番茄对有机氮的吸收及土壤可溶性有机氮行为特性研究
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摘要
可溶性有机氮(SON)是土壤氮素中最活跃的组分之一,移动性强,可随水分运移而发生径流或淋溶损失,污染水体。由于受传统矿质营养理论的影响,关于生态系统N有效性和植物N营养的认识几乎完全建立在无机氮(NH_4~+-N、NO_3~--N)的基础上,而在很大程度上忽视了植物对SON的吸收及其在生态系统中的作用。为探讨植物氨基酸态氮营养机理及SON在农业生态系统中的行为和功能,本文以完全无菌水培、~(13)C-~(15)N双标记有机氮的土壤原位试验探讨不同番茄品种(申粉918、沪樱932)的氨基酸态氮营养效应,研究了NH_4~+-N、NO_3~--N和Gly-N对水培番茄幼苗生长、碳氮积累、根系分泌特性及氮代谢等的影响。运用土壤溶液超速离心技术研究不同土壤SON含量及特性;探讨通气培养条件下SON的动态变化,揭示土壤氮素的矿化特性;采用14C标记氨基酸、多肽的室内培养法系统研究氨基酸、多肽在土壤中的吸附、矿化、吸收动力学等动态生化特性。
本文主要研究结果如下:
1甘氨酸态氮对番茄幼苗生长、碳水化合物及碳氮积累的影响
在NH_4~+-N、NO_3~--N和Gly-N存在的营养介质中,番茄幼苗在处理前期(如处理后8或16d)的株高、生物量、植株全碳积累量、总氮量等各处理差异不显著,而其后则表现为NO_3~--N>Gly-N>NH_4~+-N,处理间差异显著(p<0.05)。与NO_3~--N处理相比,Gly-N、NH_4~+-N处理均显著提高了叶片可溶性糖、叶片和根系游离氨基酸及各器官氮素含量,而降低了叶片淀粉含量和各器官氮素积累量。Gly-N处理显著提高了番茄根系可溶性糖、淀粉和叶绿素含量。番茄基因型差异是否表现与氮素形态有关,以植株根系干重为指标的基因型差异在供应NO_3~--N时不表现;在Gly-N处理下,沪樱932的生长显著优于申粉918。
2甘氨酸态氮对番茄根系分泌物和木质部及韧皮部汁液组分的影响
营养液中NH_4~+-N、NO_3~--N和Gly-N显著影响不同番茄品种根系分泌物、木质部和韧皮部汁液各组分含量。NO_3~--N处理的番茄根系分泌物、木质部和韧皮部汁液中的NO_3~--N和P含量最高,供Gly-N、NH_4~+-N其次,对照最低;而供Gly-N、NH_4~+-N时铵态氮、游离氨基酸、可溶性糖含量一般均高于处理NO_3~--N和对照。番茄根际吸收不同氮素对根际pH亦产生不同的影响,吸收NO_3~--N和Gly-N后,根际pH显著升高,而吸收NH~+_4-N后则降低。在供应NO_3~--N、NH_4~+-N和Gly-N及对照时,沪樱932的根系活力均显著大于申粉918(p<0.05)。
3无菌水培条件下无机氮和氨基酸态氮对番茄幼苗生长和氮代谢的影响
无菌水培条件下,与无氮处理相比,NH_4~+-N、NO_3~--N和Gly-N处理都显著提高了植株干物质和总吸氮量。Gly-N处理的番茄干物质重和吸氮量显著高于缺氮及NH_4~+-N处理;NH~+_4-N和Gly-N处理显著提高了地上部、根系FAA含量,而NO_3~--N处理对地上部、根系FAA含量无显著影响。可溶性蛋白含量则表现为:Gly-N >NH_4~+-N>NO_3~--N>CK处理。与无氮处理相比,各处理都显著降低了地上部和根部GS活性;而Gly-N处理显著促进了地上部、根系NADH-GDH、GOT和根系GPT活性,NO-3-N处理显著降低了地上部GPT活性。因此,番茄吸收甘氨酸后,能够通过转氨酶的作用,最终合成蛋白质,促进植株生长,氨基酸态氮(Gly-N)也可以成为番茄生长的良好氮源。
4无菌水培条件下甘氨酸浓度对番茄幼苗生长和氮代谢的影响
无菌水培条件下,番茄能够吸收各种浓度的Gly-N,且营养液中甘氨酸浓度(0-6.0 mM)与番茄植株干物重、总氮量等呈显著正相关(R~2> 0.905**)。随营养液中甘氨酸浓度的增加,两个番茄品种的地上部和根系FAA、可溶性蛋白、地上部可溶性糖含量增加。与无氮对照处理相比,各处理都显著降低了地上部淀粉含量,营养液中甘氨酸浓度对根系淀粉含量无显著影响。甘氨酸态N浓度对氮代谢关键酶有不同的影响。随营养液中甘氨酸态N浓度的增加,两个番茄品种的地上部、根系NR、NADH-GDH、GPT和GOT活性均提高。被吸收的氨基酸可通过GOT和GPT转氨酶主要在根中进行同化,最终合成蛋白质,促进植株生长。
5土壤微区培养条件下番茄幼苗对~(13)C,~(15)N-甘氨酸分子的吸收研究
不同形态的~(15)N注射48h后,相对于对照、~(15)N-NH_4Cl和~(15)N-KNO_3处理来说,供2-[~(13)C], [~(15)N]-glycine时,两个番茄品种的地上部、根系~(13)C、~(15)N原子超均显著提高,且根系~(13)C原子超显著大于地上部。申粉918、沪樱932从土壤中吸收的氮分别有12.5%和21%是以完整的甘氨酸分子直接吸收的,番茄根系吸收甘氨酸态N的能力比土壤微生物弱。甘氨酸态N对沪樱932的氮营养贡献率显著大于申粉918(p<0.05)。
6不同土壤SON的含量及特性
供试土壤SON平均含量表现为灰壤土(62 mg kg-1)>腐殖黑钙土(29.1 mg kg-1)>有机土壤(15.8 mg kg-1)>常规土壤(12.9 mg kg-1)>转换期土壤(7.8 mg kg-1)。有机、转换期、常规土壤、灰壤土、腐殖黑钙土SON含量占TSN的比例分别为48.3%、21.9%、30.9%、93.7%、83.4%;而FAA含量分别占TSN的1.4%、2.6%、2.4%、1.1%、2.4%,占SON的2.9%、11.8%、7.8%、1.2%、2.8%。相关分析结果表明,TSN、SON以FAA与全氮、全氮、硝态氮、铵态氮等各养分之间均有极显著的相关性。用盐溶液提取的SON含量大约是水提取的SON或DON的3-6倍。
