| The dry farming area of the Loess Plateau is an important food production area in China.Promoting the coordinated and use of light,water,and fertilizer and realizing the sustainable production are important challenges in arid areas.Intercropping is a technology to optimize the planting system of the Loess Plateau and ensure strategic position in food security.Reasonable intercropping combinations are widely used in arid and semi–arid regions due to the advantages of high yield,stable yield,and efficient use of resources.At present,most of intercropping researches focus on the productivity and intercropping advantages of food crops with large areas,there is little information on the physiological and ecological response of minor crops.Therefore,a research on the inter–species competition under the cereal/legume intercropping,exploring the cooperative utilization of light,water,and fertilizer resources and the driving mechanism of soil microbial community,would provide a basis for the sustainable development of agriculture in the Loess Plateau.Field experiment was performed in 2017–2020 at Northwest A&F University’s experimental site to analyze the common restriction of light,water and nutrient factors on grain yield and the uncertainty that drives crop productivity.The experiment was arranged in a randomized block design and designed to cover four intercropping patterns（2P2M:two rows of proso millet alternated with two rows of mung bean;4P2M:four rows of proso millet alternated with two rows of mung bean;4P4M:four rows of proso millet alternated with four rows of mung bean;and 2P4M:two rows of proso millet alternated with four rows of mung bean）.Sole proso millet（SP）and sole mung bean（SM）were used as controls.Furthermore,a split–plot experiment was designed:the main plot factor was the cropping system,consisting of SP or 2P4M and the subplot factor was the nitrogen（N）fertilizer（urea,N>46%）application rate,consisting of 0（N0）,60（N60）,120（N120）,or 180（N180）kg N ha-1 for proso millet.The interspecific competition between the two crops under intercropping systems,and the physiological and ecological response mechanisms of dominant crops,and the canopy structure characteristics were conducted,in order to improve the coordinated utilization of light,water and fertilizer resources and the drive force of soil microbial community as well as the growth and development,and the sustainable productivity.Main contributions of this work are as follows:（1）In the proso millet/mung bean system,the intercropped proso millet as the dominant species significantly increased dry matter accumulation,while as competitive inferior crop,the dry matter accumulation of mung bean was lower than that of traditional monocropping.Intercropping improved the horizontal and vertical growth and distribution of the root systems of the two crops,leading to significant increase in plant root length density,root surface area density,and root volume density in the 0–50 cm soil layer to allow to absorb more soil water and nutrients.Compared with the monocultures,the grain yield of the intercropped proso millet and mung bean increased by 32.2%and decreased by 41.6%,respectively.2P4M is recommended,as this system,although reducing mung bean yields,increases proso millet yields and enables the greatest land–use efficiency in arid regions of Northwest China.（2）In the field conditions,the intercropping system significantly improved the photosynthetic characteristics of proso millet leaves during the growth period.The SPAD,net photosynthetic rate,transpiration rate and stomatal conductance was 3.5%–14.1%,8.0%–35.8%,9.7%–24.6%and 6.0%–19.2%higher than SP,respectively,which enhanced the carbon fixation and carboxylation ability of the blade.Intercropping regulated the photochemical electron transfer efficiency and activity of the photoreaction center of proso millet.The maximum photochemical efficiency of PSⅡ,the photochemical quenching coefficient of PSⅡand the actual photochemical efficiency of PSⅡwas 1.8%–5.4%,8.0%–29.4%and 6.9%–20.3%higher than SP,promoting the energy capture and adaptability to the high–light environment.In addition,the activities of glutamine synthetase and nitrate reductase of the leaves under the intercropping treatment increased by 8.2%–25.3%and6.8%–15.9%,and the soluble protein content and free amino acid content increased by13.5%–24.7%and 9.1%–22.4%,further indicating that the increase of N nutrients and enzyme activity was conducive to the enhancement of plant N assimilation and delaying senescence.（3）The intercropping of proso millet and mung bean significantly improved the light distribution in the canopy.The diurnal changes of photosynthetic active radiation in2017–2019 increased by 12.0%–45.9%,6.7%–18.3%,and 9.4%–22.5%,respectively.This effect caused the increased leaf area index of the upper,middle and lower layers to make full use of light energy.The soil water content of the middle layer（40–160 cm）decreases more than the upper layer（0–40 cm）and the lower layer（160–200 cm）.Water use efficiency under 2P2M,4P2M,4P4M and 2P4M treatments increased by 31.3%,17.6%,37.6%and52.9%,respectively.In addition,compared with sole proso millet,intercropping increased the soil nitrate,ammonium,available phosphorus,and available potassium content by51.6%–138.3%,16.1%–32.3%,16.8%–70.7%,and 31.9%–112.2%,indicating that the increase in soil N content was much greater than that of phosphorus and potassium.Soil urease,catalase and invertase activities were increased by intercropping,further enhanced the stability and productivity of the ecosystem.（4）The microbial diversity and community structure of the rhizosphere soil of the proso millet was influenced by intercropping.The Shannon index of bacteria under 4P4M and2P4M treatments increased by 13.9%and 13.2%,and Chao1 was increased by 56.2%and39.6%,compared with SP,respectively.The abundance of the Proteobacteria at the bacterial phylum increased significantly,while the Actinobacteria declined.The increase in the abundance of the Proteobacteria community was conducive to promoting the N cycle of the plant–soil system and enhancing the accumulation and utilization of nutrients based on the redundancy analysis and correlations among the soil microbial compositions,soil properties,and soil enzyme activities.For fungi,the intercropping increased the abundance of the Ascomycota,accelerating the metabolism of organic substrates deposited in the rhizosphere,and decreased the abundance of the Hypocreales at the order level.These results provided evidences for intercropping to reduce the occurrence of pests and diseases.（5）The N fertilizer had significant effects on plant growth and leaf photosynthetic characteristics during the growth period of proso millet.With the increase of N application rate,the plant height,leaf area index,dry matter accumulation and leaf photosynthetic parameters of proso millet under traditional single cropping significantly increased,and N3treatment reached the maximum.In the intercropping system,the leaf area index under N1,N2,and N3 treatments increased by 9.3%,22.1%,and 19.9%,and the dry matter accumulation increased by 16.7%,31.5%,and 24.0%,SPAD increased by 3.1%,9.1%and10.3%,the net photosynthetic rate increased by 8.6%,21.4%and 16.8%,the transpiration rate increased by 8.4%,19.8%and 15.9%,and stomatal conductance increased by 13.9%,22.4%and 16.3%respectively,compared with N0.These results indicated that N2 treatment achieved the suitable growth state;in addition,the chlorophyll fluorescence parameters of the leaves showed similar results,demonstrating that appropriate N input reduction in the intercropping system still ensured the quantum efficiency of electron transfer and reduced the ineffective dissipation of light energy.（6）The combination of intercropping and N fertilizer increased the photosynthetic active radiation interception and radiation utilization efficiency,which was 6.9%and 14.8%higher than the control treatment,further enhancing the efficient use of soil water,and promoting the N absorption of the proso millet,causing a significant increase in the productivity.However,for the intercropping,under the N3 treatment,the growth of proso millet was weaker,the resource utilization efficiency was low,and the advantage of intercropping was significantly inhibited.The yield of proso millet and mung bean treated with N2 reached the maximum values under intercropping,which was 32.0%and 18.6%higher than N0 in 2018–2020.At the same time,N2 treatment achieved the maximal harvest index of the two crops and the land equivalent ratio of the system.Taken together,intercropping combined with moderate N inputs reduces the chemical fertilizer application rate and increases crop productivity and is considered to be a reasonable strategy for field management. |
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