Water Regulation Mechanism To Increase The Productivity Of Legume-grass Mixed-species Forage In The Northern Foot Of Qilian Mountains Area

Improving the productivity and sustainable development of artificial grassland plays an important role in maintaining regional biodiversity,soil and water conservation,and ecological balance.However,Northwest China is facing the problem of rigid constraints on water resources.It is the key to promoting the intensive and standardized production of artificial grassland by clarifying the relationship between improving the productivity and water use efficiency of artificial grassland and water regulation and planting patterns,as well as their regulation mechanisms.Therefore,this study takes the ecological migration area(Minghua Township,Sunan Yugu Autonomous County,Zhangye,Gansu)at the northern foot of the Qilian Mountain as the research area.Through 2-year(from April 2019 to September 2020)of field experiments,we explored the influence of 4 kinds of water regulations and 5 kinds of planting patterns on soil nutrient environment,forage growth physiological characteristics,yield,quality and water use.The 4 kinds of water regulation were full irrigation,mild water deficit,moderate water deficit,and severe water deficit,and the 5 kinds of planting patterns were brome(a gramineous forage)single sowing,alfalfa(a leguminous forage)single sowing,red bean grass(a leguminous forage)single sowing,mixed sowing of brome and alfalfa,and mixed sowing of brome and red bean grass.By carrying out this research,we clarified the influence intensity of water regulation and planting pattern on the soil nutrient environment,forage growth physiological characteristics,yield,quality and water use,and determined the key factors affecting the yield and quality of forage.We comprehensively evaluated the effects of different water regulations and planting patterns on the production of artificial grasslands,revealed the water regulation mechanism for improving the productivity of mixed legume and grass grasslands,and proposed a management strategy for efficient water use of legume and grass mixed grasslands.The main conclusions are as follows:(1)The treatments of mixed sowing of brome and alfalfa and mixed sowing of brome and red bean grass combined with mild water deficit were better in improving the soil nutrient environment and the growth and physiological status of forage.Under the same water regulation,mixed sowing of legume and grass was more beneficial to increase soil organic matter content,nitrate nitrogen content,ammonium nitrogen content,plant height,leaf area index and chlorophyll content,and reduce the photosynthetically active radiation of the lower canopy than single sowing.Under the same planting pattern,soil moisture content decreased and soil organic matter content,nitrate nitrogen content and ammonium nitrogen content increased first and then decreased with the intensification of water deficit.Similarly,plant height,leaf area index and chlorophyll content increased first and then decreased and the photosynthetically active radiation of the lower canopy increased with the intensification of water deficit.(2)The treatments of mixed sowing of brome and alfalfa and mixed sowing of brome and red bean grass combined with mild water deficit could improve forage productivity and stability.Under the same water regulation,the annual yield,sustainable yield index,and yield stability of mixed sowing of legume and grass were increased by 13.96%～170.45%,6.87%～179.17%,and 2.08%～440.63%,respectively,compared with the three kinds of single sowings.Mixed sowing of legume and grass could increase the crude protein content and relative feeding value and reduce the acidic and neutral detergent fiber content of leguminous/gramineous forages.Under the same planting pattern,the annual yield,sustainable yield index,crude protein content and relative feeding value of mild water deficit increased by-6.63%～51.18%,-8.06%～56.25%,1.04%～31.02% and 1.93%～16.11%,respectively,and the content of acid and neutral detergent fibers of LW decreased by 1.13%～16.29% and0.00%～10.30%,respectively,compared with the other three water patterns.(3)The treatments of mixed sowing of brome and alfalfa and mixed sowing of brome and red bean grass combined with mild water deficit could obtain higher yield and crude protein water use efficiency.Under the same water regulation,the yield and crude protein water use efficiency of leguminous forage were significantly higher than those of grass forage,and the yield and crude protein water use efficiency of mixed forage were significantly higher than those of single forage.Compared with brome single sowing,the yield water use efficiency of mixed sowing of brome and alfalfa and mixed sowing of brome and red bean grass was increased by 118.97%～224.49% and245.00%～340.00%,respectively,and the crude protein water use efficiency of mixed sowing of brome and alfalfa and mixed sowing of brome and red bean grass was increased by 19.35%～448.39% and 0.00%～378.13%,respectively.Under the same planting pattern,the yield and crude protein water use efficiency were higher under mild water deficit for single sowing and mixed sowing of brome and alfalfa.Compared with the other three kinds of water regulation,the yield water use efficiency of mild water deficit increased by-8.87%～34.29%,-1.48%～93.06%,and-1.01%～94.19%,respectively,and the crude protein water use efficiency was higher under mild water deficit for single sowing.(4)Water regulation and planting pattern affected forage yield and quality through direct/indirect effects.Structural equation model showed that the direct/indirect effects of water regulation and planting pattern explained 19% of the variance in forage yield and 15% of the variance in quality,respectively.In terms of yield effect,the planting pattern showed a positive effect(0.77),the water regulation showed a negative effect(-0.26),and the indirect effect of photosynthetically active radiation in the lower canopy was the largest(-0.35).In terms of quality effect,the planting pattern showed a negative effect(-0.31),the water regulation showed a positive effect(0.30),and the indirect effect of plant height(0.22)and water consumption(0.11)was significant.In summary,water regulation and planting pattern significantly affected soil nutrient environment,forage growth,productivity,stability and water use efficiency.Compared with full irrigation,mild water deficit could significantly reduce the amount of irrigation and water consumption,while maintaining higher soil nutrients and obtaining better forage yield,quality and water use efficiency.Compared with the three types of single sowing,mixed sowing of legume and grass significantly increased forage yield and quality,and mixed sowing of brome and alfalfa was better than mixed sowing of brome and red bean grass as a whole.The combination of mixed sowing of brome and alfalfa and mild water deficit was a better water regulation and planting pattern in the study area.The results have great theoretical value and practical significance for realizing water saving,production increase,high efficiency and green production of artificial grassland.It can also provide a reference for the intensive and standardized production of artificial grassland in the interleaving area of agriculture and animal husbandry.