亚热带不同类型林地土壤可溶性有机氮及其营养功能研究
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摘要
国外的研究已证实，土壤可溶性有机氮（SON）可能是植物可直接吸收的一种重要的土壤氮素组分，这对传统的陆地生态系统土壤氮素循环理论构成极大挑战。此外，由于土壤SON移动性强，会直接影响到陆地生态系统中各种营养物质的流动性与生物有效性以及污染物的迁移特性，从而影响到流域的水质，故土壤SON已成为陆地生态系统土壤氮素循环研究的热点。然而，作为土壤可溶性氮的重要组成部分，SON在很多生态系统中尚未被很好地理解，关于森林生态系统中土壤SON的来源、动力学性质、化学本质和生态学功能仍方面，还了解甚少，目前还不明确土壤SON库的大小对植物的氮营养贡献到底有多大。国内外关于土壤SON的研究报道主要集中于温带和寒温带地区，而对热带和亚热带森林生态系统土壤SON，尤其是SON的组成、季节动态及生态功能的研究仍很少，土壤生化性质等诸多要素对SON的综合影响也尚少见研究报道。为此，本研究以中亚热带发育于相同母质的三种相邻的不同覆被类型林地（针叶林、针阔混交林和阔叶林）为研究对象，在分析其相关理化和生化性质的基础上，研究了不同林地类型土壤SON的含量、组成及其差异以及季节动态变化规律，利用灰色关联分析法深入探讨了环境因子对SON的综合影响，借助稳定同位素示踪法研究了樟树和松树幼苗对土壤SON的直接吸收能力及其差异，旨在揭示不同林地生态系统土壤SON的组成、季节动态、主要影响因素以及SON的营养功能，为完善陆地生态系统特别是森林生态系统土壤氮循环理论提供科学依据。
研究结果表明：
1、供试阔叶林、针阔混交林和针叶林土壤均呈强酸性，上层（0-20cm）与下层（20-40cm）土壤间pH值无显著差异，土壤质地类型相近，砂、粉和粘粒含量差异不显著；上层土壤阳离子交换量和总氮（TN）含量高低次序为阔叶林＞针阔混交林＞针叶林，总有机碳含量（TC）和C/N比率均表现为针阔混交林＞阔叶林＞针叶林，但下层土壤TC、TN和C/N比率均以针叶林最高。
2、供试三种林地上层和下层土壤中真菌数量、细菌数量、放线菌数量和土壤微生物相对生物量高低均表现为针阔混交林＞阔叶林＞针叶林，土壤微生物区系均以细菌占优势，占微生物总量的69.92~77.08%，真菌和放线菌分别占微生物总量的20.64~30.01%和3.45~3.65%，针阔混交林土壤细菌的多样性均高于阔叶林和针叶纯林、阔叶林土壤则高于针叶纯林。供试针阔混交林和阔叶林土壤脲酶、蛋白酶和天门冬酰胺酶活性之间无显著差异，但均显著高于针叶林；三种类型林地上层土壤脲酶、蛋白酶和酰胺酶活性均均显著高于下层土壤。
3、供试三种林地土壤采用冷水、热水和KCl提取剂测定的上层土壤SON含量（SONw、SONhw和SONKCl）均显著高于下层。上、下层土壤SONw含量分别占土壤总可溶性氮的（TSNw）含量的58~71%和55~64%，SONhw含量分别占TSNhw含量的67~78%和58~71%，SONKCl含量分别占TSNKCl含量的67~78%和58~71%，表明SON是林地生态系统土壤可溶性氮的重要组成部分。针阔混交林与阔叶林上层或下层土壤SONhw和SOCKCl含量之间差异均不显著，但均显著高于针叶林上下层土壤，三种人工林之间土壤SONw含量差异不显著。
4、相同覆被类型下或相同土层中，三种提取液测定的土壤SON含量依次为SONw 5、供试三种林地不同土层中土壤SON含量季节动态均呈“夏季最高、秋季最低、冬季回升，与春季相近”的变化规律，故应特别关注林地生态系统夏季土壤SON的形成、迁移及其生态功能的研究。
6、供试三种林地上、下层土壤游离氨基酸总量（TFAA）及其占SON的比例均以阔叶林土壤最高，针阔混交林次之，针叶林最低，而非氨基酸的其它SON（OSON）含量以及其占SON的比例则均以针叶林土壤最高，阔叶林次之，针阔混交林最低。土壤TFAA含量与有机质、SON和碱解氮含量、脲酶、蛋白酶和酰胺酶活性之间均呈极显著正相关。
7、供试阔叶林、针阔混交林和针叶林土壤氨基酸均以中性氨基酸为主，在上层土壤中的含量分别占TFAA的83.75%、81.71%和84.94%，在下层土壤中则分别占TFAA的86.92%、82.59%和86.16%，碱性和酸性氨基酸含量次之，含硫氨基酸含量最低。针阔混交林土壤中酸性、碱性和中性氨基酸含量均显著高于阔叶林和针叶林，但阔叶林和针叶林之间差异不显著。
8、供试阔叶林和针阔混交林土壤游离氨基酸（FAA）均以丙氨酸、苏氨酸、亮氨酸和缬氨酸含量较高，丝氨酸、甘氨酸、苯丙氨酸、异亮氨酸和精氨酸含量次之，针叶林土壤FAA则以苏氨酸、亮氨酸、缬草氨酸、丝氨酸和甘氨酸为主。针叶林上层土壤中丙氨酸含量（仅占TFAA的6.72%）显著低于阔叶林和针阔混交林土壤，但丝氨酸和甘氨酸占土壤TFAA的比例则相对较高（分别达10.61%和9.51%）。
9、供试针阔混交林和针叶林上层土壤TFAA含量显著高于下层，而阔叶林上层土壤TFAA含量则低于下层，但差异不显著。阔叶林上层土壤中天冬氨酸、谷氨酸、组氨酸、精氨酸和苯丙氨酸含量显著高于下层，赖氨酸、丝氨酸、缬氨酸、异亮氨酸、酪氨酸、胱氨酸、甲硫氨酸略低于下层，但差异不显著；苏氨酸、脯氨酸、甘氨酸、丙氨酸、亮氨酸含量则显著低于下层。针阔混交林上层土壤中酪氨酸、组氨酸和脯氨酸含量低于下层，而其余氨基酸组分均显著高于下层。针叶林下层土壤中甲硫氨酸、赖氨酸含量低于上层，但上层土壤中其余15种氨基酸含量均高于下层。
10、灰色关联度分析结果显示，全氮含量、砂粒含量、粉粒含量、微生物生物量、细菌生物量、蛋白酶活性和脲酶活性是影响供试林地土壤SON含量的关键因子，有机质含量、SOC含量、粘粒含量、天门冬酰胺酶活性、含水量、真菌生物量、碱解氮含量和土壤pH值是影响供试林地土壤SON含量的一般因子。
11、单标记铵态氮和硝态氮处理对供试樟树和松树根系吸收无机氮无显著影响，碳氮双标记甘氨酸处理下，樟树和松树根系13C和15N均产生增量，说明供试樟树和松树根系均能从土壤中吸收完整的甘氨酸分子。
12、施肥72h时，樟树对铵态氮、硝态氮、甘氨酸态氮的吸收量均显著高于松树。樟树对铵态氮的吸收量低于硝态氮的吸收量，但极显著高于对甘氨酸态氮的吸收量，松树对铵态氮的吸收量高于对硝态氮和甘氨酸态氮的吸收量，差异均达极显著水平，表明不同树种对不同形态可溶性氮的吸收量存在明显差异。
13、在铵态氮处理中，樟树和松树地下部对15N的吸收率呈现出随施肥时间的延长而逐渐下降的趋势。在硝态氮处理中，随施肥时间的延长，樟树地下部中15N的吸收率呈现出先升高后下降的变化趋势，松树地下部对15N的吸收率则呈逐渐下降的变化趋势。在甘氨酸态氮处理中，随施肥时间的延长，樟树和松树地下部中15N的吸收率呈现出先升高后降低的变化趋势，表明经一定施肥时间后，不同树种对可溶性氮的吸收量均呈下降趋势。
1)14、在甘氨酸态氮施肥72h后，樟树地上部分13C的增量是松树地上部分13C的增量的2.31倍，樟树地下部分13C的增量显著低于松树地下部，表明樟树根系吸收甘氨酸态氮后向地上部的运输能力高于松树；樟树和松树地下部分13C的增量极显著地高于地上部。在甘氨酸态氮处理中，供试樟树和松树对13C的吸收率表现出相同的变化趋势，即均随施肥时间的延长而逐渐下降，且樟树对13C的吸收率高于松树。
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 (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.
