The Effect Of PH And Combined Nitrogen Factors On Alfalfa Bunching And Nitrogen Fixation

The leguminous symbiotic fixation was the main way of biological fixation nitrogen, the alfalfa(Medicago sativa L) as the high quality leguminous pasture was generally inoculated rhizobium toincrease its symbiotic fixation ability. But many environment factors frequently affected leguminoussymbiotic fixation effect. In this research, double-decked earthen bowl structure pot experiment wasperformed, two envkonment factors including pH and combined nitrogen were adopted, differentgradient and combined nitrogen concentration were set, physiological and biochemical indexes suchas dry weight, chlorophyll content, total nodule number, nitrogen fixation enzyme activity, totalnitrogen content had been individually determined. And the effect of these two environment factorson alfalfa-rhizobium symbiotic fixation had been observed. The main results follew as below:1. The influence of pH change on the growth of the rhizobium was remarkable, which mainlyrestrained the invasion of rhizobium to the plant root, higher or lower pH inhibated the growth ofrhizobium, When pH＜4.5 and pH＞10.5, the rate of rhizobium growth approached to zero and wasbasically at the stationary state. The optimal range of pH for the growth of rhizobium is 6.5～7.5.2. When pH was less than 6.5, dry weight of plant, the total nitrogen content and thenitrogenase activity increased along with the pH. When pH was higher than 7.5, these three indexesexhibited the drop tendency. The differences between each level were remarkable, which indicatedthat pH had remarkable influence on these three indexes; there also were remarkabe differencebetween different treatments of same level, which indicated that different match of alfalfa varietyand rhizobium had different tolerance to acid and alkali. The match of Rhizobium SM1 and SM5with AmeriStand 201 shew certain acid resistance, the match of ruizobium SMland USDA1002with AC Caribou shew certain resistanceto alkali.3. The low concentration of combined nitrogen played a positive role on the symbiotic fixation,when the concentration of combined nitrogen over 5mmol/L, the invasion of rhizobium to root hairwas restrained, the distortion rate of root hair was reduced, and the nitrogenase activity was reduced,while dry weight and the total nitrogen increased along with the combined nitrogen, which indicatedthat, as the main nutrient, combined nitrogen played a positive role on the growth of plant, but hadsuppressive effect on the nitrogen fixation。4. Because respiratory consumption of NH₄⁺-N process was less than that of NO₃^--Nassimilation process, which weaken the competition of the pogresses of symbiotic nitrogen and combinated nitrogen assimilation for photosynthesis, resulted in that the dry weight and totalnitrogen disposed by NH₄⁺-N were more than that disposed by NO₃^--N. Because key enzyme -NRthat assimilated NO₃^--N and the nitrogen fixation enzyme had competition for molybdenum,reduced the formation of haemoglobin, and reduced the symbiotic fixation nitrogenase activity andresulted in that the suppression to the nitrogen activity of NO₃^--N was stronger than that of NH₄⁺-N.5. With different conditions of pH, the change of chlorophyll was regular, but was notremarkable with the dry weight, the total nitrogen and the nitrogenase activity. At the same time,with different conditions of combinated nitrogen, the chlorophyll increased along with theconcertration of the combinated nitrogen, which indicated that the combinated nitrogen had apositive effect on plant growth. But the chlorophyll was not remarkable with the dry weight, thetotal nitrogen content and the nitrogenase activity, which indicated that chlorophyll was not themain index with the condition change of these two factors.