Effects Of Increasing Plant Density And Decreasing Nitrogen Rate On Yield And Quality Of Cotton,and Physiological Mechanisms Of Nitrogen Efficient Utilization

Plant density and nitrogen?N?application are two important factors affecting cotton yield.However,with the adjustment of planting structure and the improvement of production level of cotton in China,there are great changes in dominant regions,main varieties,plant density and N application.At present,there is little known of different plant densities and N applications on cotton yield and quality in the cotton stubble return field.To achieve this goal,the study was conducted from 2016 to 2017 in Experimental Bases of Shandong Agricultural University.They are located in Zhenxing village,Chiping,and Huangheya village,Decheng of Shandong Province.The field experiment was conducted using Liaomian 6 with the treatments of three plant densities(5.25,6.75 and 8.25 myria plants ha^-1),and five nitrogen rates(0,105,210,315 and 420 kg ha^-1).The pot experiments were conducted simultaneously in 2016.The effects of different plant density and nitrogen rate on cotton yield and quality,nitrogen use efficiency,and their physiological basis were studied.In order to clarify the effects of plant density and nitrogen application rate on cotton growth and development,yield and quality,nitrogen transport,nitrogen fate and nitrogen use efficiency.Our objective was to provide theoretical basis and technical support for cotton high yield,excellent quality in the cotton area of the Yellow River valley.The results as follows:1.Effects of increasing plant density and decreasing nitrogen rate on dry matter and nitrogen accumulation,and distribution.With the increase of plant density and nitrogen rate,the accumulation of dry matter and nitrogen in different growth stages showed an upward trend.They had obtained the maximum value at D_8.25N₄₂₀.Squaring and blooming stage were the key periods of accumulation.The accumulation amount accounted for 79.24%and 83.69%,78.04%and 78.71%of the total accumulation,respectively.The distribution of dry matter and nitrogen accumulation to reproductive organs decreased,while D_8.25N₄₂₀ was the lowest.Compared with D_5.25N₃₁₅,the accumulation of dry matter and nitrogen increased form 12.98%to 19.82%,from 1.21%to5.18%,respectively in D_6.75N₂₁₀.The distribution amount of reproductive organs increased by9.77%¹5.12%,3.23%⁴.66%,respectively.Besides,the maximal speed of accumulation increased by 7.94%⁹.46%,6.94%⁸.94%,respectively.At the same time the starting date of rapid accumulation period for nitrogen was 4.10⁶.40 days earlier than for dry matter,the maximal speed of nitrogen accumulation appeared earlier by 1.40⁶.60 d,and the peak time of nitrogen absorption was 1.40⁶.60 d ahead of time.As could be seen that the nutrient absorption of cotton was earlier than dry matter accumulation.Our results indicated that nitrogen accumulation was the basis of dry matter accumulation.2 Effects of increasing plant density and decreasing nitrogen rate on yield and nitrogen use efficiency.The lint yields of D5.25N315,D6.75N210 and D6.75N105 in 2016 as well as D5.25N315 and D_6.75N₂₁₀ in 2017 were significantly higher than that of other treatments.The interaction effect of plant density and nitrogen application amount significantly affected cotton yield.Increasing plant density and reducing nitrogen application amount could obtain high yield.It is recommended for this region that the cotton plant density should be increased from 5.25 to6.75plants hm^-2,and the amount of nitrogen application decreased from 300 kg ha^-1to 105 kg ha^-11 in the first year,and then to 210 kg ha^-1 the next year.Nitrogen rate can be optimized if plant density is increased appropriately.The significant increase of boll number after nitrogen optimized is an important guarantee for cotton to maintain high yield.Regression analysis showed that when the plant density increased to D_6.75,the amount of nitrogen application decreased to 222 kg ha^-1 in 2016 and 231 kg ha^-1 in 2017.If 95%of the maximum yield was taken as the actual target,the amount of nitrogen application decreased to 100.57 kg ha^-11 and110.65 kg ha^-1 in two years.The interaction of plant density and nitrogen rate had a significant effect on nitrogen use efficiency.The NRE was significantly higher at D_6.75N₁₀₅and D_8.25N₁₀₅ than other treatments,and the maximal aNUE and pNUE were obtained under D6.75N105 treatment.The iNUE of D5.25N0,D6.75N0,D6.75N105 and D8.25N0 was significantly higher than that of other treatments.Compared with D_5.25N₃₁₅,the NRE increased significantly by 85.61%⁹1.16%,43.49%⁶4.07%at D_6.75N₁₀₅05 and D_6.75N₂₁₀.As well as the aNUE increased significantly by 86.75%¹20.97%,23.39%³6.49%at the two treatments.Our research showed that nitrogen fertilizer utilization was more efficient after increaseing plant density and decreaseing nitrogen rate.3 Effects of increasing plant density and decreasing nitrogen rate on spatial and temporal distribution of cotton bolls.In terms of time distribution,hot-boll was the main yield component.With the increase of plant density,the proportion of hot-boll decreased,while that of after hot-boll increased.The proportion of cotton bolls was hot-boll>before hot-boll>after hot-boll,and hot-boll accounted for more than 