Effect Of Water And Nitrogen Application On High Quality And Low Carbon Of Alfalfa In Ningxia Irrigation Area Of Yellow River

Alfalfa plays an important role in improving the ecological environment and adjusting the structure of agriculture and animal husbandry in Northwest China.Unreasonable irrigation and nitrogen application not only affect the yield and quality of alfalfa,but also cause serious waste of resources and environmental pollution.Meanwhile,Irrigation and nitrogen application will also have a certain impact on soil CO2 emission.A field experiment using the split plot design was conducted a filed manipulation experiment to evaluate the response of alfalfa yield and quality and grassland soil CO2 emissions to different water and nitrogen supply from April 2017 to 2019 in an alfalfa grassland of Northwest China.There were three irrigation levels(600,675 and 750mm·yr-1)in main plots and four nitrogen levels(0,60,120,180 kg·hm-2.yr-1)in sub-plot.The main results of the study were as follows:(1)Irrigation and nitrogen application affect the growth characteristics of alfalfa and grassland microclimate.Compared with the non-fertilization treatment,fertilization decreased the air temperature,soil temperature,illumination,but improved the relative humidity among alfalfa.The microenvironment of alfalfa was different under different irrigation amount,with the increase of irrigation amount,the relative humidity among alfalfa increased gradually,and the cooling effect of air temperature and soil temperature of alfalfa were more obvious.(2)Nitrogen application increased the total nitrogen(TN),available nitrogen(AN),available potassium(AK),soil organic matter(SOM),catalase(CAT)and urease activities(URE)to a certain extent,and reduced the soil pH,soil salt(ST)and alkaline phosphatase activity(ALP).Increasing the amount of drip irrigation improved the available phosphorus(AP),available potassium(AK),the activity of catalase(CAT),invertase(INV)and urease activity(URE),while reducing the soil total nitrogen(TN),organic matter content(SOM)and alkaline phosphatase activity(ALP).(3)The effect of nitrogen application on the average annual hay yield of alfalfa in three years reached an extremely significant level(P<0.01),When the nitrogen application amount was 0-120 kg·hm-2,the annual hay yield of alfalfa was changed with nitrogen application amount increased with the increase of the amount,and it decreased slightly when the application of nitrogen fertilizer continued to exceed the N2 level;irrigation amount,water and nitrogen interaction had no significant effect on the average yield of alfalfa hay in three years(P>0.05).(4)Nitrogen application significantly increased the annual average crude protein,ash and relative feed value of alfalfa(P<0.05),but significantly reduced the annual average acid detergent fiber and neutral detergent fiber content of alfalfa(P<0.05).Increasing irrigation amount significantly increased the annual average crude protein and ash content of alfalfa(P<0.05).(5)The irrigation water use efficiency(IWUE)and water use efficiency(WUE)decreased gradually with the increase of irrigation amount and decrease of nitrogen application rate.With the increase of nitrogen application rate,the partial productivity of alfalfa nitrogen fertilizer and the agronomic efficiency of nitrogen fertilizer decreased significantly(P<0.05).(6)The soil respiration rate of alfalfa grassland showed a significant seasonal variation feature under different water and nitrogen supply.The peak value appears in late July;the lowest value occurs in mid-December and lasts until the end of February of the following year.During the growing season of alfalfa,soil respiration rate of alfalfa grassland increased with the increase of nitrogen application rate(P<0.05),but had no significant effect on the soil respiration rate during the non-growing season of alfalfa(P>0.05).Increasing the irrigation amount had no significant effect on the soil respiration rate in the growing season and the non-growing season of alfalfa(P>0.05).(7)According to statistical analysis,it indicated that soil respiration rate had a significant exponential positive relationship with soil temperature in the growing season,non-growing season and whole year of alfalfa under different water and nitrogen supply(P<0.05).Soil temperature(T)and soil moisture(W)interacted with each other and ultimately affected soil respiration(Rs),and using the two-factor linear model of soil temperature and soil moisture(Rs=a+bW+cT)could better fit the change of soil respiration rate.There was a parabolic relationship with an upward opening between Rs and hay yield during the growing season of alfalfa.There was no significant correlation between underground biomass and soil respiration(P>0.05).According to the correlation matrix analysis between soil respiration and soil biochemical properties during the growth period of alfalfa,it showed that Rs was significantly negatively correlated with soil pH and INV(P<0.05),and it was significantly positive with soil SOM,TN and URE(P<0.05).(8)Nitrogen application increased the carbon footprint of alfalfa,and it increased with the increase of nitrogen application(P<0.01).Irrigation amount had a very significant effect on the carbon footprint of alfalfa(P<0.01).With the increase of the irrigation amount,the carbon footprint of alfalfa showed a trend of increasing first and then decreasing(W2>W1>W3).In the composition of alfalfa carbon footprint,CO2 emission had the highest contribution to alfalfa carbon footprint(91.2%-95.9%),and followed by irrigation electricity,accounting for 2.9%-4.2%.(9)In comprehensive consideration of hay yield,quality and low carbon,W2N3(675 mm,120kg·hm-2)was suggested to be best combination treatment for alfalfa in this area.The configuration combination was conducive to improve the yield and quality of alfalfa hay and reduce the carbon footprint of alfalfa.The research results can provide a theoretical basis for the promotion of high-yield,high-quality and low-carbon cultivation of alfalfa under drip irrigation in Ningxia irrigation area of Yellow River.