Impacts of warming and fertilization on nitrogen-fixing microbial communities in the Canadian High Arctic

Abstract

The impacts of simulated climate change (warming and fertilization treatments) on diazotroph community structure and activity were investigated at Alexandra Fiord, Ellesmere Island, Canada. Open Top Chambers, which increased growing season temperatures by 1-3 degrees C, were randomly placed in a dwarf-shrub and cushion-plant dominated mesic tundra site in 1995. In 2000 and 2001 20N:20P2O5:20K2O fertilizer was applied at a rate of 5 gm(-2) year(-1). Estimates of nitrogen fixation rates were made in the field by acetylene reduction assays (ARA). Higher rates of N fixation were observed 19-35 days post-fertilization but were otherwise unaffected by treatments. However, moss cover was significantly positively associated with ARA rate. NifH gene variants were amplified from bulk soil DNA and analyzed by terminal restriction fragment length polymorphism analysis. Non-metric multidimensional scaling was used to ordinate treatment plots in nifH genotype space. NifH gene communities were more strongly structured by the warming treatment late in the growing season, suggesting that an annual succession in diazotroph community composition occurs.

Long-term experimental warming alters nitrogen-cycling communities but site factors remain the primary drivers of community structure in high arctic tundra soils

“…Long-term disturbance of forest soils due to clearcutting led to distinctly different nifH communities with new dominant TRFs and losses of previously dominant genotypes and additionally, these communities exhibited atypical seasonal variation (Shaffer et al, 2000). Deslippe et al (2005) also noted that nifH communities were strongly structured by OTC warming, with NMS ordinations showing differences between control plots and OTCs at lowland sites that developed over the growing season. Sequences obtained from GenBank.…”

“…However, the link between shifts in microbial community structure (measured by changes in gene frequency, abundance and richness) and corresponding changes in the processes mediated by these communities has not been satisfactorily demonstrated. Deslippe et al (2005) found little relationship between nifH genotype diversity and nitrogen-fixation rates. In addition, although excess nitrogen should negatively affect nitrogen fixation (Zehr et al, 2003), the diversity of the nitrogen-fixing community assessed using the functional gene nifH has not been shown to diminish in such cases (Piceno and Lovell, 2000).…”

“…Degenerate primer pairs were previously designed, tested and optimized for nosZ (Throbäck et al, 2004), and nifH (Deslippe, 2004;Deslippe et al, 2005).…”

“…The nifH protocol used Nh21F (5 0 -GCIWTI TAYGGNAARGGNGG-3 0 ) and WidNhR (5 0 -GCRT AIABNGCCATCATYTC-3 0 ) for the primary PCR reaction (Widmer et al, 1999) and Nh428R (5 0 -C CRCCRCANACMACGTC-3 0 ) for the second amplification (Deslippe 2004;Deslippe et al, 2005) (sequences follow standard IUPAC notation for mixed bases). The reverse primer Nh428R was labeled with Light Sabre Green (D3) dye.…”

“…Techniques such as terminal restriction fragment length polymorphism (T-RFLP), which can be used to measure the presence and/or relative abundance of functional genes, are ideal for assessing microbial community structure (Tiedje et al, 1999;Zehr et al, 2003;Deslippe et al, 2005;Wallenstein et al, 2006). Assessments of microbial community structure based upon functional genes may track differences in potential gene function based upon larger scale environmental change, although T-RFLP is most effective where species diversity is low or moderate (Engebretson and Moyer, 2003).…”