Effects Of Straw Mulching In Autumn And Reducing Nitirogen Rate On Yield,Water And Nitrogen Utilization Of Winter Wheat In Hilly Dryland Of Sichuan

In order to solve the problem of low water and nitrogen utilization efficiency caused by water deficit resulting from the drought condition from winter to spring,which restricted water and nitrogen absorption of root system,a two-factor split plot experiment was carried out at Renshou experiment station of Sichuan Agriculture University（29°60’N,104°00’E）,Sichuan Province,China,a typical hilly dryland under the winter wheat-summer maize rotation system from 2016 to 2018.The maize straw mulching was used as the main plot（SM:straw mulching;NM:no-straw mulching）and the nitrogen application rate(N₀:0 kg ha^-1 N;reducing nitrogen rate（RN）:120 kg ha^-1 N and conventional nitrogen rate（CN）:180 kg ha^-1 N)was used as sub-plot with three replications.The main purpose was to explore the effects of maize straw mulching and reducing nitrogen application rate on the soil water and heat balance,nitrogen balance,root system configuration,dry mater accumulation,yield,water and nitrogen utilization of winter wheat.In order to explore the effects of straw mulching and reducing nitrogen rate on the ferlizer-N absorption,residue and loss in the soil,a microzone experiment was further adopted.In the micro-plot experiment,¹⁵N labeled urea was used as fertilizer nitrogen source,and three fertilization schemes were set up,including:（1）base fertizer labeled:¹⁵N labeled urea was used for base fertilizer,and the ordinary urea was used for jointing fertilizer;（2）jointing fertilizer labeled:¹⁵N labeled urea was jointing fertilizer,and ordinary urea was used for the base fertilizer;（3）¹⁵N labeled urea was used for both the base and jointing fertilizer.The results were as follow:1.Effects of straw mulching and reducing nitrogen rate on soil water storage,soil water distribution,soil temperature and soil water utilizationSM improved the soil water content of the 0-10cm,10-20cm and 20-40cm soil layer,and the soil water storage of 0-100cm soil layer during the growth period were significantly promoted,which could effectively meet the water demand of wheat from sowing to booting stage.The heat preservation effect of straw mulching reduced the diurnal temperature difference of soil,and the difference of soil temperature decreased with the increase of soil depth.Compared with NM,SM decreased soil temperature in the daytime and kept soil warm at night in trefoil stage with high temperature.In the jointing stage,the temperature at 15cm and 25cm soil depth were significantly increased under SM,while which was significantly decreased during and after the booting stage,and the SM played a buffering role for the drastic change of temperature.Compared with NM,SM significantly increased the ET in the stage of sowing-jointing and jointing-anthesis by 21.2%and 18.4%,but decreased the ET of anthesis-maturity stage,which not only meet the water demand of wheat growth under winter and spring drought,but also reduced the ineffective water consumption after flowering.SM effectively improved the total ET and the WUE by11.7%and 74.5%during the whole growth period.The RN showed no significantly negative effect on the ET of sowing-jointing,jointing-anthesis and anthesis-maturity,total ET and WUE,as compared with CN under SM.2.Effects of straw mulching and reducing nitrogen rate on the distribution of inorganic nitrogenSM significantly increased the ammonium nitrogen content in 0-10cm soil layer at jointing,while which significantly reduced the nitrate nitrogen content in the soil layers of0-10cm and 10-20cm and the ammonium nitrogen content in the 0-40cm soil layer in the anthesis and maturity stage.Compared with NM,the SM significantly reduced the soil inorganic nitrogen content in 0-10cm soil layer in the anthesis stage and in 0-40cm soil layer in maturity stage.Under SM,RN effectively decreased the inorganic nitrogen content in the 0-40cm soil layer,as compared with CN.SM significantly increased the soil N output,while reduced the apparent N loss amount and loss ratio,compared with that under NM.Compared with CN,the RN significantly reduced the nitrogen output and nitrogen loss.3.Effects of straw mulching and reducing nitrogen rate on root architecture,dry matter accumulation and translocationThe root biomass and root/shoot ratio were significantly reduced under SM,whereas the plant height,leaf length,leaf width,leaf area index and aboveground biomass were effectively promoted under SM at trefoil stage.However,the lateral and longitudinal growth of wheat root were significantly improved under SM in jointing stage with thicker