Soil aggregation returns to a set point despite seasonal response to snow manipulation

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.