| A large area of Loess Plateau in China is practicing dryland farming.Winter wheat?Triticum aestivum L.?is a major crop in this area,and its yield and quality are often limited by water and nutrients.Straw mulching is an economical and effective agronomic practice for soil moisture conservation on dryland.However,the effects and regulating mechanisms of straw mulching on winter wheat yield and water use are still not clearly elucidated.In this study,we established three field experiments under straw mulching in the semiarid Loess Plateau where winter wheat was growth to investigate:1)effects and physiological mechanisms of nitrogen regulation on winter wheat population size,water use and yield;2)effects and mechanisms of population regulation on winter wheat yield and water use;3)effects and mechanisms of coupling regulation of population size,nitrogen and soil water at planting on winter wheat yield and water use.Three field experiments included:1)nitrogen regulation field experiment?2012.9-2016.6?involved two nitrogen levels(150 and 200 kg N ha-1)and three nitrogen application times?1,2 and 3?,six treatments in total;2)population regulation field experiment?2012.9-2016.6?involved two soil management practices,i.e.conventional tillage and straw mulching,and three seeding rates?low,medium and high?,and four population regulation measures at medium and high seeding rates under straw mulching,i.e.winter root pruning,spring root pruning,winter defoliation and spring defoliation,fourteen treatments in total;3)field experiment of coupling regulation of population,nitrogen and pre-sowing soil water?2013.9-2016.6?involved three pre-sowing soil water levels?low:rainfed;medium:rain+67 mm irrigation before sowing;high:rain+133 mm irrigation before sowing?,two nitrogen levels(150 and 200 kg N ha-1),and three population regulation measures,i.e.untreated control,spring root pruning and spring defoliation,eighteen treatments in total.The main results and conclusions obtained were as follows:1. Effect and physiological mechanism of nitrogen regulation on wheat population size,water use and yield under straw mulchingDuring the four seasons,winter wheat grain yield varied with season,as follows:2012-2013(3464 kg ha-1).Nitrogen rate and application times had no significant effect on winter wheat population size,grain yield,aboveground biomass?at maturity?,harvest index,evapotranspiration?ET?and water-use efficiency?WUE?,which was partially related to the fact that they did not affect physiological characteristics?soluble protein and MDA?of flag leaf after flowering.However,high nitrogen rate distinctly reduced grain weight compared with low nitrogen rate.The interaction between nitrogen rate and application times also had no obvious effect on wheat yield,yield components,ET and WUE.These results indicated that the tested low nitrogen rate(150 kg ha-1)have met the demand for wheat growth,and one application of nitrogen before sowing is feasible for the studied area under straw mulching.2. Effect of straw mulching and seeding rate on wheat yield and water-use efficiencyDuring the four seasons,winter wheat grain yield ranged from 2851 to 6981 kg ha-1 and WUE from 5.3 to 16.2 kg ha-1 mm-1.Grain yield was significantly affected by the interaction between straw mulching and season.Conventional practice showed significantly higher mean harvest index?HI?,grain yield,and WUE than straw mulching in favorable seasons.However,grain yield was significantly higher under straw mulching than under conventional tillage in very dry season.Generally,soil water storage was higher and soil temperature was lower under straw mulching than under conventional practice,especially in wet seasons.Evapotranspiration was higher and water-use efficiency was lower under straw mulching than under conventional practice.Seeding rate had no effect on winter wheat grain yield,but low seeding rate increased HI compared with high seeding rate.It is concluded that low seeding rate?75%of the recommended?would be suitable understraw mulching.3. Responses of wheat yield and water-use efficiency to root pruning under mulching,and interactions with seeding rate,pre-sowing soil water level and nitrogen rateThe results across experiments indicated that root pruning improved winter wheat grain yield by 7%,harvest index by 6%and water-use efficiency by 11%across environmental conditions under straw mulching,and grain yield increased more in low-yielding than high-yielding conditions.Root prunning in spring produced significantly higher wheat grain yield than root pruning in winter under straw mulching.In addition,spring root pruning improved wheat grain yield at both recommended and higher seeding rates,but with larger effects at high seeding rate across four seasons.Spring root pruning under straw mulching improved wheat grain yield at both tested nitrogen rates,but grain yield under spring root pruning was similar at both low and high nitrogen rates.Season and pre-sowing soil water and spring root pruning also interacted improving grain yield in low-yielding conditions,and low or medium pre-sowing soil water levels but not in high pre-sowing soil water level.Moreover,spring root pruning improved apparent translocation amount?16%?and ratio?9%?of stem water soluble carbohydrates.This is one of the important reasons for the increase in grain yield under root prunning.Thus,it is concluded that spring root pruning is a viable option to improve wheat yield and water-use efficiency under straw mulching in semiarid environment.4. Responses of wheat yield and water-use efficiency to defoliation under mulching,and interactions with seeding rate,pre-sowing soil water level and nitrogen rateThe results across experiments indicated that defoliation was largely neutral for grain yield and water-use efficiency,but improved harvest index by 7%,apparent translocation ratio of stem water soluble carbohydrates by 8%and net income by 15%across environmental conditions under straw mulching.Defoliation in winter produced similar grain yield as that in spring but defoliation in winter significantly reduced grain yield relative to the control under high yield conditions.Defoliation in spring always returned higher net income than defoliation in winter.Defoliation had similar effect on grain yield regardless of seeding rate,but defoliation in spring generally obtained higher net income at recommended than at high seeding rate.Defoliation in spring generally returned higher grain yield and net income at recommended nitrogen rate and high pre-sowing soil water level.Moreover,the interaction between season and defoliation significantly affected winter wheat grain yield and water-use efficiency.Thus,it is concluded that defoliation in spring is an effective way to improve farmers'income under straw mulching in semiarid environment.In summary,there are three ways in increasing wheat yield or income and water-use efficiency under straw mulching on the Loess Plateau,i.e.?1?wheat grows at low seeding rate?75%of the recommended?in combination with recommended nitrogen rate with one application before sowing;?2?wheat grows at recommended nitrogen rate and seeding rate in combination with root pruning in spring or?3?defoliation in spring.The increases in wheat yield or net income and water-use efficiency through above three ways is related to improve population structure,water use,and apparent translocation amount and ratio of stem water soluble carbohydrates. |
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