Physiological Mechanism Of Efficient Nutrient Uptake In Continuous Cropping Soybean Cultivars

Continuous cropping is one of the important factors affecting soybean production in China.How to improve continuous cropping soybean yield is a hot issue.Different soybean cultivars have different responses to continuous cropping.Soybean cultivars with tolerance to continuous cropping can achieve higher yields under continuous cropping,but the physiological mechanism is still unclear.It is great significance to explore how to efficiently absorb and use nutrients of soybean cultivars with tolerance to continuous cropping to improve the grain yield of continuous cropping.In this paper,localization experiment and sand culture test were used to study reveal the morphological and physiological mechanisms of nutrient absorption and utilization in continuous cropping soybean cultivars.（1）The effects of cultivar improvement and fertilizer application on the productivity and nutrient utilization of continuous cropping soybean were studied by using 15 soybean cultivars with different year of release in the localization experiment,and the continuous-cropping-tolerant soybean cultivar（Liaodou 14,Kottman）and continuous-cropping-sensitive soybean cultivar（Liaodou10,Dennison）were screened out.The changes of soil microorganisms in different continuous cropping soybean cultivars were further studied;（2）Under sand culture conditions,the continuous-cropping-tolerant soybean cultivars and continuous-cropping-sensitive soybean cultivars were selected as test materials,the different nitrogen level treatments were set up,the response of continuous cropping soybean cultivars to nitrogen nutrition was clarified.The main research results are as follows:1.The results of localization experiment showed that the leaf area index（LAI）,net photosynthetic rate（Pn）,crop growth rate（CGR）,radiation use efficiency（RUE）,dry matter weight,nitrogen content,nitrogen requirement of 100g grain and yield of contemporary cultivars were all higher than those of earlier cultivars,while the grain nitrogen concentration was opposite.Under continuous cropping,the two-year average yield of Liaodou 14 was 427kg·hm^-2 and 243 kg·hm^-2 higher than that of Liaodou 10 and Dennison,respectively,and the two-year average yield of Kottman was 438 kg·hm^-2 and 277 kg·hm^-2 higher than that of Liaodou 10 and Dennison,respectively.LAI,Pn,CGR,RUE,nitrogen,phosphorus and potassium content,dry matter weight,nitrate reductase（NR）activity,glutamine synthetase（GS）activity and yield of fertilized soybean were higher than those of unfertilized soybean,while glutamate dehydrogenase（GDH）activity was opposite.Cultivar improvement and fertilization increased photosynthetic capacity,dry matter accumulation,nutrient absorption capacity and yield of soybean,among which the yield of the new cultivar with compound fertilizer was the highest.2.Under continuous cropping treatment,the yield of continuous-cropping-tolerant soybean cultivar（Liaodou 14）was higher than that of continuous-cropping-sensitive soybean cultivar（Liaodou 10）,which was 17.0%higher in 2019,16.4%higher in 2020 and 27.2%higher in2021.Under continuous cropping treatment,soil nutrient content（NH4⁺-N,NO3^--N,AP,DOM and TK）,p H value,nitrogen cycling enzyme（URE,NAG,LAP）activities,phosphorus cycling enzyme（ALP,NPA,ACP）activities,copy numbers of nitrogen functional genes（AOA,AOB,nir K,nir K）and copy numbers of phosphorus functional genes（pho A,pho B）in Liaodou14 was higher than Liaodou 10 at the R1 and R6 growth stages.Under continuous cropping treatment,cultivar was an important factor affecting bacterial community structure and functional structure.The relative abundance of Proteobacteria,Bacteroidota,Acidobacteriota and Verrucomicrobiota in Liaodou 14 was significantly higher than that in Liaodou 10,and the complexity of soil bacterial community co-occurrence network in Liaodou 14 was higher than that in Liaodou 10.3.The results of sand culture test showed that leaf Pn,leaf color value,superoxide dismutase（SOD）activity,peroxidase（POD）activity of four soybean cultivars with different continuous cropping tolerance were gradually increased with nitrogen level under the three soil leaching liquor treatments.However,leaf catalase（CAT）activity and malondialdehyde（MDA）content showed a tendency to decrease gradually with the increase of nitrogen level under the three soil leaching liquor treatments.The leaf Pn,leaf color value,SOD activity,POD activity and MDA content of