Revegetation is an effective approach for restoring extremely degraded grassland (DG) in the Qinghai-Tibetan Plateau (QTP). However, little is known about its effects on permafrost stability. Our study investigated changes in the characteristics of DG and revegetated grassland (RG) in alpine permafrost regions of the QTP by means of in situ monitoring and sampling. Compared with DG, soil temperature was lower in warm months and slightly higher in cool months both at 2 and 10 cm depths after revegetation, while soil moisture generally decreased. Revegetation advanced the onset and increased the duration of completely frozen stage. The number of freeze-thaw days decreased at 2 cm but increased at 10 cm depth. The freeze-thaw strength weakened at 2 cm depth in spring and autumn, and at 10 cm depth in autumn, but increased at 10 cm depth in spring. The thawing index at the two depths and active layer thickness in RG were also significantly lower than those in DG. Revegetation significantly affected the particle size distribution and stability of soil aggregates by increasing the proportion of large macroaggregates. Thus, revegetation can effectively improve the permafrost stability of degraded grassland in the QTP and enhance the service functions of alpine grassland ecosystems.

As the basic units of soil structure, soil aggregate is essential for maintaining soil stability. Intensified freeze-thaw cycles have deeply affected the size distribution and stability of aggregate under global warming. To date, it is still lacking about the effects of freeze-thaw cycles on aggregate in the permafrost regions of the Qinghai-Tibetan Plateau (QTP). Therefore, we investigated the effects of diurnal and seasonal freeze-thaw processes on soil aggregate. Our results showed that the durations of thawing and freezing periods in the 0-10 cm layer were longer than in the 10-20 cm layer, while the opposite results were observed during completely thawed and frozen periods. Freeze-thaw strength was greater in the 0-10 cm layer than that in the 10-20 cm layer. The diurnal freeze-thaw cycles have no significant effect on the size distribution and stability of aggregate. However, 0.25 mm) and reduced aggregate stability. Our study has scientific guidance for evaluating the effects of freeze-thaw cycles on soil steucture and provides a theoretical basis for further exploration on soil and water conservation in the permafrost regions of the QTP.

Most climate models predict that the timing, magnitude, and duration of snow cover will change over much of the Northern Hemisphere. Because snow cover effectively buffers soil against changes in air temperature, fluctuations in snowpack could alter freeze-thaw cycling, resulting in shifts in macroaggregate stability and subsequent detachment. Moreover, vegetation type could modify these effects; however, these interactions remain unexplored. In this study, we experimentally manipulated snow cover in an agricultural field and in an adjacent 13-year-old restored prairie to assess changes to soil aggregation and detachment over a three-winter period (November-April 2014-17). Treatments consisted of complete snow removal, natural snow cover, and a sustained snowpack simulated via straw insulation. Averaged over the course of the study, snow removal resulted in a 5% and 15% over-winter reduction in wet-aggregate stability (WAS) and mean weight diameter (MWD), respectively. Conversely, natural snow cover and straw insulation resulted in a 3% and 15% over-winter increase in WAS and MWD, respectively. However, over-winter changes to WAS and MWD did not persist but instead appeared to return to a set point by the end of each growing season regardless of vegetation type. In addition, we found an offset in WAS; it was approximately 11% higher in the prairie than in the agricultural field, likely due to increased root and microbial activity in the prairie. No similar offset was observed in MWD between vegetation types. These responses in soil aggregation did not result in significant springtime changes to soil critical shear stress, measured as a proxy for soil detachment potential. The results of this study suggest that future investigations into over-winter soil processes should consider vegetation type, temporal soil aggregation dynamics, and more detailed quantification of freeze-thaw cycling.

Throughout most of the northern hemisphere, snow cover decreased in almost every winter month from 1967 to 2012. Because snow is an effective insulator, snow cover loss has likely enhanced soil freezing and the frequency of soil freeze-thaw cycles, which can disrupt soil nitrogen dynamics including the production of nitrous oxide (N2O). We used replicated automated gas flux chambers deployed in an annual cropping system in the upper Midwest US for three winters (December-March, 2011-2013) to examine the effects of snow removal and additions on N2O fluxes. Diminished snow cover resulted in increased N2O emissions each year; over the entire experiment, cumulative emissions in plots with snow removed were 69% higher than in ambient snow control plots and 95% higher than in plots that received additional snow (P < 0.001). Higher emissions coincided with a greater number of freeze-thaw cycles that broke up soil macroaggregates (250-8000 A mu m) and significantly increased soil inorganic nitrogen pools. We conclude that winters with less snow cover can be expected to accelerate N2O fluxes from agricultural soils subject to wintertime freezing.