Do our soils sleep during winter?
- Details
- Published on Wednesday, November 18, 2015
By Elmer Kaskiw, MAFRD
The Neepawa Press
The above normal temperatures through much of early November have kept soils from freezing and has resulted in the nutrient ban that is typically implemented on Nov. 10 to now be extended to Nov. 19.
Without a variance from Manitoba Conservation and Water Stewardship, the standard ban would have begun on Nov. 10 and would have stayed in place till April 10 of next year. These restrictions are designed to ensure nutrients are not applied to frozen soils where they become susceptible to runoff, thereby reducing nutrient loading into our waterways and lakes.
These extended frost free periods that run into mid and late November are certainly beneficial to our soils in regards to continued microbial breakdown and mineralization of organic matter. These so called “open falls” have led researchers to investigate more closely what is actually happening within our soils during the shoulder months of our winter.
In essence, the question becomes, “Do our soils sleep during the winter months?”. Certainly it would be logical to assume that our soils are essentially dead or are at least in a deep coma during our winter months given the sub zero soil temperatures. Recent research indicates that biological activity does indeed continue during at least a portion of our winters, depending upon snow cover and depth of frost, with the majority of this activity likely occurring in early winter and early spring depending upon freeze/thaw cycles.
What this research has found is that as temperatures cool to freezing, there is a shift in microbial populations or communities from summer to winter and that these “winter microbes” continue to grow in frozen soils but only at a much slower rate.
The mechanism for this activity seems to revolve around an increase in the composition of unsaturated fatty acids and in the production of glycerol within the lipid membranes of these microbes. Glycerol is best known as an antifreeze medium but can also serve as a food source for some of these microbes. The two measurements that are used to indicate microbial activity in frozen soils revolve around the measurement of carbon dioxide levels from the breakdown of organic matter and in the release of nitrous oxide as a result of denitrification.
The rate of winter soil respiration will vary depending upon snow cover, soil moisture and the number of freeze/thaw cycles. Standing stubble with adequate soil moisture and good early snow cover prior to deep freezes can see soil temperatures remain relatively stable in the -5 to -10°C range for much of the winter. Soil temperature probes placed this fall into seeded winter wheat fields in South Parkland have found the average soil temperature in November to be somewhere around 2 to 4.5°C. This gradual cooling of soil temperatures are ideal for the shift in microbial populations, allowing them to become more easily accustomed to cooler soil temperatures.
The food source for these cold soil microbes can be found in the form of intact root systems that have gone through a cold hardening process. Under field conditions, cold hardened root systems can have a sucrose content which is eight times higher during fall and winter than that measured during the spring and summer. This was particularly evident this fall by the leaf color change that saw many trees and shrubs turn their leaves from green to brilliant red, orange and yellows indicating high sugar content within the leaves of these trees. This food source along with more moderate winter soil temperatures and more frequent freeze/thaw cycles will increase the level of winter microbial activity and ultimately impact the level of nutrients available for spring growth. This winter microbial activity is likely why we see winter crops and fall planted shrubs, trees and perennials get such a jump on spring growth when compared to being spring planted. These fall plantings see an accumulation of nutrients in close proximity to their root systems as a result of this winter microbial activity, allowing these fall plantings to have immediate access to these nutrients once soils warm in the spring.
This information certainly puts a renewed emphasis on being able to regulate winter soil temperatures at levels which will help maximize winter soil microbial activity. Moderating soil temperatures and increasing the level of freeze/thaw cycles at the front and back end of our lengthy winters by even 60 to 90 days can have a significant impact on the level of nutrients available for our relatively short growing season.