Response Of Nitrogen Fixation Of Several Legumes To Different Environmental Factors

Nitrogen is essential in plant growth and it participated in the synthesis of protein.Nitrogen can promote plant growth and the leaf nitrogen concentration of some species is positively corelated with photosynthetic rate.Moreover,nitrogen is also related to leaf life span and leaf senescence,and nitrogen can also regulate gene related to leaf senescence.Nitrogen fixation of legumes is strongly affected by soil nitrogen and phosphorus.Nitrogen fixation of legumes can be one of the nitrogen source for plants,and is strongly affecting agriculture and ecosystems.Soil nitrogen,especially soil nitrate can inhibit the formation of nodules and nitrogen fixation.Phosphorus is necessary in nodule formation and metabolism.Moreover,soil moisture can directly affect the migration and availability of nitrogen,this nitrogen and water coupling relationship can also affect nitrogen fixation.The%Ndfa and total nitrogen fixed by different legume species are also different,which is an important factor that affecting nitrogen fixation.Nowadays human activities and global climate change have strongly changed nature environment.Increased nitric oxide and ammonia in the atmosphere change the biodiversity and community structure.In recent decades global climate change have led to an temporal and special alteration of precipitation distribution.Moreover,alteration of precipitation pattern can change the availability of nitrogen in soil.Seasonal change,phenology and soil condition are also important factors that affecting plant metabolism and plant traits.As nitrogen fixing legumes can both absorb nitrogen from soil and fix nitrogen from atmosphere,the metabolism of nitrogen in legumes are more sophisticated.Therefore,it is very important to study the response of legume nitrogen fixation and legume species growth on different environmental factors,and compare the difference of legume metabolism under different environmental factors,as well as the influence of legume nitrogen fixation in the ecosystem.In our research,we compared the response of typical nitrogen-fixing legume species Robinia pseudoacacia and typical non-nitrogen-fixing legume species Sophora japonica and Senna surattensis under different nitrogen depositon level,to reveal the effect of nitrogen fixation under different nitrogen level;we compared the response of R.pseudoacacia to various precipitation patterns and nitrogen levels to reveal the responding mechanism of nitrogen fixation to coupling relationship of nitrogen and water;we compared%Ndfa and eco-physiological traits of R.pseudoacacia under different nitrogen levels during seasonal change,to verify the relationship between nitrogen fixation and nitrogen supply under seasonal change;we compared the cluster root and nodule formation of nodulated and non-nodulated Lupinus albus to reveal the relationship between nitrogen fixation and absorbing phosphorus.We tested the effects of N fertilisation on one N-fixing(R.pseudoacacia)and two non-N-fixing(S.aponica and S.surattensis)woody legume species,which were subjected to five different N levels(0,1.5,2.9,5.9 and 11.4 mg N per plant day-1)under greenhouse conditions.Results show that the growth of the two non-N-fixing species S.japonica and S.surattensis was promoted by N supply,while the growth of R.pseudoacacia was unaffected.Among the three legume species,R.pseudoacacia had the largest specific leaf area and total chlorophyll concentration,S.japonica had the largest root-to-shoot ratio and main root-to-lateral root ratio,while S.surattensis had the largest leaf N and phosphorus concentrations.The N-fixing species was mostly unaffected by N supply.The growth and metabolic indexes of the non-N-fixing legume species were promoted by N supply.The N-fixing legume species showed better growth in low-N environments,while under increased N deposition,its growth was similar to that of the non-N-fixing legume species.We conducted an experiment to determine the response of black locust to different precipitation pattern at different N supplies,a widely planted N2-fixing legume tree species.Seedlings were grown in a glasshouse at a combination of four N levels;0,1.5,5.9 and 11.9 mg N per plant day-1(N1,N2,N3 and N4),three irrigation amounts 90,270 and 445 ml per plant day-1(W1,W2 and W3),and two irrigation frequencies,watered once a day and watered once ever five days(F1 and F2).After 