| It is of great concern worldwide that nitrous oxide (N2O) and nitrogen oxides (NOx,including NO and NO2) emissions in the global nitrogen cycle contribute to regional and global-scale environmental issues. Agro-ecosystem plays an important role in the land-atmosphere flux of greenhouse gases, which is considered as a major anthropogenic source of greenhouse gases emission. Although a lot of studies have been done on N2O and NO emissions from staple crops (e.g. rice, wheat, corn), few researches have focused on simultaneously measuring N2O and NOx emissions from greenhouse vegetable systems.Meanwhile, more and more people are interested in using biochar to agro-ecosystems as an effective method to reduce N2O and NO emissions from agricultural soils in recent years.However, the effect of biochar on N2O and NO emissions may vary largely with biochar application rates, soil type and the feedstock of biochar.Incubation and field experiments were conducted in our study, the primary objective of the study was to explore the mitigation potential of biochar on different pH agricultural soils, then further to illuminate the effects of biochar on N2O and NOx emissions from greenhouse vegetable systems in China. For the incubation experiment, in order to explore the mitigation potential of biochar amendment on agricultural soils, four different pH soils with N and biochar addition or not were set in the incubation study. The incubation experiment has analyzed the relationship between soil pH and N2O emissions. For the field experiments, a typical plastic greenhouse vegetable system located in suburb Nanjing was selected in the present study to measure N2O, NO and NO2 emission fluxes simultaneously with static opaque chamber - gas chromatograph (GC)/NOx method. The field experiments explored the effects of N fertilizer and biochar application on N2O, NO and NO2 emissions from greenhouse vegetable systems, then furthermore revealed N2O, NO and NO2 emission patterns and its relationship with soil factors.The main results of this study are as follows:1) The incubation experiment results showed that N2O emissions had a negative relationship with soil pH when without N input, while had a positive relationship with soil pH when with N input. Meanwhile, N, biochar and soil type influenced N2O emissions significantly. Biochar amendment could reduce N2O emissions significantly from agricultural soils, especially in acid and slightly acid soils.2) N2O, NO and NO2 emissions were influenced significantly by vegetable types and rotation systems, which showed a distinctly seasonal and annual variation characteristic.During the pepper-tomato-Chinese cabbage rotation system in 2012.8-2013.8, more than 80% of the annual N2O emissions occured in pepper and tomato growing seasons.Cumulative N2O emissions under N fertilization were ranged from 13.25 to 17.14 kg N2O-N ha-1. N2O background emissions were 4.48 kg N2O-N ha-1. During the tomato-Chinese cabbage-green soybean rotation system in 2013.8-2014.8,seasonal cumulative N2O emissions were green soybean > tomato > Chinese cabbage, while seasonal NO emissions were tomato > green soybean > Chinese cabbage. Cumulative N2O and NO emissions under N fertilization were 10.13-16.47 kg N2O-N ha-1 and 5.79-10.81 kg NO-N ha-1, respectively. N2O and NO background emissions were 4.41 kg N2O-N ha-1 and 1.26 kg NO-N ha-1, respectively. Meanwhile, the greenhouse vegetable soils showed as a sink of NO2 emissions.3) N2O and NO emission factors were influenced significantly by vegetable types and rotation systems, which showed an obviously seasonal and annual variation characteristic.During the pepper-tomato-Chinese cabbage rotation system in 2012.8-2013.8, seasonal N2O emission factors were ranged from 0.76% to 2.98%, with different vegetable types showed as tomato > pepper > Chinese cabbage. Annual N2O emission factors were 1.33-1.92%. During the tomato-Chinese cabbage-green soybean rotation system in 2013.8-2014.8, the seasonal N2O emission factors were 0.64-5.00%, with different vegetable types showed as green soybean > tomato > Chinese cabbage. Meanwhile, the seasonal NO emission factors were 0.62-2.64%, with different vegetable types showed as green soybean > tomato > Chinese cabbage. Annual N2O and NO emission factors were 1.07-1.91% and 0.96-1.35%, respectively.4) Soil moisture and soil mineral N were two key factors influencing N2O and NO emissions in greenhouse vegetable systems. N2O emissions showed a significantly exponent relationship with soil moisture, while NO-N/(N2O+NO)-N and NO-N/N2O-N showed a significantly linear relationship with soil moisture. Our study showed that NO emissions would exceed N2O emissions when soil moisture lower than 61%WFPS, which shown NO-N/(N2O+NO)-N or NO-N/N2O-N could be used as an indicator of the relative importance of nitrification and denitrification processes in producing NO and N2O.Meanwhile, N2O and NO emissions showed significantly linear relationship with soil mineral N.5) Biochar application reduced N2O and NO emissions significantly in greenhouse vegetable systems, while had no significantly influence on NO2 emissions. During the pepper-tomato-Chinese cabbage rotation system in 2012.8-2013.8, 40 t ha-1 biochar application rate reduced annual N2O emissions significantly by 22.69%, with 31.27% in Chinese cabbage growing season. Meanwhile, annual N2O emission factors were reduced by 31%. During the tomato-Chinese cabbage-green soybean rotation system in 2013.8-2014.8, 40 t ha-1 biochar application rate at different N application rates reduced N2O and NO emissions by 10-19% and 13-19%, respectively. The best mitigation effects occurred in tomato growing season while green soybean season was the least. Meanwhile,the biochar mitigation effects on N2O at different application rates shown as F+B2 (20 t ha-1) > F+B3 (40 t ha-1) > F+B1 (10 t ha-1) and on NO mitigation effects were F+B1 (10 t ha-1) > F+B2 (20 t ha-1) > F+B3 (40 t ha-1), respectively. In general, 20 t ha-1 biochar application rate showed the best mitigation potential on N2O and NO emissions from greenhouse vegetable systems and has enhanced vegetable yields.6) N2O and NO emissions showed a significantly positive linear relationship with N fertilization, while NO2 emissions showed a significantly negative linear relationship with N fertilization. During the tomato-Chinese cabbage-green soybean rotation system in 2013.8-2014.8, the sum of N2O and NO emissions showed the least and vegetable yields showed the highest at reducing 1/3 N fertilization treatment (F-L), which means the local conventional input of chemical N fertilizer could be partially reduced to attain high yield of vegetable and low N2O and NO emissions in greenhouse vegetable cropping systems in China.First, in conclusion, N2O emissions had a negative relationship with soil pH when without N input while had a positive relationship with soil pH when with N input.Meanwhile, biochar amendment could significantly reduce N2O emissions from agricultural soils, especially in acid and slightly acid soils. Second, N2O, NO and NO2 emissions were influenced significantly by vegetable types and rotation systems, which showed a distinctly seasonal and annual variation characteristic. Soil moisture and soil mineral N were two key factors influencing N2O and NO emissions in greenhouse vegetable fields. Thirdly, N2O and NO emissions showed a significantly positive linear relationship with N fertilization,while NO2 emissions showed a significantly negative linear relationship with N fertilization.The sum of N2O and NO emissions showed the least and vegetable yields showed the highest at reducing 1/3 N fertilization treatments (F-1/F-L+B). Finally, biochar amendment reduced N2O and NO emissions significantly in greenhouse vegetable systems, while has no significantly influence on NO2 emissions. Meanwhile, 20 t ha-1 biochar application rate has the best mitigation potential on N2O and NO emissions from greenhouse vegetable systems and has enhanced vegetable yields. |
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