7不同土壤中氨基酸、多肽的生物化学动态特性
氨基酸、多肽在土壤中的吸附量表现为Val-Pro-Pro> Glu> Val> Glu-Phe。而不同土壤对氨基酸、多肽的吸附能力则为有机土壤>转化期土壤>常规土壤。氨基酸、多肽在土壤中的矿化周转速率迅速,Glu、Glu-Phe、Valine、Val-Pro-Pro在供试土壤中的半衰期(D_(50))分别为5.6±1.1、7.7±1.3、14.8±3.6和10.5±2.0 h。培养24h后,大约有近(76±3)%的氨基酸或多肽被微生物细胞利用,而仅(24±2)%的氨基酸或多肽以CO2的形式释放出来。外加Ala-Ala和NH_4NO_3等氮源能显著提高氨基酸、多肽在土壤中的矿化。氨基酸、多肽等SON在土壤中的矿化是一个完全依靠微生物参与的生物降解过程。
8不同园艺生产系统土壤氮素的矿化特性
随着通气培养过程的进行,不同园艺生产系统土壤NO_3~--N、SON的含量均明显增加, FAA含量呈先升高后下降的趋势。土壤矿化培养过程中生成一定数量的SON;在整个培养过程中,SON占TSN比例均大幅度下降。评价土壤氮素矿化特性时仅仅测定矿化的无机氮数量,可能会低估土壤氮素矿化潜力和氮素损失的数量和效应。施入土壤的~(14)C-氨基酸、多肽在土壤中的矿化迅速,不同园艺生产系统土壤~(14)C-氨基酸、多肽的矿化速度表现为:有机土壤>转化期土壤>常规土壤。在矿化过程中,影响SON含量的瓶颈不是微生物利用低分子量SON的速率,而是SON从生物体释放进入土壤的速率。
9温度对土壤氨基酸、多肽的矿化及其吸收动力学的影响
温度显著影响着氨基酸、多肽在土壤中的矿化及其吸收动力学特性。随着温度的升高,氨基酸和多肽在土壤中的矿化速度加快。在1、15和25°C下,三种供试土壤中Val的平均半衰期分别为28.4、17.9和10.1h;Glu则分别为21.6、10.7和8.1h;Val-Pro-Pro分别为13.1、10.6和8.6h;Glu-Phe分别为33.9、11.1和6.8h。氨基酸、多肽的吸收速率随着施入土壤的氨基酸和多肽浓度的增加而增加。氨基酸、多肽的动力学参数Vmax、Km、Vh值随温度的升高而增大。在0-2.5 mM浓度范围内,氨基酸的吸收动力学曲线遵循经典的米氏动力学曲线,而多肽的则表现为线性模式。三种不同园艺生产系统土壤的氨基酸、多肽的周转速率、吸收动力学参数(Vmax、Km和Vh)均表现为有机土壤>转化期土壤>常规土壤。
综上所述,营养液中不同氮源(NO_3~--N、NH_4~+-N和Gly-N)对番茄幼苗的生长、根系分泌特性、碳水化合物及氮素积累等有不同的影响。番茄具有直接吸收利用氨基酸态氮的能力,其吸收Gly-N后,能够通过转氨酶的作用,最终合成蛋白质,促进植株生长,但氨基酸进入植物体内后,通过不同部位的不同转氨酶同化的能力可能不同。番茄体内的NADH-GDH活性的变化说明了番茄吸收甘氨酸后能为脱氨基作用提供碳骨架。这些可能是植物氨基酸态氮营养效应机理。SON在不同园艺生产系统土壤中的吸附、矿化及其吸收动力学等生物化学特性不同,但氨基酸、多肽等SON在土壤中的矿化周转速度表现为有机土壤>转化期土壤>常规土壤。氨基酸、多肽在土壤中的矿化周转速率迅速,在氮素矿化过程中,氨基酸态或多肽态SON转变为NH_4~+-N以及NH_4~+-N转变为NO_3~--N不会限制土壤N的矿化速率;影响SON含量的瓶颈不是微生物利用低分子量SON的速率,而是SON从生物体释放进入土壤的速率。本研究的这些发现,扩大和丰富了植物有机营养理论,为补充和完善现有的氮循环模型和理论提供了重要的科学依据。
Soluble organic nitrogen (SON) has been recognized as one of the labile forms of N available to microorganisms and plants, it is prone to losing from ecosystems to deteriorate the water quality. Therefore, SON plays an important role in N cycling in most ecosystems. However, the classical paradigm of the terrestrial N cycle has considered that organic N must be converted into inorganic N (NO_3~--N, NH_4~+-N) by soil microorganisms, prior to becoming available to plant root, therefore, more attention has often been focused on the behaviour of inorganic nitrogen in agricultural soils and overlooked the roles of SON in soil nitrogen cycles; and little information is known about SON absorbed by plants and its significance in some natural ecosystems.
