Effects Of Nitrogen Addition At Different Levels On Soil Carbon Limits In Semi-arid Grassland

Soil microorganisms are the Main driving force of carbon,nitrogen and other elements cycling in soil.Coordination of carbon-to-nitrogen ratio is the basis of soil respiration and soil microbial activity.Whether,and to what extent,soil respiration is limited after nitrogen addition,the differences in different in different ecological system.In the experiment,The soil sample of 8 nitrogen treatment(0,1,2,4,8,16,24 and 32 g N m-2)in semi-arid grassland were added with glucose(0.65 mg C·g-1 soil,6 mg C·g-1 soil)to determine the low-carbon and high-carbon level for indoor incubation.By measuring the soil respiration rate,soil microbial biomass carbon(MBC),soil microbial biomass nitrogen(MBN),soluble organic carbon(SOC),dissolved organic carbon(DOC),total nitrogen(TN),p H value and carbon nitrogen ratio(C/N),and other soil index,we studied under low and high carbon,soil respiration rate and the changing rule of the soil microbial biomass.The aim of this study was to determine the effects of carbon limiting of soil respiration and microbial biomass under nitrogen addition.The main results were as follows:(1)Nitrogen addition reduced soil respiration rate as the whole.Soil p H decreased with the increase of nitrogen addition as a whole,SOC declined under low-nitrogen treatment(0,1,2,and 4 g N m-2)and raise under high-nitrogen treatment(8,16,24 and 32 g N m-2),soil MBC showed a trend of increasing first and then decreasing with the addition of nitrogen,and the content of MBN increased with the addition of nitrogen,which showed a trend of increasing first and then decreasing with the addition of nitrogen.(2)Carbon addition significantly improved soil respiration rate(P<0.05).With the increase of glucose,soil SOC and TN showed a trend of increasing first and then decreasing,while soil MBC and DOC showed a trend of gradually increasing,and the effect of high carbon treatment was more significant(P<0.05).(3)Carbon addition significantly improved soil respiration rate after nitrogen addition(P<0.05),while high-carbon treatment was significantly higher than low-carbon treatment(P<0.05),indicating that carbon addition had a more significant promoting effect on soil respiration rate after nitrogen addition.With the increase of culture days,the overall respiration rate of soil treated with low-carbon gradually decreased,while that of soil treated with high-carbon increased first and then decreased.MBC,DOC,SOC,C/N of soil increased with the addition of glucose,while MBN showed a decreasing trend.The total nitrogen of low-nitrogen treatment(0,1,2 and 4 g N m-2)increased,while that of high-nitrogen treatment(8,16,24 and 32 g N m-2)decreased.High-carbon treatment increased soil microbial activity and significantly enhanced respiration intensity,indicating that grassland soil respiration was limited by carbon source after nitrogen addition.(4)The correlation analysis of carbon and nitrogen addition showed that MBC and soil respiration rate were significantly positively correlated(P<0.01),and the soil respiration rate and MBC significantly increased under carbon-addition treatment(P<0.05),indicating that the soil was limited by carbon source and vulnerable to exogenous carbon input.The MBN treated with carbon was reduced(P>0.05),which indicated that the biomass of soil microorganisms was not only limited by carbon,but also restricted by nitrogen and phosphorus.Correlation analysis showed that soil respiration rate was positively correlated with MBC,DOC,SOC and C/N(P<0.01),MBC was positively correlated with respiration rate,SOC and DOC(P<0.01),and MBN was positively correlated with MBC and DOC(P<0.05).The comprehensive analysis showed that soil is limited by the carbon,the carbon limit state of soil was temporarily lifted after the addition of carbon,and the results of the high-carbon treatment was more significant than the low-carbon treatment.