Altered soil carbon and nitrogen cycles due to the freeze-thaw effect: A meta-analysis

Global climate change may result in changes in snow cover, which may enhance freeze-thaw phenomena in mid and high latitude and high elevation ecosystems, especially in the northern hemisphere, in the future. As a common non-biological stress, the freeze-thaw process can substantially alter soil carbon and nitrogen cycles. However, a comprehensive understanding of nutrient pools and dynamics in response to freeze-thaw cycles is not available. Here, we evaluated the effect sizes of the responses of 18 variables related to soil carbon and nitrogen cycles to the freeze-thaw effect from 46 papers. Seventeen studies that reported field observations and 28 studies that reported results from laboratory experiments were included, as well as one paper that used both methods to explore freeze-thaw processes. We used a random-effects model to examine whether soil origins, effect phases (including initial and long-term effects), methods and soil horizons affect the magnitudes of the responses to freeze-thaw events. The soil sources include forest, shrubland, grassland/meadow, cropland, tundra and wetland. We used meta-regression to explore possible relationships among effect sizes with freezing temperature, soil pH, soil C/N ratios and other factors. Our results suggest that the freeze-thaw process causes microbial N and the microbial C/N ratio to decrease by 12.2% and 8.5%, respectively. Soil solution dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) are enhanced by 27.5% and 37.3%, respectively. The freeze-thaw effect increases the concentrations of NH4+, NO3− and dissolved inorganic nitrogen (DIN) by 84.1%, 29.6% and 35.4%, respectively. N2O emissions are also increased by 95.0% in freeze-thaw treatments. Laboratory measurements resulted in contrasting responses in terms of mineralization, nitrification and respiration. Freeze-thaw events promote turnover of fine roots but have no effect on the long-term aboveground biomass of grassland and heath. The results of this meta-analysis help to achieve a better understanding of the overall effects of freeze-thaw events on soil carbon and nitrogen cycles and their modulation across different environments.