Nitrogen (N) is an important limiting nutrient for plant production in arid and semiarid ecosystems. Nitrogen (N) exists in the atmosphere as N₂, a form that is not useable by vascular plats. N₂ must first be “fixed”, or reduced, to ammonia (NH₄⁺) by prokaryotic organisms. Biological soil crusts, formed by different combination of mosses, lichens, liverworts, algae, fungi, cyanobacteria and bacteria, may be important in arid and semi-arid ecosystems because of their ability to fix atmospheric nitrogen (N₂). This is especially true for regions where rainfall and anthropogenic inputs of N are low. Estimations of N₂ fixation by soil crusts in arid and semi-arid areas vary widely. It is difficult to compare these values because they were reported with different units (from hourly to annual rates), and at different spatial scales (from cm2 to hm2). Laboratory studies were demonstrated that N fixation rates in individual species of soil crust lichens and cyanobacteria were controlled by species composition, moisture, temperature, and light. As crust organisms are only active when wet, moisture is the most limiting factor. Much of the N fixed by crustal species is released soon after fixation. N compounds released include small amounts of amide, peptides, and free amino acids, with most as nitrate and ammonium. Released N was shown to be utilized by surrounding organisms, including vascular plants, fungi, actinomycetes, and bacteria. Previous studies showed that soil surface disturbance such as trampling by animals could greatly reduce or eliminate N fixation in biological soil crusts. Disturbance of biological soil crusts also decreases soil stability and albedo, both of which can influence N inputs and N cycles. Thus, reductions in N inputs from crusts which reflected in surrounding plant tissue N levels could have large implications for N cycles in places dominated by biological soil crusts. A quantification of the inputs of atmospheric N₂ by biological soil crusts under field conditions is important for the estimation of the potential impacts on the N budget. Given the high rates of N input but low levels of soil N in deserts, much of the fixed N must be leached downwards into the soil or lost to the atmosphere in gaseous phase. Little is known about either pathway from biological soil crusts in these ecosystems.
Biological soil crusts serve as important biological factors contributing to the soil stability and fertility due to their large coverage in the Gurbantunggut Desert, northern Xinjiang. Although biological soil crusts were widely studied in many deserts of the world, their importance in the Gurbantunggut Desert had not been well analyzed. Previous studies regarding nitrogenase activity (NA, indicative of N₂ fixation) in this desert were related to legumes, ignoring the possible role of biological crusts in N₂ fixation. Soil surface disturbances associated with human activities were repeatedly shown to convert species-rich biological soil crusts, dominated by lichen and moss, to species-poor crusts dominated by cyanobacteria. The objective of this study was to quantify the nitrogen-fixing activity in different types of biological soil crusts in the Gurbantunggut Desert using acetylene reduction assays (ARA). We measured potential N fixation rates of algae crusts, lichen crusts and moss crusts at different time. The results suggested that NA for each type of crusts were highly variable. From March to May，algae crusts reached the highest NA (2.26×103 nmolC₂H₄m^-2h^-1), lichen crusts the midium (6.54×10² nmolC₂H₄m^-2h^-1) and moss crusts the lowest (6.38×10² nmolC₂H₄m^-2h^-1).From June to October, all types of crust reached their highest NA respectively, especially for lichen crusts and moss crusts (p<0.01). NA for algae crusts(9.81×103 nmolC₂H₄m^-2h^-1) was higher than lichen crusts (9.06×103 nmolC₂H₄m^-2h^-1) and moss crusts (2.03×103 nmolC₂H₄m^-2h^-1), indicated that species composition was critical when estimating N inputs in desert ecosystems. In addition, all three types of crusts generally responded in a similar tendency to weather conditions. From November to next February, low temperature (<0℃) are responsible for reduction of NA in crusts, especially in algae crusts (4.18×10² nmolC₂H₄m^-2h^-1) and moss crusts(5.43×10² nmolC₂H₄m^-2h^-1) (p<0.01). Statistical analysis was conducted using SPSS statistical package (Chicago,IL,USA). Tests of between-subjects effects showed that NA levels were significantly influenced by sampling time and crust type (p<0.01). The presence of N fixation activity in all crusts may help keeping fertile in sparsely vegetated areas and provide surrounding vascular plant with fixed nitrogen in the Gurbantunggut Desert.