In former studies it has been observed that for example N 2O emissions occur in hotspots during hot moments, but to our knowledge this has not been described for N r emissions, and the underlying small-scale processes have not been analyzed in detail. (18,20,32) A drawback of flux measurements is, however, that they only give the integrated balance of gas exchange at the sample scale (∼cm), whereas they do not allow to draw conclusions at the scale relevant to microorganisms, substrate concentrations at microsites and processes occurring at pore scales. (18,20,37) For biocrusts, a high variability in the total flux values was observed. (36) In former studies, HONO and NO emissions were analyzed by means of continuous flux measurements from bulk soil, (23) soil bacteria, (21) and biocrusts. (18,20,21,23) In another study, emissions of NO and HONO were shown to also occur at high water contents. (35) In most studies, emissions from drying soil and biocrusts were lowest at high water holding capacity (100% WHC) and reached maximum values around 20–30% WHC. NO and HONO emissions are known to be strongly related to the water content. Our results illustrate simultaneous, spatially separated aerobic and anaerobic nitrogen transformations, which are critical for N r emissions, but might be impacted by future global change and land management. During further desiccation they decreased, while emissions of HONO and NO increased, reaching maximum values around 20% WHC. Nitrite concentrations (∼6 to 800 μM) and pH values (∼6.5 to 9.5) were highest around 70% WHC. Initial oxygen saturation was mostly low (∼30%) at full water holding capacity, suggesting widely anoxic conditions, and increased rapidly upon desiccation. Microsensor measurements showed strong gradients of pH, oxygen, and nitrite, nitrate, and ammonium ion concentrations at micrometer scales in both vertical and lateral directions. X-ray microtomography and fluorescence microscopy revealed mixed pore size structures, where photoautotrophs and cyanobacterial polysaccharides clustered irregularly in the uppermost millimeter. Here, we explore small-scale gradients in chemical concentrations related to structural characteristics and organism distribution. Recent investigations suggested that aerobic and anaerobic microbial nitrogen transformations occur simultaneously upon desiccation of biocrusts, but the spatio-temporal distribution of seemingly contradictory processes remained unclear. Biocrusts covering drylands account for major fractions of terrestrial biological nitrogen fixation and release large amounts of gaseous reactive nitrogen (N r) as nitrous acid (HONO) and nitric oxide (NO).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |