The Study Of Soil Organic Nitrogen With Adsorbed Amino Acid Distribution Characteristics

With the fast development, especially the rising of some new branches of Plant Nutrition, such as molecular physiology, molecular biology and environmental biology et al., it has been confirmed that higher plants can directly adsorb amino acids. But the feature of soil amino acids in natural or farmland ecosystems still uncleared. Pot experiments were conducted in a naturally glasshouse with rice (Oryza sativa L.) and pakchoi (Brassica chinensis L.) as test crops to investgate the adsorption desorption characteristics of different amino acids in soils (red soil and silt soil), the better method to assess the concentration of extractable amino acids in soils (red soil, paddysoil and silt soil), and the distribution characteristics of organic nitrogen, especially extractable amino acids in the rhizosphere and bulk soils under traditional agricultural planting conditions or at different growth stages of rice. This will help to state the nature of soil nitrogen fertility comprehensively, definite the role and significance of plant organic nutrition, and enrich the theory of plant nutrition. The main results were as follows:1. Results of the adsorption-desorption characteristics of different amino acids in soils showed that:Gly, Glu and1:1Gly/Glu amino acids can bind to the soil’s solid phase.The sorbed amounts of amino acids were similar at concentrations lower than100mg/L. At higher than100mg/L concentration, the amino acid sorption rates followed the order of Gly>1:1Gly/Glu> Glu. In addition, sorption appeared to be near saturation at a concentration of approximately700mg/L for three amino acids. The maximum sorption capacities for both soils followed the same order of Gly>1:1Gly/Glu> Glu. Langmuir equation fitted well to the amino acids sorption process in both soils. Amino acids at low concentrations can be readily desorbed in soils, while amino acids at high concentrations were sorbed stronger to the soil solid phase and had low desorbability. 2. Results of the better method to assess the concentration of extractable amino acids in soils showed that0.5mol/L K2SO4with a5times extraction was a better method to assess the concentration of extractable amino acids in soils.3. Results of the distribution characteristics of organic nitrogen, especially amino acids in the rhizosphere and bulk soils confirm that EON is a major contributor to TEN pools in soils. Although only a small proportion of the EON was present in the form of water-extractable AA, the results presented here indicate that K2SO4-extractable AA accounted for25%of the K2SO4-extractable EON pool and nearly10%of the TEN pools in both rhizosphere soils. K2SO4-extractable AA and the released amino acids from soil exchangeable sites might be an important organic N source for plant growth. Overall, both K2SO4-extractable EON and AA contents were positively related to the TEN contents. The results show that the amounts of acidic hydrolyzable amino acid varied from0to24.5mg/kg in soils. The amount of aspartic acid, glutamic acid and glycine was higher among17amimo acids. The order of the total amounts of acidic hydrolyzable amino acid as follows:neutral amino acids> acidic amino acids> basic amino acids> sulfur-containing amino acid. Our findings suggest that the contents of the organic N were significantly greater in rhizosphere than in bulk soils. But the TEN contents with no N application were lower in the rhizosphere than in bulk soils.4. Results of the distribution characteristics of organic nitrogen, especially amino acids in the rhizosphere and bulk soils confirm that EON is a major contributor to TEN pools in paddy soil. During the growth stage of rice, the amounts of K2SO4-EON in paddy soil was highest at tillering stage, reduced at jointing stage, then increased at heading stage and mature stage. Although only a small proportion of the EON was present in the form of water-extractable AA, the results presented here indicated that K2SO4-extractable AA accounted for27%of the K2SO4-extractable EON pool and nearly11%of the TEN pools in both rhizosphere soils. K2SO4-extractable AA and the release of amino acids from soil exchangeable sites might be an important source organic N for plant growth. During the growth stage of rice, the amount of amino acids in paddy soil increased at jointing stage, then reduced at heading stage and mature stage gradually. Overall, both K22SO4-extractable EON and AA contents were positively related to the TEN contents. The results show that the amounts of acidic hydrolyzable amino acid varied from0to30mg/kg in soils. The amount of aspartic acid, glutamic acid or glycine was higher among17amimo acids. The order of the total amounts of acidic hydrolyzable amino acid as follows:neutral amino acids> acidic amino acids> basic amino acids> sulfur-containing amino acid. Our findings suggest that the contents of the organic N were significantly greater in rhizosphere than in bulk soils. But the TEN contents with no N application were lower in the rhizosphere than in bulk soils.