| The soil in karst area is poor and the soil and water loss is serious.Its water retention and fertilizer capacity and permeability are low,leading to the deterioration of the structure.How to improve the quality of the soil,there has been no breakthrough.It is an important research direction to improve the soil in karst rocky desertification area by planting forage,but how to improve the yield and quality of forage is an urgent problem to be solved at present.Compared with monoculture,mixed sowing of different grass species can make full use of the advantages of each forage,maximize the nutrient utilization efficiency,and is conducive to the improvement of grassland productivity and soil quality.Gramineous forage and leguminous forage with different nitrogen fixation capacities have different effects on forage yield and soil improvement.Therefore,it is necessary to explore suitable forage planting patterns to promote the restoration of degraded ecosystem services.Current studies on forage planting patterns mostly focus on the improvement of forage yield,but lack of research on the driving mechanism of element flow in soilplant interaction system under different forage species allocation patterns.Based on this,our study designed monoculture and intercropping patterns using ryegrass(Lolium perenne),leguminous forage with high nitrogen fixation capacity-alfalfa(Medicago sativa),and leguminous forage with low nitrogen fixation capacity-white clover(Trifolium repens).Field experiments,root interaction experiments(pot experiments)and rhizobia inoculation experiments were conducted to investigate the distribution,absorption and transfer mechanism of mineral elements in plants and soil under different planting modes and different root interaction intensities.At the same time,the microecological effect of rhizosphere soil inoculated with rhizobium in mixed cropping and intercropping system and the mechanism of carbon,nitrogen and phosphorus co-variation were investigated.This study is helpful to reveal the coupling effect of carbon,nitrogen and phosphorus co-variation between plants and soil and forage yield under different planting patterns,aiming to provide theoretical and data support for exploring suitable forage planting and soil improvement in karst areas.The main results are as follows:(1)The intercropping pattern of ryegrass and alfalfa(high nitrogen fixation capacity)and white clover(low nitrogen fixation capacity)had high yield and element utilization rate.The land equivalent ratio of ryegrass-alfalfa in intercropping S2R2 mode reached the maximum value,and that of ryegrasswhite clover in S2R1 reached the maximum value.Compared with monoculture and mixed cropping,intercropping can promote the increase of C,N and P contents in shoot and root of ryegrass and alfalfa,which had an accumulation advantage of C,N and P compared with the corresponding monoculture.The nitrogen and phosphorus dilution curve showed that alfalfa and ryegrass had stronger nitrogen and phosphorus uptake capacity under intercropping.Compared with monoculture,ryegrass mixed with white clover can promote the increase of nitrogen content of forage,but has an inhibitory effect on phosphorus.In conclusion,the adjustment of leguminous forages with different nitrogen fixation capacity,planting mode,species combination,density and sowing ratio can optimize the community structure and obtain high yield forage,and the intercropping has a significant promoting effect on the growth of forage.(2)The intercropping patterns of ryegrass and legume forages with different nitrogen fixation capacities had significant effects on the improvement of soil physical structure,element content and biological activity.Planting forage can significantly reduce soil p H,increase soil electrical conductivity,reduce soil bulk density,ultimately improve soil structure.The soil clay content in intercropping and mixed planting modes was significantly higher than that in CK and monocropping respectively,and the increasing effect of alfalfa with high nitrogen fixation capacity was stronger than that of white clover with low nitrogen fixation capacity.The mixed cropping and intercropping of grasses and leguminous forages can increase the contents of ammonia nitrogen and available phosphorus in soil,improving the nutrient enrichment rates of rhizosphere soil.The soil enzyme activities under mixed sowing and intercropping patterns were higher than those under monoculture.The activities of soil sucrase,acid phosphatase and amylase can better explain the differences of soil carbon,nitrogen and phosphorus nutrients in karst area.