Effects Of Winter And Growing Season Warming On Carbon And Nitrogen Processes In Grassland Ecosystems

There are apparent seasonal differences in climate warming magnitude,while the warming rate in winter and spring is higher than that in summer and autumn.It is still unclear whether there are differences in the impact characteristics and mechanisms of temperature changes on carbon and nitrogen processes in terrestrial ecosystems in different seasons.This study investigated the ecological effects of winter and growing season warming using a meta-analysis combined with field control experiments.To identify the characteristics of the effects of winter warming experiments on carbon and nitrogen processes of ecosystem,we summarized the experimental data of global winter warming in the past 30 years.With a multilevel metaanalysis,we evaluated the carbon and nitrogen processes of terrestrial ecosystems and the response laws of different environmental factors to winter warming,especially in grassland ecosystem.In the Yunwu-Mountain grassland ecosystem in Ningxia,we also conducted a warming experiment in the growing season of plants.We explored the changing law for the different indicators,included: community aboveground biomass,soil organic carbon,total nitrogen,available phosphorus,microbial biomass carbon,microbial biomass nitrogen,nitrate nitrogen,ammonium nitrogen underground 0-5 cm,5-10 cm,10-15 cm soil layers.The result indicated that:(1)Winter warming accelerates carbon and nitrogen processes in terrestrial ecosystems.Winter warming significantly increased gross primary productivity(23%),soil organic carbon(2%),ecosystem respiration in winter(30%)and in the following growing season(22%,P<0.05).In addition,winter warming affected aboveground primary productivity(71%),aboveground biomass(4%),foliar%C and foliar%N content(96% and 24%),foliar C: N(37%),net ecosystem exchange(24%)、Soil inorganic N(1%)was also promoted,but did not reach a significant level.However,winter warming decreased soil microbial carbon content(13%,P>0.05).The study further explored the response characteristics of the indicators listed above in different ecosystems to winter warming,and the results showed that the tundra ecosystem responded more strongly to winter warming,which was a positive response;However,there is no uniform response law in grassland and other ecosystems.(2)Growing season warming increased plant above-ground biomass(122.87%)of Loess Plateau.The warming treatment in the growing season increased the average soil temperature at different depths by 2.03℃-2.93℃,especially in the surface layer(0-5 cm),which was higher than the lower layer.Increased soil temperature leads to a decrease in soil moisture content.The response of surface soil nutrients to warming was stronger than that of the lower layer,and warming significantly decreased soil organic carbon content of the surface soil by 18.47%(P<0.05).In addition,warming during the growing season significantly increased the content of available phosphorus in 0-5 cm soil and decreased the content of microbial biomass nitrogen(P<0.01).Different from the top soil response,the changes of soil nutrient indexes were not significant in 5-10 cm soil.In 10-15 cm soil,soil nitrate nitrogen content increased significantly by 23.89% after warming treatment(P<0.05),but the changes of other soil nutrient indexes did not show a significant change trend.The study also found that in the 0-5 cm layer of soil,soil available phosphorus content was significantly positively correlated with aboveground biomass(r=0.89,P<0.05).Soil organic carbon,total nitrogen,nitrate,ammonium nitrogen and microbial carbon and nitrogen were negatively correlated with aboveground biomass and soil available phosphorus.(3)The study compared the differences in the ecological effects of warming in winter and growing seasons.The main findings include: growing season warming significantly reduced soil organic carbon content,contrary to the results of winter warming experiments and meta-analysis.The response of soil inorganic nitrogen content to warming in different seasons was quite different.The results showed that both the winter and the growing season warming decreased the nitrate nitrogen-content(P<0.05).On the contrary,the ammonium nitrogen content increased significantly after winter warming(P>0.05).Plant aboveground biomass showed an increasing trend after warming both in winter and in the growing season.In conclusion,there are differences in the ecological effects of warming in winter and growing seasons.Winter warming has a positive effect on most indicators in winter and subsequent growing seasons,and warming accelerates carbon and nitrogen processes in terrestrial ecosystems,especially in tundra ecosystems.The effect of warming in the growing season on soil nutrients was mainly in the surface soil.Warming significantly increased the content of soil available phosphorus,while the content of organic carbon and microbial biomass nitrogen showed an opposite trend.Plant aboveground biomass showed an upward trend after warming both in winter and growing seasons.However,comparative studies on the ecological effects of warming in different seasons are still lacking.The unification of test indicators will help to improve the level of analysis in this field,which needs to be strengthened urgently.Research on climate change in winter is relatively scarce,and most winter research focuses on high-altitude areas such as tundra.Winter research on other ecosystems also needs to be strengthened.