75 days,plant growth,biomass and leaf nutrient concentrations were measured.Results show that higher levels of precipitation amount and frequency promoted plant growth.Compared with day 30,the relative growth rate of height of plants at day 45,watered once a day,with 445 ml per plant day-1,with no N supplied was significantly higher,while it was less with 1.5,5.9 and 11.4 mg N per plant day-1.A higher N supply significantly decreased biomass allocation to nodules.Higher precipitation amounts and frequency promoted plant growth.Precipitation pattern altered the effect of N supply on the growth of R.pseudoacacia.There was likely a shift to acquire N mainly from fixation to acquisition of N from soil as N supply was increased.An experiment was conducted to determine the effect of N supply under seasonal change on photosynthesis,biomass allocation and plant traits of R.pseudoacacia,a widely used N-fixing legume species.Seedlings of R.pseudoacacia was provided with 5 different levels of 15N-labelled NH4N03 seedlings of S.japonica as reference plants.Plant were harvested 4 times(every 15 days)to measure%Ndfa,photosynthetic rate,biomass of different plant parts and other plant traits.Plants with low N supply had high%Ndfa only at the early growing stage.Total biomass,biomass allocation,relative growth rate and total N concentration were similar under all N supply at four harvest times,though specific leaf area was slightly affected by N supply.The 8th compound leaf tend to have higher photosynthetic rate than 4th,and the leaves still maintained considerable photosynthetic rate when height growth ceased.Plants tend to allocate more biomass to leaf at the early growing stage and to stem and root at late growing season.Nitrogen supply did not affect leaf photosynthesis and leaf senescence,mainly due to 1)a similar high%Ndfa under all N supplies,2)N fixation existed under all N levels.To test the relationship between nitrogen fixation and phosphorus absorption of L.albus,seedlings of L.albus,either inoculated with rhizobia or non-inoculated,were grown in nutrient solution with a low phosphorus supply.According to the preNon-inoculated plants were provided with the same amount of nitrogen in the form of nitrate as the nodulated ones acquired,from both air and nutrient solution,based on preliminary experiments.We measured biomass,phosphorus and nitrogen concentrations as well as photosynthesis just prior to each harvest.Nodulated plants and non-nodulated control plants produced the same amount of biomass.Nodulated plants had,on average,86%more cluster roots than non-nodulated ones at the four harvests.As hypothesised,nodulation significantly promoted cluster-root formation;it also enhanced rates of photosynthesis.Nodulation promoted cluster-root formation and photosynthesis,presumably because nodules are significant sinks for phosphorus and photosynthates.Our results do not provide evidence for a trade-off between investment of resources in nodules and cluster roots.In all,the ecological strategies of dealing with nitrogen deposition in nitrogen-fixing R.peudoacacia and non-nitrogen-fixing S.japonica and S.surattensis are not the same.Nitrogen deposition promoted the growth of non-fixing legume species,though nitrogen regulated nitrogen fixation,nitrogen did not promote growth of R.pseudoacacia.Different plant traits of different legume species reflected different strategies of dealing with different environmental factors.Nitrogen supply did not significantly affect the nitrogen need and nitrogen acquisition of R,pseudoacacia except under high precipitation amount and frequency.The nitrogen acquisition strategy changed under different nitrogen supply.R.pseudoacacia is more dependent on nitrogen fixation when getting bigger,and we conclude that nitrogen fixation is the main strategy of getting nitrogen for R.pseudoacacia.On the other hand,photosynthesis of R.pseudoacacia did not significantly affect by nitrogen fixation,but seasonality.Phosphorus played an important role in nitrogen fixation,and phosphorus deficiency would cause cessation of nitrogen fixation.Robinia pseudoacacia with high%Ndfa,also had high leaf phosphorus concentration.We confirmed that nitrogen fixation can promote phosphorus absorption in the experiment of L.albus,and the formation of cluster root of L.albus is also promoted by nodulation.This explained the mutual promotion between nitrogen and phosphorus absorption in nitrogen-fixing legumes.