Effect Of Different Slow-release Urea Fertilization Patterns On Yield Of Winter Rapeseed And Soil Carbon And Nitrogen Transformations

Winter rapeseed（Brassica napus L.）has a high nitrogen（N）demand,and N fertilizer application is the most direct and effective way to improve its yield and economic benefits.However,the phenological period of winter rapeseed is longer,and the traditional urea with one-time base application could not meet the N demand of winter rapeseed in the whole growth period.Moreover,excessive inputs of N fertilizers will increase N losses,reduce the N use efficiency,inducing a direct negative impact on the the entire agroecological environment.Slow-release urea basal application has been used to improve crop yield and nitrogen use efficiency.However,little is known about the mechanism of plant-soil carbon and nitrogen transformation in winter rapeseed under different slow-release urea regimes.A Field experiment about the effects of zero N application（N0）,conventional urea at high-N rate（C100）,slow-release urea at high-N rate（SR100）,slow-release urea at medium-N rate（SR80）,slow-release urea at low-N rate（SR60）,slow-release urea combined conventional urea at high-N rate（SRC100）,slow-release urea combined conventional urea at medium-N rate（SRC80）,and slow-release urea combined conventional urea at low-N rate（SRC60）,slow-release urea combined organic fertilizer at high-N（SRO100）,slow-release urea combined organic fertilizer at medium-N rate（SRO80）,and slow-release urea combined organic fertilizer at low-N rate（SRO60）on the yield of winter rapeseed and the soil carbon（C）and nitrogen transformations was carried out in two winter rapeseed planting seasons from 2016 to 2018.The effects of different fertilization treatments on growth and photosynthetic characteristics,nitrogen uptake,nitrogen use efficiency,soil carbon and nitrogen content,enzyme characteristics and soil microbial functional diversity were investigated.The major constraints for soil carbon emission and yield formation were analyzed,and the fertilization effects of different slow-release urea fertilization regimes were comprehensively evaluated to achieve a effective N management for high yield and N use efficiency of winter rapeseed,so as to provide theoretical basis for scientific fertilization of winter rapeseed in hilly areas of Southwest China.The main conclusions are as follows:（1）Compared with N0 treatment,N fertilization significantly increased leaf area index（LAI）,dry matter weight and root morphology.The values of LAI were not significantly different between high-N treatments and medium-N treatments,but the values of LAI of high-N treatments were significantly lower than those of low-N treatments（P<0.05）.Relative to conventional urea,slow-release urea applications significantly improved the growth parameters of winter rapeseed.SRO100 and SRO80 treatments significantly improved the root morphology,in comparison to conventional urea in two continuous cropping seasons of winter rapeseed.The contents of Chla,chlb and Chl in N0 and low nitrogen treatments were significantly lower than those in high-and medium-N treatments.N0 and low-N treatments were not conducive to maintaining higher chlorophyll content in leaves,which reduced net photosynthetic rate,water use efficiency and chlorophyll fluorescence parameters.Unlike other photosynthetic parameters,the non photochemical quenching coefficient（NPQ）and photosynthetic nitrogen-use efficiency（PNUE）decreased with increasing N supply.The results of correlation analysis showed that the leaf N content was significantly negatively correlated with NPQ and PNUE,however,leaf N content was significantly positively correlated with LAI and chlorophyll content（P<0.01）.Further,NPQ,photochemical quenching coefficient（q P）,PSII potential photochemical efficiency（F_v/F_o）,the ratio of regulated energy dissipation yield to non-regulated energy dissipation yield（Y（NPQ）/Y（NO））had higher correlation coefficients with leaf N content.