| Foreign studies have confirmed that soil soluble organic nitrogen (SON) as animportant component of soil nitrogen can be directly absorbed by plants, which is agreat challenge to the traditional theory of soil nitrogen cycle in terrestrial ecosystem.In addition, due to its strong mobility, soil SON can directly affect the effectivenessand mobility of nutrients, migration of contaminants and water quality characteristicsin ecosystem, so research on soil SON has become a focus of soil nitrogen cycling interrestrial ecosystems. However, many aspects about soil SON, such as the sources,kinetic properties, chemical nature, ecological functions and the contribution of SONto plant nitrogen nutrition in forest ecosystem remains unclear. The exist reports onsoil SON focused on temperate and boreal regions, few attention was payed to thetropical and he content, composition, seasonal dynamics and ecological functions ofSON in subtropical forest ecosystems. Moreover, the combined effect of factors suchas biochemical properties of soil on SON is still rare reported. Therefore, in thepresent study, by taking soils which were developed in the same parent material inthree adjacent forests (coniferous forest, coniferous–broadleaf mixed forest and broadleafforest) in subtropical as the research object, the relevant physicochemical andbiochemical properties of soil were analysed, and the contents, composition andseasonal dynamics of soil SON in the different types of forest were studyed. And therelations between environmental factors and SON contents were anlaysed using thegray correlation analysis. Moresover, the capacity of Cinnamomum camphora (L.) andpinus massonian seedling on directly absorbing soil SON and its differences werestudyed using stable isotope tracer. The object of this study is to reveal thecomposition, seasonal dynamics, main factors and nutritional functions of soil SON inthe different forest ecosystems, and provide a scientific basis for further improvingthe theory on soil nitrogen cycle in terrestrial ecosystems.The main results are as follows:(1) Soils in the broad-leaved forest, coniferous–broadleaf mixed forest and coniferous forest were strongly acidic, there was no significant differences between pH inthe0-20cm layer and that in the20-40cm layer. The soil textures are similar,and no significant differences was found in the contents of sand, powder andclay between the different layers. The soil cation exchange capacity (CEC) andtotal nitrogen (TN) concentration in broad-leaved forest were the highestfollowed with that in coniferous–broadleaf mixed forest, and that in coniferousforest were the lowest. The total organic carbon (TC) contents and C/N ratios inthe0-20cm layer were the highest in the coniferous–broadleaf mixed forestfollowed with that in the broadleaf forest, and that in the coniferous forest werelowest. But the contents of TC, TN and C/N ratios in the20-40cm layer werehighest in the coniferous forest.(2) The descending order of soil microorganisms, fungi, bacteria and actinomycetescounts were the coniferous–broadleaf mixed forest> broadleaf forest> coniferousforest. Bacteria are dominant in soil microbial flora, accounting for69.92~77.08%of total microbial biomass, fungi and actinomycetes accounted for20.64~30.01%and3.45~3.65%of total microbial respectively. The diversity of soilbacteria was higher in the coniferous–broadleaf mixed forest than that in thebroadleaf and coniferous, and that in the broadleaf forest was as well higher thanthat in the coniferous forest. The activity of soil urease, protease andasparaginase in the coniferous–broadleaf mixed forest was not significantlydifferent with that in the broadleaf forest, but were significantly higher than thatin the coniferous forest. However, the activity of soil urease, protease andamidase in the0-20cm layer were significantly higher than that in the20-40cmlayer.(3) Soil SON content extracted in cold water, hot water and KCl (SONw, SONhwand SONKCl) in the0-20cm layer were significantly higher than that in the20-40cm layer. The SONwcontents in the0-20cm layer and the20-40cm layerrespectively accounted for58~71%, and55~64%of the total soluble nitrogen(TSNw) content, and SONhwcontents accounted for67~78%and58~71%TSNhw content, SONKClcontent accounted for67~78%and58~71%of TSNKClcontent, which indicating that the SON is an important part of soilsoluble nitrogen in ecosystems. There was not significant differences betweensoil SONhwand SOCKClcontents in the0-20cm layer and in the20-40cm layerin the coniferous–broadleaf mixed forest and that in the broadleaf forests, but weresignificantly higher than that in the coniferous forest, and soil SONwcontentbetween the three kinds of plantations were no significantly different.(4) The decending order of soil SON contents soluble in the three extracts eitherunder the same plant cover or in the same layer were SONw<SONhw<SONKCl.It is an ideal extraction method to extract SON with hot water (70℃) whichcan better reflect effects of plant cover on soil SON.(5) The soil SON content in different layer soil showed obvious seasonal dynamics,which was the highest in the summer and lowest in the autumn, and recovery inthe winter close to that in the spring. Thus, special attention should payed to thecomposition, migration and ecological functions of soil SON in forestecosystem.