Effects of Freeze-Thaw Cycles on the Soil Nutrient Balances, Infiltration, and Stability of Cyanobacterial Soil Crusts in Northern China

Aims: Freeze–thaw fluctuation is a natural phenomenon, which is frequently encountered by biological soil crusts (BSCs) in late autumn and early spring in cold deserts. The objective of our study was to investigate the effects of freeze–thaw cycles (FTCs) on the soil nutrient balances, infiltration, and stability of cyanobacterial soil crusts (CSCs) in the temperate desert region.

Methods: A controlled incubation experiment was carried out to study the effects of diurnal freeze–thaw cycles (FTCs) on total soil carbon (TC), total soil nitrogen (TN), soil TC/TN, hydraulic conductivity, and strength of light and dark cyanobacterial crusts, respectively. Six successive diurnal FTCs were applied as three temperature regimes (i.e., six successive mild FTCs (mild), six successive severe FTCs (severe), three successive mild FTCs followed by three successive severe FTCs (medium)). The experiment intended to simulate natural temperature changes in one of the temperate regions of northern China.

Results: Compared with dark CSCs cores, light CSCs cores lost a greater proportion of nitrogen. For both crust cores, severe FTCs decreased TC and TN more than mild FTCs. However, TC and TN remained relative constant when CSCs cores were treated with severe FTCs after experiencing mild FTCs. TC and TN of both CSCs cores decreased in the earlier FTCs and then remained stable in the later FTCs. TC/TN increased significantly for light CSCs, but only changed slightly for dark CSCs after successive FTCs. The effects of FTCs on the hydraulic conductivity and strength of CSCs were not consistent with our expectations that FTCs would increase hydrological conductivity and decrease strength. These effects depended on crust type, FTC number, and freeze/thaw intensity. Increase in hydraulic conductivity and decrease in strength only occurred in severe treatment in the dark CSCs during the later FTCs.

Conclusions: Light CSCs are more sensitive to FTCs than dark CSCs. Mild FTCs decrease less TC and TN than severe FTCs and mostly increase the stability of the CSCs. However, severe FTCs may decrease TC and TN drastically, thereby, degrading the BSCs.