Variation in symbiotic N2 fixation rates among Sphagnum mosses

Abstract

Biological nitrogen (N) fixation is an important process supporting primary production in ecosystems, especially in those where N availability is limiting growth, such as peatlands and boreal forests. In many peatlands, peat mosses (genus Sphagnum) are the prime ecosystem engineers, and like feather mosses in boreal forests, they are associated with a diverse community of diazotrophs (N 2-fixing microorganisms) that live in and on their tissue. The large variation in N 2 fixation rates reported in literature remains, however, to be explained. To assess the potential roles of habitat (including nutrient concentration) and species traits (in particular litter decomposability and photosynthetic capacity) on the variability in N 2 fixation rates, we compared rates associated with various Sphagnum moss species in a bog, the surrounding forest and a fen in Sweden. We found appreciable variation in N 2 fixation rates among moss species and habitats, and showed that both species and habitat conditions strongly influenced N 2 fixation. We here show that higher decomposition rates, as explained by lower levels of decomposition-inhibiting compounds, and higher phosphorous (P) levels, are related with higher diazotrophic activity. Combining our findings with those of other studies, we propose a conceptual model in which both species-specific traits of mosses (as related to the trade-off between rapid photosynthesis and resistance to decomposition) and P availability, explain N 2 fixation rates. This is expected to result in a tight coupling between P and N cycling in peatlands.

Chronic Atmospheric Reactive Nitrogen Deposition Suppresses Biological Nitrogen Fixation in Peatlands

“…The mean BNF rate of 12.2 nmol N gDW ha –1 yr –1 found in laboratory incubations of peat from a forested peatland of Austria with an Nr deposition of 21 kg N ha –1 yr –1 , fell within the range of the BNF rates of peat we found in Fenn’s & Whixall and Migneint (0.3–37 nmol N gDW –1 d –1 ). Looking at the median BNF rates by species (Figure ) we found BNF values close to those reported by van den Elzen et al (2020) from samples collected in different peatland habitats of southern Sweden regarding S. capollifolium subsp. rubellum/ S. fuscum (open bog 0.4–17.5 nmol N gDW –1 d –1 ) and S. fallax (Lagg fen 17.5–66.3 nmol N gDW –1 d –1 ).…”

“…rubellum/ S. fuscum (open bog 0.4–17.5 nmol N gDW –1 d –1 ) and S. fallax (Lagg fen 17.5–66.3 nmol N gDW –1 d –1 ). Across the moss species, S. fallax had the highest BNF rates, which is commensurate with the findings of van den Elzen et al (2020) . Both Sphagnum mosses and peat collected from hollows, had higher BNF rates than species in hummocks (70% and 67%, respectively).…”

Phosphorus supply affects long-term carbon accumulation in mid-latitude ombrotrophic peatlands

“…At the same time, elevated nutrient deposition increases the size, activity and diversity of the microbial pool in the acrotelm 24,25 . Lin et al 26 showed P availability imposes a limiting effect on the microbial communities of decomposers and N2 fixers, and there is strong evidence for direct effects of P on N2 fixation rates from fertilisation plots [33][34][35]38 and surface peat measurements 36 . This would accelerate decomposition of the more recalcitrant compounds that usually become buried in the catotelm.…”

“…Bogs that accumulate more P are characterised by higher N:C (lower ΔN:C) ratios, indicative of more efficient decomposition. We propose this is a widespread and largely overlooked biogeochemical characteristic of ombrotrophic peatlands, whereby P supply influences the intensity of organic matter decay by promoting N fixation [33][34][35][36][37] and microbial activity [24][25][26] . Furthermore, bogs that experience proportionally greater burial of P relative to N seem to sequester carbon more slowly (r 2 = 0.64, p = 0.003; Figure 4C), showcasing a direct link between nutrient stoichiometry and long-term carbon cycling.…”

“…Provided ample phosphorus is available 32 , terrestrial ecosystems also acquire N through biological N2 fixation. There is good evidence from field manipulations for a direct influence of P on N2 fixation in peatlands [33][34][35][36][37] , probably by modulating the nitrogenase activity of diazotrophs 38 . But field studies span hours to years 23,30,39,40 , whereas carbon burial has been on-going for thousands of years 41 .…”