(6) Total free amino acids (TFAA) contents and its proportion to SON were thehighest in the broadleaf forest, followed with that in theconiferous–broadleaf mixedforest, and that in the coniferous were minimum, while the other amino acidsSON (OSON) content and its proportion to SON were the highest in theconiferous forest, followed with that in the broad-leaved forest, and lowest inthe coniferous–broadleaf mixed forest. The contents of TFAA showed a highlysignificant between positive correlation to soil organic matter content, SON andnitrogen content, urease, protease and amidase activity.(7) The amino acid in the broad-leaved forest, coniferous–broadleaf mixed forest andconiferous forest were dominated by neutral amino acid, accounted for83.75%,81.71%and84.94%to TFAA contents in the0-20cm layer, respectively, andaccounted for86.92%,82.59%and86.16%of TFAA in the20-40cm layer,followed by alkaline and acidic amino acids, and sulfur-containing amino acidcontent were the lowest. Contents of acidic, basic and neutral amino acids in theconiferous–broadleaf mixed forest were significantly higher than that in the broadleaf and coniferous forests, but no significant differences between thecontents of acidic, basic and neutral amino acids in the broadleaf forest and thatin the coniferous forests were found.(8) The soil free amino acids (FAA) were majored with contents of alanine,threonine, leucine and valine in the broadleaf forest and the coniferous–broadleafmixed forest, following with contents of serine, glycine, phenylalanine,isoleucine and arginine. Soil FAA was dominated by threonine, leucine, valerianacid, serine and glycine in the coniferous forest. Contents of alanine (only6.72%of TFAA) in0-20cm layer in the coniferous forest was significantlylower than that in the broadleaf forest and the coniferous–broadleaf mixed forest,but the proportion of serine and glycine to the total TFAA (respectively10.61%and9.51%) were relatively higher.(9) Contents of TFAA in the0-20cm layer were significantly higher than that in the20-40cm layer in the coniferous forest and in the coniferous–broadleaf mixedforest, otherwise, that in the broad-leaved forest were opposite, but thedifference was not statistically significant. Contents of aspartic acid, glutamicacid, histidine, arginine and phenylalanine in the0-20cm layer weresignificantly higher than that in the20-40cm layer in the broadleaf forest, andcontents of lysine, serine, valine, isoleucine, tyrosine acid, cystine, methioninewere slightly lower than that in the20-40cm layer, and contents of threonine,proline, glycine, alanine, leucine content in the0-20cm layer was significantlylower than that in the20-40cm layer. Contents of tyrosine, histidine and prolinein the0-20cm layer in the coniferous–broadleaf mixed forest were less than that inthe20-40cm layer, while contents of the remaining amino acid compositions inthe0-20cm layer were significantly higher than that in the20-40cm layer.Contents of methionine and lysine in the0-20cm layer in the coniferous forestwere lower than that in the20-40cm layer, but contents of the remaining15amino acids were opposite.(10) The results of gray correlation analysis showed that total nitrogen content, sandcontent, silt content, microbial biomass, bacterial biomass, protease and urease activity are the key factors affecting soil SON content in the three forests, andsoil organic matters contents, SOC content, viscosity clay content, asparaginaseactivity, moisture content, fungal biomass, nitrogen content and soil pH aregeneral factors affecting soil SON content in the three forests.(11) The single marker ammonium and nitrate nitrogen treatments had no significanteffect on absorption of inorganic nitrogen by roots of Cinnamomum camphoraand Pinus massoniana. There were generating incremental of13C and15N underdouble-labeled glycine treatment in the roots of C. camphora and P. massoniana,indicating that C. camphora and P. massoniana can directly absorb intactglycine molecule from the soil.(12) C. camphora had a higher absorption of ammonium, nitrate, glycine nitrogenthan P. massoniana after72h under the treatment. The absorptions ofammonium nitrogen in C. camphora roots were lower than the absorption ofnitrate nitrogen, but significantly higher than absorptions of glycine nitrogen.However, the absorptions of ammonium nitrogen in P. massoniana roots weremajorly higher than absorptions of nitrate nitrogen and glycine nitrogen. Theresluts suggested that uptake of soluble nitrogen by different tree species variedsignificantly.(13) With the extension of treatment, the absorption rate of15N by roots of C.camphora and P. massoniana under the ammonium nitrogen treatment descend,and the absorption rate of15N by roots of C. camphora increased and thendecreased under the nitrate nitrogen treatment, and that in roots of P.massoniana gradually declined.. And the absorption rate of15N by roots of C.camphora and P. massoniana increased and then decreased with the extension oftreatment under glycine nitrogen treatment.(14) Increments of13C in leaves of C. camphora was2.31times to that of P.massoniana after72h under glycine nitrogen treatment, but the Increments of13C in the roots of P. massoniana was significantly lower than that of P.massoniana, which indicated that transport capacity of glycine nitrogen of C.camphora was stronger than that of P. massoniana. The absorption rate of13C by roots of C. camphora and P. massoniana decreased with the extension oftreatment under glycine nitrogen treatment, and the absorption of C. camphorawere higher than that of P. massoniana. |
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