In this study, wholly sterile hydroponics culture and ~(13)C-~(15)N dual labeled organic nitrogen was use to determine the influences of inorganic and amino acid nitrogen on growth, accumulation of carbon and nitrogen, the characteristic of root exudation, N assimilation and the contribution of organic nitrogen uptake to the nitrogen nutrition in tomato seedling. Chemical extracts and soil solution drain-centrifuge methods to assess SON pools in different soils. The ~(14)C-labeled incubation experiments were carried out to study the behavior and role of SON (such as dynamics of adsorption, mineralization, biodegradation, biology uptake kinetics etc.).
The main results were indicated as follows:
1 Effects of amino acid-N on growth, accumulation of carbohydrate, carbon and nitrogen in tomato seedling under hydroponic culture
Supplied with inorganic nitrogen (NH_4~+-N, NO_3~--N) and amino acid-N (Gly-N), no significant differences in plant height, dry biomass, plant carbon and total N amount appeared in treatments of 8 or 16d, but significant differences were observed in treatments of 24 and 32 d, the order was NO_3~--N> Gly-N>NH_4~+-N. Significant differences were observed between 2 tomato cultivars. Compared with the NO_3~--N treatment, soluble sugar content of leaf, total free amino acid and N content both in root and in shoot were significantly increased treated with NH_4~+-N and Gly-N. However, leaf starch content, N concentration in different organs were decreased significantly, and root soluble sugar, starch and Chlorophyll content were enhanced in Gly-N treatment. Among the N forms applied, and the increasing effects of the NH_4~+-N treatment were larger than that of the Gly-N. The growth of Shenfen918 in NH~+_4-N and Gly-N solution were more inhibited than that of Huying932. The genotypical difference depended on N forms, and such difference could only be found in the NH_4~+-N and Gly-N treatments.
2 Influence of amino acid N supply on the composition of tomato root exudates, xylem and phloem sap grown in hydroponic culture
After 8 days of growth with the different N forms, root dry weight followed the series: NO_3~--N > Gly-N > CK > NH_4~+-N while root volume followed the series Gly-N > NO_3~--N > NH_4~+-N > CK in both cultivars. Root enzyme activities was the highest in the NO_3~--N or Gly-N treatment followed by NH_4~+-N treatment with CK showing the lowest activities. The concentration of NO_3~- and phosphate was highest in the NO_3~--N treatment followed by Gly-N and NH_4~+-N with CK showing the lowest concentrations. Ammonium, free amino acid and soluble sugar content of phloem and xylem sap in the Gly-N and NH_4~+-N treatments were higher in both cultivars followed by NO_3~--N and CK treatment. Rhizosphere pH was elevated by the NO_3~--N and Gly-N treatment, but decreased in the NH_4~+-N treatment. Soluble protein content in root exudates followed the series: NO_3~--N> Gly-N > NH_4~+-N > CK; however, different patterns were observed in xylem and phloem sap (Gly-N >NO_3~--N > NH_4~+-N > CK). Root enzyme activities were significantly different (p < 0.05) between the two cultivars. Root enzyme activities varied in the two different tomato cultivars with activities being highest in Huying 932 in all four N treatments.
3 Influence of inorganic and amino acid nitrogen on the growth and nitrogen metabolism of tomato seedlings under aseptic hydroponic cultivation
Total N and biomass of the 2 cultivars treated with inorganic and amino acid-N was significantly greater (P< 0.05) than the control treatment. Tomato could grow well in conditions of glycine supply. There had significant differences between two cultivars. After 21 days of sterile incubation, NH_4~+-N and Gly-N treatments increased total free amino acid in roots and leaves sharply, while there’s no significant difference in NO_3~--N treatments. All treatments significantly enhance soluble protein content in shoot and root compared with control treatment. Compared to control treatment; NH_4~+-N, NO_3~--N and Gly-N treatments all significantly decreased GS activity, NO_3~--N treatment significantly decreased shoot GPT activity, while NH_4~+-N and Gly-N treatments had no significant effect on it. While Gly-N treatment significantly increased NADH-GDH, GOT and GPT activity in root compared with NO_3~--N, NH_4~+-N treatment. Tomato could effectively use organic nitrogen (eg. Gly-N) directly. Growth of tomato response to organic nitrogen supply might be related to genotypes. Our results clearly demonstrate the intrinsic capacity for tomato plants to directly use organic nitrogen as a sole source of N.
4 Influence of glycine-N concentration on the growth and nitrogen metabolism of tomato seedlings under aseptic hydroponic cultivation
We grew two genotypes of tomato in sterile, hydroponic culture with different N concentration with 0, 1.5, 3.0 and 6.0 mM organic-N in the form of glycine. Our results showed that biomass production, N content, free amino acid and soluble protein content in both shoot and root and soluble sugar content in shoot of both genotypes were increased significantly when Gly-N concentration in the nutrient solution increased. In addition, plant biomass production and N content were positively correlated with the rate of glycine-N supply (R~2 >0.905**), however, the magnitude of the response was genotype dependent. Although addition of Gly-N to the solution, decreased shoot starch content compared to the no nitrogen (CK) treatment. Concentrations in the nutrient unaffected starch content in root. No significant difference in CK, 1.5Gly and 3.0Gly treatments was observed in root soluble sugar content. The concentration of Gly-N supply also significantly affected N assimilatory enzyme activities in roots and shoots. For example, glycine addition increased the activity of NR, NADH-GDH, GOT and GPT activity in roots and shoots compared with the no nitrogen (CK) treatment. Further studies should be carried out to clarify the functional significance of DON in horticultural systems under non-sterile conditions.
5 Absorption of ~(13)C,~(15)N-glycine by tomato seedlings under soil microcosms culture conditions
We selected two genotypes of tomato (Shenfen918 and Huying932) by using plants growing in‘rhizotube’(microcosms) with natural field soil. The results showed that significant increases in ~(13)C and ~(15)N in plants (shoots and roots), indicating part of the glycine N was taken up in the form of intact amino acid by two cultivars tomato 48h after injection. Regression analysis of excess ~(13)C against excess ~(15)N showed that about 12.5% and 21% of glycine-N was taken up intact by Shenfen918 and Huying932 respectively. Atom% excess ~(15)N and ~(13)C in the roots was higher than that in any of the shoots. Results also indicated that the poor competitive ability of tomato roots to absorb amino acids from the soil solution. This implies that tomato can take up organic nitrogen in intact form from the soil, despite the rapid turnover of organic N usually found under such conditions, and also uptake capacity varies widely among genotype dependent.
