Nitrogen Cycling in Well-Developed Cyanobacterial Biological Soil Crusts Under Varied Moisture Conditions.

Biological soil crusts (BSC) are one of the most important contributors of nitrogen to soil in arid ecosystems. With 36% of the Earth’s land surface covered in desert, human disturbance of BSC can have a significant effect on the global nitrogen cycle. Understanding BSC and their influence on their surroundings is fundamental to preserving desert ecosystems. There is a strong relationship between moisture availability and the nitrogen cycle as regulated by BSC. Any human-induced alteration of water distribution patterns will have a direct effect on this process. The current scientific consensus is that alternating wet and dry cycles common in desert environments result in a greater nitrogen fixation per unit “moist time” than ecosystems with consistently hydrated soils. Until now, no study has exclusively tested this hypothesis. Shifts in pools of ammonium and nitrate regulated by microorganisms associated with dark cyanobacterial crusts (dominated by Microcoleus, Nostoc, Scytonema, and Schizothrix) of Moab, Utah were measured under varying moisture conditions. Cation and anion resin membranes were buried directly under crust populated soil samples to measure ammonium and nitrate levels before and after moisture treatments. Ammonium and nitrate levels were compared between samples brought to field capacity once a day during the study period and samples brought to field capacity only once at the onset of the study and left to desiccate for the remaining study period. It is hypothesized that cyanobacterial BSC will cycle more nitrogen when wetted and allowed time to desiccate, than crusts on consistently hydrated soils. In arid environments where BSC most commonly occur, water is only available in quick pulses, rather than steadily throughout the rainy season. Increased nitrogen cycling of cyanobacterial-dominated BSC in such conditions may suggest an evolutionary foundation for the colonization of BSC in deserts.