| Nitrogen(N)is the most important and limiting element in crop production and it is applied excessively to soils to meet the high food demand resulting from the rapid growth of the world’s population.Meanwhile,long-term excessive application of nitrogen fertilizers has caused many ecological and environmental problems and reduces crop yield and nitrogen use efficiency.Straw return could partly eliminate the negative effects of excessive nitrogen application.Therefore,the study was conducted in a long-field experiment started in 2013 with five-year different inorganic N rates(N0,no-N application;N75,75 kg ha-1;N150,150 kg ha1;N225,225 kg ha-1;N300,300 kg ha-1 and N375,375 kg ha-1)in Dingxi,Gansu province.Before planting of potato in 2018,four of the six nitrogen rates were selected and each plot was divided into two parts.Maize straw(S,9 t ha-1)was incorporated into one part,while the other part received no straw addition to assess the effects of nitrogen rate and straw return on potato production,soil physiochemical and carbon pool characteristics.The experiment included eight treatment combinations,namely no-N application(control,CK),no-N application with straw(CK+S),low-N application(LN,75 kg N ha-1),low-N application with straw(LN+S,75 kg N ha-1),medium-N application(MN,150 kg N ha-1),medium-N application with straw(MN+S,150 kg N ha-1),high-N application(HN,300 kg N ha-1)and high-N application with straw(HN+S,300 kg N ha-1).The treatments were arranged in a randomized complete block design with four replications.The key findings of the study are summarized below:1.Appropriate nitrogen fertilization rate and maize straw return significantly improved the growth,tuber yield,tuber quality,and nitrogen use efficiency(NUE)of potato.a)Among the N rates,the MN treatment produced the highest biological and economic tuber yield,profit margin,and nitrogen use efficiency of potato in both the 2018 and 2019 seasons.Compared to the CK,the MN treatment stimulated biological tuber yield(31.3%and 32.8%),economic tuber yield(36.9%and 35.0%),and profit margin(59.5%and 60.8%)in 2018 and 2019 seasons,respectively.The HN treatment reduced the biological tuber yield,economic tuber yield and profit margin by 2.0%,1.1%,and 4.9%in the 2018 season and by 2.9%,1.5%,and 4.2%in the 2019 season compared with the MN treatment.b)Treatments with straw decreased the biological tuber yield,economic tuber yield,and profit margin by 0.2%,0.8%,and 1.9%,respectively,compared to treatments without straw in 2018;however,treatments with straw increased the biological tuber yield,economic tuber yield,and profit margin by 3.0%,5.6%,and 6.6%,respectively,in the 2019 season compared to those obtained in treatments without straw.c)The treatment MN+S stimulated biological tuber yield by 29.8%in 2018 and 34.7%,economic tuber yield by 34.7%in 2018 and 40.7%in 2019,and profit margin by 55.5%in 2018 and 63.0%in 2019,respectively,compared to the control.2.Nitrogen fertilization rate and straw return stimulated the SOM fractions,carbon pool management index(CPMI)and humification characteristics.The correlation analysis showed that TOC,SOCS and CPMI were positively and significantly related to the labile SOC components and humic substances.a)Among the four nitrogen fertilization rates,the HN treatment had the highest SOM fractions,CPMI,and humic substances;however,it was comparable to those obtained in the MN treatment.Compared to the CK,treatment MN increased the total soil organic carbon(TOC),readily oxidizable carbon(ROC),dissolved organic carbon(DOC),microbial biomass carbon(MBC),CPMI,humic acid carbon(HAC)and humin carbon(HMC)at 18 months of the experiment by 7.9%,13.9%,20.5%,24.1%,15.0%,5.8%and 3.0%,respectively at 0-20 cm depth.b)Straw treatments increased the TOC by 11.4%,ROC by 14.8%,DOC by 35.0%,MBC by 34.9%,CPMI by 15.4%,HAC by 17.0%and HMC by 8.9%after 18 months of the experiment compared to treatments without straw at 0-20 cm depth.Straw return stimulated carbon sequestration by 63.79%,70.80%and 70.16%at 6,12 and 18 months,respectively,compared to treatments without straw at 0-20 cm soil depth.c)For the interaction effect,the treatments MN+S and HN+S had statistically similar SOM fractions,CPMI,and humic substances,as both treatments provided sufficient nutrients that met the microbial N requirements for straw decomposition,mineralization,and humification.Compared to the control,the treatment MN+S stimulated the content of TOC,ROC,DOC,MBC,CPMI,HAC and HMC by 18.4%,26.7%,51.9%,52.6%,15.4%,21.7%and 10.3%,respectively,at 0-20 cm depth after 18 months of the experiment.3.Nitrogen fertilization rate and straw return significantly stimulated microbial residues and amino sugar concentrations,CO2 emission,and soil physicochemical properties.The CO2 emission peaked at the 1st and 14th day of incubation under treatments with straw and without straw,respectively.a)Compared to the CK,treatment HN increased the cumulative CO2 emission(20.2%),total nitrogen(22.6%),SOM(9.2%),nitrate(74.2%),and decreased soil pH(1.1%)and bulk density(0.2%)compared to treatment CK at 0-20 cm depth.The HN treatment stimulated the nitrate accumulation in the soil by 75.1%,74.2%,77.9%,75.9%and 77.4%at 0-20,20-40,40-60,60-80 and 80-100 cm depths,respectively compared to the CK.b)Straw treatments stimulated cumulative CO2 emission,total nitrogen,and SOM by 62.9%,2.0%,and 11.4%,respectively,while soil pH decreased by 0.2%and bulk density by 1.7%at 0-20 cm compared to treatments without straw.Straw return reduced nitrate content by 29.11%,23.91%,28.75%,33.94%and 36.10%compared to treatments without straw at 0-20,20-40,40-60,60-80 and 80-100 cm depths,respectively.c)Total amino sugar content was significantly higher in treatments HN+S and MN+S,where N requirement was sufficient for microbial proliferation.Compared to the control,treatment HN+S stimulated the cumulative CO2 emission,total nitrogen,SOM and nitrate by 68.1%,19.6%,24.2%and 75.1%,respectively while soil pH was reduced by 0.6%and bulk density by 2.0%at 0-20 cm.In conclusion,nitrogen fertilizer should be applied after maize straw is returned to the field to increase tuber yield and NUE of potato and improve soil physicochemical properties.Straw return should be encouraged in soils with high nitrate concentrations to prevent groundwater contamination that affects human health.Nitrogen fertilization after straw return is an effective and sustainable practice to increase tuber yield and NUE of potato and improve soil fertility to promote sustainable agriculture.However,nitrogen fertilization after straw return increases CO2 emissions,which has a negative impact on the environment. |