6 Assessing soluble organic nitrogen pools in soils
Mean extractable SON (DON) as a proportion of total soluble N was smaller in three horticultural soils (36.3%) than in two grassland (74.5%) and generally followed the series Haplic podzol > Eutric cambisol > Organic soil > Conventional soil > Transitional soil. SON comprised 48.3%、21.9%、30.9%、93.7%、83.4% of the total soluble N ( TSN) in organic, transitional, conventional soil, Haplic podzol and Eutric cambisol, respectively. FAA comprised 1.4%、2.6%、2.4%、1.1%、2.4% of TSN and comprised 2.9%、11.8%、7.8%、1.2%、2.8% of SON in organic, transitional, conventional soil, Haplic podzol and Eutric cambisol, respectively. Correlation analysis showed that TSN, SON and FAA were significantly correlated with total soil N, NO_3~--N, indicating the important role of SON and FAA in the supply of N in these soils. Results also indicated that, the greatest amount of SON was observed when extracted with phosphate buffer, the least with water and the centrifugal-drainage technique, and intermediate with KCl, K2SO4 and CaCl2. We conclude that SON represents a significant N pool in all agro-ecosystems but its amount is less sensitive to agricultural use system than dissolved inorganic nitrogen.
7 Characterization of amino acid and peptide dynamics in soil
The amount of amino acids and peptides sorbed was concentration dependent but followed the series Val-Pro-Pro> glutamate> valine> Glu-Phe in both Chinese horticultural and UK grassland soils. The degradation of amino acids and peptides was soil dependent but that mean half-life of amino acids (glutamate, valine) at 18±0.5°C was 10.7±1.4h, whilst of peptides (Glu-Phe, Val-Pro-Pro) was 9.1±0.3h. On average of (24±2)% the amino acid-C was respired as CO_2 whilst (76±3)% was utilized for new cell biomass. In generally, a greater proportion of the glutamate, valine, Glu-Phe and Val-Pro-Pro were used for new biomass production producing yields of 0.73, 0.81, 0.69 and 0.82μmol biomass-Cμmol amino acid-C~(-1) respectively. Peptides and amino acids decomposition were hypothesized to be a purely biological process as CHCl3 fumigation and autoclaving resulted in no observable mineralization. The presences of 10 mM either phenylalanine or Ala-Ala significantly increased either the amino acid and peptides uptake or mineralization rate, while the presences of 10 mM glutamic acid, Pro-Pro-Pro and NH_4NO_3 depressed the utilization of glutamate and Glu-Phe sharply (P<0.05).
8 Dynamics of nitrogen mineralization in soils from organic, transitional and conventional production systems
The dynamics of N speciation was significantly affected by mineralization and immobilization. The amount of SON, total free amino acid (TFAA) and NH_4~+-N are kept at very low levels and did not accumulate, whereas NO_3~--N gradually accumulated in these soils. The conversion of insoluble organic N to low molecular weight (LMW) SON and not LMW-SON to NH_4~+-N or NH_4~+-N to NO_3~--N represents a main constraint to N supply. Free amino acids and peptides were rapidly mineralized in soil by the microbial community, and that consequently they do not accumulate in soil. Turnover rates of additional amino acids and peptides were soil dependent, and generally followed the series: organic soil, transitional soil and conventional soil. High molecular weight (HMW) SON turns over very slowly and represents the major SON loss to freshwaters. Further studies investigating organic nitrogen degradation pathway and its bottleneck are warranted.
9 Temperature dependence of amino acid and peptide kinetics and mineralization in soil
The degradation of amino acid and peptides was soil dependent but the Val mean half-life at 1, 15, 25°C was 28.4, 17.9 and 10.1h respectively, Glu mean half-life at 1, 15, 25°C was 21.6, 10.7 and 8.1h; Glu-Phe mean half-life at 1, 15, 25°C was 33.9, 11.1 and 6.8h; Val-Pro-Pro mean half-life at 1, 15, 25°C was 13.1, 10.6 and 8.6h. Amino acid and peptide half-life of subsoil (20-60cm) was much greater than that of topsoil (0-20cm), this probably reflects the higher microbial biomass and activity in topsoil (0-20cm). Kinetics of soil microbial uptake of amino acid and peptide showed that: Amino acid and peptide uptake was concentration and temperature dependent, and amino acid conformed to a single Michaelis-Menten equation, however, peptide conformed to linear regression. Kinetic parameters of amino acid and peptide (Vmax, Km and Vh) were increased by the temperature. The turnover rate and the value of Vmax, Km and Vh of amino acid and peptide generally followed the series organic soil> transitional soil> conventional soil.
From all above, we can draw a conclusion that different N forms (NO_3~--N, NH_4~+-N and Gly-N) in hydroponic culture affected the growth, characteristic of root exudation and the accumulation of carbohydrate and nitrogen in tomato seedling. Tomato (Solanum lycopersicum) can uptake intact amino acid-N. Compared to inorganic nitrogen (NO_3~--N and NH_4~+-N), amino acid-N significantly increased GOT and GPT activity in root, indicating that amino acid was transaminated in root at first, and then transformed into other amino acids, finally synthesized protein. This may be the mechanisms of organic nitrogen uptake by plants. The behaviour of SON (such as adsorption, mineralization, biodegradation, biology uptake kinetics etc.) was different characteristic in different horticultural soils has different, however, the turnover rate of amino acid and peptide in soil was followed the order: organic soil>transitional soil>conventional soil. Amino acids and peptides were rapidly mineralized in soil by the microbial community, and that consequently they do not accumulate in soil. During the N mineralization period, the conversion of insoluble organic N to low molecular weight (LMW) SON and not LMW-SON to NH_4~+-N or NH_4~+-N to NO_3~--N represents a main constraint to N supply. The bottleneck which affected the content of SON was not the absorption rate of LMW-SON by microorganism, but the rate of SON released from soil by organism. The findings of the present study may provide a theoretical base for plant organic nutrition. The widespread importance of organic nitrogen uptake by plants has important ramifications for our understanding of ecological processes. The role of organic nitrogen is critical to our understanding of how ecosystems function and how they will be affected by environmental changes. The N cycle in many ecosystems needs to be revisited with a new perspective.
