Soil Respiration Characteristics Of Pinus Tabuliformis And Picea Crassifolia Forests In Helan Mountains,China

Soil respiration is an important part of the carbon cycle of terrestrial ecosystems,and its dynamic variations are important for global carbon balance and carbon cycle.we investigated soil respiration rate,soil temperature and soil moisture content in two typical forest types which consist of Pinus tabuliformis forest and Picea crassifolia forest at Helanshan,Ningxia.In order to analyze the soil respiration dynamic characteristics and its relationship with environmental factors.At the same time,the effects of increased precipitation and nitrogen deposition on soil respiration of Pinus tabuliformis were studied by experiments of increasing precipitation and nitrogen deposition.The main results are as follows:（1）The soil respiration rate had obvious single peak diurnal and seasonal variations of Pinus tabuliformis and Picea crassifolia forest.The maximum of diurnal variation occurred at 14:00,and the monthly variation occurred in June-July.Soil respiration rate of Picea crassifolia forest is always higher than Pinus tabuliformis forest（P<0.05）.The seasonal variation of soil respiration was mainly controlled by soil temperature and soil moisture content in Pinus tabuliformis forest,which can explain the seasonal variation of 94.15%;however,the seasonal variation of soil respiration was mainly affected by soil temperature and which can explain 88.0%in Picea crassifolia forest.The Q₁₀of Picea crassifolia forest was significantly higher than that of Pinus tabuliformis Q₁₀,which was 4.95 and 4.01 respectively,indicating soil respiration of Picea crassifolia forest was more sensitive to temperature changes.（2）The soil respiration rate was significantly positively correlated with soil organic carbon,microbial biomass carbon and fine root biomass,and negatively correlated with soil bulk density in Pinus tabuliformis and Picea crassifolia forests（P<0.05）,there was no significant correlation with dissolved organic carbon,total nitrogen,microbial biomass nitrogen,and p H.The difference of soil respiration was mainly related to soil organic carbon,fine root biomass and soil moisture content.（3）The diurnal and seasonal variations of soil respiration rate showed a single peak curve at different slope positions of Pinus tabuliformis forests.There has a difference in the time when the maximum diurnal variation occurs,and the monthly variation occurred from June to July.The mean and accumulation of soil respiration rate showed lower slope>upper slope>middle slope,and there was a significant difference of soil respiration at different slope positions（P<0.05）.（4）The soil respiration rate was exponentially positively correlated with soil temperature at different slopes of Pinus tabuliformis forest,the soil temperature could explain the seasonal variation of soil respiration 67.4%-75.6%,there was positively correlated in between soil respiration rate and soil moisture content in the upper slope（P<0.05）,and no significant correlation in the lower slope and middle slope of Pinus tabuliformis.The temperature sensitivity Q₁₀value was between 2.72-4.01,the Q₁₀value of the middle slope（4.01）is the largest,indicating that the soil respiration in the middle slope is more sensitive to temperature changes;The two-factor model of soil temperature and soil moisture content can explain 85.2%-93.6%of soil respiration seasonal variation at different slope positions.（5）The soil physical and chemical properties were significantly different at different slopes of Pinus tabuliformis,there was a significant positive correlation between soil respiration rate and dissolved organic carbon and microbial biomass carbon（P<0.05）,and without significant correlation between soil organic carbon,total nitrogen,microbial biomass nitrogen,soil bulk density,p H and fine root biomass.The difference of soil respiration was mainly related to soil dissolved organic carbon at different slopes of Pinus tabuliformis.（6）Increasing precipitation significantly increased soil respiration rate by10.44%-32.09%and accumulation by 10.73%-32.65%in Pinus tabuliformis.Increasing precipitation reduced the effect of soil moisture on soil respiration,and no significant effect on the relationship between soil respiration and soil moisture content.Increasing precipitation reduced the Q₁₀value and comprehensive effect of soil temperature and soil moisture content on soil respiration.（7）Nitrogen deposition had no significant effect on soil respiration.Low nitrogen treatment increased the effect of soil temperature on soil respiration,while high nitrogen treatment was showed the opposite trend.Nitrogen deposition increased the effect of soil moisture content on soil respiration,but compared with soil temperature,soil moisture content has less effect on soil respiration.Soil respiration Q₁₀values of control,low nitrogen and high nitrogen treatment were 3.86,3.94 and3.22,respectively.Low nitrogen treatment increased the comprehensive effect of soil temperature and soil moisture content on soil respiration,while high nitrogen treatment reduced the effect.