Effects Of Plastic Film Mulching And Straw Retention On Fertilizer Nitrogen Fate And Budget In Dry-land Winter Wheat

Nitrogen is one of the essential nutrients for plant growth,and it is also one of the elements that are deficiency in farmland ecosystems.Nitrogen fertilization is the most effective way to obtain high grain yield.At present,the problems of agronomy,environment and economic disadvantages caused by imbalanced fertilization have become increasingly prominent.Global climate change has led to the limited water and soil resources and low nitrogen use efficiency at dry farming areas of the Loess Plateau.Therefore,mitigating the losses of N fertilizers in farmland,improving nitrogen fertilizer utilization,and sustaining the development of farmland ecological environment have become the main scientific and practical problems which urgently need to be solved in Northwest China.Thus,we carried out a research on the winter wheat-summer fallow cropping system,and used the isotopic,nitrogen budget and structural equation modeling to systematic investigate the influence on N crop uptake,soil residue,and nitrogen loss under different tillage and nitrogen fertilizer rate.This work aimed at finding out the main loss ways of fertilizer nitrogen about winter wheat fields,and providing theoretical and technical support to reduce the losses of N fertilizer in farmland and improve the utilization of nitrogen fertilizer.Field experiment was conducted at“Cao Xinzhuang experimental farm”of the Northwest Agricultural and Forestry University,Yangling,Shannxi Provence in 2017?2021wheat growing seasons.The field experiment was a split-plot design,where combinations of three tillage systems(conventional tillage with residue removed,CT;plastic film mulching,PFM;straw residue incorporated into soil,SR)as main plot and three N application rates(0kg hm^-2,N0;144 kg hm^-2,N144;180 kg hm^-2,N180)as sub plot were applied.The fate of N fertilizer and nitrogen use efficiency(NUE)of winter wheat under different conditions were studied using stable isotope labeling.The nitrogen reduction potential of dryland winter wheat based on yield and farmland nitrogen budget was explored.The farmland ammonia volatilization,nitrous oxide emissions and the distribution of N fertilizer in the 0?300 cm soil profile were tested to explore the ways and amounts of N fertilizer loss.The structural equation model was used to investigate the effects of soil water,heat conditions and cultivation on the fate of N fertilizer and explore the mechanism of N fertilizer loss.The main results were:1.The average of grain yield,grain number and spike density in PFM and SR treatments during four wheat seasons were 8.95%?12.57%,2.45%?3.38%and 0.46%?5.49%higher than CT,and the coefficient of variation was also lower than CT.The application of nitrogen fertilizer can significantly increase the yield of winter wheat.There had no significant difference between 144 kg hm^-2(reduced nitrogen treatment)and 180 kg hm^-2(conventional fertilization)treatments in grain yield,grain number,spike density,and thousand grain weight,which indicated that the local winter wheat field has a certain potential for nitrogen reduction.2.PFM could increase the mineral nitrogen content of the cultivated soil,increase the nitrogen absorption,nitrate reductase and glutamine synthetase activities of winter wheat at various growth stages,and increase the apparent utilization of fertilizer nitrogen.While,SR increased the biological fixation of fertilizer nitrogen(especially the first season),reduced the mineral nitrogen content of the plow layer,and reduced the apparent utilization rate of fertilizer nitrogen.3.The effects of nitrogen application and tillage on soil labile nitrogen pool were mainly in the 0?20 cm soil layer at the pre-winter tillering stage,and with the increased of soil layer depth,the nitrogen pool content gradually decreased.Compared with CT,SR treatment in0?20 cm soil layer could significantly increase the microbial biomass nitrogen,and promote the immobilization.SR also reduced the nitrate nitrogen in the 0?60 cm soil layer at the reviving and jointing stage(critical period),which limited the crops absorption of mineral nitrogen.PFM could significantly increase soil water content and soil temperature,stimulate soil microbial activity.PFM also promoted the activity of nitrogen assimilation enzymes in plant leaves,which was beneficial to the growth of crops and the crop absorption of mineral nitrogen.4.Soil ammonia volatilization and nitrous oxide emissions in winter wheat fields mainly occured in one month after fertilization,and could reach peak within two weeks after fertilization.Tillage and nitrogen application had significant effects on ammonia volatilization and nitrous oxide.Soil ammonia volatilization was directly affected by soil ammonium nitrogen content,controlled by urea hydrolysis.While nitrous oxide emissions was affected by soil water content and soil nitrate nitrogen content,controlled by nitrification and denitrification.The flux and cumulation of ammonia volatilization and nitrous oxide emission loss were related to precipitation and temperature which increasd with the nitrogen input rate.Compared with CT,PFM significantly reduced the amount of soil ammonia volatilization accumulation and fertilizer nitrogen loss rate,but increased the nitrous oxide emissions potential.SR treatments increased the proliferation of microorganisms,caused the trade-off effect which effected the soil ammonia volatilization and nitrous oxide emissions had not significant different between CT.5.Crop uptake and soil residue are the main fate of nitrogen fertilizer in rain-fed winter wheat fields at Loess Plateau,the tillage would profound impact on it.The proportions of plant absorption,soil residue,ammonia volatilization and unknown fertilizer nitrogen fate in the first season were 33.92%?46.32%,25.90%?41.18%,2.27%?3.14%and 12.15%?28.28%,respectively.After three consecutive seasons of winter wheat planting,the cumulative nitrogen utilization rate could reach 44.00%?54.43%.Meanwhile,11.04%?26.89%of fertilizer nitrogen remained in the 0?300 cm soil profile,and ¹⁵N was not detected in the ammonia volatilization at following two seasons.Compared with CT,PFM treatment could reduce ammonia volatilization,soil nitrate nitrogen leaching and proportion of unknown nitrogen,increase nitrogen residues in the soil and NUE.The SR treatment increased the N fertilizer residual in the soil,while reduced the N uptake of plants and NUE.The residual¹⁵N component in the soil was mainly as organic nitrogen.The ¹⁵N isotope showd that ammonia volatilization only accounted for a small part of the nitrogen fertilizer loss in farmland,and about 20%of the fertilizer nitrogen was still unknown.From the distribution of nitrate nitrogen in the soil profile,deep nitrate nitrogen leaching may be the main loss way of fertilizer nitrogen.6.Tillage and fertilization treatments could significantly affect the nitrogen budget.The research showed that the N deficiency of the treatments without nitrogen application was91.4?186.2 kg hm^-2,especially the PFM treatment.However,there was N surplus in fertilization treats(N144 and N180).The more N was input,the more N was surplus.There were 126.1?169.1 kg hm^-2 N surpluses in the N144 and N180 treatments,respectively in SR treatment.In conclution,at Loess Plateau crop absorption and soil residue are the main fate of fertilizer nitrogen.Soil ammonia volatilization was not the main loss way of fertilizer nitrogen,while nitrate nitrogen leaching might be.It has 20%nitrogen reduction potential.PFM has the effect of increasing absorption and nitrogen use efficiency,reducing ammonia volatilization and nitrate nitrogen leaching of fertilizer nitrogen.Straw residue incorporated into soil for long time could increase the residue of fertilizer and the nitrogen surplus of farmland,which was beneficial to increase the soil organic matter and total nitrogen content.However,it should be paid attention to the nitrogen immobilization processes effect which would be influenced on yield.