Characteristics Of CO₂ Emission Flux In Natural Grassland Communities In Loess Hilly-gully Region And Its Response To Rainfall

Grassland ecosystem CO₂ efflux is an important component of carbon cycle in terrestrial ecosystem.Rainfall is a major factor affecting vegetation growth and soil temperature-humidity dynamics in semi-arid areas,which drives changes in CO₂ flux in natural grassland communities as an important factor.To clarify the short-term response of grassland CO₂ efflux dynamics to rainfall pulse in natural grassland communities in loess hilly-gully region is of great significance to evaluate the carbon budget of the degraded grassland ecosystem in the semi-arid area of the Loess Plateau.In this study,the natural grassland community dominated by Artemisia gmelinii and Lespedeza davurica was selected,and bare land as the control.CO₂ efflux,soil temperature and soil water content in top soil layer were monitored.Four simulated rainfall amounts?5 mm,10 mm,20 mm and 40 mm?and three simulated rainfall frequency?40 mm×1,20 mm×2 and 10 mm×4?were carried on from June to September,2016,and the measurements were taken one day before and seven days after the simulated rainfall.The main results were as follows:1.The CO₂ flux and soil respiration rate of the natural grassland community showed a single peak curve.The peak value appeared between 12:00¹6:00.The dynamic change of CO₂ flux in natural grassland was significantly correlated with soil temperature and soil water content.In A.gmelinii and L.davurica community,CO₂ flux in September was significantly lower than that in other months.In A.gmelinii community,the maximum value of CO₂ flux appeared in August(0.40 g·m-2·h^-1),and in L.davurica community,the maximum appeared in June(0.54 g·m-2·h^-1).The change of CO₂ flux in bare land during growing season was 0.01g·m-2·h^-10.28 g·m-2·h^-1,and there was no significant difference between different months.2.Simulated rainfall had a significant pulse effect on CO₂ flux,but the increase in CO₂ flux varied in different periods of the growing season and in different types of communities,and the amounts that arouse the pulse effect were also different.40 mm rainfall amount could significantly increase the CO₂ flux in the growing season in A.gmelinii community,but the increase was lower at the end of the growing season.Under the same rainfall conditions,the increase of CO₂ flux and the average of the monitoring period were significantly higher thanthose of the L.davurica community and bare land.In June,cumulative CO₂ emission from the three communities were higher than those in the growing season,and were lowest in September.The cumulative CO₂ emission under the 40 mm rainfall was the highest.3.Month,community type and their interaction effect significantly affected CO₂ flux of natural grassland.Soil water content were significantly different in different months,rainfall frequency and community types?p<0.05?.In different month of the growing season,the CO₂ flux of A.gmelinii community was significantly increased after a single 40 mm rainfall event,but there was no significant pulse effect on the L.davurica community and bare soil.Under different rainfall frequency,the cumulative CO₂ emission of A.gmelinii and L.davurica community was the highest under 20 mm × 2 treatment,and the continuously small rainfall treatment inhibited the CO₂ emissions in bare land.There was a significant correlation between CO₂ flux and water status index in grassland community,indicating that the difference of dynamic change of CO₂ flux at different rainfall frequency was closely related to soil temperature and humidity conditions before rainfall event.It is necessary to consider the community types for accurately evaluating the carbon budget of rehabilitated grassland communities in the region under rainfall pattern change phenomenon.