The Study On Short Term Nitrogen Transfer In Maize/Alfalfa Intercropping System

Intercropping of leguminous and non leguminous crops can broaden their nitrogen ecological niche,and the nitrogen transfer in intercropping system can stimulate nitrogen fixation of legumes.Transferred nitrogen is one of the nitrogen sources of non leguminous crops,which can promote the growth of non leguminous crops and thus lead to the generation of intercropping advantages.In the intercropping system,the transfer rate of nitrogen from leguminous crops to non leguminous crops is related to intercropping distance,intercropping row ratio,soil nitrogen content and fertilizer application,mycorrhizal colonization status etc.Previous studies mainly focused on the annual legume/grass intercropping system in pot experiments,but few studies focus on short term nitrogen transfer and its influencing factors in perennial legumes/non leguminous plants under field conditions.Maize and alfalfa are used as the important food crop and excellent leguminous forb in the northeast farming and pastoral area.So maize/alfalfa intercropping system has promising prospects.In this study,field experiment was carried out to investigate the relationship between nitrogen transfer and its influencing factors（mycorrhizal colonization,root exudates,rhizosphere soil physical and chemical properties of maize and alfalfa）,and to investigate their effects on yield and nitrogen use efficiency.It can not only enrich the theories of nitrogen transfer in intercropping systems,but also provide a theoretical basis for the promotion of maize/alfalfa intercropping system in the northeast farming and pastoral area.The experiment was carried out on the platform of maize/alfalfa intercropping system in a random split-plot design with four replications in Songnen Grassland Ecological Research Station of Northeast Normal University.The main plots included no nitrogen（N0）and nitrogen fertilizer(N1,Maize:225.0 kg hm^-2 CO（NH₂）₂;Alfalfa:53.0 kg hm^-2 CO（NH₂）₂).The sub plots contains five planting patterns:maize monoculture（MM）;alfalfa monoculture（MA）;maize and alfalfa intercropping with 4:6 row ratio,spacing 30 cm（IMA43）;maize and alfalfa intercropping with 4:6 row ratio,spacing 20 cm（IMA42）;maize and alfalfa intercropping with 2:6 row ratio,spacing 30 cm（IMA23）.The nitrogen transfer rate,mycorrhizal colonization,root exudates,rhizosphere soil water content and N content were determined by ¹⁵N leaf-labeling method,microscopy and ion chromatography,respectively.Our results are shown as follows:（1）Nitrogen levels and planting patterns had significant effects on the yield of maize and alfalfa,but there was no significant interaction between these two factors.Nitrogen application significantly increased the yield of maize and alfalfa.Compared with monoculture,the yield of intercropping maize decreased slightly,while the yield of alfalfa in intercropping patterns increased significantly,with the rate of 11.0%-45.9%.In addition to IMA23 under N0 level,the other intercropping patterns all showed the advantages of intercropping（LER>1）.The LER of IMA43 was higher than IMA42 and IMA23 by5.29%-17.34%and 7.48%-29.73%.This indicated that IMA43 was the best intercropping pattern.（2）There existed nitrogen transfer between alfalfa and maize in the intercropping system,and the transfer rate varied from 5.52%to 10.49%.Nitrogen levels and planting patterns had significant effects on the rate of nitrogen transfer,but there was no significant interaction between these two factors.Nitrogen application significantly reduced the rate of nitrogen transfer.Among these different intercropping patterns,the highest nitrogen transfer rate was found in IMA42.Nitrogen transfer from alfalfa to maize promoted nitrogen use efficiency in intercropping system.Therefore,compared to monoculture,the nitrogen utilization efficiency in intercropping patterns was increased.In addition,there was a positive correlation between nitrogen transfer rate and the nitrogen use efficiency of maize and alfalfa.（3）In the intercropping system,the F%of maize and alfalfa in five cropping patterns reached to 100%,indicating that all of the plant roots had mycorrhizal colonization sites.Nitrogen application restricted the mycorrhizal colonization rate of plant roots and the hyphal density in soil.Intercropping enhanced the mycorrhizal colonization rate of alfalfa roots,however,there was a decreasing trend of mycorrhizal colonization rate in maize roots.There was a positive correlation between mycorrhizal fungi colonization and nitrogen transfer rate,indicating soil hyphal network could help N transfer between plants.（4）Nitrogen application promoted the secretion of total soluble sugar and organic acids from the roots of maize and alfalfa.In different cropping patterns,intercropping significantly increased the total soluble sugar content in root exudates of maize.From the organic acids contents,intercropping increased the contents of tartaric acid,formic acid,acetic acid,citric acid and total organic acid in root exudates of maize and they reached a significant level in IMA42,but the content of lactic acid decreased.Intercropping increased the contents of formic acid,acetic acid,citric acid and total organic acid in the root exudates of alfalfa,but the tartaric acid content decreased significantly.The contents of total organic acid and citric acid in root exudates of maize and the contents of total organic acid,citric acid and lactic acid in root exudates of alfalfa were positively correlated with nitrogen transfer rate.（5）The physical and chemical properties of rhizosphere soil also had impacts on nitrogen transfer rate.There was a positive correlation between rhizosphere soil water content of alfalfa and nitrogen transfer rate,but the rhizosphere nitrogen content of maize was negatively correlated with nitrogen transfer rate.However,soil pH and EC had minor influence on nitrogen transfer rate.In conclusion,the nitrogen use efficiency of maize/alfalfa intercropping system was improved and the group yield was increased due to the existence of nitrogen transfer and root interaction.In the intercropping system,mycorrhizal fungi colonization,root exudates and soil physical and chemical properties of rhizosphere soil can significantly affect the nitrogen transfer from alfalfa to maize.The 4:6 row ratio of maize and alfalfa was better than that of2:6 row ratio in intercropping systerm,and the spacing of 30 cm intercropping was higher than that of 20 cm in nitrogen transfer amount and group yield.However,the nitrogen transfer rate was higher when maize and alfalfa intercrop in the space of 20 cm than that of30 cm space.