本文主要研究结果如下:
1甘氨酸态氮对番茄幼苗生长、碳水化合物及碳氮积累的影响
在NH_4~+-N、NO_3~--N和Gly-N存在的营养介质中,番茄幼苗在处理前期(如处理后8或16d)的株高、生物量、植株全碳积累量、总氮量等各处理差异不显著,而其后则表现为NO_3~--N>Gly-N>NH_4~+-N,处理间差异显著(p<0.05)。与NO_3~--N处理相比,Gly-N、NH_4~+-N处理均显著提高了叶片可溶性糖、叶片和根系游离氨基酸及各器官氮素含量,而降低了叶片淀粉含量和各器官氮素积累量。Gly-N处理显著提高了番茄根系可溶性糖、淀粉和叶绿素含量。番茄基因型差异是否表现与氮素形态有关,以植株根系干重为指标的基因型差异在供应NO_3~--N时不表现;在Gly-N处理下,沪樱932的生长显著优于申粉918。
2甘氨酸态氮对番茄根系分泌物和木质部及韧皮部汁液组分的影响
营养液中NH_4~+-N、NO_3~--N和Gly-N显著影响不同番茄品种根系分泌物、木质部和韧皮部汁液各组分含量。NO_3~--N处理的番茄根系分泌物、木质部和韧皮部汁液中的NO_3~--N和P含量最高,供Gly-N、NH_4~+-N其次,对照最低;而供Gly-N、NH_4~+-N时铵态氮、游离氨基酸、可溶性糖含量一般均高于处理NO_3~--N和对照。番茄根际吸收不同氮素对根际pH亦产生不同的影响,吸收NO_3~--N和Gly-N后,根际pH显著升高,而吸收NH~+_4-N后则降低。在供应NO_3~--N、NH_4~+-N和Gly-N及对照时,沪樱932的根系活力均显著大于申粉918(p<0.05)。
3无菌水培条件下无机氮和氨基酸态氮对番茄幼苗生长和氮代谢的影响
无菌水培条件下,与无氮处理相比,NH_4~+-N、NO_3~--N和Gly-N处理都显著提高了植株干物质和总吸氮量。Gly-N处理的番茄干物质重和吸氮量显著高于缺氮及NH_4~+-N处理;NH~+_4-N和Gly-N处理显著提高了地上部、根系FAA含量,而NO_3~--N处理对地上部、根系FAA含量无显著影响。可溶性蛋白含量则表现为:Gly-N >NH_4~+-N>NO_3~--N>CK处理。与无氮处理相比,各处理都显著降低了地上部和根部GS活性;而Gly-N处理显著促进了地上部、根系NADH-GDH、GOT和根系GPT活性,NO-3-N处理显著降低了地上部GPT活性。因此,番茄吸收甘氨酸后,能够通过转氨酶的作用,最终合成蛋白质,促进植株生长,氨基酸态氮(Gly-N)也可以成为番茄生长的良好氮源。
4无菌水培条件下甘氨酸浓度对番茄幼苗生长和氮代谢的影响
无菌水培条件下,番茄能够吸收各种浓度的Gly-N,且营养液中甘氨酸浓度(0-6.0 mM)与番茄植株干物重、总氮量等呈显著正相关(R~2> 0.905**)。随营养液中甘氨酸浓度的增加,两个番茄品种的地上部和根系FAA、可溶性蛋白、地上部可溶性糖含量增加。与无氮对照处理相比,各处理都显著降低了地上部淀粉含量,营养液中甘氨酸浓度对根系淀粉含量无显著影响。甘氨酸态N浓度对氮代谢关键酶有不同的影响。随营养液中甘氨酸态N浓度的增加,两个番茄品种的地上部、根系NR、NADH-GDH、GPT和GOT活性均提高。被吸收的氨基酸可通过GOT和GPT转氨酶主要在根中进行同化,最终合成蛋白质,促进植株生长。
5土壤微区培养条件下番茄幼苗对~(13)C,~(15)N-甘氨酸分子的吸收研究
不同形态的~(15)N注射48h后,相对于对照、~(15)N-NH_4Cl和~(15)N-KNO_3处理来说,供2-[~(13)C], [~(15)N]-glycine时,两个番茄品种的地上部、根系~(13)C、~(15)N原子超均显著提高,且根系~(13)C原子超显著大于地上部。申粉918、沪樱932从土壤中吸收的氮分别有12.5%和21%是以完整的甘氨酸分子直接吸收的,番茄根系吸收甘氨酸态N的能力比土壤微生物弱。甘氨酸态N对沪樱932的氮营养贡献率显著大于申粉918(p<0.05)。
6不同土壤SON的含量及特性
供试土壤SON平均含量表现为灰壤土(62 mg kg-1)>腐殖黑钙土(29.1 mg kg-1)>有机土壤(15.8 mg kg-1)>常规土壤(12.9 mg kg-1)>转换期土壤(7.8 mg kg-1)。有机、转换期、常规土壤、灰壤土、腐殖黑钙土SON含量占TSN的比例分别为48.3%、21.9%、30.9%、93.7%、83.4%;而FAA含量分别占TSN的1.4%、2.6%、2.4%、1.1%、2.4%,占SON的2.9%、11.8%、7.8%、1.2%、2.8%。相关分析结果表明,TSN、SON以FAA与全氮、全氮、硝态氮、铵态氮等各养分之间均有极显著的相关性。用盐溶液提取的SON含量大约是水提取的SON或DON的3-6倍。
7不同土壤中氨基酸、多肽的生物化学动态特性
氨基酸、多肽在土壤中的吸附量表现为Val-Pro-Pro> Glu> Val> Glu-Phe。而不同土壤对氨基酸、多肽的吸附能力则为有机土壤>转化期土壤>常规土壤。氨基酸、多肽在土壤中的矿化周转速率迅速,Glu、Glu-Phe、Valine、Val-Pro-Pro在供试土壤中的半衰期(D_(50))分别为5.6±1.1、7.7±1.3、14.8±3.6和10.5±2.0 h。培养24h后,大约有近(76±3)%的氨基酸或多肽被微生物细胞利用,而仅(24±2)%的氨基酸或多肽以CO2的形式释放出来。外加Ala-Ala和NH_4NO_3等氮源能显著提高氨基酸、多肽在土壤中的矿化。氨基酸、多肽等SON在土壤中的矿化是一个完全依靠微生物参与的生物降解过程。
8不同园艺生产系统土壤氮素的矿化特性
