Mechanism Of Plant Density Effects On Nitrogen Use In Barley/pea Intercropping System

The cereal-legume intercropping systems could improve the nitrogen fixation of legume crops,enhance the nitrogen use efficiency and reduce the accumulation of inorganic nitrogen in the soil,so as to increase resource ultilization rate.For the modern crop cultivation techniques,the increase of density always meant high grain yield.However,there is little information about the mechanism on how the planting density of crops regulates the nitrogen use in cereal-legume intercropping system,in agricultural production.Therefore,this study took barley-pea intercropping as sample,discussed the effects of barley planting density on nitrogen use characteristics and related physiological or ecological index of intercropped and monoculture barley and pea,via root barrier and 15 N labeling technique.The main results are follows:(1)The high barley planting density significntly improved the nitrogen accumulation and nitrogen use efficiency of intercropped barley and pea,and boosted the amount of biological nitrogen fixation of pea with the root interaction approach.The high barley planting density significantly increased the nitrogen accumulation by 4.2% of the barley-pea intercropping system;the root interaction could enhance the positive effect of nitrogen accumulation in intercropping system via increasing planting density of intercropped barley,and improved by 6.1%.The biological nitrogen fixation of pea occupied by12.8% to 43.9% of aboveground nitrogen accumulation amount,and the high barley planting density increased biological nitrogen fixation of pea by 17.4% and 39.7% in nitrogen application and no nitrogen treatment,with root interaction.The high level of barley planting density and root interaction significantly increased the nitrogen use efficiency of the intercropping system,and the high barley planting density increased the nitrogen use efficiency by 59.8% with root interaction treatment.(2)The high barley planting density can strengthen the effects of nitrogen application and root interaction on the root activity,leaf protein content,nitrate reductase and nitrogenase activity in intercropped barley and pea.The root activity of barley was the highest at the barlry tillering stage,and the high barley planting density increased the root activity of barley by 6.0% to 11.0% under the root interaction approach.Meanwhile,the root activity of pea was the highest at the pea filling stage,and the root interaction increased the root activity of pea by 13.8% to 26.0% under the same level of barley planting density and nitrogen application.The heading stage of barley was the key period on the regulation of nitrate reductase,dense planting of barley improved the activity of nitrate reductase,and root interaction enhanced the positive effect on improving the activity of nitrate reductase of barley under the dense planting.The bud stage of pea was the key period on the regulation of the protein content and nitrogenase activity of pea.The high barley planting sensity decreased the protein content of pea under root interaction approach;however,the high barley planting density increased the protein content of pea under root barrier approach.The high barley planting density increased the nitrogenase activity of pea by 23.8% with no nitrogen application,under root interaction approach.Therefore,root interaction could enhance the positive effect of the nitrogenase activity via increasing the barley planting density.(3)The high barley planting density can enhance interspecific competition and mutual benefit of intercropped barley and pea,with nitrogen application and the root interaction approach.Also,the high barley planting density can enhance the subtractive effect of nitrogen depression and increase the nitrogen transfer amount from fixed nitrogen of pea to barley.The barley crop was the dominant species of nitrogen competition;high level of barley planting density significantly increased the nitrogen competition ratio,but nitrogen application decreased barley nitrogen competition ratio with dense planting.High planting density contributed to 50.7% and 47.0% of the nitrogen competition ratio in no fertilizer and nitrogen application treatment,respectively.A positive relationship was observed between the nitrogen use efficiency of intercropping system and the nitrogen competition ratio of barley versus pea.The nitrogen competition ratio of barley to pea was the highest at the barlry heading stage,and the correlation coefficient was the highest between the nitrogen competition ratio and grain nitrogen content of intercropping system.Therefore,the barlry heading stage was the key period on the regulation of the nitrogen competition in the barley-pea intercropping system.The root interaction had a subtractive effect of nitrogen repression for legume crop.Compared with root barrier treatment,high level of barley planting density could improve the subtractive effect of “nitrogen repression” under nitrogen application treatment,and increased by 3.6% to 16.1%.With the root interaction,the nitrogen fixed by legume crops transferred to barley crops,with the amount of 57.5% under the high level of barley planting treatment.With the nitrogen application,the barley seed amount enhanced by 3.2% to 6.3% with the increase of planting density,which indicated that the barley crops uptook the nitrogen fixed by legume plants and the nitrogen transfer ratio increased under the high level of planting density.The results showed that proper nitrogen fertilizer application and surficiant root interaction could support the high planting density in intercropping system,optimize the the nitrogen competition ratio of barley and legume intercropping system,increase the nitrogen accumulation amount and nitrogen use efficiency,enhance biologic nitrogen fixation of legume plants,subtrate the “nitrogen depression” of legume and improve the transfer of nitrogen.The barley plants are more sensitive to the regulation in each treatment and heading stage is considered as optimazied timing for regulation.