roots on the upper and slenderer roots in subsoil layer,and the root length density,root surface area density and root volume density were significantly promoted.Furthermore,the mean root diameter of 0-10cm soil layer was effectively improved,whereas the mean root diameter of 10-40cm soil layer was significantly decreased under SM.There was no significantly difference in root architecture between RN and CN.SM significantly improved the photosynthetic performance of wheat by increasing the leaf length,leaf width and leaf area index,and significantly delaying the chlorophyll degradation in the post-antheis stage.Under SM,RN showed no significant effect on the photosynthetic performance of winter wheat.Compared with NM,SM significantly increased the dry matter accumulation of anthesis stage and the dry matter accumulation after anthesis stage by 71.5%and 114.5%.The dry matter translocation amount increased by 54.8%,and contribution rate of dry matter of post-anthesis to grain increased from 38.1%to 48.0%.4.Effects of straw mulching and reducing nitrogen rate on nitrogen utilization,and the balance of fertilizer nitrogen and inorganic nitrogen（1）SM significantly improved the nitrogen accumulation rate and peak value of nitrogen accumulation rate in the whole wheat growth period,and the peak value under SM advanced by 14-19d.As a result,the nitrogen accumulation under SM at jointing,anthesis and maturity stage was 120.2%,108.1%and 73.1%higher than that under NM.Compared with NM,SM promoted the translocation of nitrogen by significantly increased the translocation amount and proportion of the nitrogen stored before anthesis,and effectively increased the contribution rate of nitrogen translocation to grains by 13.8percentage,which reached to 72.0%,thus significantly improving the agronomic efficiency of N fertilizer and nitrogen partial factor productivity by 367.1%and 83.7%,and nitrogen recovery efficiency increased by 44.4 perncentage under SM.（2）SM significantly increased the fertilizer nitrogen accumulation of winter wheat with the fertilizer nitrogen absorption efficiency reached to 43.4%,which was 15.4percentage more than that under NM,and SM also significantly reduced residual amount of fertilizer nitrogen and loss amount of fertilizer nitrogen by 26.5%and 18.2%,and the residual ratio and loss ratio of fertilizer nitrogen were 6.1 percentage and 9.3 percentage less under SM compared with that under NM.The absorption rate of fertilizer nitrogen from both base fertilizer and jointing fertilizer was significantly increased under SM.In addtion,compared with CN,RN significantly reduced the residual and loss of fertilizer nitrogen in the soil,whereas increasing the absorption efficiency of fertilizer nitrogen under SM.SM significantly increased soil nitrogen output and decreased apparent nitrogen loss and loss ratio.Under SM,compare with CN,RN significantly reduced the soil nitrogen output and loss.5.Effects of straw mulching and reducing nitrogen rate on the yield and yield components of winter wheat.SM significantly increased the tiller capacity and earbearing tiller percentage of winter wheat,leading to the significantly increase in effective spikes.The number of spikelet per spike,number of fertile florets per spikelet,and the seed setting rate of fertile florets were significantly improved under SM,thus to significantly improve the grain number per spike.Therefore,the grain yield of winter wheat under SM was significantly promoted by 92.9%（averaged across years）,compared with that under NM.No significant reduction was observed on the grain yield in RN,compared with that in CN under SM.In summary,in winter and spring drought condition in southwest China,the water retention effect of SM could last until the booting stage of winter wheat,which effectively met the water demand of winter wheat before booting to promote the root and aboveground growth,improve the absorption of soil water and nitrogen by root,delay the senescence of leaves,promote the flowering and seed setting,and finally significantly increase effective spikes,kernels per spike and wheat yield.Reducing nitrogen application rate without significant yield reduction under SM was owing to that the water retention effect of SM significantly improved the distribution of root in the topsoil and deep soil layer,while increased the absorption efficiency of fertilizer nitrogen and soil nitrogen,so as to meet the nitrogen demand of wheat growth.These results have important theoretical value and great significance for efficient utilization of precipitation and nitrogen and the sustainable development of agriculture of winter wheat in hilly dryland of southwest China.