Characteristics in Rates of Nitrogen Fixation and Ammonia Oxidation in Biocrusts Under Different Snow Cover Depths

Abstract

Biocrusts (biological soil crusts) regulate nutrient cycling, soil structure, and ecosystem function in arid environments. However, the effects of seasonal variations and snow cover treatments on nitrogen (N2) cycling processes, such as N2 fixation and ammonia oxidation (AO), are not fully understood. We aimed to fill this knowledge gap by investigating how biocrusts of different types (Moss, Lichens, and Cyanobacteria) respond to seasonal changes (April and October) and snow treatments (Double Snow (DS), Remove Snow (RS), and Control (CK)) in winter in the Gurbantunggut Desert. We measured nitrogenase activity (NA) and AO across the seasons and treatments while also analyzing their correlations with soil organic carbon (SOC), Soil water content (SWC), Nitrate Nitrogen (NO3−-N), Ammonium Nitrogen (NH4+-N), and Electrical Conductivity (EC). NA and AO were significantly higher in October than in April across all biocrust types and treatments, particularly under the DS treatment. Specifically, NA in Moss biocrusts increased from 2 µg N g-1 day-1 in April to 6 µg N g-1 day-1 in October (p < 0.01), with AO increasing from 0.8 to 1.5 µg N g-1 day-1 (p < 0.05) during DS treatment. Cyanobacteria biocrusts under DS treatment exhibited the highest microbial activity in April, with NA reaching 6.2 µg N g-1 day-1 and AO 1.6 µg N g-1 day-1 in October. Strong correlations between SOC and SWC in October suggest that environmental factors like moisture retention, pH, temperature, and nutrient dynamics enhance microbial activity, especially in DS treatment. This promotes nitrate availability by supporting mineralization. In contrast, NO3−-N correlations with TN and SOC, significant in April, weakened in October, reflecting a shift in nutrient cycling toward AO. In most cases, NA and AO decreased along with reduced depths of snow treatments in April, while no significant effects were found in October.

Keywords: Biocrusts, Nitrogen Fixation, Ammonia Oxidation, Seasonal Variation, Soil Moisture, Nutrient Cycling