\section{Abstract}
Biological soil crusts (BSC) are key components of ecosystem productivity in
arid lands and they cover a substantial fraction of the terrestrial surface. 
In particular, BSC N$_{2}$-fixation contributes significantly to the
nitrogen (N) budget of arid land ecosystems.  In mature crusts,
N$_{2}$-fixation is largely attributed to heterocystous cyanobacteria, however,
early successional crusts possess few N$_{2}$-fixing
cyanobacteria and this suggests that microorganisms other than cyanobacteria
mediate N$_{2}$-fixation during the critical early stages of BSC development.
DNA stable isotope probing (DNA-SIP) with $^{15}$N$_{2}$ revealed that
\textit{Clostridiaceae} and \textit{Proteobacteria} are the most common
microorganisms that assimilate $^{15}$N$_{2}$ in early successional crusts. The
\textit{Clostridiaceae} identified are divergent from previously characterized
isolates, though N$_{2}$-fixation has previously been observed in this family.
The Proteobacteria identified share $>$98.5 \%SSU rRNA gene sequence identity
with isolates from genera known to possess diazotrophs (e.g.
\textit{Pseudomonas}, \textit{Klebsiella}, \textit{Shigella}, and
\textit{Ideonella}). The low abundance of these heterotrophic diazotrophs in
BSC may explain why they have not been characterized previously. Diazotrophs
play a critical role in BSC formation and characterization of these organisms
represents a crucial step towards understanding how anthropogenic change will
affect the formation and ecological function of BSC in arid ecosystems.\\ 

\textbf{keywords:}  microbial ecology / stable isotope probing / nitrogen
fixation / biological soil crusts \\
