Pitfalls and New Mechanisms in Moss Isotope Biomonitoring of Atmospheric Nitrogen Deposition

Moss N isotope (delta N-15(bulk)) has been used to monitor N deposition, but it remains questionable whether inhibition of nitrate reductase activity (NRA) by reduced dissolved N (RDN) engenders overestimation of RDN in deposition when using moss delta N-15(bulk). We tested this question by investigation of delta N-15(bulk) and delta(NO3-)-N-15 in mosses under the dominance of RDN in N depositions of Guiyang, SW China. The delta N-15(bulk) of mosses on bare rock (-7.9 parts per thousand) was unable to integrate total dissolved N (TDN) (delta N-15 = -6.3 parts per thousand), but it reflected delta N-15-RDN (-7.5 parts per thousand) exactly. Moreover, delta N-15-NO3- in mosses (-1.7 parts per thousand) resembled that of wet deposition (-1.9 parts per thousand). These isotopic approximations, together with low isotopic enrichment with moss [NO3-] variations, suggest the inhibition of moss NRA by RDN. Moreover, isotopic mixing modeling indicated a negligible contribution from NO3- to moss delta N-15(bulk) when the RDN/NO3- reaches 3.8, at which maximum overestimation (21%) of RDN in N deposition can be generated using moss delta N-15(bulk) as delta(15) N-TDN. Moss delta N-15-NO3- can indicate atmospheric NO3- under distinctly high RDN/NO3- in deposition, although moss delta N-15(bulk) can reflect only the RDN therein. These results reveal pitfalls and new mechanisms associated with moss isotope monitoring of N deposition and underscore the importance of biotic N dynamics in biomonitoring studies.