（2）In accordance with dry matter weight,N accumulation of winter rapeseed increased with the increase of N fertilizer application.Compared with the high-N treatments,decreased N accumulations of winter rapeseed were observed under medium-N treatments in 2016-2017.Medium-N treatments significantly improved the N-use efficiency of winter rapeseed as a result of the reduction in fertilizer application,.The nitrogen uptake efficiency（NUp E）of SRO80 treatment was the highest,which was 24.72%and 29.78%higher than SRO100 and C100 treatment in2016-2017,respectively.The recovery efficiency of applied N（RE_N）of SRO80 treatment was59.74%higher than that of conventional urea treatment.SRO60 treatment significantly increased partial factor productivity from applied N（PFP_N）,compared with conventional urea treatment.However,as the soil N mineralization rate was affected by seasonal drought in 2017-2018,the contents of total nitrogen and mineral nitrogen in soil decreased,that is the content of available nitrogen of plants decreased,leading to the N accumulation decreased considerably.The N accumulations of winter rapeseed were the highest under high nitrogen treatments,and reducing fertilizer applications did not significantly improve N-use efficiency of winter rapeseed.However,the accumulations of nitrate nitrogen（NO₃^--N）in 40～60 cm soil layer and apparent N loss were significantly reduced under medium-and low-N treatments.Compared with conventional urea treatment,SR100 and SRO100 treatments could effectively reduce NO₃^--N accumulations in 40～60 cm soil layer and apparent N loss.The contents of total nitrogen and NO₃^--N accumulation in 0～60 cm soil layer under high nitrogen treatments were higher than those in low-N treatments,while total nitrogen and mineral nitrogen in 40～60 cm soil layer was higher in the treatment with conventional urea.There was no measurable effect on total nitrogen and ammonium nitrogen in most cases between different N fertilization treatments,while NO₃^--N accumulation was significantly affected by N applications（P<0.05）.Additionally,the content of NO₃^--N in soil was closely related to N accumulation and N-use efficiency of winter rapeseed.Besides,the activities of enzymes involving in soil N transformation also significantly affected nitrogen uptake and nitrogen use efficiency of winter rapeseed.The activities of soil Urease,nitrate reductase and nitrite reductase decreased with the decrease of N fertilizer application.Soil Urease activity was greatly affected by different fertilization treatments.The activities of soil Urease in SRO100 and SRO80 treatments increased by 22.37%and 28.05%,respectively,compared with conventional urea in 2016-2017.During the growth season of 2017-2018,the Urease activity of SRC100 treatment was significantly higher than those of conventional urea and SR100 treatments,and the Urease activity of SRC100 treatment increased by 18.79%,in contrast to conventional urea treatment.（3）Compared with N0 treatment,the total soil organic carbon content（SOC）in stem elongation stage increased by 5.16%-16.90%（2016-2017）and 13.57%-37.96%（2017-2018）,respectively.At the stem elongation stages of the two planting seasons,the contents of soil readily oxidized organic carbon（ROC）and water-soluble organic carbon（WSOC）were significantly affected by different N fertilization treatments（P<0.05）.The contents of SOC,ROC and WSOC were lower under low-N treatments,while the contents of soil organic carbon components were the highest in SRO100 plots.Before flowering stage of 2016-2017,compared with N0 treatment,high-N fertilization treatments significantly increased soil CO₂ emissions.However,N0 treatment increased significantly in seed filling and maturation stages,while high-N fertilization treatments decreased.There was no significant difference in total CO₂ emission during the whole growth season of winter rapeseed under different fertilization treatments in 2016-2017.However,the soil CO₂ emissions were higher under N0 and low-N treatments at each growth stage of winter rapeseed in 2017-2018.SRC100 treatment is an effective N management regime to improve soil C sequestration and mitigate the CO₂ emission.The soil CO₂ emissions showed obvious seasonal variation trends during the whole growth period of winter rapeseed in both two planting seasons.Soil CO₂ emissions increased significantly at stem elongation and flowering stages,and there were significant differences in soil CO₂ emission between different N fertilization treatments.There was a significant negative correlation between plant dry matter weight and soil CO₂ emission（P<0.01）.Additionally,there was a significant positive correlation between soil CO₂ emission and soil temperature（P<0.01）,and the relationship between soil CO₂ emission and soil temperature was best simulated by binomial fitting.SOC,WSOC and ROC contents were significantly negatively correlated with soil CO₂ emission（P<0.01）.Soil CO₂ emission was significantly positively correlated with soil