Leaching Of Nitrogen From Above-ground Parts Of Maize And Wheat And The Affecting Factors

A series of simulated rainfall experiments were conducted in2008and2009toinvestigate the Nitrogen （N） leaching from above-ground parts of maize and wheat, and theeffects of N application, irrigation, canopy characteristics and rainfall pH on N leaching lossas well as its changes over the growing season. Also, the effect of different levels of Nitrogen（N） and water supply on the accumulation and distribution of dry matter and N in maize leafand straw-sheath was investigated in pot experiments. In additional, the effects of N and waterstress on stomatal morphology characteristics （including stomatal length, width, area andperimeter）, stomatal density and area of stomatal pore in wheat were studied. The main resultswere as follows:（1） Effects of N and water supply on dry matter and N accumulation and distribution inmaize （Zea mays L.）:The results showed that accumulation of dry matter and N in leaf and straw-sheath wereasynchronous, and the accumulation curves varied with different levels of water and N supply.Leaf was the main part of N accumulation, but the main part of dry matter accumulationvaried with N application and growing stage. N application and irrigation both significantlyincreased the dry matter and N content in maize canopy, and the interactive effect betweenirrigation and N application was significant, in addition, the effect of N application was higherthan irrigation in our experiment.（2） Effects of N application and rainfall pH on nitrate leaching from canopy of maize:The amount of nitrate （NO₃^--N） leaching from maize canopy mainly affected by canopyN content which is closely related with N application when rainfall pH was above5, Napplication significantly increased the amount of canopy NO₃^--N leaching loss, nevertheless,when rainfall pH was4, both rainfall pH and canopy N content had significant effects onNO₃^--N leaching loss. The change characteristics of NO₃^--N leaching loss with decreasingrainfall pH varied at different growth stages, rainfall pH had more significant effect onNO₃^--N leaching in early growth stage than in later growth stage. Under the same pH rainfallcondition, the amount of canopy NO₃^--N leaching loss decreased with maize growing:11-leafstage>silking stage> grain filling stage, and the amount of leaching loss in early growthperiod was obviously higher than in later period. In conclusion, both canopy N content and rainfall pH can affect NO₃^--N leaching loss, but the canopy N content was relatively moreimportant factor. NO₃^--N can be significantly leached by simulation rainfall from maizecanopy, especially in early growth period. Thus the amount of NO₃^--N leaching from canopyshould be taken into account in studying the canopy N cycle, flux and losses.（3） Effects of drought stress and rainfall pH on nitrate leaching from canopy of maize:When rainfall pH was7, drought significantly decreased the amount of nitrate （NO₃^--N）leaching from maize canopy. However, when rainfall pH was5or4, the amount of NO₃^--Nleaching from maize canopy suffered from drought stress was comparatively higher than theNO₃^--N leaching from the maize with irrigation. Rainfall pH had more significant influenceon NO₃^--N leaching in early growth stage than in later growth stage. The change trends ofNO₃^--N leaching loss with decreasing rainfall pH varied at different growth stages because ofthe influences of soil water content and rainfall pH. Under the simulated rainfall with thesame pH value, the amount of canopy NO₃^--N leaching loss decreased with maize growingstage:11-leaf stage>silking stage> grain filling stage, and the amount of leaching loss inearly growth stage was obviously higher than in later stage. The amount of NO₃^--N leachingfrom maize canopy was positive and linear correlated with canopy N content when rainfall pHwas7. But the linear correlation relationship was not found between the content of NO₃^--Nleaching from canopy and the canopy N content when rainfall pH was5and4. Stomatalconductivity, transpiration rate and photosynthetic rate were not the mainly influencingfactors for NO₃^--N leaching loss from maize canopy.（4） The amount of ammonium （NH₄⁺-N） leaching from maize canopy was mainlyaffected by canopy N content which is closely related with N application and irrigation.Stomatal conductivity, transpiration rate and photosynthetic rate were not the maininfluencing factors for NH₄⁺-N leaching loss from maize canopy. In addition to the effect ofsoil N application and irrigation, NH₄⁺-N leaching was also affected by rainfall pH. Butoverall, the amount of NH₄⁺-N leaching from maize canopy was mainly affected by Napplication rate and irrigation.