随着通气培养过程的进行,不同园艺生产系统土壤NO_3~--N、SON的含量均明显增加, FAA含量呈先升高后下降的趋势。土壤矿化培养过程中生成一定数量的SON;在整个培养过程中,SON占TSN比例均大幅度下降。评价土壤氮素矿化特性时仅仅测定矿化的无机氮数量,可能会低估土壤氮素矿化潜力和氮素损失的数量和效应。施入土壤的~(14)C-氨基酸、多肽在土壤中的矿化迅速,不同园艺生产系统土壤~(14)C-氨基酸、多肽的矿化速度表现为:有机土壤>转化期土壤>常规土壤。在矿化过程中,影响SON含量的瓶颈不是微生物利用低分子量SON的速率,而是SON从生物体释放进入土壤的速率。
9温度对土壤氨基酸、多肽的矿化及其吸收动力学的影响
温度显著影响着氨基酸、多肽在土壤中的矿化及其吸收动力学特性。随着温度的升高,氨基酸和多肽在土壤中的矿化速度加快。在1、15和25°C下,三种供试土壤中Val的平均半衰期分别为28.4、17.9和10.1h;Glu则分别为21.6、10.7和8.1h;Val-Pro-Pro分别为13.1、10.6和8.6h;Glu-Phe分别为33.9、11.1和6.8h。氨基酸、多肽的吸收速率随着施入土壤的氨基酸和多肽浓度的增加而增加。氨基酸、多肽的动力学参数Vmax、Km、Vh值随温度的升高而增大。在0-2.5 mM浓度范围内,氨基酸的吸收动力学曲线遵循经典的米氏动力学曲线,而多肽的则表现为线性模式。三种不同园艺生产系统土壤的氨基酸、多肽的周转速率、吸收动力学参数(Vmax、Km和Vh)均表现为有机土壤>转化期土壤>常规土壤。
综上所述,营养液中不同氮源(NO_3~--N、NH_4~+-N和Gly-N)对番茄幼苗的生长、根系分泌特性、碳水化合物及氮素积累等有不同的影响。番茄具有直接吸收利用氨基酸态氮的能力,其吸收Gly-N后,能够通过转氨酶的作用,最终合成蛋白质,促进植株生长,但氨基酸进入植物体内后,通过不同部位的不同转氨酶同化的能力可能不同。番茄体内的NADH-GDH活性的变化说明了番茄吸收甘氨酸后能为脱氨基作用提供碳骨架。这些可能是植物氨基酸态氮营养效应机理。SON在不同园艺生产系统土壤中的吸附、矿化及其吸收动力学等生物化学特性不同,但氨基酸、多肽等SON在土壤中的矿化周转速度表现为有机土壤>转化期土壤>常规土壤。氨基酸、多肽在土壤中的矿化周转速率迅速,在氮素矿化过程中,氨基酸态或多肽态SON转变为NH_4~+-N以及NH_4~+-N转变为NO_3~--N不会限制土壤N的矿化速率;影响SON含量的瓶颈不是微生物利用低分子量SON的速率,而是SON从生物体释放进入土壤的速率。本研究的这些发现,扩大和丰富了植物有机营养理论,为补充和完善现有的氮循环模型和理论提供了重要的科学依据。
Soluble organic nitrogen (SON) has been recognized as one of the labile forms of N available to microorganisms and plants, it is prone to losing from ecosystems to deteriorate the water quality. Therefore, SON plays an important role in N cycling in most ecosystems. However, the classical paradigm of the terrestrial N cycle has considered that organic N must be converted into inorganic N (NO_3~--N, NH_4~+-N) by soil microorganisms, prior to becoming available to plant root, therefore, more attention has often been focused on the behaviour of inorganic nitrogen in agricultural soils and overlooked the roles of SON in soil nitrogen cycles; and little information is known about SON absorbed by plants and its significance in some natural ecosystems.
In this study, wholly sterile hydroponics culture and ~(13)C-~(15)N dual labeled organic nitrogen was use to determine the influences of inorganic and amino acid nitrogen on growth, accumulation of carbon and nitrogen, the characteristic of root exudation, N assimilation and the contribution of organic nitrogen uptake to the nitrogen nutrition in tomato seedling. Chemical extracts and soil solution drain-centrifuge methods to assess SON pools in different soils. The ~(14)C-labeled incubation experiments were carried out to study the behavior and role of SON (such as dynamics of adsorption, mineralization, biodegradation, biology uptake kinetics etc.).