microbial diversity indexes,the utilization intensity of carboxylic acids,polymers and phenolic acids,and activities of soil amylase andβ-glucosidase（P<0.01）.The activities of key enzymes involved in C cycle were higher under high-and medium-N treatments.SRO100 and SRO80 treatments increased the activities of soil amylase,invertase,polyphenolase,α-glucosidase andβ-glucosidase.The application of slow-release urea and organic fertilizer（SRO100 and SRO80）achieved high soil diversity index in the continuous winter rapeseed planting season,and the utilization intensity of degradable carbon sources（such as carbohydrates and carboxylic acids）was higher.On the contrary,N0 treatment and single nitrogen treatment could not maintain high soil microbial carbon source utilization capacity in continuous two planting seasons of winter rapeseed.Soil C and N contents accounted for 23.8%and35.6%of the variation of microbial functional diversity and soil enzyme activities in 2016-2017,32.8%and 10.4%in the stem elongation stage and maturation stage in 2017-2018,respectively.（4）There was no significant difference in the yield and yield components of winter rapeseed between high-N and medium-N treatments,but high-N treatments were significantly higher than those of low-N and N0 treatments.Compared with 2016-2017,the yield and yield components of winter rapeseed in each treatment decreased in 2017-2018.From 2016 to 2017,N fertilization treatments significantly increased the number of pods per plant.SR100,SRO100 and SRO80treatments had the higher number of pods per plant,which was significantly higher than that of SRC100 treatment,but there was no significant difference between SRC80 and SR100.However,in2017-2018,there was no significant difference in the actual yield of winter rapeseed between all high-N fertilization treatments.Moreover,the actual yield and pod number per plant of SRO60treatment was the lowest,which were significantly lower than those of SR60 treatment.From 2016to 2017,although the SR100 treatment increased the fertilizer input,the net income was the highest(9702 yuan·hm^-2)due to the higher yield,which was 5.74%higher than that of conventional urea treatment.However,SRO100 application has a narrow advantage in increasing net income because of its higher fertilizer input.For SRC and SRO treatment,since the fertilizer input of medium-N treatments decreased,the net income of medium-N treatments increased by 24.54%and 17.71%,in contrast to high-N treatments,respectively.In 2017-2018,due to the impact of seasonal drought,yield and net income decreased.SRC100 treatment had the highest net income,which was 28.75%higher than that of conventional urea treatment.Compared with the SR treatment,the slow-release urea combined with conventional urea（SRC）was beneficial to increase the net income of winter rapeseed because of the lower proportion of slow-release urea input.The oil content of seed decreased with the increase of fertilizer application,,but the opposite occurred in protein content.Compared with N0 treatment,the protein content was significantly increased by N fertilization.There was no significant difference in protein content between high-N treatments and medium-N treatments（P>0.05）.The contents of erucic acid and glucosinolate were significantly affected by different N fertilization treatments（P<0.05）.,and the contents of erucic acid and glucosinolate were higher in N0 and low-N treatments.Therefore,the optimized fertilization measures could effectively improve the seed quality of winter rapeseed.Among the growth indexes,leaf area index,root surface area and root tips number accounted for the highest proportion of total yield variation（R²=0.813）.Among the photosynthetic parameters,the content of chlorophyll a,q P,F_v/F_o and NPQ accounted for the highest proportion of total yield variation（R²=0.767）.The yield of winter rapeseed was highly correlated with the activities of S-β-GC,S-AL and Urease,soil nitrate nitrogen content,readily degradable organic carbon content,soil microbial diversity index U and utilization intensity of polymer.The results of grey correlation analysis and principal component analysis showed that:SRO100 treatment ranked second according to the soil comprehensive fertilization score in 2016-2017,and in 2017-2018,SRO100treatment ranked first.Therefore,slow-release urea plus organic fertilizer at high-N rate（SRO100）is the best fertilization pattern to continuously improve the yield of winter rapeseed and soil fertility.