| The relationship between nitrogen and zinc is an important issue in plant nutrition field. However, little work has been done about it, especially for it's mechanism. Therefore, pot experiment, sand culture experiment combined with 15N trace technique, electron microscopy examination, neutron activation, biochemical determination and regular soil and plant analysis were carried out to investigate the effects of nitrogen and zinc combined application on herbage growth, absorption and concentration of nitrogen and zinc in plant, N-fixation and N-transfer from white clover to associated ryegrass, and the changes of forms and availability of zinc in soil, as well as their mechanisms. The main findings are as following:1. The effects of nitrogen and zinc combined application on herbage growth and the absorption of nitrogen and zinc as well as their physiological-biochemical characteristics were different in different plant and at various levels of nitrogen and zinc. There were synergism effects on ryegrass growth and absorption of nitrogen and zinc whether at zinc deficiency or at zinc sufficiency condition when nitrogen and zinc applied together. In the condition of zinc deficiency, the ability of nitrogen metabolism and eliminating active oxygen, the content of IAA, GAs and SOD decreased and the content of ABA, MDA increased. Therefore, the growth, nitrogen and zinc absorption of white clover showed antagonism effect with the increasing rate of nitrogen supply. However, in the condition of zinc sufficiency, the ability of nitrogen metabolism and eliminating active oxygen, the content of IAA, GAs and SOD increased and the content of ABA, MDA decreased by suitable amount of nitrogen supply. Consequently, the growth, nitrogen and zinc absorption of white clover displayed synergism effect. Nevertheless, when high amount of nitrogen supply, the growth, nitrogen and zinc absorption of white clover displayed antagonism effect.2. Applied nitrogen significantly decreased the percentage of N-fixation of white clover after fertilization, but suitable amount of applied nitrogen significantly increased it in the late growth stage. Suitable amount of applied nitrogen could promote root and nodule development, maintain N-fixation capability, and alleviate the decay of nodule. Suitable amount of applied zinc significantly increased the nodule number, N-fixation percentage, nodule weight and N-fixation enzyme activity. Suitable amount of nitrogen and zinc applied together had synergism effect on nodule development and N-fixation. The transfer of N-fixed by white clover to associated ryegrass was decreased by applied nitrogen, but suitable amountof zinc applied promoted it, which had remarkable correlation with nodule declination.3. The ultrastructure of nodule cell and chloroplast of mesphyll cell of white clover showed that: (1) In the condition of zinc deficiency, with the incre'asing rate of applied nitrogen, the structure of chloroplast ruined, in which the number of the grana and grana lamellae as well as stroma lamellae were decreased and disintegrated, which resulted in the declination of photosynthesis capacity of chloroplast. In the condition of sufficient supply of zinc, the number of the grana, grana lamellae and stroma lamellae as well as the order of the combination of them in chloroplast were the best at middle rate of nitrogen applied; However, the volume of chloroplast reduced and the system of lamellae swelled and couldn'n form grana when high rate of applied nitrogen. (2) hi the condition of zinc deficiency or heavy applied, the cell wall and membrane of nodule become malformation, and the vacuole enlarged in nodule cell, as well as the number and volume of bacteriod decreased. Compared with suitable amount of zinc supplied, the bacteriod become malformation and declined and its peribacteriod membrane space (PBS) enlarged. Higher rate of applied nitrogen enlarged PBS and reduced the number of bacteriod compared with suitable amount of nitrogen applied. Nitrogen and zinc applied togethe...
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