| Plant organic nutrition, as one of the important development direction of plant nutrition, is the prolongation and in-depth study of traditional plant nutrition. Lots studies have inverstigated the topics of whether plants can absorb mineral nutrition or organic nutrition, since Liebig funded the theory of plant mineral nutrition in1840. Now it has been well accepted that plant could directly absorb the intact molecular amino acids on the basis of widely applications of modern analytical instruments, such as the Isotope Ratio Mass Spectrometer (IRMS), Nuclear Magnetic Resonance (NMR), Gas Chromatography-Mass Spectrometer (GC-MS) and so on. And this phenomenon is not the individual, but the universal rule that exists in the different ecosystems. So the mineralization of organic N is not the "Key step" that control N cycle in the whole ecosystem. For a long time amino acids were considered as a non-significant N source for plant growth, mainly due to its low concentration in soil solution (only0.1～150μmol L-1) and low competitiveness in utilization by plants relative to soil microbes.Recently, some researchers have reported that amino acids, peptides, and NH4+could be readily adsorbed on the soil solid phase. The content of adsorbed amino acids may account for as high as88%～92%of the total amino acids in soil. However, the researchs about amino acids distribution and bioavailability were mainly focused on soil free amino acids in natural or low N input ecosystems, such as the arctic regions, boreal forests, alpine ecosystems and heath lands. Of the studies to date, people still know little about the quantity of soil adsorbed amino acids. Combined the technology of sterile cultivation and stable isotope tracer, the purpose of our study was to research the plant uptake mechanism of amino acids, contribution of amino acid nitrogen nutrition to plant and zonal distribution of soil amino acids. The main results were as follow: 1. Effects of partial replacing NO3--N with amino acid on yield, quality and root secretion of pakchoi(Brassica chinensis L.):Compared to the single nitrate treatment, the20%partial replacing NO-3-N with amino acids could significantly reduce biomass with decreasing of nitrate and soluble starch content in the edible part of pakchoi after24day’s treatment. The nitrate and soluble starch contents in the treatment of20%replacing of NO-3-N by Glu are lowest at1490.8mg kg-1and1.77mg kg-1, while the contents of soluble protein, soluble sugar, free amino acid and total N in leaves and roots are improved. The treatments of partial replacing NO-3-N could change the root morphology structure and root activity, and the contents of NH+4-N, free amino acids, total organic C, total N and soluble protein are also significantly increased in the treatments of20%Gly and20%of mixed amino acid replacing NO-3-N. Compared to the treatment of single nitrate, the ratios of organic nitrogen to total nitrogen in the root secretions are increased by5.6%,11.2%and12.8%in the treatments of20%Gly,20%Glu and20%of mixed amino acids replacing NO-3-N, respectively. Therefore, partial replacement of NO-3by amino acids could improve the quality of pakchoi, there is a negative relationship between the biomass of pakchoi and the root secretion (0.7<r<0.83, p<0.05), and the types of amino acid could significantly affect biomass, root morphology structure and the amount of root secretion.2. Effects of nitrogen rate and nitrogen form on glycine uptake by pakchoi {Brassica chinensis L.) under sterile culture:Our data showed plant glycine uptake and glycine nutritional contribution were significantly affected by N rate, N form and their interactions. Plant glycine uptake was positively related to amino acid N status. Plant low N status induced more amino acid uptake under the low amino acids concentration. NO3-, NH4+, and their interactions with glycine all significantly inhibited plant glycine uptake and its nutritional contribution for plants. Amount of glycine uptake was highest in the treatment that simultaneously offered low inorganic N and high glycine content. Glycine nutritional contribution to plant total N uptake was significantly higher in the treatments with high glycine concentration (13.4%～35.8%) than that with low glycine concentration (2.2%～13.2%). The high nutritional contribution indicated that amino acids could serve as an important nitrogen source for plant growth under the high organic-N and low inorganic-N input ecosystems.3. Preferential uptake of glycine, nitrate and ammonium by pakchoi (Brassica chinensis L.) under sterile hydroponics:Plant uptake for glycine, nitrate and ammonium was influenced by N concentration, differences in N form and N uptake patterns under the sterile hydroponics. Plant N uptake was positively related to its N concentration (R=0.89-0.99, p<0.11), while N uptake efficiency had a negative relation with N concentration (R=-0.64～-0.77, p<0.36). Regression analysis showed