continuous-cropping-tolerant soybean cultivars（Liaodou 14 and Kottman）were higher than those of continuous-cropping-sensitive soybean cultivars（Liaodou 10 and Dennison）.However,leaf CAT activity of continuous-cropping-tolerant soybean cultivars was lower than those of continuous-cropping-sensitive soybean cultivars.The increase of nitrogen level can improve the photosynthetic capacity and antioxidant enzyme activity of soybean cultivars with different continuous cropping tolerance,and the photosynthetic capacity and antioxidant enzyme activity of leaves of continuous-cropping-tolerant soybean cultivars were higher than those of continuous-cropping-sensitive soybean cultivars.4.Leaf and fine root NR activity,nitrite reductase（Ni R）activity,GS activity,glutamate synthetase（GOGAT）activity,the nitrogen transport-related genes（Gm NRT1.1 and Gm NRT2.5）expressions and the nitrogen assimilation-related genes（Gm INR1,Gm INR2,Gm NIR,Gm GS1,Gm GDH1 and Gm Fd-GOGAT）expressions of four soybean cultivars with different continuous cropping tolerance were gradually increased with nitrogen level under the three soil leaching liquor treatments.However,leaf and fine root GDH activity and Gm NADH-GOGAT expression showed a tendency to decrease gradually with the increase of nitrogen level under the three soil leaching liquor treatments.The leaf and fine root NR activity,Ni R activity,GS activity,GOGAT activity,nitrogen transport-related genes expressions and nitrogen assimilation related genes expressions of continuous-cropping-tolerant soybean cultivars were higher than those of continuous-cropping-sensitive soybean cultivars.However,leaf and fine root GDH activity and Gm NADH-GOGAT expression of continuous-cropping-tolerant soybean cultivars were lower than those of continuous-cropping-sensitive soybean cultivars.The increase of nitrogen level can improve nitrogen metabolism capacity of soybean cultivars with different continuous cropping tolerance,and nitrogen metabolism capacity of continuous-cropping-tolerant soybean cultivars was higher than that of continuous-cropping-sensitive soybean cultivars.5.Dry matter weight and nitrogen,phosphorus and potassium content of above-ground parts and roots of four soybean cultivars with different continuous cropping tolerance showed a gradual increase trend with the increase of nitrogen level under the three soil leaching liquor treatments.The dry matter weight,nitrogen,phosphorus and potassium content in ground parts and roots of continuous-cropping-tolerant soybean cultivars were higher than those of continuous-cropping-sensitive soybean cultivars.The increase of nitrogen level can promote the accumulation of dry matter and nutrient in soybean plants,and the dry matter weight and nutrient content of continuous-cropping-tolerant soybean cultivars were higher than those of continuous-cropping-sensitive soybean cultivars.6.The root morphological indexes（total root length,root surface area,root volume,root diameter）,root activity,nodule number at the R1 growth stage and nitrogenase activity at the R6 growth stage of four soybean cultivars with different continuous cropping tolerance showed a gradual increase trend with the increase of nitrogen level under the three soil leaching liquor treatments.However,nitrogenase activity at the R1 growth stage showed a gradual decrease trend with the increase of nitrogen level under the three soil leaching liquor treatments,nodule number at the R6 growth stage increased first and then decreased.The root morphological indices,azotofixase activity,root activity,nodule number of continuous-cropping-tolerant soybean cultivars were higher than those of continuous-cropping-sensitive soybean cultivars.The increase of nitrogen level can promote the root growth and nitrogen fixation ability of soybean cultivars with different continuous cropping tolerance.The root morphological indexes and nitrogen fixation ability of continuous-cropping-tolerant soybean cultivars were higher than those of continuous-cropping-sensitive soybean cultivars.In summary,continuous-cropping-tolerant soybean cultivar could improve photosynthetic capacity,nitrogen metabolism capacity,nutrient absorption capacity and root growth,recruit more beneficial bacteria,change soil microbial community structure and functional structure,improve soil productivity and nitrogen and phosphorus cycle related enzyme activities,improve the yield potential of continuous cropping soybean and alleviate the obstacles of continuous cropping soybean.