Effects of nitrogen fertilization on diazotrophic activity of microorganisms associated with Sphagnum magellanicum

Abstract

Background and aims In pristine ombrotrophic Sphagnum-dominated peatland ecosystems nitrogen (N) is often a limiting nutrient, which is replenished by biological N 2 fixation and atmospheric N deposition. It is, however, unclear which impact long-term N deposition has on microbial N 2 fixing activity and diazotrophic diversity, and whether phosphorus (P) modulates the response. Therefore, we studied the impact of increased N deposition and N depletion on microbial N 2 fixation and diazotrophic diversity associated with the peat moss Sphagnum magellanicum, and their interaction with P availability. Methods Nitrogenase activities of S. magellanicumassociated microorganisms were determined by acetylene reduction assays (ARA) and 15 N 2 tracer methods on mosses from two geographically distinct locations with different N deposition histories, high or low N deposition, and in samples depleted in N (grown 3 years in the greenhouse) versus recent field samples. The short-term response to increased N deposition was tested for mosses differing in N and P fertilization histories. In addition, diversity of diazotrophic microorganisms was assessed by nifH gene amplicon sequencing of N-depleted mosses. Results We showed distinct and persistent differences in diazotrophic communities and their activities associated with S. magellanicum from sites with high versus low N deposition. Initially, diazotrophic activity was six times higher for the low N site. During incubation and repeated ARA, however, this activity strongly decreased, while it remained stable for the high N site. Activity for the high N site could not be increased by long-term experimental N deprivation. Short-term, experimental N application had an inhibitory effect on N 2 fixation for both sites, which was not observed in mosses with high indirect P availability. Conclusions We conclude that although N deposition negatively affects N 2 fixation as also shown in previous studies, long-term effects of N deprivation on the diazotrophic activity and community are more complex. Furthermore, our results indicated that P availability might be an important factor in modulating the response of Sphagnum-associated diazotrophs to N deposition.

Bacterial, fungal, and plant communities exhibit no biomass or compositional response to two years of simulated nitrogen deposition in a semiarid grassland

“…There are a number of potential explanations. First, the N inputs used here were based upon levels of N deposition recorded in the study area (∼3 kg N ha −1 year −1 ), and while these amounts are more realistic for our area and are in line with many newer studies that use smaller inputs (Gilliam et al ., ; Kox et al ., ; Tulloss and Cadenasso, ), the inputs are much lower than the levels of enrichment often used in nutrient limitation studies and in areas of exceptional deposition, such as some parts of Europe (Holland et al ., ). Second, as with most N deposition experiments, we applied the N in pulses that joined smaller daily rates into a compound amount, in our case applied twice yearly.…”

Experimental warming reduces the diversity and functional potential of theSphagnummicrobiome

“…Nitrogen fixation activity and temperature were negatively correlated which may be due to plant-specific tolerance to water stress and desiccation given that nitrogen fixation associated with moss is influenced by moisture (Zielke et al , 2002; Sorensen et al , 2006; Sorensen & Michelsen, 2011). Additionally, oxygen level, photosynthetic activity (Warren et al , 2017), and phosphorous (Rousk et al , 2017) or nitrogen availability (Kox et al , 2016) have also been found to limit diazotrophy in Sphagnum (Warren et al , 2017). However, we observed a reduction in diazotroph absolute abundance indicating diazotrophs were not inactive but rather undetectable with our methods in elevated temperature treatments.…”

Symbiosis revisited: phosphorus and acid buffering stimulate N<sub>2</sub> fixation but not <i>Sphagnum</i> growth

“…N 2 fixing microorganisms (diazotrophs) live on the surface and inside dead hyaline cells of Sphagnum (Opelt et al, 2007;Bragina et al, 2012;Larmola et al, 2014), forming a symbiosis with their host. A highly diverse microbial community, including Proteobacteria, Verrucomicrobia and Cyanobacteria, has been found to colonize peat mosses (Bragina et al, 2014), and many of these microorganisms have the capacity to fix N 2 (Bragina et al, 2013;Kox et al, 2016). Also, in other bryophytes, like Hylocomiaceae (feather mosses), such a symbiotic relationship can be found with N 2 fixing cyanobacteria, supplying up to 50 % of the total N input in boreal forests (Rousk et al, 2013).…”