Influence Mechanism Of Warming Or Adding Snow During The Dormancy Period On Productivity Of Enclosed Meadow In Loess Plateau

Much of previous work considering the impacts of climate change on grassland productivity has focused on the variability in temperature and precipitation during the growing season.Very few reports are available on responses of productivity to climate variation during the dormancy period,a critical period for plants to avoid frost damage and to maintain growth in the next year.Significantly Increased temperature in winter on a global scale has become an indisputable fact,which will profoundly affect the structure and function of terrestrial ecosystems,but its characteristics and effect mechanism is not clear at present.Based on long-term records of grassland productivity and the corresponding daily climatic data on the Loess Plateau during 1992-2011,temperature and precipitation change trends during the dormancy period(November-March)were analyzed.Impacts of variation in daily temperature and precipitation on grassland productivity were also estimated using Partial Least Squares(PLS)regression.At the same time design experiments of increasing temperature and adding snow to verify the accuracy of the preliminary statistical analysis results,and analysis impact mechanism of climate change on productivity by measuring the change of soil temperature and humidity and soil nutrient content.Our results indicated that:1.During the past 20 years,temperature during January-March increased significantly.Warming trends also occurred in November and December,however,it was not significant(P>0.1).Monthly rainfall averaged 5 mm,and no obvious inter-annual variation trend was found.Mean temperature during the dormancy period increased significantly(0.09°C per year),while precipitation did not show a statistically significant trend.Increased temperature during the dormancy period was negatively correlated with grassland productivity.Impacts of precipitation were negligible.Negative correlations between increased temperature during the dormancy period and grassland productivity could be explained by decreased snow depth,and variations in soil micro-environment and microbial metabolic caused by winter warming.The rare precipitation during the dormancy period did not have significant effects on productivity.As alternative form of precipitation during the dormancy period,impacts of snow variation on grassland productivity need further investigations.2.The average temperature increased significantly--about 2.8 ?by using Open-top chamber during the dormancy period,the warming effect is better during the day,an average increase of 5?.Based on adding snow three times,the average temperature decreased in the day,while increased at night,but both of them had no significant effect.Open-top chamber warming significantly improved the soil average temperature of underground 0-20 cm.compared with obvious warming in the day,the open-top chamber has slight influence on the soil average temperature at night.Adding snow significantly raised the soil average temperature of underground 10-15 cm,while the other soil layers were less affected.Soil relative ambient humidity was dramatically reduced during the dormancy period under warming and adding snow conditions.3.The soil organic carbon,total nitrogen content,nitrate nitrogen content were significantly decreased by open-top chamber warming or adding snow.Soil microbial biomass carbon content were increased under the dormant warming conditions,while decreased by adding snow.4.During the dormancy period Open-top chamber warming had a inhibitory effect to the grassland productivity in the early growing season,but this effect decreased with times went,and there was no apparent difference in grass maximum productivity.Adding snow during dormant significantly reduced the early growing season grassland productivity and the grass maximum productivity,which may be related to the early soil nutrient content.