Effects Of Fertilization Depth On Water And Fertilizer Utilization And Nitrogen Balance In Upland Maize Field

High-yield,high-efficiency as well as green and sustainable production are not only the important goals for the agricultural development in China,but also an inevitable requirement to ensure national food security and agro-ecological security.At present,fertilization is still one of the main measurements of the formation and improvement of farmland crop yield in China.However,the inappropriate fertilizer application methods,on one hand,have increased the greenhouse gas emission and on the other hand,have restricted the improvement of the crop yield and fertilizer use efficiency.How to both achieve the goals of food production and environmental protection has become an important challenge in current agricultural production under this background.Therefore,this study was started with the depth of fertilization and took maize as the research object to explore the synergistic effect of different fertilization depths on the water and fertilizer utilization as well as greenhouse gas emission reduction in maize field.In this study,four fertilization depth treatments of 5 cm（D5）,15 cm（D15）,25 cm（D25）and 35 cm（D35）were set up,and field experiment were conducted in Yangling,Shaanxi Province for two consecutive years（2019-2020）.The effects of different fertilization depths on maize root distribution,leaf senescence,photosynthetic performance,grain filling,maize yield and soil gaseous nitrogen loss（NH₃ and N₂O）were analyzed.The research results can provide theoretical reference for optimizing maize fertilization management in dry farming areas.The main findings of this study are as follows:（1）The regulatory effect of fertilization depth on maize root development was proved.Optimizing the fertilization depth changed the maize root structure and promoted the growth of lateral roots,which had a significant regulatory effect on the growth and development of the root system.D25 had the strongest regulatory effect,followed by D15,but D35 had no obvious regulatory effect.Compared with D5,the total root length under D25 at the vertical,inter-row and within-row location（0-100 cm soil layer）of maize was significantly increased by 20.22%,14.49%and 13.73%,respectively,and the total surface area was significantly increased by 25.67%,14.65%%and 17.97%,respectively.Meanwhile,compared with D5,D25 significantly increased the root length density and root surface area density in the 0-100 cm soil layer,which was followed by D15.The optimization the fertilization depth significantly improved the ability of roots in transporting nutrients to the shoot after maize silking stage.Compared with D5,D25 significantly increased the root bleeding sap rate as well as the nitrate and ammonium nitrogen concentrations in the root bleeding sap by 15.66%,29.78%and 38.72%,respectively,during the silking,grain filling and maturity stage.（2）The physiological mechanism of optimizing fertilization depth to delay leaf senescence and promote grain filling and yield formation was elucidated.The antioxidant and photosynthetic properties of leaves were D25>D15>D5≈D35.Compared with D5,D25 observably reduced the accumulation of reactive oxygen species in leaves after maize silking stage,and enhanced the antioxidant capacity and photosynthetic performance.Specifically speaking,the content of hydrogen peroxide（H₂O₂）and superoxide radical（O₂^-）under D25 were significantly reduced by 21.50%and 13.98%,the superoxide dismutase（SOD）activity and net photosynthetic rate（Pn）increased significantly by 16.73%and 32.89%,while the content of malondialdehyde（MDA）and the concentration of intercellular CO₂（Ci）decreased significantly decreased by 11.10%and 30.71%.The optimization the fertilization depth significantly increased the biomass after maize silking stage and increased the dry matter distribution ratio of grains.The biomass of D25 was significantly higher than that of D5 by 12.65%.Compared with D5,the average and maximum grain filling rates under D25 were significantly increased by 4.96%and 6.37%,respectively.The yield of D25 was significantly increased by 14.00%and 13.66%compared with D5 in 2019 and 2020.D15 comes the second,and there was no significant difference between D35 and D5.Therefore,the optimization of fertilization depth can improve leaf photosynthetic performance by delaying leaf senescence,in order to promote dry matter accumulation and grain filling to increase maize yield.