（5） Effects of N application and rainfall pH on organic N leaching from canopy of maize:The amount of organic N leaching from maize canopy at silking stage was much higherthan in11-leaf and grain-filling stage, and the amount of organic N leaching loss was affectedby not only N application but also rainfall acidity in the growing season of maize, furthermore,N application had more significant influence on organic N leaching loss at silking stage andgrain filling stage than at11-leaf stage. When rainfall pH was7and5, N application had thedominant influence on organic N leaching loss, and the amount of organic N leaching fromcanopy was significantly increased with N application, nevertheless, when rainfall pH was4, rainfall pH and N application both had significant influence on organic N leaching, and theformer’s effects played the dominant role. In addition, whether N were applied or not, theamount of organic N leaching loss when rainfall pH was5was much higher than that whenrainfall pH was7, and rainfall pH had more significant influence on organic N leaching atearly growth stage than at later growth stage.N in maize canopy can be significantly leached by simulated rainfall, despite the largequantities of inorganic nutrients which can be leached, organic substances account for themajor quantity of leached materials. During the whole growing season, when simulatedrainfall pH was7,5and4, organic N leaching from the maize canopy with N application andirrigation was49%,63%and61%, respectively, N leaching from the maize canopy withirrigation but no N application was61%,55%and71%, respectively.（6） Effects of N application and rainfall pH on total N leaching from canopy of maize:During the early portion of the growing season, when the pH value of simulated rainfallwas7and5, N fertilizer application increased total N leaching from the canopy of maizecompared with the control, but when rainfall pH was4, the amount of total N leaching fromcanopy of maize with N application was not increased but decreased compared with thecontrol. During the middle and late portion of the growing season, no matter what the rainfallpH was, N application increased total N leaching from the canopy of maize compared withcontrol. So, N application was one of the main factors affecting the N leaching from canopyof maize, but it was not the only factor, and its effect on N leaching was closely related withthe pH of rainfall.During the whole growing season, rainfall with pH of5increased the amount of Nleaching from canopy no matter whether N fertilizer was applied or not. When rainfall pHwas decreased from5to4, the N leaching from control significantly increased, but the Nleaching from N application treatment decreased compared to the leaching of total N whenrainfall pH was5. In addition, the effect of rainfall pH on N leaching from maize canopy wasrelated to the growth period, and the effect was greater in the early portion of the growthseason than the middle and late portions. The trend of N leaching from maize canopy didn’tcomply with the trend of N accumulation in canopy. N in maize canopy can be significantlyleached by simulated rainfall. Thus, the amount of N leaching from canopy should be takeninto account in studying the canopy N cycle, flux and losses.（7） Response of leaf stomatal size, shape, density and area of stomatal pore to water andN stress in wheat:Both water and N in soil are important factors affecting stomatal size and shape. Underthe conditions of drought stress, lack of N fertilizer resulted in reduction of size of stomata located in upper and lower epidermis of wheat, but if sufficient irrigation water were supplied,lack of N fertilizer increased stomatal size. Not only N fertilizer application but also droughtstress can reduce the area of stomatal pore, and increase stomatal density. The interaction ofwater and N application was significant, and the effect of drought on stomata was moresignificant than N stress. Significantly differences were found between the stomata located inupper epidermis and lower epidermis. The stomata located in upper epidermis were smallerthan lower epidermis, and the stomatal density of upper epidermis was higher than that oflower epidermis. The same changing trends of stomatal response to water and N stress wereobserved between the upper epidermis and lower epidermis.（8） N leaching from above-ground parts of wheat and the affecting factorsA simulated rainfall experiment was conducted to investigate N leaching fromabove-ground parts of wheat and the affecting factors. The results showed that N fertilizerapplication increased total N leaching from above-ground parts of wheat compared with thecontrol. Also, irrigation increased total N leaching. N leaching significantly correlated withabove-ground biomass, especially when rainfall pH value was7and5. The change trends ofN leaching loss with decreasing rainfall pH varied because of different irrigation and Napplication. A significant positive correlation was found between N leaching and stomatalwidth, a negative correlation was found between N leaching and stomatal length, nosignificant correlation was found between N leaching and the area and density of stomata.N application significantly decreased the total N leaching per unit dry weight fromabove-ground parts of wheat and maize. If N fertilizer were applied, drought stress decreasedN leaching per unit dry weight biomass from maize. But if no N fertilizer were applied,drought increased N leaching per unit dry weight biomass. The quantity of N leaching per unitdry weight biomass from maize at spinning stage was significantly higher than that of wheatat booting stage.