Synergetic Effects Of Decreased Irrigation Quota,Nitrogen Rate And High Plant Density On Nitrogen Use Of Maize In The Oasis Irrigation Area Of China

Nitrogen plays a key role in crop production.Grain and biomass yields are linked to nitrogen application.Inappropriate nitrogen applications on farms to meet human and animal food security will worsen climate change.The objectives were to determine which nitrogen rate,plant density,and irrigation level reduce nitrogen losses through ammonia and nitrous oxide emissions and nitrate leaching;optimize ammonium,nitrate and total nitrogen accumulation in the soil and;improves grain and biomass yields,yield components,water,and nitrogen use efficiencies（WUE and NUE）.To achieve this objective,three nitrogen rates of urea（46–0–0 of N–P₂O₅–K₂O）(N₀=0 kg N ha^-1,N₁=270 kg N ha^-1,and N₂=360 kg N ha^-1)were applied to three maize plant densities(D1=75,000 plants ha^-1,D2=97,500 plants ha^-1,and D₃=120,000 plants ha^-1),and two irrigation levels（W₁=405mm and W₂=324mm）applied in triplicates each on plot measuring 7×5 m separated by 80 cm with a 50 cm ridge built between plots and the parameters measured according to standard procedures.The results showed that factors and their interactions generally had significant effect on the ammonium,nitrate nitrogen,soil total nitrogen at the various soil depths across the durations after planting in the two cropping seasons.Generally,the ammonium accumulation relatively decreased with an increase in irrigation level for the first three durations after planting but increased with increase in irrigation for the last two durations after planting in2018 but the reverse was the case for the 2019 growing season for almost all the soil depths.Ammonium and nitrate nitrogen accumulation and soil total nitrogen generally increased with an increase in the nitrogen fertilizer rate but decreased with an increase in plant density.Nitrate and soil total nitrogen accumulation decreased as the irrigation level is higher.Nitrate accumulation generally decreased with an increase in duration after planting in the first season but increased as the duration increased in the second season at the first two soil depths.Ammonium nitrogen generally increased as duration after planting increased except at harvest where there was a decline for both seasons while nitrate accumulation decreased as duration after planting increased.Soil total nitrogen decreased after the second and fourth durations after planting in the first and second seasons.The ammonia emission was significantly affected by irrigation level,nitrogen application rate and plant density,and it is increased as the duration after planting increased.Emission of ammonia decreased as the irrigation level increased but relatively increased as the nitrogen fertilizer rate increased from(0 to 270 to 360 kg N ha^-1),at the third and last months in 2018and at first,third and last durations after planting in the 2019.Ammonia emission was relatively higher at the lower plant densities(D1=75,000 plants ha^-1).The emission was higher in the second growing season than in the first season.All the factors and their interactions greatly affected nitrous oxide emission regardless of the duration after planting and the season of cultivation.Nitrous oxide emission increased with decreases in irrigation level and plant density but increased correspondingly with an increase in nitrogen fertilizer level.Nitrate leaching was markedly influenced by the all factors and their interactions.The conventional irrigation caused more leaching of nitrate in 2018 season.In the second season the decreased irrigation caused more leaching than conventional irrigation treatment which might be attributed to differences in climate factors.Nitrate leaching increased with an increase in the level of nitrogen fertilizer applied.The lowest plant density recorded the highest leaching.Biomass yield,and nitrogen use efficiency increased while grain yield,harvest index,500-grain weight and kernel per ear of maize decreased with an increase in the irrigation level,regardless of the growing season.Water use efficiency and number of ears per hectare increased proportionally with irrigation level in the first season but behaved inversely in the following season.The amount of nitrogen fertilizer applied significantly affected all the yield components irrespective of the year of cultivation.An increase in the nitrogen level positively impacted all the yield components of the maize,irrespective of the grow ing season.Yield components were higher in the second season.The number of ears/ha and the number of kernels per ear generally increased as the plant density increased except marginal declines at second highest plant density in 2018 and at the highest plant density in 2019,respectively.The harvest index decreased as the plant density increased in the first season but increased correspondingly with the plant density in second season.The 500-grain weight,however,remained fairly unchanged,regardless of the plant density.From the results,to reduce nitrogen losses through emission and leaching,the conventional irrigation level,the decreased nitrogen rate(270 kg N ha^-1)and the highest plant density(120,000 plants ha^-1)of maize should be used.For maximum grain yield and biomass,the conventional irrigation level,with conventional nitrogen application rate coupled with120,000 plants ha^-1is ideal when cultivating maize on similar soils.However,for water and nitrogen use efficiencies and most of the yield components,the decreased irrigation level with360 kg N ha^-1 coupled with 97,500 plants ha^-1may be ideal for optimum results.Nitrogen management in previous cropping seasons can influence the proceeding season crop performances.Yearly cropping when coupled with adequate water and nutrient management will cause nitrogen depletion in the soil over time.It can therefore be concluded that optimized irrigation level,reduced nitrogen rate and higher plant density are vital for optimum grain and biomass yields,reduce nitrogen losses,improves NUE,and WUE in maize cropping in the Oasis irrigation region and other regions with similar climatic conditions.