Responses Of Soil-plant Carbon And Nitrogen Stable Isotopes And Their Nutrients To Nitrogen And Phosphorus Additions In Northern Subtropical Evergreen Broad-leaved Forest

In recent years,with the development of industry and modern agriculture,the globalatmospheric nitrogen deposition has intensified,and the rapidly increasing nitrogen deposition has had a huge impact on the forest ecosystem.Nitrogen and phosphorus are essential nutrients for plant growth.In this study,a long-term nitrogen and phosphorus addition test plot was set up in the Zhawan Nature Reserve of Qimen County,Anhui Province to study the impact of nitrogen and phosphorus addition on the forest ecosystem.Three treatments of control（CK）,high nitrogen(HN;100 kg N·hm^-2·a^-1),and high nitrogen+phosphorus(HN+P;100 kg N·hm^-2·a^-1+50 kg P·hm^-2·a^-1)were set on the middle slope and the top of the slope,and 14 plant leaf and soil samples were collected from evergreen broad-leaved forests in the northern subtropical zone.The stable carbon and nitrogen isotope,nutrient content,and soil physical and chemical properties are analyzed,and the stable carbon and nitrogen isotope technology is used to explore the process and mechanism of nutrient cycling in forest ecosystems,and to provide references and theories for the protection and scientific management of subtropical evergreen broad-leaved forests in accordance with.The main research is as follows:（1）Soil carbon and nitrogen isotopes(δ¹³C,δ¹⁵N)and soil carbon turnover rateThe addition of nitrogen and phosphorus had extremely significant effects on theδ¹³C andδ¹⁵N values of subtropical deciduous forest soils（P<0.01）.Both HN and HN+P treatments increased theδ¹³C value of the soil on different slope positions.The addition of nitrogen and phosphorus has different effects on the soilδ¹⁵N value on different slope positions.On the middle slope,HN+P and HN treatments reduced the soilδ¹⁵N value;on the top of the slope,HN+P and HN treatments increased the soilδ¹⁵N value.In the middle slope,HN treatment accelerated the soil carbon turnover rate;on the top of the slope,both HN and HN+P treatments accelerated the soil carbon turnover rate.（2）Soil physical and chemical properties,nutrients and stoichiometryThe addition of nitrogen and phosphorus has extremely significant effects on soil p H,EC,total phosphorus（TP）,SOC/TN,SOC/TP,TN/TP（P<0.05）,and has no significant effect on soil organic carbon（SOC）and total nitrogen（TN）（P>0.05）.In the middle slope and the top of the slope,the addition of nitrogen and phosphorus reduces the soil p H and TP content,and increases the soil EC value.The soil SOC/TN,SOC/TP,TN/TP values respond differently to the addition of nitrogen and phosphorus on different slope positions.Different slope positions have significant effects on soil p H,EC,SOC,TN,SOC/TN,SOC/TP,TN/TP（P<0.05）,but have no significant effects on soil TP（P>0.05）.The soil EC,TP,C/P and N/P values of the top of the slope should be greater than that of the mid-slope,and the p H and C/N values of the top of the slope should be less than that of the mid-slope.The soil layer has a significant impact on soil p H,EC,SOC,TN,TP,SOC/TN,SOC/TP,TN/TP（P<0.05）;except for p H,all indicators increase and decrease with the increase of soil layer.（3）Leaf carbon and nitrogen isotopes(δ¹³C,δ¹⁵N)and water use efficiency（WUE）The addition of nitrogen and phosphorus had significant effects on theδ¹³C andδ¹⁵N values of plant leaves in subtropical deciduous forest（P<0.05）.HN treatment and HN+P treatment reduced theδ¹³C value of plant leaves and increased theδ¹⁵N value of plant leaves;Different life forms did not significantly change theδ¹³C value andδ¹⁵N value of the leaves of the plant（P>0.05）.The addition of nitrogen and phosphorus significantly affected plant leaf water use efficiency（P<0.05）in subtropical deciduous forest.HN treatment and HN+P treatment reduced plant leaf water use efficiency（WUE）;Different life forms have no significant effect on the water use efficiency of plants without significant changes（P>0.05）.（4）Leaf nutrients and their stoichiometryThe addition of nitrogen and phosphorus significantly affected the P content of plantleaves in the subtropical evergreen broad-leaved forest（P<0.05）,but had no significant effect on the C and N content of plant leaves（P>0.05）.HN+P and HN treatments increased the P content of plant leaves.Different life forms significantly affect the C and P content of plant leaves（P<0.05）,but do not significantly affect the N content of plant leaves（P>0.05）.The C content of leaves is all expressed as:tree>shrub>herb,and the P content of leaves is all expressed as:Herbs>Shrubs>Trees.The addition of nitrogen and phosphorus has a significant effect on the C/P and N/P values of plant leaves（P<0.05）,but has no significant effect on the C/N of plant leaves（P>0.05）.Plant leaves C/N,C/P,N/The P values are all expressed as:CK>HN>HN+P.Different life forms significantly affect the C/N,C/P,and N/P values of plant leaves（P<0.05）.The C/N,C/P,and N/P values of plant leaves are all expressed as:tree>shrub>herb.