Increased freeze-thaw cycles in winter partially offsets stimulatory effects of growing season soil warming on microbial activity in northern hardwood forests
Mean winter air temperatures have risen by 2.5 deg C over the last fifty years in the northeastern U.S., reducing the maximum depth of winter snowpack ~26 cm and the duration of winter snow cover by about four days per decade. Forest soils in this region are projected to experience a greater number of freeze-thaw cycles and colder winter soil temperatures as the depth and duration of winter snow cover continues to decline. Rising air temperatures are also likely to result in higher soil temperatures during the growing-season. In our experiment, we removed snow from forested plots at Hubbard Brook Experimental Forest, NH during winter to induce winter soil freezing. Buried-heating cables were used to induce freeze-thaw events during winter and also to warm soils 5 deg C above ambient soil temperature during the growing-season. We sought to determine whether soil microbial biomass and activity are affected similarly by soil temperature changes occurring in the winter versus the growing season.
Increasing the frequency of freeze-thaw events resulted in a two-fold reduction in proteolytic enzyme activity prior to tree-leaf out during spring, potentially limiting nitrogen (N) availability for plant and microbial production. However, these reductions were transient and did not persist through the growing-season. Similarly, increasing the number of freeze-thaw cycles led to a ~ 45% reduction in phenol oxidase activity across the growing-season. Because phenol oxidase activity has been associated with soil carbon (C) storage, fungal abundance and fungal diversity, this finding suggests that an increased number of freeze-thaw cycles over the next century might lead to sustained effects on soil C cycling and the soil fungal community. Warming soils 5 deg C above ambient during the growing-season did not increase rates of potential microbial respiration or net N mineralization measured in lab incubations. However, increased frequency of winter freeze-thaw cycles reduced the temperature-sensitivity of microbial respiration and net N mineralization following the snowmelt period and prior to tree leaf-out. We show that declining snow cover and increased soil freeze-thaw cycles are likely to contribute to reduced microbial activity in northern hardwood forests and may partially offset the stimulatory effect of soil warming on soil microbial activity.