Response strategies of N-fixation by epiphytic bryophytes to water change in a subtropical montane cloud forest

Abstract

Water changes are predicted to regulate physiological activities of bryophytes characterized by poikilohydric gametophytes. In montane forest ecosystems, nitrogen(N)–fixing bryophyte–cyanobacteria associations are main N resources. The aim of this study was to assess how bryophyte–associated microbiomes and their nitrogenase activity response to instant or long-term water content changes. We investigated the cyanobacterial colonization and nitrogenase activity of four epiphytic bryophyte species in a subtropical montane cloud forest during the dry and rainy seasons in Ailao Mountains, Yunnan, southwestern China. We also evaluated the nitrogenase activity of bryophyte–cyanobacteria associations in response to different water contents in laboratory experiment. The degree of cyanobacterial colonization was evaluated using ultraviolet fluorescence microscopy, and nitrogenase activity of bryophyte–cyanobacteria associations were measured using the acetylene reduction assay (ARA). Cyanobacteria showed an association with all four bryophyte species, with 1.04–3.37% area colonized of the shoot and 10.16–20.21 nmol C2H4·g−1·h−1 average nitrogenase activity. Nitrogenase activity was positively related to cyanobacterial colonization (R = 0.742; P = 0.0349). The relationship between water content and nitrogenase activity was unimodal, and both water surplus and shortage inhibited N fixation. Furthermore, long–term drought conditions reduced the degree of cyanobacteria colonization on bryophyte shoots, resulting in decreased nitrogenase activity in the dry season. These results indicate that different response strategies of N fixation operate in bryophyte–cyanobacteria associations to cope with instant and long–term changes in water availability. Our data suggest that both extreme precipitation and drought have a negative impact on N fixation of cyanobacteria–bryophyte associations.