| Maize-soybean intercropping has the characteristics of increasing land output and reducing nitrogen dependence.As a green and sustainable development technology,maize-soybean intercropping has been employed all over the world.Although numerous researchers studied the biological nitrogen fixation in intercropping,the characteristics of biological nitrogen fixation of soybean in the relay intercropping need to be further revealed.Improving the efficient utilization of nitrogen in intercropping is conducive to the realization of high yield and efficiency of agriculture.In this study,from 2018 to 2020,two experiments were conducted to study how intercropping regulates soybean nodule growth,antioxidant metabolism and nitrogen utilization.In experiment one,a two-factor split-plot experimental design was adopted,the main factors were nitrogen rates,including no nitrogen（N0）,45 kg N hm-2（N1）,60 kg N hm-2（N2）,120 kg N hm-2（N3）,and the sub-factor were cropping patterns,including soybean monoculture and maize-soybean intercropping.In experiment two,a two-factor random block design was used.The main factors were soybean varieties,including nitrate-tolerant supernodulation mutant nts1007,shade-tolerant soybean Nandou 12（ND12）and shade-tolerant soybean Guixia 3（GX3）.The sub-factors were cropping patterns,including soybean monoculture,soybean intercropping with different interspecific row spacing:45cm（I45）,60cm（I60）,75cm（I75）.In addition,root separation treatments were performed for Nandou 12 to investigated the characteristics of soybean leaves at root nodules,carbon supply capacity of root nodules,antioxidant metabolism characteristics of root nodules and so on.The main results are as follows:1.In the relay intercropping system,a synergistic response between soybean leaves and nodules was observed under different N inputs.At the V5 stage of soybean,intercropping reduced leaf area per plant（67.3%）by reducing leaf number per plant（51.9%）and unit leaf area（32.1%）.Similarly,intercropping reduced root nodule dry weight（38.1%）by reducing average nodule diameter（3.5%）and nodule number per plant（33.1%）.At the R5 stage of soybean,intercropping compensated for the decrease of unit leaf area（14.5%）by increasing the number of leaves（47.2%）,and then increased the leaf area per plant（25.9%）.Similarly,intercropping increased the dry weight of root nodules at 24.7%through increasing nodule diameter（7.7%）and number（13.5%）.Compared with the nitrogen-free treatment（N0）,at V5 stage,nitrogen application increased the leaf area per plant by increasing the number of leaves and unit leaf area.On the contrary,nitrogen application decreased the dry weight of root nodules by reducing the number and diameter of root nodules.At the R5 stage,nitrogen application increased leaf area per plant by increasing leaf number and unit leaf area.However,the increase of nitrogen fertilizer application led to the decrease of root nodule number and diameter,thus reducing root nodule dry weight.The results of principal component analysis and correlation analysis showed that there was cooperation between nodule characteristics and leaf characteristics.2.In the relay intercropping system with different soybean cultivars and configurations,a synergistic response between soybean leaves and root nodules,which was not affected by interspecific interaction intensity.The characteristic parameters of soybean leaves and nodules were decreased in intercropping contrast in sole cropping reduced.The importance of machine learning parameters indicated that the traits of root nodule dry weight is mainly caused by aboveground interaction rather than belowground interaction.At the R5 stage,ND12 increased the total leaf are at 16.6%through compensatory growth and increasing leaf number by 21.3%and compensated for the decrease in the unit leaf area by 4.1%.Then increased the nodule dry weight at27.5%through increase nodule diameter by 12.3%and number by 12.8%.At the R5stage,even though the leaf area per plant,leaf number and unit leaf area of intercropped GX3 decreased by 56.7%,24.1%and 42.8%respectively,the dry weight,number and average diameter of root nodules increased by 12.8%,1.2%and 8.8%,respectively.On the contrary,due to the lack of compensatory growth of nts1007,the characteristics of leaves and nodules at the R5 stage were lower than those of monoculture.When