(3)Ryegrass mixed with legume forage with different nitrogen fixation ability could increase the accumulation of seven mineral elements except aluminum(Al)in the forage,and the elements contents of soil were significantly decreased compared with the CK.The accumulation of potassium(K),calcium(Ca),magnesium(Mg),manganese(Mn)and copper(Cu)in the ryegrass-alfalfa system,which reached the maximum in the intercropping S2R2.The accumulation of K,Ca,Mg and Cu in the ryegrass-white clover system reached the maximum in S1R1.The vertical distribution of different elements was different.With the increase of soil depth,the contents of K and Ca were significantly decreased,and the contents of Mg and Mn were significantly increased(P<0.05).The contents of Ca,Mg,Fe,Cu and Zn in the soil were different in different planting modes.The contents of Ca,Mg,Fe,Cu and Zn reached the maximum in the CK,and decreased significantly after planting forage.The enrichment coefficient of mineral elements in ryegrass and alfalfa under different planting patterns showed Ca>K>Mg>Zn>Mn>Cu>Al>Fe.The enrichment coefficient of mineral elements in ryegrass and white clover under different planting patterns showed Ca>K>Mg>Zn>Mn?Cu>Al>Fe,and the enrichment coefficients of K,Ca and Mg element were greater than 1,which suggested the forage had a higher accumulation effect on these three elements.(4)Root interaction changed the quantitative characteristics of plant and soil elements in the mixed cropping and intercropping of ryegrass and leguminous forages with different nitrogen fixation capacities.The carbon,nitrogen and phosphorus accumulation value of ryegrass under the three root interaction modes was that no barrier was the largest,followed by the mesh barrier,and the solid barrier reached the minimum value of carbon accumulation.The accumulation value of carbon,nitrogen and phosphorus reached the maximum in S2R2 under the three root interaction modes and the equivalent ratio of C,N and P in the solid barrier was less than 1.The carbon and phosphorus accumulations of alfalfa and white clover were the highest in the no barrier treatment,followed by the mesh barrier and the lowest in the solid barrier.The nitrogen accumulation reached the maximum value in the mesh barrier treatment.The interaction between gramineae and legumes can significantly increase the soil carbon content in the rhizosphere of forage,and the carbon content in the rhizosphere soil was significantly decreased with the increase of the degree of root separation.The no barrier can promote the increase of N and P in the rhizosphere soil of grasses,but decreasing the N and P contents in the rhizosphere soil of legumes.The soil available nutrients and enzyme activities with no barrier were significantly higher than those with mesh barrier and solid barrier.The element content of plant and soil,available nutrients and the activities of soil enzymes were significantly affected by planting mode,root separation mode and their interactions(P<0.01).(5)The increase of nitrogen and phosphorus in the mixed cropping and interplanting modes of ryegrass and white clover which with low nitrogen fixation capacity was more sensitive to the response of inoculation with rhizobia,and the symbiosis of rhizobia improved the element content in the rhizosphere soil and the utilization efficiency of the forage.The contents of ammonia nitrogen,nitrate nitrogen and available phosphorus in the rhizosphere soil of leguminous forage inoculated with rhizobia were reached the maximum values in the S2R2 in ryegrass-alfalfa system and the ryegrass-white clover system.The yield of leguminous forages intercropped with grasses showed a positive U-shaped curve with the root C/N ratio,which first increased and then decreased with the increase of the root C/N ratio.The yield of leguminous forage intercropped with grasses showed a quadratic function relationship with the soil C/N ratio,and the yield first decreased and then increased with the increase of the soil C/N ratio and C/P ratio.Nitrogen and phosphorus dilution curves showed that the intercropping system of alfalfa and white clover with inoculation of rhizobia had higher absorptive capacity for nitrogen and phosphorus.Rhizobium treatments,legume forages with different nitrogen fixation capacities,planting patterns and their interactions had significant effects on the C,N,P contents and soil stoichiometric ratio of forage and rhizosphere soil.To sum up,the effects of ryegrass and legume forage with different nitrogen fixing abilities are different.Plant growth,the utilization of elements as well as the improvement of soil environment effect is best when the seed ratio of ryegrass and alfalfa is 1:1 and rows ratio is 1:1(S2R2),and when the seed ratio of ryegrass and white clover is 1:1 and row ratio is 3:1(S2R1),which has the advantage of land use compared to their monoculture.Intercropping can promote the uptake of mineral elements such as K,Ca and Mg,and the symbiosis of rhizobia can improve the uptake and utilization capacity of the intercropping system,and affected the stoichiometric ratio of C,N and P,which changed the coupling relationship between the intercropping system and biomass.The results provided an important basis for guiding grassland vegetation restoration and grassland management in karst areas. |
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