The Difference And Mechanism Of Nitrogen Use Efficiency Under Winter Wheat-Summer Maize Cropping System On Different Soil Fertility On Loess Soil

Soil fertility level has a significant impact on nitrogen use efficiency?NUE?.Therefore,it is of paramount importance to fully understand the mechanism responsible for the effect,on which optimizing N application rates through improving soil fertility could be based.As a result,we can improve NUE,decrease nitrogen losses and eventually achieve the desired yields of crops.In this thesis,we studied the effects and the mechanisms of soil fertility level on NUE by conducting a ¹⁵N micro-plot experiment and a short-term field experiment under winter wheat-summer maize cropping system on Loess soil.The ¹⁵N micro-plot experiment was composed of 5 sets of soils?F1,F2,F3,F4,F5?varying in fertility levels,which were established from long-term fertilization experiment treatments,and each of the soil receiving5 rates of N?N0,N1,N2,N3,N4?.We studied the effect of soil fertility on crop yields and NUE through measuring crop yields,total N and ¹⁵N uptakes in aboveground biomass and in such tissues as shoots and leaves.We examined the difference of nitrogen residues in soils under different fertility levels by testing soil mineral N?NO₃^--N and NH₄⁺-N?in 0-100 soil profiles and the contents of soil fixed-N and the fractions of organic nitrogen in 0-20 cm soil;We further studied the effect of ¹⁵N-labeled fertilizer applied on the subsequent crops by testing the amount and form of residual ¹⁵N in soil after three crops harvest.In addition,a short-term experiment was conducted to study the optimum N application rate based on achieve high crop production,high nitrogen use efficiency and low risk of environmental pollution through measuring crop yields,aboveground N uptake and NO₃^--N content in 0-200cm soil after crop harvest.The main results are as follow:1.There was no significant effect of N rate on crop yields on the high fertility soil,while the application of nitrogen significantly increased the grain yield of winter wheat by 31-287%and summer maize by 58-340%in low fertility soil.Under the same soil fertility,aboveground N uptake was increased with increasing of N rate while there was opposite for NUE;under the same N rate,aboveground N uptake and NUE both increased with increasing of N rate.Multivariate regression analysis between soil organic matter,N rate and crop yield or NUE showed that the effect of soil organic matter on crop yields and NUE both more than N rate,it indicated that improving soil fertility could both increase crop yields and NUE.Soil total N content was significantly increased with soil fertility improvement.Mineral N?NO₃^--N and NH₄⁺-N?and soil organic nitrogen both significantly increased,while there was a linear-plateau relationship between soil total N and fixed N:fixed N would not be further increased when soil total N arrived at 1.12 g kg^-1.Organic-N is main form in soil.The content of different fractions of organic nitrogen was significantly increased with soil fertility improvement,among which the biggest ranges of amino acid N,and hydrolysable N and non-hydrolysable N as follow.Principal component analysis showed that amino acid N is the major contributor to the soil mineralized nitrogen,improve soil fertility level is an efficient way to enhance the nitrogen supplying capacity of soil.In addition,the content of soil mineralized N was significantly increased with soil C/N ratio increased when C:N ranged from 7.5-10.0,it was indicated that high soil fertility could improve nutrient cycling and nitrogen supply potential.2.The results of ¹⁵N micro-plot experiment showed that the aboveground ¹⁵N uptake in wheat season ranged from 26.97-93.90 kg ha^-1,and grain account for 67-90%,which the value was decreased with soil fertility improvement.The ¹⁵NUE in wheat season ranged from 26-54%,which was increased with soil fertility improvement.The rate of residual ¹⁵N in 0-20 cm soil ranged from 16-42%,among which organic ¹⁵N,mineral ¹⁵N and fixed-¹⁵N account for58-64%,34-40%and 1-3%,respectively.The residual ¹⁵N in 0-100 cm soil ranged from 20-44%,which was decreased with N rate increased and increased with soil fertility improvement.Unaccounted ¹⁵N fertilizer under different treatments ranged from 14-48%,which was increased with N rate increased and decreased with soil fertility improvement.There was a linear-plateau relationship between ¹⁵NUE and soil organic matter,¹⁵NUE achieved maximum value at each N rate,which was decreased with N rate increased at maximum ¹⁵NUE,when soil organic matter arrived 19.0-21.0 g kg^-1.Unaccounted ¹⁵N fertilizer at each N rate significantly decreased with soil organic matter increased?P<0.05?.it was indicated that improving soil fertility could increase fertilizer N residues in soil and improve crop N uptake,reduces potential loss.3.The residual ¹⁵N fertilizer in soil after wheat harvest still available for subsequent crop seasons.Aboveground ¹⁵N uptake in second crop season?maize?and third crop season?wheat?ranged from 1.69-6.27 kg ha^-1 and 0.34-1.95 kg ha^-1,residual ¹⁵N use efficiency(R¹⁵NUE)ranged from 5-17%and 1-5%,respectively,both increased with soil fertility improvement.Total ¹⁵NUE in three crop seasons under different treatments ranged from 28-58%.Residual fixed-¹⁵N in soil after wheat harvest have been partly released during the subsequent crop seasons,the released amount was decreased with soil K⁺content increased.There still 12-31%residual ¹⁵N fertilizer in soil after the third crop harvest,which the rate of residual ¹⁵N was decreased with N rate increased and increased with soil fertility improvement.Unaccounted ¹⁵N fertilizer account for 22-53%after the third crop harvest,which the rate was increased with N rate increased and decreased with soil fertility improvement.The results showed that improve soil fertility could enhance the availability of residual ¹⁵N fertilizer to subsequent crop seasons in order to increase the total ¹⁵NUE.4.The results of short-term experiment showed that there was linear-plateau relationship between crop yields and N rate,the agronomy optimum N rate?AON?for wheat and maize ranged from 116-124 kg ha^-1and 112-180 kg ha^-1,the corresponding maximum yield ranged from 4366 to 5199 kg ha^-1 and 6945-8238 kg ha^-1,respectively.The result of N input-output in winter wheat-summer maize cropping system showed that the high yield production and NUE can be realized when N application rate low than 210 kg N ha^-1 for wheat,225 kg N ha^-1for maize,and 435 kg N ha^-1 for wheat combine maize season in low fertility soil.In middle and high fertility soil,however,more than 53%of the treatments with NUE more than90%.NO₃^--N content in 0-200 soil increasing with N application rate increased.Plateau-linear model was employed to fitted the relationship between soil NO₃^--N content and N rate,the result showed that the soil NO₃^--N buffer capacity in 0-100 cm soil for wheat and maize ranged from 12-91 kg ha^-1and 7-63 kg ha^-1,respectively.Correlation analysis showed that soil NO₃^--N buffer capacity was significant positive with soil organic matter in wheat season and significant positive with NO₃^--N content before planting in maize season,additionally,the environmental safety N rate?ESN?under different soil fertility ranged from 81-170 kg ha^-1 for wheat and 67-148 kg ha^-1 for maize,respectively.in general,ESN should be given priority in the optimization of N rate under different soil fertility.In summary,improving soil fertility is an effective way to increase crop yield and NUE meanwhile decrease potential N loss under winter wheat-summer maize cropping system in Shaanxi province.However,the threshold of NUE changes with soil organic matter ranged from 19.0 to 21.0 g kg^-1.Additionally,ESN should be given priority in the optimal N rate in this area.