Effects Of Autumn Nitrogen Application And Early Winter Irrigation On Leymus Chinensis Seed And Hay Production

Leymus chinensis,also known as alkali grass,is an important rhizome perennial forage grass in Northeast China.It is an excellent grass species for planting artificial grassland in arid and semiarid areas.However,without manual intervention,the sexual reproduction ability of L.chinensis is weak,and the seed yield cannot meet the needs of forage production and ecological construction.In this study,the effects of nitrogen application in autumn(0 g m^-2,10 g m^-2)and early winter irrigation（equivalent to 0 mm,20 mm,and 40 mm rainfall）on the reproduction of new offspring and the next year and the yield of L.chinensis were studied by randomized block.The results showed that:（1）In the early stage of the post fruit nutrition period,nitrogen addition in 2020increased the number of new offspring by 30.70%,32.16%and 41.62%in autumn;the trend of nitrogen addition in 2021 was the same as that in the previous year,but the number of new seedlings in each month in autumn increased by 65.53%,87.48%and 78.77%,respectively.The regreening rate,total density per unit area,heading number and heading rate of L.chinensis in the next year were significantly improved in both years.The treatment of irrigation during the overwintering period reached the maximum value when the irrigation amount was 20 mm,which could increase the total density,heading number and heading rate of L.chinensis by 0.45 times,1.9 times and 1.03 times.（2）The study of photosynthetic characteristics showed that the intercellular CO₂concentration,photosynthetic water use efficiency,net photosynthetic rate,stomatal conductance,leaf water vapor pressure deficit,transpiration rate and chlorophyll content of L.chinensis leaves under appropriate water and nitrogen addition treatments were improved to varying degrees compared with the control group,and the effect of nitrogen application treatment was significantly better than that of irrigation treatment.In terms of soil physical and chemical properties,irrigation has a significant effect on soil relative water content,soil porosity and carbon-nitrogen ratio.Nitrogen application had significant effects on soil total nitrogen,p H,and C/N ratio.（3）The addition of water and nitrogen significantly promoted the seed composition factors of L.chinensis.Irrigation increased the spikelet number and seed setting rate of L.chinensis.Nitrogen addition increased the number of florets by 71.61%and the number of seeds by 127.87%.The addition of water and nitrogen played a positive role in the sexual reproduction of L.chinensis and promoted the increase of seed yield of L.chinensis.The results of structural equation model showed that water and nitrogen addition,photosynthetic characteristics and heading correlation were important factors leading to the change of seed composition factors.（4）In terms of yield and forage quality of L.chinensis,irrigation treatment significantly promoted the yield,and reached the maximum at 20 mm irrigation amount,and the total yield increased by 1.01 times compared with the control.The contents of crude protein,crude fat and soluble sugar increased by 7.61%,7.81%and 8.04%compared with the control.Nitrogen addition had a significant effect on yield,crude protein,crude fat,plant total nitrogen,total carbon and carbon-nitrogen ratio.The results of structural equation model showed that nitrogen application had a direct positive effect on yield.The effect of water and nitrogen addition on forage quality is higher than that on yield,which affects forage quality by affecting plant traits.Irrigation has a direct negative impact on forage quality.Through the comprehensive score of membership function,the most favorable water and nitrogen addition treatment for L.chinensis yield and forage quality was nitrogen application+20 mm irrigation amount.In summary,in the artificially planted L.chinensis grassland,water and nitrogen addition can significantly affect the seed composition factors,hay yield and forage quality of L.chinensis in the next year.However,under different treatment conditions,nitrogen is still the main factor controlling yield and forage quality.Nitrogen application combined with moderate irrigation can improve the quality of L.chinensis grassland to alleviate the negative impact of grassland degradation.