Extreme isotopic depletion of nitrogen in New Zealand lithophytes and epiphytes; the result of diffusive uptake of atmospheric ammonia?

• Published in: Oecologia 2005-08, Vol.144 (4), p.628-635
• Main Authors: Tozer, W.C, Hackell, D, Miers, D.B, Silvester, W.B
• Format: Article
• Language: English
• Subjects: Agricultural land; Air Pollutants; Ammonia; Ammonia - metabolism; Atmosphere; Atmospherics; Biological Transport, Active; Chlorophyta - metabolism; Dairying; diffusion; Ecosystem; Epiphytes; Fiberglass; Fractionation; gas emissions; Lichens; Lichens - metabolism; Natural environment; New Zealand; Nitrogen; Nitrogen Isotopes - metabolism; Plants; Rain; Signatures; stable isotopes; Thallus; Trentepohlia; Vegetation
• ISSN: 0029-8549; 1432-1939
• DOI: 10.1007/s00442-005-0098-0

Several lichens and the terrestrial alga Trentepohlia were found to have extremely depleted ¹⁵N signatures at two sites near the Rotorua geothermal area, New Zealand. Values, typically -20[per thousand], with several extreme cases of -24[per thousand], are more isotopically depleted than any previously quoted δ¹⁵N signature for vegetation growing in natural environments. For Trentepohlia, distance from a geothermal source did not affect isotopic signature. A 100-km transect showed that the phenomenon is widespread and the discrimination is not related to substrate N, or to elevation. Rainfall NHx and atmospheric gaseous NH₃ (NH₃₍g₎) were shown to be isotopically depleted in the range -1[per thousand] to -8[per thousand] and could not, of themselves, be responsible for the plant values obtained. A simulation of Trentepohlia thallus was created using an acidified fiberglass mat and was allowed to absorb NH₃₍g₎ from the atmosphere. Mats exposed at the geothermal sites and on farmland showed a significant further depletion of ¹⁵N to -17[per thousand]. We hypothesize that the extreme isotopic depletion is due to dual fractionation: firstly by the volatilization of NH₃₍g₎ from aqueous sources into the atmosphere; secondly by the diffusive assimilation of that NH₃₍g₎ into vegetation. We further hypothesize that lithophytes, epiphytes, and higher plants, growing on strongly N-limited substrates, will show this phenomenon more or less, depending on the proportion of diffusively assimilated NH₃₍g₎ utilized as a N source. Many of the isotopically depleted δ¹⁵N signatures in vegetation, previously reported in the literature, especially epiphytes, may be due to this form of uptake depending on the concentration of atmospheric NH₃₍g₎, and the degree of reliance on that form of N.