Water And Nitrogen Utilization Efficiency Influenced By Water-Saving Irrigation For Rice Cultivation

Rice,a major Chinese food crop,consumes enormous water in its production process.Consequently,water remains an important factor in its stable production.Rice planted in Southern China accounts for 94%of the total in sown acreage and 88%of the total in production,which matters a lot to Chinese food security.However,due to the prolonged conflict between water availability and rice growth in spatial/temporal distribution,traditional rice cultivation suffers from seasonal water shortage at acreage of 16-22%,which compromises food production capacity and food security.It necessitates the use of water-saving rice production technologies.Besides water,the use of nitrogenous fertilizers has been increased in rice cropping systems worldwide as well as in China.Whereas,the efficiency of nitrogen?N?utilization is notoriously low with continuous flooding during rice cultivation.Though the confirmations are rare,and tactics to achieve the consequence of paddy water environment on nitrogen dynamics is still tacit.In this backdrop,the field and greenhouse studies were conducted to investigate the efficient utilization of water and nitrogen resources in paddy field using ¹⁵N isotopic tracer technique under water-saving irrigation.The field study was conducted at the Tuanlin irrigation research station,Jingmen city,Hubei province,China in a paddy field with silty loam soil over two consecutive years?2015-2016?.The experiment was laid out in a split plot design with water management treatments?conventional irrigation and water-saving irrigation named as“thin-shallow-moist-dry irrigation”,hereafter referred to as CI and TSMDI,respectively?as main plot and nitrogen fertilizer were applied in three rates;N₀(0 kg N ha^-1),N₁(90 kg N ha^-1)and N₂(180 kg N ha^-1,local farmer's application rate)as subplot with three replications.A column experiment was conducted in an open greenhouse?with a moveable rain-proof cover?during the summer season in 2016 at Huazhong Agricultural University,Wuhan,China.A two-factor factorial experiment based on a completely randomized design was conducted with three replications of each treatment.Irrigation factor included two levels;CI and TSMDI and nitrogen factor in the form of ¹⁵N-labeled urea?6%atom excess?in three rates;N₀(0 kg N ha^-1),N₁(90 kg N ha^-1)and N₂(180 kg N ha^-1).The main findings of the dissertation are as follow:1)The efficient utilization of water resource was conspicuous in TSMDI water-saving irrigation practice compared to conventional irrigation.At successive characteristic stages of rice,the field irrigation water utilization coefficient?FIWUC?was0.21-3.85%more pronounced under TSMDI water-saving irrigation compared to the conventional irrigation technique.The FIWUC provides information about the effective water quantity of applied irrigation,which is useful for plant growth.The trend of higher FIWUC in TSMDI water-saving irrigation might be ascribable due to a reduction in standing water depth,which favors in minimizing water percolation through the lower pressure head.Higher water savings,ranged from 16-21%,observed in TSMDI water-saving irrigation over conventional irrigation practice.The water productivity?WP_I?in terms of irrigation water was about 19.04-30.26%higher under TSMDI water-saving irrigation than in conventional irrigation regime,and the differences were significant at the 5%level.The combination of water-saving irrigation and local farmer's nitrogen application rate(180 kg N ha^-1)show that WP_I was substantially higher 25.96%,compared with CIN₂.The water productivity(WP_I+R)in accordance with irrigation plus rainfall was about 4.54-16.50%higher under TSMDI water-saving irrigation than in conventional irrigation.However,it is better to use FIWUC in conjunction with WP_I for efficient utilization of applied irrigational water in lowland paddy soil.2)The TSMDI water-saving irrigation could maintain rice grain yield,even higher sometimes.In field during 2015,the TSMDI water-saving irrigation produced 5.53-6.37%higher grain yield compared to conventional irrigation method,and in the greenhouse also 1.72-3.52%higher grain yield under water-saving irrigation than CI.The TSMDI water-saving irrigation created a healthy environment for root growth,favored to extract water and nitrogen efficiently,whereby offered more grain yield compared to CI.Nevertheless,plant growth parameters and yield components,i.e.plant height,number of tillers,leaf area index,dry biomass,number of panicles,filled grain percentage and 1000 grain weight were shown non-significant behavior between water treatments,which is evident of saving water without jeopardizing grain yield.However,it was noted that in field the difference between the N₁ and N₂ for grain yield was not too much deviated,creating an opportunity that low rates of N could also achieve desired grain yield by utilizing TSMDI water-saving irrigation practice.3)Rice with water-saving irrigation technique improved nitrogen use efficiency?NUE?by increasing nitrogen accumulation in root and grain parts.The NUE in TSMDI water-saving irrigation regime at N₁ and N₂ was 5.68-30.23%and 6.56-13.92%higher than in CI,respectively.The ¹⁵N tracer technique quantifies that the nitrogen derived from fertilizer was higher under water-saving irrigation in different plant parts,whereas significant differences observed for rice root and grain.The extensive root system in water-saving irrigation increases the N-uptake and translocate efficiently it to the aerial parts of the plant,ultimately leads to higher grain yield.The nitrogen uptake by plants with N₂ was higher than others,but NUE was lower than N₁.4)Water-saving irrigation practice had remarkable environmental effects on the reduction of total nitrogen leaching losses and decreasing water leakage from paddy soil.The successful implementation of TSMDI water-saving irrigation reduced 18.63%water percolation.A survey of leachate indicated that NH₄⁺-N was the main form of N leaching.The NH₄⁺movement in soil is greatly influenced by irrigation regimes and that the vast potential risk of NH₄⁺leaching deserves more attention.By using the method of TSMDI water-saving irrigation,the NH₄⁺leaching loss of paddy-soil was definitely decreased,while the NO₃^--N leaching loss was increased in some degrees,especially at re-irrigation after“sun-baking”.However,the T-N leaching losses were substantially reduced,indicating the positive impact of water-saving irrigation technique of minimizing groundwater contamination.5)Regarding the fertilizer N balance,the proportions of applied fertilizer for plant absorption,residue in paddy soil and nitrogen loss were managed appropriately under water-saving irrigation.Our results concluded that TSMDI water-saving irrigation enhanced fertilizer nitrogen uptake by rice plant about 10.49%and 6.56%and retention in paddy soil about 3.77%and 4.77%at N₁ and N₂,compared to conventional irrigation method.While,reduced the fertilizer nitrogen loss about 32.38%and 19.03%at N₁ and N₂,respectively,which is a good indication to minimize environmental degradation.We concluded that the TSMDI water-saving irrigation was the most rational regime in nitrogen levels studied.Moreover,the synthetical benefit in lower nitrogen level?N₁?was better than in higher nitrogen level?N₂?,because the difference in grain yield was not too much deviated.However,N₁ increased the NUE and substantially minimized the N leaching losses.These outcomes suggest that the TSMDI water-saving irrigation can be used effectively to reduce the total amount of water and fertilizer-N application in agriculture for the safe ecological environment.However,further studies are needed for the subsequent effects of TSMDI water-saving irrigation along with optimal nitrogen management strategies for sustainable agricultural systems.