The Freeze-thaw Effect On Soil Mineralization Between Various Moisture States Of Wetlands

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.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.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 simulations and field observations 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 soil moisture condition is important environmental factor in the wetland ecosystem.The wetland soil moisture can be effected by snow thawing during spring.Besides,the terrain features of wetland can make the soil moisture fluctuated with time and space.Our paper conducted a laboratory simulation using soil pillar to explore wetland soil mineralization affected by freeze-thaw cycles between various soil moisture conditions.The freeze-thaw effect can destory macro-aggregates(>2 mm),decrease the volume-weight of soil with 50%water holding capacity(WHC)and 100%WHC.The freeze-thaw also increased the soil Eh values and oxygen concentration of that two moisture conditions.However,the freeze-thaw effect had contradictory effect results to the soil with 10cm overlying water because of the buffering and consolidation of the water.The freeze-thaw effect can increase the soil DOC content,the NH4+ content,and decrease the soil DON content between various moisture conditions.The freeze-thaw effect increased the NO3-content in soils with 50%WHC and 100%WHC.However,it decreased the N03 content in the soil with 10 cm overlying water.The freeze-thaw effect significantly increased the DOC concentration,DON concentration and NH4+ concentration in leachates between various moisture conditions.The freeze-thaw effect increased the NO3-concentration in the leachate in which group the soil had 50%water holding capacity.However,the freeze-thaw effect decreased the NO3-concentration in soil leachates with 100%WHC and 10cm overlying water.The freeze-thaw inhibited the CH4 emissions of the soil with 50%WHC,and stimulated the CH4 emissions of the soil with 100%WHC and 10 cm overlying water.The freeze-thaw effect inhibited CO2 emissions between various soil moisture conditions.N2O emissions were stimulated in soils with 50%WHC and 100%WHC.However,N2O emissions were inhibited in the soil with 10cm overlying water.Moreover,the freeze-thaw effect stimulated the respiration rate and denitrification rate between various moisture conditions.The freeze-thaw effect significantly increase the gross nitrification rate of soils with 50%WHC and 100%WHC.However,the soil with 10cm overlying water had little increase of gross nitrification rate.No matter the soil changed from dry to wet or wet to dty,the freeze-thaw effect always decreased the soil DOC and DON content.When the soil was changed from dry to wet,the freeze-thaw significantly increased the soil NH4+and NO3-content.However when the soil was changed from wet to dry,the results were opposite.When the soil was changed from dry to wet,the freeze-thaw effect significantly decreased the DOC and NH4+ content in leachates.However when the soil was changed from dry to wet,the results were opposite.Besides,when the soil was changed from dry to wet,the freeze-thaw effect can increase the NO3-content in leachates.Furthermore,no matter the soil changed from dry to wet or wet to dty,the freeze-thaw effect can significantly decrease the DON content in the leachate,stimulate CH4 and N2O emissions,as well as inhibit the CO2 emissions.The results of this meta-analysis in our paper 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.Otherwise,we interpreted the freeze-thaw effect on the wetland soil mineralization between various moisture conditions.We also had a brief description of effects between dry wet flutuation and freeze-thaw on wetland soil mineralization.Our results can help understand the role of moisture,the important environmental factor,in the soil mineralization during the freeze-thaw process.