| Nitrogen is one of the most important nutrients for crop growth.However,application of high level of nitrogen fertilizer and lower nitrogen use efficiency of crops in conventional farming system not only restrict the development of agriculture,but also cause serious environmental pollution.Therefore,how to improve nitrogen uptake and utilization as well as crop yield still is a great challenge faced by the present agriculture.In this context,intercropping has received more attention for its advantage of optimizing resource use and harvesting optimal economic production.In northeast farming and pastoral area of China,alfalfa(Medicago sativa L.)is an excellent leguminous forb and it has strong ability of nitrogen fixation,maize(Zea mays L.)is an important cereal crop which can guarantee the food security,thus maize/alfalfa intercropping has brought prospective development in production.Root is the main organ for absorption of water and nutrients,and its characteristic architecture plays an important role in nitrogen uptake and utilization as well as the generation of intercropping advantage.But,there are few information on the crop root system architecture,nitrogen uptake and utilization in the maize/alfalfa intercropping system.Therefore,a field experiment was conducted to improve and enrich the underground ecological theory of intercropping advantage,which provides scientific basis for production practice of maize/alfalfa intercropping system.Intercropping of maize and alfalfa were studied at Songnen Steppes Research Station of Northeast Normal University in a split-plot design with four replications in 2015 and 2016.The main plots include no nitrogen fertilizer addition(N0)and nitrogen fertilizer addition(N1).Subplots include five cropping patterns: monoculture maize(MM),monoculture alfalfa(MA),four rows of maize intercropping with six rows of alfalfa,spacing of 20 cm(IMA42),four rows of maize intercropping with six rows of alfalfa,spacing of 30 cm(IMA43),two rows of maize intercropping with six rows of alfalfa,spacing of 30 cm(IMA23).The parameters of maize and alfalfa’s total root length,root surface area,root volume,root length density in different cropping patterns at two different nitrogen levels were measured by using the WinRHIZO Root Analysis System,and N uptake,N use efficiency,yield and land equivalent ratio(LER)were calculated.Main results are presented as follows:(1)All the intercropping patterns’ LER were greater than 1 except IMA23 at N0 level in2016,indicating that maize/alfalfa intercropping system had intercropping advantage and can improve land utilization rate.N fertilizer application significantly improved the yield of maize and alfalfa.Irrespective of N level,the yield of alfalfa in the three intercropping patterns weresignificantly higher than monoculture.The yield of alfalfa and maize in IMA43 were higher than that of monoculture by 32.5% and 4.9% respectively,and the LER of IMA43 was the largest of the three intercropping patterns,thus IMA43 is the optimal intercropping pattern.(2)The root surface area and root volume of maize were significantly affected by N levels,intercropping patterns and their interaction.The root surface area and root volume of maize in MM were the largest of all cropping patterns at N0 level,but the biggest values were observed in IMA43 when N fertilizer was added.The total root length,root surface area and root volume of maize in IMA23 were significantly lower than MM,which might be as result of the fact that the two rows of maize in IMA23 were under severe water stress by near alfalfa.Intercropping significantly improved the axial root diameters,root collar diameters and lateral root numbers of alfalfa so that N uptake ability of root was improved.In addition,under the nitrogen stress conditions,the crown root number and crown root whorl number of maize were decreased.However,root angle was increased(root became steeper)and the position of alfalfa lateral root was moved down which could extend the niche in the vertical direction;consequently increase N uptake from deep soil.(3)60%-74% of maize and 51%-63% of alfalfa roots were distributed in 0-20 cm soil layer,respectively.At N0 level,the proportion of maize and alfalfa’s root in deep soil were increased;thus more uptake of water and nutrients.In order to increase the space of N uptake,the vertical root length density of alfalfa of intercropping was significantly improved.In the horizontal direction,the roots of maize and alfalfa have extended to the first row of each other in three intercropping patterns,thus the nitrogen transfer,competition and compensation occurred.(4)The N uptake of maize grain was significantly affected by N levels,intercropping patterns and their interaction.N fertilizer significantly improved the N uptake of maize grain,and N uptake of maize grain in MM were the highest of all cropping patterns at N0 level,but the biggest was observed in IMA43 at N1 level.The N uptake of alfalfa was significantly affected by N levels and intercropping patterns,but no significant interactions were found between N levels and intercropping patterns.N fertilizer significantly improved N uptake of alfalfa,irrespective of N level,N uptake of alfalfa in three intercropping patterns were significantly higher than that of monoculture;this illustrates that intercropping could increase the ability of alfalfa N uptake.In addition,the N uptake of maize and alfalfa in IMA43 were the highest among three intercropping patterns so that this intercropping pattern has obvious advantage.N fertilizer significantly decreased the N use efficiency of maize grain.(5)The total root length,root surface area and root volume of maize were significantly positively correlated to its yield and N uptake.The axial root diameter,root collar diameter,lateral root number,root surface area and root volume of alfalfa were significantly positivelycorrelated to its N uptake.Besides,the axial root diameter,root surface area and root volume of alfalfa were significantly positively correlated to its yield,and the morphological traits,N uptake and yield of alfalfa in three intercropping patterns were higher than that of monoculture.Therefore,maize/alfalfa intercropping can improve the root surface area,root volume,the axial root diameter,root collar diameter and lateral root number of alfalfa,leading to an increase of N uptake and yield of alfalfa.Overall,the intercropping advantage of maize/alfalfa intercropping system significantly influenced by cropping patterns.The two rows of maize in IMA23 were under severe water stress by near alfalfa so that this pattern has no advantage in maize yield and morphological traits.But crop yield,output value,N uptake,and the morphological traits such as total root length,root surface area,root volume in IMA43 were much higher than those parameters in other cropping patterns,suggesting further studies are needed in larger-scale to evaluate its efficiency and finally spread this intercropping pattern in farm land scale. |
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