Recurrent soil freeze-thaw cycles enhance grassland productivity.

Ongoing global warming will increase the frequency of soil freeze-thaw cycles (FTCs) in cool-temperate and other high-latitude regions. The spatial relevance of seasonally frozen ground amounts to c. 55% of the total land area of the northern hemisphere. Evidence suggests that FTCs contribute to nutrient dynamics. Knowledge of their effects on plant communities is scarce, although plants may be the decisive factor in controlling ecosystem functions such as nutrient retention. Here, the effects are analysed of five additional FTCs in winter for the above- and below-ground productivity of experimental grassland communities and soil enzymatic activity over the following growing season. Freeze-thaw cycles increased the above-ground productivity but reduced root length over the whole subsequent growing season. In summer, no changes in soil enzymatic activities representing the carbon, nitrogen and phosphorus cycles were observed in the FTC-manipulated plots, except for an increased cellobiohydrolase activity. Changes in productivity resulting in an increased shoot-to-root ratio and shifts in timing are capable of altering ecosystem stability and ecosystem services, such as productivity and nutrient retention.