The main results were indicated as follows:
1 Effects of amino acid-N on growth, accumulation of carbohydrate, carbon and nitrogen in tomato seedling under hydroponic culture
Supplied with inorganic nitrogen (NH_4~+-N, NO_3~--N) and amino acid-N (Gly-N), no significant differences in plant height, dry biomass, plant carbon and total N amount appeared in treatments of 8 or 16d, but significant differences were observed in treatments of 24 and 32 d, the order was NO_3~--N> Gly-N>NH_4~+-N. Significant differences were observed between 2 tomato cultivars. Compared with the NO_3~--N treatment, soluble sugar content of leaf, total free amino acid and N content both in root and in shoot were significantly increased treated with NH_4~+-N and Gly-N. However, leaf starch content, N concentration in different organs were decreased significantly, and root soluble sugar, starch and Chlorophyll content were enhanced in Gly-N treatment. Among the N forms applied, and the increasing effects of the NH_4~+-N treatment were larger than that of the Gly-N. The growth of Shenfen918 in NH~+_4-N and Gly-N solution were more inhibited than that of Huying932. The genotypical difference depended on N forms, and such difference could only be found in the NH_4~+-N and Gly-N treatments.
2 Influence of amino acid N supply on the composition of tomato root exudates, xylem and phloem sap grown in hydroponic culture
After 8 days of growth with the different N forms, root dry weight followed the series: NO_3~--N > Gly-N > CK > NH_4~+-N while root volume followed the series Gly-N > NO_3~--N > NH_4~+-N > CK in both cultivars. Root enzyme activities was the highest in the NO_3~--N or Gly-N treatment followed by NH_4~+-N treatment with CK showing the lowest activities. The concentration of NO_3~- and phosphate was highest in the NO_3~--N treatment followed by Gly-N and NH_4~+-N with CK showing the lowest concentrations. Ammonium, free amino acid and soluble sugar content of phloem and xylem sap in the Gly-N and NH_4~+-N treatments were higher in both cultivars followed by NO_3~--N and CK treatment. Rhizosphere pH was elevated by the NO_3~--N and Gly-N treatment, but decreased in the NH_4~+-N treatment. Soluble protein content in root exudates followed the series: NO_3~--N> Gly-N > NH_4~+-N > CK; however, different patterns were observed in xylem and phloem sap (Gly-N >NO_3~--N > NH_4~+-N > CK). Root enzyme activities were significantly different (p < 0.05) between the two cultivars. Root enzyme activities varied in the two different tomato cultivars with activities being highest in Huying 932 in all four N treatments.
3 Influence of inorganic and amino acid nitrogen on the growth and nitrogen metabolism of tomato seedlings under aseptic hydroponic cultivation
Total N and biomass of the 2 cultivars treated with inorganic and amino acid-N was significantly greater (P< 0.05) than the control treatment. Tomato could grow well in conditions of glycine supply. There had significant differences between two cultivars. After 21 days of sterile incubation, NH_4~+-N and Gly-N treatments increased total free amino acid in roots and leaves sharply, while there’s no significant difference in NO_3~--N treatments. All treatments significantly enhance soluble protein content in shoot and root compared with control treatment. Compared to control treatment; NH_4~+-N, NO_3~--N and Gly-N treatments all significantly decreased GS activity, NO_3~--N treatment significantly decreased shoot GPT activity, while NH_4~+-N and Gly-N treatments had no significant effect on it. While Gly-N treatment significantly increased NADH-GDH, GOT and GPT activity in root compared with NO_3~--N, NH_4~+-N treatment. Tomato could effectively use organic nitrogen (eg. Gly-N) directly. Growth of tomato response to organic nitrogen supply might be related to genotypes. Our results clearly demonstrate the intrinsic capacity for tomato plants to directly use organic nitrogen as a sole source of N.
4 Influence of glycine-N concentration on the growth and nitrogen metabolism of tomato seedlings under aseptic hydroponic cultivation
We grew two genotypes of tomato in sterile, hydroponic culture with different N concentration with 0, 1.5, 3.0 and 6.0 mM organic-N in the form of glycine. Our results showed that biomass production, N content, free amino acid and soluble protein content in both shoot and root and soluble sugar content in shoot of both genotypes were increased significantly when Gly-N concentration in the nutrient solution increased. In addition, plant biomass production and N content were positively correlated with the rate of glycine-N supply (R~2 >0.905**), however, the magnitude of the response was genotype dependent. Although addition of Gly-N to the solution, decreased shoot starch content compared to the no nitrogen (CK) treatment. Concentrations in the nutrient unaffected starch content in root. No significant difference in CK, 1.5Gly and 3.0Gly treatments was observed in root soluble sugar content. The concentration of Gly-N supply also significantly affected N assimilatory enzyme activities in roots and shoots. For example, glycine addition increased the activity of NR, NADH-GDH, GOT and GPT activity in roots and shoots compared with the no nitrogen (CK) treatment. Further studies should be carried out to clarify the functional significance of DON in horticultural systems under non-sterile conditions.
5 Absorption of ~(13)C,~(15)N-glycine by tomato seedlings under soil microcosms culture conditions
We selected two genotypes of tomato (Shenfen918 and Huying932) by using plants growing in‘rhizotube’(microcosms) with natural field soil. The results showed that significant increases in ~(13)C and ~(15)N in plants (shoots and roots), indicating part of the glycine N was taken up in the form of intact amino acid by two cultivars tomato 48h after injection. Regression analysis of excess ~(13)C against excess ~(15)N showed that about 12.5% and 21% of glycine-N was taken up intact by Shenfen918 and Huying932 respectively. Atom% excess ~(15)N and ~(13)C in the roots was higher than that in any of the shoots. Results also indicated that the poor competitive ability of tomato roots to absorb amino acids from the soil solution. This implies that tomato can take up organic nitrogen in intact form from the soil, despite the rapid turnover of organic N usually found under such conditions, and also uptake capacity varies widely among genotype dependent.
