Effect Of Different Irrigation And Nitrogen Regimes On Root Growth And Nitrogen Utilization Of Winter Wheat

Excessive nitrogen(N)application combined with water shortage has a negative effect on crop production,particularly wheat production in North China Plain.Unreasonable levels of irrigation and excessive nitrogen supplementation commonly occur in this area,which leads to a lower use efficiency of water and fertilizers,higher production costs and severe pollution of the soil and underground water.Therefore,it is highly important in this region to use chemical fertilizers in a reasonable manner and to reduce the costly use of water resources.The stationary field experiments were conducted to investigate the effects of irrigation and N regimes on root development and its relationship with aboveground,and soil water and N use in different soil layers.We also analyzed nitrogen balance in soil-wheat system,flow of fertilizer N,allocation and remobilization of nitrogen,grain yield and nitrogen utilization efficiency in winter wheat.The aim of this research was to provide a basis for the formulation of appropriate scientifically-based strategies regarding water and nitrogen management for high-yield wheat production in the Huang-huai wheat production area.The main results were as follows:1.This study made clear the regulation of soil inorganic nitrogen migration,apparent soil nitrogen budget and flow of fertilizer N.N supply increased aboveground nitrogen accumulation compared to no-N treatment.The N240 treatment gave the highest efficiency in nitrogen accumulation partitioning to grain,and the higher N application(N300)had a lower efficiency in N accumulation partitioning to grain,but apparent N surplus significantly increased.With the increase of N application rates,the NO3-N move down to deep soil layers under both filed plot and 15 N labeled micro-plot experiments,and the N move doen obviously in medium fertility soil.Using 15 N isotope method,we found that 15.8%-20.6% N accumulation in aboveground at harvesting was derived from N fertilizer applied,and 79.4%-89.2% from soil in high fertility soil.In medium fertility soil,22.1%-25.0% and 75.0%-77.9% was derived from N fertilizer and soil,respectively.The N fertilizer recovery rate,N fertilizer residual rate and N fertilizer loss rate were 31.17%-33.80%?32.62%-45.36% and 20.84%-36.21%,respectively,in high fertility soil,and 33.68%-39.12%?25.49%-34.83% and 26.05%-40.83%,respectively,in medium fertility soil.The N fertilizer recovery rate and N fertilizer residual rate were less in high fertility soil than in medium fertility soil,but not with N fertilizer loss rate.2.This study expounded the regulation of root growth and its relationship with above-ground and water-nitrogen use efficiency in winter wheat.Irrigation significantly increased the root weight density(RWD)by 50.4%,with a corresponding increase in root length density(RLD)of 35.8% compared to the non-irrigation treatment(W0).The effect of nitrogen on RWD and RLD depended on soil water conditions.Under no-irrigation conditions(W0),RWD decreased with increasing rates of nitrogen application.The application of 180 kg ha–1 nitrogen significantly facilitated root growth and produced the maximum RWD values under irrigation conditions.Although further increasing the nitrogen application rate to 240 kg ha–1 slightly improved the grain yield,this treatment markedly decreased the RWD.The treatment of N180 gave the highest RLD under both irrigation conditions.Since RWD was positively correlated with GY,ET and NUE,it is also suggested that increases in root biomass cause an increase in NUE;however,a large root system also results in rapid soil water consumption.Positive relationships between RLD and SWR,and RLD and SNC were found in each soil layer for the non-irrigated and no-N treatments.SWC and SNC at upper soil layers and deep soil layers exhibited significant positive correlations with RLD for the appropriate irrigation and N regimes,while a significant correlation was only found in the upper soil profile(0-0.4 m)for the exceed irrigation and N application.Appropriate N application rates facilitated root development,which not only resulted in higher yields and WUE,but also reduced residual NO3-N.When irrigation was combined with N rates of 180-240 kg N ha-1,it favorably enhanced the use of soil water and NO3-N in both the upper and deeper soil layers by regulating root growth,which is conducive to attaining high grain yields and WUE.3.The changes of key enzyme activities in nitrogen assimilation and the key genes playing an important role in the response to nitrogen stress were clarified.The activity of NR and GS increased with soil fertility,and the maximum activity was recorded in high fertility soil.Compared to no-N treatment,N supply increased NR activity by 45.8%-65.8% in flag leaf and 51.0%-87.0% in second top leaf,respectively,in high fertility soil,by 56.7%-72.6% in flag leaf and 69.5%-103.8% in second top leaf,respectively,in medium fertility soil,and by 57.0%-76.7%% in flag leaf and 100.5%-139.3% in second top leaf,respectively,in low fertility soil.The activity of NR and GS in the second leaf was either approximated to or higher than that of the flag leaf in the late filling under optimal N application conditions,which indicated that both flag leaf and the second leaf can play an important role in amino acid synthesis and nitrogen transportation under optimal N application conditions.The transcription levels of NRT1 and NRT2 genes were measured using quantitative real-time PCR(q PCR)in flag leaf of N application under field condition.The results indicated that N stress(N0)significantly reduced the transcription levels of Ta NRT1.2 Ta NRT1.3 and Ta NRT1.8 at all sampling times,and the Ta NRT2.2,Ta NRT2.3,Ta NRT2.4 and Ta NRT2.5 at grainfilling.However,the transcription levels of Ta NRT1.5,Ta NRT1.6,Ta NRT2.1 and Ta NRT2.2 were significantly enhanced at booting stage,which plays more important role in respond to nitrogen stress in winter wheat under field condition.4.The water and nitrogen management strategies of wheat fields with different precipitation years and different soil fertility levels were established.Compared to the non-irrigated treatment(W0),a single irrigation at jointing(W1)significantly increased yield by 3.6-45.6%.With increases in water(W2,a second irrigation at flowering),ET increased,but grain yield did not increase,WUE decreased significantly during normal precipitation growing seasons.But during the drier growing seasons,grain yield,ET,and WUE increased simultaneously.In the growing seasons from 2012-2013 to 2015-2016,the wheat yield was increased by from 10.8% to 54.9% in high fertility soil,from 125.3% to 165.8% in medium fertility soil,and from 53.0% to 288.2% in low fertility soil,in the N treatments compared to the N0 control treatment,respectively.This suggesting that the effect of N fertilizer on wheat yield became greater as the experimental seasons progressed.Under dry conditions,non-N and high rates of N have a negative effect on wheat production,indicating the combined effect of water stress and deficient or excess N application.Under current Huang-Huai climate conditions,application of N at rates of 180 and 240 kg N ha-1 produced the maximum grain yield under drought stress and irrigated conditions,respectively.Application of N at rates of 180 and 240 kg N ha-1 under high and medium soil fertilizer,respectively.Under low fertility(sand soil),the early nitrogen surplus was large before jointing stage.The nitrogen should be supplied use a small amount with multiple fertilization.Therefore,N rates of 180-240 kg ha-1 with two irrigations can reduce the risk of yield loss that occurs due to reduced precipitation during the wheat growing seasons,while under better soil moisture conditions,a single irrigation at jointing was effective and more economical.