50%in 2016.However,the proportion of cotton bolls was before hot-boll>hot-boll>after hot-boll,the hot-boll accounted for more than 40%in 2017.The first two harvests of lint cotton accounted for 86.89%and 92.43%in two years.From the spatial distribution,the inner boll was the main part of yield,and the percentage of lint yield harvested by inner boll is 77.95%and 78.99%.The proportion of UFB_1² in 2016 and MFB_1²in 2017 is significantly higher than that of other parts.With the increase of nitrogen rate at D_5.25 and D_6.75,lint yields of LFB_{3^</sub>,MFB1²,MFB3^</sub>,and UFB1² increased first and then decreased.It had obtained maximum yield at D5.25N315,D6.75N105 and D6.75N210.This showed that nitrogen optimized could regulate the distribution of cotton bolls and the contribution rate of different parts of cotton bolls to yield.4 Effects of increasing plant density and decreasing nitrogen rate on cotton bolls fiber quality.In terms of time distribution,the fiber quality of cotton bolls harvested in the last two times was better than that of the first.Compared with D5.25N315,the fiber strength of D6.75N210increased significantly for the first time,decreased significantly for the third time,and did not change significantly for the second time in 2016.The other indicators did not change significantly for the three time harvests in 2016,and all indicators did not change significantly in 2017.From the spatial distribution,the fiber quality of cotton bolls from upper and middle branches was better than that from lower branches.The main part of the high quality bolls was in the inner of upper and middle branches.Compared with D5.25N315,there were no significant differences in the fiber quality of D6.75N10505 and D6.75N210.Under the high density and nitrogen application conditions,the fiber quality was decreased significantly.We discovered that nitrogen optimized had no significant effect on the fiber quality of cotton bolls.The climate conditions were the main reasons for the difference of cotton fiber quality during the two years.5 Effects of increasing plant density and decreasing nitrogen rate on physiological characteristics of functional leavesWith the increase of plant density and nitrogen rate,the leaf area index showed an upward trend.It obtained the maximum value at D8.25N420.Meanwhile,photosynthetic rate and SPAD value increased,and D8.25N315 and D8.25N42020 were significantly higher than other treatments.Compared with D5.25N315,there were no significant changes of leaf area index,photosynthetic rate and SPAD value at D6.75N210.At the later growth stage,there was a rapid decline in all indicators under N0.But the high density and nitrogen maintained higher leaf area index,photosynthetic rate and SPAD value,which indicated that nitrogen deficiency in later period would cause premature senescence of cotton plants,while excessive nitrogen would lead to late maturity.Compared with D5.25N315,NR and GS activities at D6.75N210 did not change significantly at full flowering stage,but increased significantly at late flowering stage.The results showed that nitrogen assimilation capacity of functional leaves increased after nitrogen optimized,which laid a foundation for nitrogen accumulation in later stage.6 Effects of increasing plant density and decreasing nitrogen rate on soil nitrate N contentSoil NO3^--N content in the 0⁶0 cm soil layer at harvest increased with increasing N application rates,but decreased with higher density in 2016 and 2017.No significant difference in soil NO3^--N content was found after applying N210 and N315 between 2016 and2017.Soil NO3^--N content decreased sharply after applying N0 or N105,while NO3^--N content was a large increase in the treatments with applied N420 compared with that in 2015 at all densities.The results showed that the inorganic nitrogen content of D6.75N210 could not only meet the needs of cotton plants,but also maintain the soil balance.7 Effect of nitrogen rate on ¹⁵N absorption and utilization,and fate of fertilizerWith the increase of nitrogen application rate,the amount of fertilizer ¹⁵N absorbed by cotton plants increased significantly,while the recovery rate of fertilizer ¹⁵N decreased.The amount of soil nitrogen absorbed by cotton plants increased,while the amount of nitrogen absorbed by cotton plants from soil?Ndfs?decreased.The increase of soil nitrogen absorbed by cotton plants under high nitrogen condition showed the effect of nitrogen stimulation.The proportion of nitrogen absorbed by cotton plants from soil was 66.35%-81.87%.Soil was the main and the largest source of nitrogen supply for cotton plants.Reducing the amount of nitrogen application could explore the potential of soil nitrogen supply and achieved high yield and efficiency.With the increase of nitrogen application rate,the fate of fertilizer ¹⁵N was different.The ¹⁵N residue and ¹⁵N loss were on the rise.The ¹⁵N residue and ¹⁵N loss rate were the biggest at N420.After topdressing,¹⁵N residue increased by 40.34%and 82.23%,¹⁵N loss increased by 15.69%and 73.92%,while the ¹⁵N residue rate decreased by 8.89%and29.83%,¹⁵N loss rate decreased by 13.04%and 42.16%,and the ¹⁵N absorption was accelerated in the middle and late growth stages.The recovery of ¹⁵N from cotton plant increased,while the residue rate and loss rate decreased.}