Effects Of Different Nitrogen Addition Levels On Soil Respiration Characteristics Of Saline-alkali Grasslands

With the increasing activities of human beings,such as the large use of fossil fuels,chemical nitrogen fertilizer and the development of animal husbandry,atmospheric nitrogen deposition is increasing day by day.It has a huge impact on the fragile ecosystem in the agro-pastoral ecotone.However,the effect of nitrogen deposition on soil respiration of saline-alkali grassland ecosystem in the agro-pastoral ecotone of northern China is still unknown.In order to explore the effect of nitrogen deposition on soil respiration of saline-alkali grassland ecosystem in the agro-pastoral ecotone of northern China,8 different levels of nitrogen addition(0,1,2,4,8,16,24 and 32 g N·m^-2·a^-1)were set up in April 2017 in the agro-pastoral ecotone of northern China(Youyu County,Shanxi).In this study,nitrogen was applied at the beginning of each month in the growing seasons(May to September)from 2017 to 2019.At the same time,soil respiration,10-cm deep soil temperature,0?10 cm soil moisture,0?30 cm below-ground biomass,microbial biomass carbon and nitrogen,soil carbon and nitrogen with soil inorganic nitrogen were measured.And the effects of different levels of nitrogen addition on soil respiration were analyzed.The experimental results for three consecutive years showed that:(1)Soil respiration increased gradually with time until the maximum value appeared at the end of June or the beginning of July under different nitrogen addition levels.The results showed that there was a single peak curve in all treatment,as different levels of nitrogen did not change the seasonal change soil respiration characteristics of saline-alkali grassland in the agro-pastoral ecotone.(2)The average value of soil respiration in control treatment(N₀)was 2.95±0.24 ?mol CO₂·m^-2·s^-1.There was a saturation threshold for nitrogen addition in the agro-pastoral ecosystem.and it was about 16 g N·m^-2·a^-1.When the nitrogen addition level exceeded the threshold,soil respiration tended to decline.(3)Soil pH was 9.62±0.05,and it had a significant negative correlation with nitrogen application amount in saline-alkali grassland(P<0.05).In 2018 and 2019,the soil pH was significantly lower than that in 2017 of saline-alkali grassland in the agro-pastoral ecotone(P<0.05).There was a significant negative correlation between soil respiration and soil pH(P<0.05).(4)There was a extremely significant correlation between soil respiration and soil temperature at 10 cm depth(P<0.001).With the increase of nitrogen addition,the Q₁₀ value of soil respiration increased first and then decreased,and the maximum value appeared in 8 g N·m^-2·a^-1 treatment.In conclusion,the soil respiration of the saline-alkali grassland ecosystem in the agro-pastoral ecotone showed a trend of first increasing and then decreasing with the increase of the level of nitrogen addition,and different levels of nitrogen addition had no significant effect on the soil respiration.