| Atmospheric nitrogen deposition not only increases the nitrogen content of forest soil,but also causes significant changes with soil properties,which will affect the structure and function of soil microbial community,soil respiration and carbon cycling rate,meantime inevitably affect the nitrogen utilization strategy of dominant species of forest community and the stoichiometric relationship of carbon,nitrogen and phosphorus of leaf / fine root / soil.It is very important to study the response mechanism of forest ecosystem to atmospheric nitrogen deposition in order to optimize the adaptive management of forest.In this study,We took the Larix principis-rupprechtii plantation of Longxing forest farm in Guandi Mountain Forest region and the natural Larix principis-rupprechtii in Pangquangou nature reserve as the research objects.The three-levels nitrogen addition experiment of control(CK,0 g·m-2·a-1),light nitrogen addition(LN,8 g·m-2·a-1)and heavy nitrogen addition(HN,15 g·m-2·a-1)were used to study the soil physical and chemical properties,soil microbial biomass and community structure,soil respiration rate,nitrogen and phosphorus utilization characteristics of leaves and fine roots for three consecutive years.The measurement and analysis of soil respiration rate and other indicators aims to reveal the adaptive response of soil and leaf/ fine root eco-chemical characteristics of Larix principis-rupprechtii to nitrogen deposition and its impact on the relationship between them,for providing a theoretical basis for the adaptive management of Larix principis-rupprechtii.The results revealed that:(1)Nitrogen addition and stand origin significantly affected soil TOC,TN,C/N,AN,NN,p H,SWC and BD.The response of soil TN to Nitrogen addition depended on nitrogen addition concentration(HN >LN).The contents of NN and AP in different soil layers had different responses to nitrogen addition.Nitrogen addition significantly promoted the content of NN in surface and shallow soil of planted and natural forest,while the content of AP significantly decreased.(2)Without nitrogen addition,the contents of soil microbial carbon(MBC)and microbial nitrogen(MBN)were surface layer > shallow layer > middle layer.The content of MBC and MBN to nitrogen addition was regulated by nitrogen addition concentration.Light nitrogen addition had no significant effect on MBC,while MBN increased significantly,MBC decreased significantly under heavy nitrogen addition,MBN had no significant effect.MBC/MBN decreased significantly with nitrogen addition treatment.The response of soil microbial community structure to nitrogen addition depended on nitrogen addition concentration.The amount of B,G+,G-,F,AMF,total PLFAs and F/B of plantation and natural forest soil increased significantly with light nitrogen addition.With heavy nitrogen addition treatment,B,G-and AMF of natural forests decreased significantly.The results of redundancy analysis showed that soil microbial community structure of plantation was significantly correlated with TN,p H and BD,natural forest was significantly correlated with TN and MBN.Both plantation and natural forest TN were positively correlated with 16:1ω7C and 18:1ω7C which labeled the fatty acids of gram-negative bacteria PLFAs.(3)Without nitrogen addition,the monthly dynamic change of soil respiration rate in planted and natural forest were that the peak value appeared in July or August,and the low value appeared in October.The response of soil respiration to nitrogen addition in planted and natural forests depends on the concentration of nitrogen addition.Light nitrogen addition significantly promoted soil respiration of plantation,but heavy nitrogen addition had no effect.while light and heavy nitrogen increase had no effect on soil respiration of natural forest.The seasonal change of soil respiration to N addition depended on nitrogen concentration and stand origin.The soil respiration of plantation from May to October and natural forest from May,June and August increases significantly with light nitrogen addition treatment,while the soil respiration rate increased with heavy nitrogen addition treatment,but the differences are not significant of plantation from May,August and September and natural forest from May to October(except September),The correlation analysis between soil respiration and soil properties showed that soil respiration were significantly or extremely significantly positively correlated with soil TOC,TN and SWC,and significantly or extremely significantly negatively correlated with NN,AP and p H.(4)The effects of nitrogen addition on the content of C,N,P and stoichiometric characteristics of coniferous leaves and fine roots depended on nitrogen addition concentration and stand origin.Nitrogen addition significantly increased the content of C in mature leaves of plantations,but had no significant effect on the content of C in natural forests.Nitrogen addition significantly increased of the N in mature leaves of plantation and natural forest,P decreased significantly,N/P and C/P increased significantly,and C/N decreased.Nitrogen addition significantly increased C and N contents in fine roots of plantations and natural forests,and increased N/P while decreased C/N.The response of NRE and PRE to nitrogen addition depended on nitrogen addition concentration.Nitrogen addition significantly increased PRE in leaves of plantations and natural forests,while light nitrogen addition significantly decreased NRE and NRE/PRE in plantation and natural forests.(5)Nitrogen addition significantly increased the N content in leaves and fine roots of plantations and natural forests by promoting the increase of available nitrogen content in soil.Leaves and fine roots,as the main organs of plant absorption of nutrients and water,have synergistic responses to nitrogen addition,and the N nutrients in leaves come from the absorption and transport of fine roots.There were significant positive correlations between C,N content and C/N of leaves and fine root before nitrogen addition and with light nitrogen addition.while the C,P and C/P of leaves and fine root were significantly positively correlated with heavy nitrogen addition treatment.Without nitrogen addition,there were significant linear positive correlation between the contents of C and N between leaves and soil,while there were significant negative correlation between leaves P and soil P,indicating that the supply of soil N and P can affect the contents of plant N and P;There was no significant correlation between leaves and soil stoichiometry with light nitrogen addition treatment,but there were significant linear positive correlation between leaves and soil N and N/P with heavy nitrogen addition,and a significant negative correlation between C/P.There were significant linear positive correlation between fine roots and soil C in the control plot,and there were significant linear negative correlation between fine roots and soil N and C/N with nitrogen addition treatment. |