uptake rates for nitrate, ammonium and glycine were well fitted to the Michaelis-Menten kinetics, with their affinity constant (Km)177,2000and801μmol for nitrate, ammonium and glycine, respectively. Plant uptake for nitrate, ammonium and amino acids was dominated by active uptake, while passive uptake only served as a minor role at the low N concentration. Amino acid active uptake at low concentration indicated plant physiological potential to uptake amino acids at low concentration (25μM) that is relevant to field condition. However, passive uptake should not be overlooked in future studies, especially at high N concentration.4. Rice uptake of soil adsorbed amino acids under sterilized environment: The contents of soil free and adsorbed amino acids between the two soils were significantly affected by the soil type, extrants and soil treatment. The silt loam soil from Xiaoshan site (Soil A) had higher glycine sorption capacity than clay loam soil from Xianju site (Soil B). Soil sorption capacity was closely related to soil physical and chemical properties, such as the clay content, organic C and CEC. Soil adsorbed glycine improved plant biomass compared with ammonium, and the content of adsorbed glycine had a positive effect on plant glycine uptake and glycine uptake efficiency. Plant glycine uptake was largest in the AS (Adsorption Saturation) treatment, lowest in the NAS (Adsorption Saturation puls added ammonium) treatment. About4.8%～11.6%and10.9%～32.2%of the adsorbed glycine in soil from Xiaoshan and Xianju, respectively, were found in rice seedling leaves and roots after21days of incubation. Glycine uptake accounted for about15.7%-47.3%of plant total N uptake in the treatments with different ratios of adsorbed glycine and ammonium, and the glycine nutritional contribution was independent of soil sorption capacity.5. Effects of ammonium content on rice adsorbed glycine uptake under sterile culture:In the T3treatment (treatment of glycine:ammonium=2:1at glycine half adsorption saturation point), plant biomass in soil A and soil B were largest compared with the other treatments, reached155.0and115.7mg plant-1, respectively. Plant glycine uptake was affected by the status of soil adsorbed amino acids, ammonium. Its uptake efficiency was decreased with the increasing of ammonium content, their nutritional contribution for plant varied from8.8%to22.6%after21days sterilized cultivation. Though Soil A had higher glycine sorption capacity than Soil B, its uptake efficiency in soil B (7.4%-28.7%) was significantly more than that in soil A (1.0%-7.3%). At the treatments of T4and T1(treatment of glycine:ammonium=1:2), its nutritional contributions were only8.8%and12.9%, significantly lower than that in the other treatments, indicated that high ammonium content could inhibit plant uptake of soil adsorbed glycine and its nutritional contribution for plant.6. The zonal distribution of soil adsorbed amino acids in different soils of China:The contents of extractable total dissolved N, amino acids in the vertical zonality of soil, were significantly increased with the increasing of sampling altitude. While the contents of extractable total dissolved N, nitrate, ammonium and amino acids that derived from the horizontal zonality of soil, were affected by the factors of soil type, plant species and framing systems. The contents of amino acids in woodland were significantly more than that in farmland. Correlation analysis showed that soil adsorbed amino acids was closely related to the soil free amino acids(r=0.98, p<0.001), which indicated that the former likely exists as the "pool" for the later. Soil extractable total dissolved N, soil total N, organic matter and alkaline hydrolysis N were also significantly related to soil adsorbed amino acids (r>0.65p<0.001). In conclusion, soil adsorbed amino acids showed a similar zonal distribution as the zonal distribution of soil types, and its distribution was affected by the altitude, plant species, soil types and farming systems.7. The zonal distribution of hydrolyzed amino acids in different soils of China:The average amounts of acidic hydrolyzable amino acids in the vertical zonality of soil, were increased with the increasing of the sampling altitude; its contents were significantly more than that from the horizontal zonality of soil, the former was1787.1mg N kg-1and the latter only300.2mg N kg-1. The constitution of acidic hydrolyzable was mainly as the His(16.83%), Ala(9.46%), Gly(8.39%), Glu(6.91%), Ser(6.11%), Leu(5.95%), IIe(5.77%) in the horizontal zonality of soil, accounted for about60%of the whole amino acids. While the His(30.3%), Ala(10.7%), Gly(7.0%) were the dominated aminmo acids in the vertical zonality of soils, accounted for about48%of the whole amino acids. Our study also revealed that soil acidic hydrolyzable amino acids were positively related to soil total N content (r=0.73-0.85, p<0.001). |
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