Effects Of Combined Application Of Biochar And Organic Fertilizers On Vegetable Production And N₂O Emissions

In order to obtain greater economic benefits,the growing area of intensive vegetable fields in mainland China has been expanding.But excessive fertilization of intensive vegetable fields and frequent irrigation have led to rapid growth of soil nitrous oxide（N₂O）emissions and reduced nitrogen fertilizer utilization.The decline in vegetable production and the emergence of nitrate enrichment in leafy vegetables have severely hindered and restricted the healthy and sustainable development of the intensive vegetable economy.Biochar has the effects of soil amendment and carbon sequestration and emission reduction,and is increasingly being used in the production and development of intensive vegetable fields.The combined application of organic fertilizers and chemical nitrogen fertilizers has also been proved to be an effective measure to improve soil quality,increase vegetable yields and agronomic utilization of nitrogen fertilizer.However,there are few comprehensive studies on vegetable yield and quality,N₂O emissions under the condition of adding combined application of biochar and organic-inorganic fertilizers.This experiment was carried out in the intensive vegetable field in Dou Village,Jiangning District,Nanjing City,Jiangsu Province.Five treatments were set up:no nitrogen fertilizer control（CK）,nitrogen fertilizer（SN）,nitrogen fertilizer+biochar（NC）,organic and inorganic fertilizer 1:1 combined application（M1N1）and 1:1 combined application of organic and inorganic fertilizer+biochar（M1N1C）,with three replicates for each treatment,using a random block design.Under the conditions of field greenhouse cultivation,observing the changes of soil properties,vegetable yield and protein,soluble sugar,VC,and nitrate content after each season of the vegetables were harvested with different treatments and different years;using static dark box-gas chromatography to observe the same season vegetables for four consecutive years soil N₂O dynamic emission during crop growth,measuring the abundance of functional genes related to soil N₂O emission after the vegetables were harvested.The main results are as follows:1.During the vegetable observation period from 2017 to 2020,compared with the SN treatment,NC treatment has a tendency to reduce NH₄⁺-N content for four consecutive years,but its NO₃^--N content significantly increased by 183.2%in the fourth year;the vegetables dry matter weight of the NC treatment showed an increasing trend in the first three years after the application of biochar,and their values were 2.21 t ha^-1,2.26 t ha^-1and2.49 t ha^-1;the protein content of NC treatment was significantly higher than that of SN treatment for four consecutive years,and the values were 7.9 mg kg^-1and 5.04 mg kg^-1.5.82 mg kg^-1and 5.04 mg kg^-1,respectively;the VC content of NC treatment in the fourth year was significantly higher than that of SN treatment,with a value of 69.37 mg 100g^-1.Compared with SN treatment,the M1N1 treatment had the highest denitrification activity(0.13～0.18 mg kg^-1h^-1)for four consecutive years,which was significantly higher than that of the SN treatment,and there was no significant difference among other treatments;it reduces the soil nitrate content by 45.3%,in the fourth year;the soluble sugar content of the M1N1 treatment will increase in 2020,with a value of 1.14%;for three consecutive years from 2017 to 2019,M1N1 treatment significantly reduced vegetable nitrate content compared with SN treatment;for three consecutive years from 2018 to 2020,the M1N1treatment N fertilizer agronomic utilization efficiency increased significantly,and the values were 12.6%,19.4%,and 19.0%,respectively.The total carbon content（TC）and total nitrogen content（TN）of M1N1C treatment were 23.42-24.73 g kg^-1and 2.38-2.85 g kg^-1,respectively,which were higher than NC and M1N1 treatments;compared with the NC and M1N1 treatments,the combined M1N1C treatment had the highest VC content in the first three years,87.79 mg 100g^-1,73.95 mg 100g^-1and 84.77 mg 100g^-1,respectively.2.During the 2017-2020 vegetable observation period,compared with the SN treatment,the NC treatment significantly reduced the cumulative N₂O emission in the first two years by 76.3%and 34.3%,and significantly reduced the N₂O emissions per unit of output,the values were 0.31 kg N t^-1and 0.98 kg N t^-1,respectively;by the fourth year,the cumulative N₂O emission of NC treatment increased by 18.5%compared with SN treatment,and the emission factor was the highest.Compared with SN treatment,the M1N1treatment copy numbers of nir K gene,nir S gene,nos ZⅠgene,nos ZⅡgene and Fungi nir K gene in M1N1 treatment showed an increasing trend compared with SN treatment in 2020;the M1N1 treatment significantly reduced N₂O emissions per unit output for two consecutive years from 2019 to 2020,and their values were 1.04 kg N t^-1and 1.06 kg N t^-1,respectively.Compared with the NC and M1N1 treatments,the combined M1N1C treatment has a tendency to reduce the N₂O emission flux in the second year,but there is no significant difference compared with NC treatment in the fourth year;compared with the M1N1 treatment,the cumulative N₂O emission was significantly increased by 69.6%;compared with 2017,the number of copies of AOB amo A gene treated in 2020 showed an increasing trend,which may be related to N₂O emissions;the highest N₂O emission per unit output in four years was 2.17 kg N t^-1,which was higher than that of NC treatment and M1N1 treatment.Therefore,compared with single nitrogen fertilizer application,the NC treatment of nitrogen fertilizer+biochar is the best fertilization measure to continuously increase the dry matter weight and protein content of vegetable,but it has not obvious effect of continuously reducing N₂O emissions per unit output and improving other nutritional quality of vegetables;compared with single application of nitrogen fertilizer,the combined application of organic and inorganic fertilizers M1N1 treatment is the best choice for reducing vegetable nitrate content,N₂O emissions per unit yield and increasing the N fertilizer agronomic utilization efficiency in long-term vegetable planting,but the effect of vegetable yield and other nutritional qualities is not obvious.In this study,the combined M1N1C treatment could neither continuously reduce the N₂O emissions per unit yield,nor could it continuously increase the nitrogen utilization rate and vegetable dry matter weight,and improve the nutritional quality of vegetable crops,its field application effect needs to be clarified in other vegetable fields.