6 Assessing soluble organic nitrogen pools in soils
Mean extractable SON (DON) as a proportion of total soluble N was smaller in three horticultural soils (36.3%) than in two grassland (74.5%) and generally followed the series Haplic podzol > Eutric cambisol > Organic soil > Conventional soil > Transitional soil. SON comprised 48.3%、21.9%、30.9%、93.7%、83.4% of the total soluble N ( TSN) in organic, transitional, conventional soil, Haplic podzol and Eutric cambisol, respectively. FAA comprised 1.4%、2.6%、2.4%、1.1%、2.4% of TSN and comprised 2.9%、11.8%、7.8%、1.2%、2.8% of SON in organic, transitional, conventional soil, Haplic podzol and Eutric cambisol, respectively. Correlation analysis showed that TSN, SON and FAA were significantly correlated with total soil N, NO_3~--N, indicating the important role of SON and FAA in the supply of N in these soils. Results also indicated that, the greatest amount of SON was observed when extracted with phosphate buffer, the least with water and the centrifugal-drainage technique, and intermediate with KCl, K2SO4 and CaCl2. We conclude that SON represents a significant N pool in all agro-ecosystems but its amount is less sensitive to agricultural use system than dissolved inorganic nitrogen.
7 Characterization of amino acid and peptide dynamics in soil
The amount of amino acids and peptides sorbed was concentration dependent but followed the series Val-Pro-Pro> glutamate> valine> Glu-Phe in both Chinese horticultural and UK grassland soils. The degradation of amino acids and peptides was soil dependent but that mean half-life of amino acids (glutamate, valine) at 18±0.5°C was 10.7±1.4h, whilst of peptides (Glu-Phe, Val-Pro-Pro) was 9.1±0.3h. On average of (24±2)% the amino acid-C was respired as CO_2 whilst (76±3)% was utilized for new cell biomass. In generally, a greater proportion of the glutamate, valine, Glu-Phe and Val-Pro-Pro were used for new biomass production producing yields of 0.73, 0.81, 0.69 and 0.82μmol biomass-Cμmol amino acid-C~(-1) respectively. Peptides and amino acids decomposition were hypothesized to be a purely biological process as CHCl3 fumigation and autoclaving resulted in no observable mineralization. The presences of 10 mM either phenylalanine or Ala-Ala significantly increased either the amino acid and peptides uptake or mineralization rate, while the presences of 10 mM glutamic acid, Pro-Pro-Pro and NH_4NO_3 depressed the utilization of glutamate and Glu-Phe sharply (P<0.05).
8 Dynamics of nitrogen mineralization in soils from organic, transitional and conventional production systems
The dynamics of N speciation was significantly affected by mineralization and immobilization. The amount of SON, total free amino acid (TFAA) and NH_4~+-N are kept at very low levels and did not accumulate, whereas NO_3~--N gradually accumulated in these soils. The conversion of insoluble organic N to low molecular weight (LMW) SON and not LMW-SON to NH_4~+-N or NH_4~+-N to NO_3~--N represents a main constraint to N supply. Free amino acids and peptides were rapidly mineralized in soil by the microbial community, and that consequently they do not accumulate in soil. Turnover rates of additional amino acids and peptides were soil dependent, and generally followed the series: organic soil, transitional soil and conventional soil. High molecular weight (HMW) SON turns over very slowly and represents the major SON loss to freshwaters. Further studies investigating organic nitrogen degradation pathway and its bottleneck are warranted.
9 Temperature dependence of amino acid and peptide kinetics and mineralization in soil
The degradation of amino acid and peptides was soil dependent but the Val mean half-life at 1, 15, 25°C was 28.4, 17.9 and 10.1h respectively, Glu mean half-life at 1, 15, 25°C was 21.6, 10.7 and 8.1h; Glu-Phe mean half-life at 1, 15, 25°C was 33.9, 11.1 and 6.8h; Val-Pro-Pro mean half-life at 1, 15, 25°C was 13.1, 10.6 and 8.6h. Amino acid and peptide half-life of subsoil (20-60cm) was much greater than that of topsoil (0-20cm), this probably reflects the higher microbial biomass and activity in topsoil (0-20cm). Kinetics of soil microbial uptake of amino acid and peptide showed that: Amino acid and peptide uptake was concentration and temperature dependent, and amino acid conformed to a single Michaelis-Menten equation, however, peptide conformed to linear regression. Kinetic parameters of amino acid and peptide (Vmax, Km and Vh) were increased by the temperature. The turnover rate and the value of Vmax, Km and Vh of amino acid and peptide generally followed the series organic soil> transitional soil> conventional soil.
From all above, we can draw a conclusion that different N forms (NO_3~--N, NH_4~+-N and Gly-N) in hydroponic culture affected the growth, characteristic of root exudation and the accumulation of carbohydrate and nitrogen in tomato seedling. Tomato (Solanum lycopersicum) can uptake intact amino acid-N. Compared to inorganic nitrogen (NO_3~--N and NH_4~+-N), amino acid-N significantly increased GOT and GPT activity in root, indicating that amino acid was transaminated in root at first, and then transformed into other amino acids, finally synthesized protein. This may be the mechanisms of organic nitrogen uptake by plants. The behaviour of SON (such as adsorption, mineralization, biodegradation, biology uptake kinetics etc.) was different characteristic in different horticultural soils has different, however, the turnover rate of amino acid and peptide in soil was followed the order: organic soil>transitional soil>conventional soil. Amino acids and peptides were rapidly mineralized in soil by the microbial community, and that consequently they do not accumulate in soil. During the N mineralization period, the conversion of insoluble organic N to low molecular weight (LMW) SON and not LMW-SON to NH_4~+-N or NH_4~+-N to NO_3~--N represents a main constraint to N supply. The bottleneck which affected the content of SON was not the absorption rate of LMW-SON by microorganism, but the rate of SON released from soil by organism. The findings of the present study may provide a theoretical base for plant organic nutrition. The widespread importance of organic nitrogen uptake by plants has important ramifications for our understanding of ecological processes. The role of organic nitrogen is critical to our understanding of how ecosystems function and how they will be affected by environmental changes. The N cycle in many ecosystems needs to be revisited with a new perspective.
引文
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