A δ15N assessment of nitrogen deposition for the endangered epiphytic orchid Laelia speciosa from a city and an oak forest in Mexico

Atmospheric nitrogen deposition poses a major threat to global biodiversity. Tropical epiphytic plants are especially at risk given their reliance on atmospheric sources of nutrients. The leaf, pseudobulb, and root carbon and nitrogen content, C:N ratio, as well as the nitrogen isotopic composition were studied for individuals of Laelia speciosa from a city and from an oak forest in Mexico. The nitrogen content of leaves was similar between the city and the oak forest, reaching 1.3 ± 0.2 % (dry mass). The δ¹⁵N of leaves, pseudobulbs, and roots reached 5.6 ± 0.2 ‰ in the city, values found in sites exposed to industrial and vehicular activities. The δ¹⁵N for plant from the oak forest amounted to –3.1 ± 0.3 ‰, which is similar to values measured from sites with low industrial activities. Some orchids such as Laelia speciosa produce a single pseudobulb per year, i.e., a water and nutrient storage organ, so the interannual nitrogen deposition was studied by considering the ten most recent pseudobulbs for plants from either site formed between 2003 and 2012. The C:N ratio of the ten most recent pseudobulbs from the oak forest, as well as that of the pseudobulbs formed before 2010 for plants in the city were indistinguishable from each other, averaging 132.4 ± 6.5, while it was lower for the two most recent pseudobulbs in the city. The δ¹⁵N values of pseudobulbs from the oak forest averaged ‒4.4 ± 0.1 ‰ for the entire series. The δ¹⁵N ranged from 0.1 ± 1.6 ‰ for the oldest pseudobulb to 4.7 ± 0.2 ‰ for the pseudobulb formed in the city from 2008 onwards. Isotopic analysis and the C:N ratio for L. speciosa revealed that rates of nitrogen deposition were higher in the city than in the forest. The δ¹⁵N values of series of pseudobulbs showed that it is possible to track nitrogen deposition over multiple years.