the interspecific row spacing increased from 45cm（I45）to 75cm（I75）,it was beneficial to the compensatory growth of intercropping soybean under 60cm（I60）,and the leaf area was increased mainly by increasing the number of leaves,while the root nodule dry weight was increased by increasing the diameter and number of root nodules.The root separation treatment only decreased the leaf and nodule characteristics of ND12 under I45.Principal component and correlation analysis showed that the characteristics of leaves and root nodules changed synergistically,which was affected by both environment and soybean genotypes.3.The relay intercropping increased the antioxidant capacity of root nodules,while the increase of nitrogen fertilizer input decreased its antioxidant capacity.The compensatory growth of intercropped soybean at the R2-R4 stages promoted the accumulation of aboveground dry matter and potentially increased sucrose partition and starch accumulation in root nodules.Intercropping enhanced the buffering capacity of nodule antioxidant system at compensatory growth stages.Compared with the monoculture at the R4 stage,the activities of nodules superoxide dismutase（SOD）and catalase（CAT）in intercropping increased by 47.1%and 12.5%,respectively,while the contents of reduced glutathione（GSH）,oxidized glutathione（GSSG）,ascorbic acid（As A）and legume hemoglobin（Lb）decreased by 8.0%,15.9%,19.9%and 34.8%,respectively.The relative expression of CAT4 and SOD2 in intercropping nodules was insignificant compared with the monoculture,but the relative expression of glutamine synthase（GS）and urea invertase（UR9）in root nodules was up-regulated.The the accumulation of ureide and nitrogen in shoot was increased.The increase of nitrogen application rate promoted the compensatory growth of intercropping,but inhibited the compensatory growth of monoculture soybean.At the same time,the increase of nitrogen fertilizer led to the decrease of sucrose content and starch accumulation in nodules,reducing the activities of antioxidant enzymes SOD and CAT and the relative expression of coding genes,decreasing the contents of antioxidants GSH,GSSG,As A and Lb,and the content of malondialdehyde（MDA）in nodules.The relative expression of nitrogen transformation related genes（GS and UR9）in root nodules was down-regulated.Finally,the increase of nitrogen fertilizer will reduce the ureide and nitrogen accumulation of soybean.With a amount of N input at 180 kg N hm-2,the N accumulation of intercropped soybean was the highest,which increased soybean productivity and resources use efficiency.4.The antioxidant capacity of soybean nodules was limited by soybean genotypes.Intercropping can improve the antioxidant capacity of root nodules and expand the interspecific row spacing to 60cm,which was beneficial to the strengthening of antioxidant capacity of root nodules.Through the compensatory growth of intercropping soybean,the sucrose content and starch accumulation of root nodules at the R5 stage were significantly increased by 37.6%and 11.9%,respectively.Although compensatory growth can improve the buffering capacity of antioxidant system in intercropped soybeans’nodules,this ability is limited by genotypes.The performance of ND12 was better than that of GX3 and nts1007.At the R4 stage,the activity of SOD and CAT of intercropped ND12 root nodules increased by 34.3%and 3.7%respectively.Compared with monoculture,the contents of GSH,GSSG and Lb decreased by 9.0%,14.8%and increased by 56.7%,respectively.Root separation only significantly affected the antioxidant capacity of root nodules under I45.At the R5 stage,the relative expressions of CAT4,SOD2,GS and UR9 in root nodules of intercropped ND12 were up-regulated than those of monoculture.Finally,the antioxidant and buffering capacity of ND12 was the best under the interspecific row spacing of 60（I60）.At the mature stage,the aboveground N accumulation of ND12 in I60 was the highest and obtained higher land productivity and resources use efficiency compared with the monoculture.In summary,intercropped soybean increased the leaf area to increase the sucrose supply of root nodules,enhanced the antioxidant metabolism of root nodules to delay the senescence of root nodules and increased the supply of nitrogen to the shoot through compensatory growth. |
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