研究结果表明：
1、供试阔叶林、针阔混交林和针叶林土壤均呈强酸性，上层（0-20cm）与下层（20-40cm）土壤间pH值无显著差异，土壤质地类型相近，砂、粉和粘粒含量差异不显著；上层土壤阳离子交换量和总氮（TN）含量高低次序为阔叶林＞针阔混交林＞针叶林，总有机碳含量（TC）和C/N比率均表现为针阔混交林＞阔叶林＞针叶林，但下层土壤TC、TN和C/N比率均以针叶林最高。
2、供试三种林地上层和下层土壤中真菌数量、细菌数量、放线菌数量和土壤微生物相对生物量高低均表现为针阔混交林＞阔叶林＞针叶林，土壤微生物区系均以细菌占优势，占微生物总量的69.92~77.08%，真菌和放线菌分别占微生物总量的20.64~30.01%和3.45~3.65%，针阔混交林土壤细菌的多样性均高于阔叶林和针叶纯林、阔叶林土壤则高于针叶纯林。供试针阔混交林和阔叶林土壤脲酶、蛋白酶和天门冬酰胺酶活性之间无显著差异，但均显著高于针叶林；三种类型林地上层土壤脲酶、蛋白酶和酰胺酶活性均均显著高于下层土壤。
3、供试三种林地土壤采用冷水、热水和KCl提取剂测定的上层土壤SON含量（SONw、SONhw和SONKCl）均显著高于下层。上、下层土壤SONw含量分别占土壤总可溶性氮的（TSNw）含量的58~71%和55~64%，SONhw含量分别占TSNhw含量的67~78%和58~71%，SONKCl含量分别占TSNKCl含量的67~78%和58~71%，表明SON是林地生态系统土壤可溶性氮的重要组成部分。针阔混交林与阔叶林上层或下层土壤SONhw和SOCKCl含量之间差异均不显著，但均显著高于针叶林上下层土壤，三种人工林之间土壤SONw含量差异不显著。
4、相同覆被类型下或相同土层中，三种提取液测定的土壤SON含量依次为SONw
6、供试三种林地上、下层土壤游离氨基酸总量（TFAA）及其占SON的比例均以阔叶林土壤最高，针阔混交林次之，针叶林最低，而非氨基酸的其它SON（OSON）含量以及其占SON的比例则均以针叶林土壤最高，阔叶林次之，针阔混交林最低。土壤TFAA含量与有机质、SON和碱解氮含量、脲酶、蛋白酶和酰胺酶活性之间均呈极显著正相关。
7、供试阔叶林、针阔混交林和针叶林土壤氨基酸均以中性氨基酸为主，在上层土壤中的含量分别占TFAA的83.75%、81.71%和84.94%，在下层土壤中则分别占TFAA的86.92%、82.59%和86.16%，碱性和酸性氨基酸含量次之，含硫氨基酸含量最低。针阔混交林土壤中酸性、碱性和中性氨基酸含量均显著高于阔叶林和针叶林，但阔叶林和针叶林之间差异不显著。
8、供试阔叶林和针阔混交林土壤游离氨基酸（FAA）均以丙氨酸、苏氨酸、亮氨酸和缬氨酸含量较高，丝氨酸、甘氨酸、苯丙氨酸、异亮氨酸和精氨酸含量次之，针叶林土壤FAA则以苏氨酸、亮氨酸、缬草氨酸、丝氨酸和甘氨酸为主。针叶林上层土壤中丙氨酸含量（仅占TFAA的6.72%）显著低于阔叶林和针阔混交林土壤，但丝氨酸和甘氨酸占土壤TFAA的比例则相对较高（分别达10.61%和9.51%）。