（3）The response characteristics of soil water distribution and utilization to fertilization depth were elucidated.The appropriate increase of fertilization depth affected the distribution and utilization of soil water in maize fields.D15 and D25 mainly reduced the soil water content in 0-60 cm and 40-140 cm at jointing and silking stage,respectively.Compared with D5 and D35,D15and D25 increased the soil water consumption in the whole growth period.The soil water consumption amount under D15 in the seeding stage to joining stage and D25 in the jointing stage to silking stage increased by 11.13-16.52%and 18.07%-31.51%,compared with other treatments.There was no significant difference in water productivity between D5 and D35during the silking to maturity stage,and D25 dramatically increased by 43.29%and 39.64%regarding to the previous ones,and D15 followed.From the perspective of the whole growth period,approximately increasing the fertilization depth to 25 cm on the basis of D5 can significantly improve the water use efficiency（WUE,11.89%）and water productivity（WP,10.40%）.The increasing rate of D15 is inferior to that of D25,and there was no significant difference between D5 and D35.（4）The function regulation of optimizing fertilization depth to drive plant nutrient accumulation and efficient utilization of nitrogen and phosphorus was elucidated.With the increase of fertilization depth,the nutrient absorption and fertilizer use efficiency of maize plants increased first and then decreased.The appropriate fertilization depth improved the nutrient transport capacity of nutritive organs and the nutrient assimilation capacity after silking,which significantly improved the distribution ratio of nitrogen and phosphorus in grains at maize maturity stage.D25 had the strongest nutrient absorption and utilization ability,followed by D15,and there was no significant difference between D35 and D5.Compared with D5,D25 significantly increased nitrogen and phosphorus accumulation at maturity by 17.41%and 43.42%,respectively.D25 had the highest nitrogen use efficiency（43.63%）and phosphorus use efficiency（22.02%）,which were significantly higher than those of D5 by 38.37%and 59.96%,respectively.（5）The effect of deep placement fertilizer on reducing the loss of gaseous nitrogen was elucidated.The cumulative diffusion flux of N₂O in the 0-20 cm soil layer under D5,D15,D25 and D35 accounted for 79.21%,73.73%,58.09%and 48.37%of that in the 0-40 cm soil depth,respectively.Deep placement fertilizer can reduce the production rate and diffusion flux of N₂O in the soil layer by reducing the NO₃^--N content in the 0-20 cm soil depth.The surface N₂O emission flux was significantly positive correlated with the N₂O diffusion flux and production rate in the 0-20 cm soil layer.Compared with D5,the total surface N₂O cumulative emissions under D25 and D35 were dramatically decreased by 30.84%and59.29%,and the N₂O-N emission factor were decreased by 41.64%and 80.05%,respectively.Therefore,deep placement fertilizers transferred the N₂O production position to deeper soils to reduce the surface N₂O emissions.Deep placement fertilizers reduced the NH₃volatilization rate by reducing the NH₄⁺-N content and urease activity,thereby it decreased the cumulative NH₃ volatilization amount.Compared with D5,the cumulative NH₃volatilization amount under D15,D25 and D35 was significantly decreased by 29.69%,43.82%and 54.95%,respectively.（6）The effect of fertilization depth on nitrogen footprint and soil nitrogen dynamic balance was verified.Deep placement fertilizer significantly decreased the cumulative reactive nitrogen loss and nitrogen footprint throughout the maize production process.Compared with D5,the cumulative reactive nitrogen loss under D15,D25 and D35 was significantly reduced by11.27%,17.32%and 20.97%,respectively,and the nitrogen footprint was significantly reduced by 16.04%,27.36%and 18.99%,and the nitrogen footprint of D25 was significantly lower than other treatments.Compared with D5,the cumulative nitrate nitrogen residues in the 0-100 cm soil layer after harvest under D15 and D25 decreased by 5.97%and 7.95%,while D35 increased by 3.90%.Appropriately increasing the fertilization depth to 15 and 25cm on the basis of D5 can significantly increase the cumulative target nitrogen output while significantly reducing nitrogen loss and promoting nitrogen balance.The cumulative nitrogen output under D25 was significantly higher than that under D15 and nitrogen loss is significantly reduced.There was no significant difference on the cumulative target nitrogen output under D35 and D5,but D35 significantly increased the nitrogen losses.In conclusion,deep placement fertilizer at 25 cm soil depth one-time before maize sowing promoted maize growth,delayed leaf senescence,promoted grain filling capability,improved yield and water and fertilizer use efficiency,and significantly reduced cumulative N₂O emissions and NH₃ volatilization,which reduced reactive nitrogen loss and nitrogen footprint.Therefore,appropriately increasing the fertilization depth to 25 cm is an effective fertilization strategy in the maize planting system in dry farming areas,which can both achieve the goals of crop production and environmental production and ensure the sustainable development for agricultural system.