored with trees whose isotopic values increase with saturation. Although given ecophysiological limitations of different organisms, particular studies should be conducted in each area of interest to determine the most suitable biomonitors. Overall, biomonitors can provide an integrative approach for characterizing nitrogen deposition in regions where the deployment of automated instruments or passive monitoring is not feasible or can be complementary.
Highlights
Nitrogen is one of the essential elements for life and the most abundant in the terrestrial atmosphere, 80% of which is composed of N2 (Soderlund, 1976)
Atmospheric nitrogen deposition is the third largest cause of global biodiversity loss, with rates that have more than doubled over the past century. This is especially threatening for tropical regions where the deposition may soon exceed 25 kg of N ha−1 year−1, well above the threshold for physiological damage of 12–20 kg of N ha−1 year−1, depending on plant species and nitrogenous compound
The tissue nitrogen content responds to the rate of deposition, especially for mosses, whose nitrogen content increases by 1‰ per kilogram of N ha−1 year−1
Summary
Nitrogen is one of the essential elements for life and the most abundant in the terrestrial atmosphere, 80% of which is composed of N2 (Soderlund, 1976). Due to the high chemical stability derived from its strong triple bond, this molecule can only be divided by processes involving large quantities of energy or through the action of specialized nitrogen-fixing microorganisms (Galloway et al, 2003) For this reason, in the pre-industrial age, more than 99% of the atmospheric. Agriculture, industry and the use of automobiles are the main sources of a complex of chemical species known as reactive nitrogen (Nr), originated from the splitting of N2 (Galloway et al, 2008) Such an increased deposition of atmospheric nitrogen has adverse effects on biodiversity. We explore the advantages and disadvantages of using different types of biomonitors such as mosses and vascular plants, as well as their particular responses to the different forms of nitrogen
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