Response Of Sandy Grassland Plant Community Characteristics And Soil Quality To Nitrogen And Phosphorus Nutrient Addition

Nitrogen(N)and phosphorus(P)are the key nutrients for plant growth.In most terrestrial ecosystems,N limits plant productivity,but P is equally important.The dynamics of N and P have a great impact on ecosystem processes,especially on the carbon cycle process regulating global climate change.Under the interference of human activities,such as the extensive use of fossil fuels and chemical fertilizers,atmospheric N and P deposition has increased significantly worldwide,and the availability of N and P for plant growth has increased.Therefore,how the ecosystem structure and functional processes respond to the increase of these nutrients has become a hot scientific issue.This study took Horqin sandy grassland,a typical ecologically fragile area in north China as the research object,two experiments were conducted in the field,experiment 1:CK(control group),N5(5 g N·m^-2·yr^-1),N10(10 g N·m^-2·yr^-1),N15(15 g N·m^-2·yr^-1),P5(5 g P·m^-2·yr^-1),N10P5(10 g N·m^-2·yr^-1+5 g P·m^-2·yr^-1)and the six nutrient addition experiments to study the effects of N and P nutrient addition and their synergistic effects on plant communities and soil quality,Experiment 2:CK(control group),N20(20 g N·m^-2·yr^-1),P10(10 g P·m^-2·yr^-1)and N20P10(20 g N·m^-2·yr^-1+10 g P m^-2 yr^-1)and the four nutrient addition experiments to study the effects of N and P nutrient addition and their synergistic effects on soil respiration.It is expected to provide scientific basis and practical support for the formulation of sustainable restoration measures for the degraded ecosystem of Horqin Sandy Land,the improvement of carbon sequestration and sink enhancement functions,and the response to global change risks.The main results are as follows:(1)In Horqin sandy grassland,the addition of N and P nutrients has a significant impact on the biomass of plant communities.Among them,the application of N fertilizer alone and P fertilizer alone,and the mixed application of N?P fertilizer(NP)significantly increased the above-ground biomass(P<0.05),while application P fertilizer alone significantly reduced the amount of litter(P<0.05);and Horqin sandy grassland has a critical value of N saturation,when the N application rate exceeds 10 g N·m^-2·yr^-1,vegetation growth will be inhibited.(2)Application of N fertilizer and P fertilizer alone increased the average plant height,but the effect was not significant(P>0.05),NP mixed addition significantly increased the average plant height(P<0.05);application of N fertilizer alone and NP mixed application had a significant effect on coverage Significant increase(P<0.05),application of P fertilizer alone increased community coverage,but not significant(P>0.05);application of N fertilizer alone,application of P fertilizer alone and NP mixed application had no significant effect on species diversity index(P>0.05).It shows that the plant community structure of Horqin sandy grassland is mainly affected by the combined effects of N and NP,while the addition of N and P nutrients has no significant effect on the composition of the plant community.(3)Application of N fertilizer significantly decreased soil p H(P<0.05),but had no significant effect on soil electrical conductance(EC),Soil organic carbon(SOC),total nitrogen(TN),total phosphorus(TP)and nitrogen/phosphorus(N/P)ratio(P>0.05);P addition significantly increased soil EC(P<0.05),but had no obvious effect on soil p H,SOC,TN,TP and N/P ratio(P>0.05);NP addition significantly decreased soil p H,and significantly increased soil EC(P<0.05),but had no significant impact on soil p H,SOC,TN,TP and N/P ratio(P>0.05).The results showed that in Horqin sandy grassland,N and P only had significant effects on soil p H and EC,but had no significant effects on other soil physical properties and total nutrients.(4)5 g·m^-2·yr^-1 and 10 g·m^-2·yr^-1 N addition and P fertilizer significantly reduced the soil ammonium nitrogen(NH₄⁺-N)content(P<0.05),15 g·m^-2·yr^-1 significantly increased the nitrate nitrogen(NO₃^—N)content(P<0.05),and significantly reduced the available phosphorus(AP)content(P<0.05);the addition of P alone and the mixed application of NP had no significant effect on the soil NO₃^--N content(P>0.05),but significantly increased the AP content(P<0.05);NP mixed addition significantly increased the soil NH₄⁺-N and AP content(P<0.05).It shows that in Horqin sandy grassland,soil available nutrients are mainly affected by the combined effects of NP,of which NO₃^--N is mainly affected by the combined effects of low N content and P fertilizer,and NH₄⁺-N is mainly affected by the combined effects of high N content and NP.AP mainly affected by high N content,NP combined action and P fertilizer.(5)Application of N fertilizer alone and application of P fertilizer alone have a significant increase in soil microbial biomass carbon(MBC),among the three N additions,only 15 g·m^-2·yr^-1 N addition significantly increases soil MBC,but soil MBC with the increase of N addition,there was a gradual increase trend;soil microbial biomass nitrogen(MBN)was not affected by the addition of N and P nutrients.(6)From the beginning of growing season to the end of growing season,the change trend of soil temperature with the addition of N and P nutrients was the same,showing a"W"shaped change;the changing trend of soil respiration rates between different treatments was also the same,showing a"three-peak curve"change,however,there is no correlation between soil temperature and soil respiration rate;The addition of N and P nutrients has no significant effect on soil temperature and soil respiration(P>0.05),and there was no significant difference in soil temperature and soil respiration rate between N and P treatments(P>0.05).In summary,the nutrient limitation of Horqin sandy grassland is mainly restricted by N,followed by P limitation.N and P jointly limit the vegetation growth of Horqin sandy grassland.The mixed addition of N and P nutrients is the best fertilization method to promote the restoration of the degraded and enclosed Horqin sandy grassland,and the N addition amount should not exceed 10 g·N m^-2·yr^-1.