| Biological soil crusts (BSCs) are cm-thin, cryptic, topsoil organo-sedimentary microbial communities that develop where plant cover is restricted, and attain maximal extent in arid lands. They stabilize soils against erosion, and are thought to play a significant role in local biogeochemistry, particularly regarding N-cycling in desert areas, since they are primary microsites for biological dinitrogen-fixation. Yet, N-limitation is characteristic of BSCs, indicating that net N exports are important in their biogeochemistry. I attempted to ascertain the fate of this unaccounted N, and the mechanism behind it, by studying N-transformations and the bacterial populations responsible in model crusts from the Colorado Plateau.; I conducted microchemical, microbiological and geochemical analyses at scales commensurate with their small size. Biomass, oxygen, pH and nitrogenous compounds varied strongly with depth at the mm scale. Microbial populations were also organized vertically. Potential dinitrogen-fixation rates were high, and congruent with previous reports. Ammonium oxidation, though limited by oxygen supply, accounted for a significant proportion of net N-inputs in all crusts tested, and occurred preferentially 1--3mm below the surface. Recoverable populations of ammonium-oxidizing bacteria were vertically stratified, peaking at depths of maximal activity. Molecular fingerprinting using the amoA gene identified a previously unknown Nitrosospira-like organism as the crust's principal nitrifier. This organism was found in crusts from E Oregon to S Arizona. Phylogenetic analyses revealed a marked biogeographical separation among crust Nitrosospiras, which constitutes an unprecedented example of bacterial biogeographic differentiation in the absence of apparent dispersal barriers.; Despite theoretically optimal conditions, denitrification rates in BSCs were extremely low and thus were irrelevant for N cycling. Congruently, recovered denitrifier populations were small and lacked vertical stratification. While the reason for this break in the N-cycle remains unclear, the lack of denitrification implies that N losses to the atmosphere are trivial. By contrast, downward diffusional export of dissolved ammonium and nitrate were comparable to N-inputs.; My results indicate that BSCs act as net exporters of dissolved N downwards. Given their great horizontal expanse, this speaks for a major role of BSCs in the fertility of arid lands in general, and provides grounds for renewed efforts in their preservation. |
