Researchers Reveal Biocrust Diazotrophs and Bacteria rather than Fungi are Sensitive to Chronic Low N Deposition

Over the last century, global nitrogen (N) emissions from anthropogenic sources (e.g., chemical fertilizer application and fossil fuel combustion) have increased by 71.3% with deposition projected to further increase to 100% over the next thirty years. The level of N deposited on soils often exceed a critical load aboveground resulting in a decrease in microbial species diversity and function and system stability. Even low levels of N may reach a critical loading threshold altering biocrust constituents and function.  

Establishing a critical threshold for N deposition that dramatically alters biocrust form and function will provide key ecological knowledge to maintain the integrity of existing drylands that cover the globe. 

To identify the impact of chronic and continuous low levels of N deposition on biocrusts, scientists from Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences created a realistic gradient mirroring anthropogenic N addition rate (2:1 NH₄⁺: NO₃^- rates: 0.3, 0.5, 1.0, 1.5, 3 g N m^-2 yr^-1) and measured the response of bacteria and fungi within cyanobacterial-dominated biocrusts over eight years in a temperate desert, the Gurbantunggut Desert, China. They also measured microbial biomass, functional potential of communities, and the environmental factors (e.g., pH, Ec, inorganic N, TK, and soil moisture) structuring community following their experimental N additions. 

Results show that once N deposition reaches 1.5 g N m^-2 yr^-1 biocrust bacterial communities, including diazotrophs, are altered while no such tipping-point exists for fungi. Above the threshold, bacterial richness is enhanced, the relative abundance of Chloroflexi, FBP, and Gemmatimonadetes is elevated, and diazotrophs shift from being dominated by Nostocaceae and Scytonemataceae (Cyanobacteria) to free-living Bradyrhizobiaceae (Alphaproteobacteria). Alternatively, the relative recovery of a few fungal species within the Lecanorales, Pleosporales and Verrucariales become either enriched or dimished due to N deposition. The chronic addition of N result in a dense and interconnected bacterial co-occurrence network that accentuate a functional shift from networks dominated by phototrophic species within the Nostocaceae, Xenococcaceae, Phormidiaceae and Scytonemataceae (Cyanobacteria) to ammonia-oxidizing species within the Nitrosomonadaceae (Betaproteobacteria) and nitrifying bacteria (i.e., Nitrospiraceae [Nitrospirae]). The effects of N additions on biocrust constituents are imposed through indirect effects on pH, soil electrical conductivity, and  ammonium concentrations.  

In summary, biocrust constituents are generally insensitive to chronic low levels of N depositions until rates reach above 1.5 g N m^-2 yr^-1 with diazotrophs being the most sensitive biocrust constituents followed by bacteria and finally fungi. Ultimately once the threshold is reached N deposition favors biocrust constituents utilizing inorganic N and other C sources over relying on phototrophic and/or N-fixing cyanobacteria for C and N.  

The research results titled " Biocrust diazotrophs and bacteria rather than fungi are sensitive to chronic low N deposition" was published by Environmental Microbiology. 

Article link: https://articlereview.pubmate.in/#/?templateID=d1bbeab9367143c2a13ab7831509c50e145711185 

Fig. General patterns of microbial beta-diversity in cyanobacteria crusts under long-term N addition.  

Fig. Co-occurrence networks of crust communities affected by N addition in the low and high N added soils.