Study On The Characteristic Of Absorption Of Organic Nitrogen By Tomato And The Behavior Of Soluble Organic Nitrogenin Soil

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 (NO₃^--N, NH₄⁺-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 ¹³C-¹⁵N 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 ¹⁴C-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 cultureSupplied with inorganic nitrogen (NH₄⁺-N, NO₃^--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 NO₃^--N> Gly-N>NH₄⁺-N. Significant differences were observed between 2 tomato cultivars. Compared with the NO₃^--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 NH₄⁺-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 NH₄⁺-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 NH₄⁺-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 cultureAfter 8 days of growth with the different N forms, root dry weight followed the series: NO₃^--N > Gly-N > CK > NH₄⁺-N while root volume followed the series Gly-N > NO₃^--N > NH₄⁺-N > CK in both cultivars. Root enzyme activities was the highest in the NO₃^--N or Gly-N treatment followed by NH₄⁺-N treatment with CK showing the lowest activities. The concentration of NO₃^- and phosphate was highest in the NO₃^--N treatment followed by Gly-N and NH₄⁺-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 NH₄⁺-N treatments were higher in both cultivars followed by NO₃^--N and CK treatment. Rhizosphere pH was elevated by the NO₃^--N and Gly-N treatment, but decreased in the NH₄⁺-N treatment. Soluble protein content in root exudates followed the series: NO₃^--N> Gly-N > NH₄⁺-N > CK; however, different patterns were observed in xylem and phloem sap (Gly-N >NO₃^--N > NH₄⁺-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 cultivationTotal 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, NH₄⁺-N and Gly-N treatments increased total free amino acid in roots and leaves sharply, while there's no significant difference in NO₃^--N treatments. All treatments significantly enhance soluble protein content in shoot and root compared with control treatment. Compared to control treatment; NH₄⁺-N, NO₃^--N and Gly-N treatments all significantly decreased GS activity, NO₃^--N treatment significantly decreased shoot GPT activity, while NH₄⁺-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 NO₃^--N, NH₄⁺-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 cultivationWe 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 (R² >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 ¹³C,¹⁵N-glycine by tomato seedlings under soil microcosms culture conditionsWe 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 ¹³C and ¹⁵N 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 ¹³C against excess ¹⁵N showed that about 12.5% and 21% of glycine-N was taken up intact by Shenfen918 and Huying932 respectively. Atom% excess ¹⁵N and ¹³C 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 soilsMean 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, NO₃^--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 soilThe 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 CO₂ 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 NH₄NO₃ 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 systemsThe dynamics of N speciation was significantly affected by mineralization and immobilization. The amount of SON, total free amino acid (TFAA) and NH₄⁺-N are kept at very low levels and did not accumulate, whereas NO₃^--N gradually accumulated in these soils. The conversion of insoluble organic N to low molecular weight (LMW) SON and not LMW-SON to NH₄⁺-N or NH₄⁺-N to NO₃^--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 soilThe 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 (NO₃^--N, NH₄⁺-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 (NO₃^--N and NH₄⁺-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 NH₄⁺-N or NH₄⁺-N to NO₃^--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.