Responses Of Dryland Winter Wheat Yield Formation And Soil Carbon And Nitrogen Pools To Topdressing Under Straw Mulching

With the development of the mulching management in rainfed field,numerous studies have reported that straw mulching could not increase winter wheat yield,and even decrease wheat yield in recent years.The high C/N ratio of wheat straw tends to lead to the lack of soil nutrient availability during the decomposition process.Soil nitrogen deficiency in the later stages of crop growth may be the main reason for the reduction of wheat yield.Based on a 10-year straw mulching field experiment,we tested the 2-year?2018 and 2019?effect of late topdressing on wheat yield and its components,soil carbon and nitrogen dynamic change process during the growing stages and wheat photosynthetic characteristics during flowering-filling period.The purpose is to deeply explore the stable yield mechanism of winter wheat under straw mulching in dryland.Four treatment included as no mulching?CK?,high straw mulching at a rate of 9000 kg·hm^-2 during growing season?HSM?,low straw mulching at a rate of 4500 kg·hm^-2 during growing season?LSM?and straw mulching at a rate of 9000 kg·hm^-2 during summer fallow season?FSM?.Topdressing 50kg·hm^-2 during the flowering-grouting periods.The main results of the study are as follows:?1?After long straw mulching experiment,soil organic carbon content was in a dynamic balance state,In the past two years soil total nitrogen content decreased by 15.61%compared with 10-year straw mulching experiment.In the past two years,the total soil nitrogen has decreased by 15.61%compared with 10-year long straw mulching experiment.The content of nitrate nitrogen and ammonium nitrogen in soil decreased significantly during the flowering-grouting periods,and the nitrogen utilization rate of wheat was significantly weaker.Soil organic carbon and total nitrogen increased by 12.98%and25.08%respectively in the 0-20 cm soil layer after topdressing in the late growing period.Soil organic carbon and total nitrogen with half amount of straw mulching increased the most compared with no mulching.During the late growing stages,soil ammonium nitrogen and nitrate nitrogen content under topdressing increased by 17.08%and 58.40%when compared with no topdressing in the 0-40 cm soil layer.Under topdressing,the effect of full straw mulching on increasing soil ammonium nitrogen and nitrate nitrogen is the most obvious when compared with other treatments.Adequate nitrogen supply improved the transformation of nitrogen between soil and crops,and provided guarantee for increasing grain yield.?2?Under without topdressing,long-term straw mulching significantly decreased the photosynthetic parameters of wheat during the flowering-filling period,and the photosynthetic rate under straw mulching during the growth period decreased by 15.07%compared with no mulching.Transpiration rate and stomatal conductance under straw mulching were decreased by 8.93%and 3.41%compared with no mulching treatment,respectively.The intercellular CO₂ concentration was greater with straw mulching than no mulching,increased by 2.17%.The application of nitrogen fertilizer in the later growth period significantly enhanced wheat photosynthesis,and the photosynthesis rate of the full straw mulching treatment was 23.61%higher than no mulching.The transpiration rate and stomatal conductance of the whole straw mulching treatment were 33.30%and 20.74%higher than no mulching treatment,and the utilization rate of intercellular CO₂ was improved.Straw mulching combined with topdressing optimized the daily variation curve of photosynthetic characteristic parameters.Topdressing increased the maximum photosynthetic rate in all treatments,but did not change the time of peak photosynthetic rate.The combination of full straw mulching and topdressing stimulated the emergence of the second peak of transpiration rate and stomatal conductance.?3?Under without topdressing,the grain yield decreased by 8.14%on average in the past2-years?2017-2019?compared with 10 years straw mulching experiment.The lack of soil nitrogen content under straw mulching would result in a decrease in crop yield.In addition to the seedling number,the other yield components also declined to different degrees in the past 2-years?2017-2019?compared with 10 years straw mulching experiment.After the supplemental application of nitrogen fertilizer in the later growth period,the straw mulching increased wheat yield when compared no mulching.The plant height,biomass,and 1000-grain weight were significantly improved.The yield and harvest index of the whole straw mulching were 30.27%and 1.54%higher than that of no mulching treatment.In conclusion,Straw mulching combined with topdressing increased the carbon and nitrogen content in the soil,improved soil fertility,and improved crop nitrogen utilization efficiency.The application of nitrogen fertilizer in the later growth period effectively improved the photosynthetic characteristics of winter wheat and increased the photosynthetic rate of winter wheat during the flowering-filling period.Straw mulching combined with topdressing treatment increased winter wheat yield and yield components to varying degrees,achieving the effect of stabilizing and increasing winter wheat yield.