9、供试针阔混交林和针叶林上层土壤TFAA含量显著高于下层，而阔叶林上层土壤TFAA含量则低于下层，但差异不显著。阔叶林上层土壤中天冬氨酸、谷氨酸、组氨酸、精氨酸和苯丙氨酸含量显著高于下层，赖氨酸、丝氨酸、缬氨酸、异亮氨酸、酪氨酸、胱氨酸、甲硫氨酸略低于下层，但差异不显著；苏氨酸、脯氨酸、甘氨酸、丙氨酸、亮氨酸含量则显著低于下层。针阔混交林上层土壤中酪氨酸、组氨酸和脯氨酸含量低于下层，而其余氨基酸组分均显著高于下层。针叶林下层土壤中甲硫氨酸、赖氨酸含量低于上层，但上层土壤中其余15种氨基酸含量均高于下层。
10、灰色关联度分析结果显示，全氮含量、砂粒含量、粉粒含量、微生物生物量、细菌生物量、蛋白酶活性和脲酶活性是影响供试林地土壤SON含量的关键因子，有机质含量、SOC含量、粘粒含量、天门冬酰胺酶活性、含水量、真菌生物量、碱解氮含量和土壤pH值是影响供试林地土壤SON含量的一般因子。
11、单标记铵态氮和硝态氮处理对供试樟树和松树根系吸收无机氮无显著影响，碳氮双标记甘氨酸处理下，樟树和松树根系13C和15N均产生增量，说明供试樟树和松树根系均能从土壤中吸收完整的甘氨酸分子。
12、施肥72h时，樟树对铵态氮、硝态氮、甘氨酸态氮的吸收量均显著高于松树。樟树对铵态氮的吸收量低于硝态氮的吸收量，但极显著高于对甘氨酸态氮的吸收量，松树对铵态氮的吸收量高于对硝态氮和甘氨酸态氮的吸收量，差异均达极显著水平，表明不同树种对不同形态可溶性氮的吸收量存在明显差异。
13、在铵态氮处理中，樟树和松树地下部对15N的吸收率呈现出随施肥时间的延长而逐渐下降的趋势。在硝态氮处理中，随施肥时间的延长，樟树地下部中15N的吸收率呈现出先升高后下降的变化趋势，松树地下部对15N的吸收率则呈逐渐下降的变化趋势。在甘氨酸态氮处理中，随施肥时间的延长，樟树和松树地下部中15N的吸收率呈现出先升高后降低的变化趋势，表明经一定施肥时间后，不同树种对可溶性氮的吸收量均呈下降趋势。
1)14、在甘氨酸态氮施肥72h后，樟树地上部分13C的增量是松树地上部分13C的增量的2.31倍，樟树地下部分13C的增量显著低于松树地下部，表明樟树根系吸收甘氨酸态氮后向地上部的运输能力高于松树；樟树和松树地下部分13C的增量极显著地高于地上部。在甘氨酸态氮处理中，供试樟树和松树对13C的吸收率表现出相同的变化趋势，即均随施肥时间的延长而逐渐下降，且樟树对13C的吸收率高于松树。
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
(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.
引文
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