The Characteristics, Sources And Impact Factors Of N₂O Emissions From Vegetable Fields

Both field experiment and laboratory incubation were used to do the research. Four different fieldsand totally seven treatments, including①a maize field (maize),②a newly established open-groundvegetable field converted from a maize field four years earlier (OV4) with four different fertilizationtreatments of2a) A no fertilization control (OV4_CK),2b) Manure amendment only (OV4_M),2c)Combination of manure and chemical fertilizers (OV4_FM), and2d) Dicyandiamide (DCD) additionbased on treatment2c (OV4_FM+DCD),③an established open-ground vegetable field converted froma maize field more than20years ago (OV20), and④an established sunlight heated greenhousevegetable field converted from a maize field more than20years ago (GV20), were set in the fieldexperiment to measure the yearly N2O emission fluxes and analysis the emission factors of inputchemical and manure nitrogen (N) and relevant impact factors.Laboratory incubation was adopted to calculate the N2O emission fluxes and the relevantcontribution sourced from different pathway such as nitrifier nitrification (NN),nitrifierdenitrification(ND), denitrifierdenitrification (DD), and other processes (heterogenousnitrification and chemical nitrification) after ammonium N was applied from different vegetable soils--a calcareous vegetable soil (pH=8.0、Ec=285μS/cm), an acidified vegetable soil (pH=5.62), a calcareousvegetable soil with higher salinity (Ec=1000μS/cm). Different gas inhibitors including pure oxygen (O),0.06%acetylene (A),0.06%acetylene+pure oxygen (AO), pure helium (H)were used to identify theN2O emission contribution of different processes in laboratory incubation. The content of soil mineral Nand soil pH ammonium were also measured in the incubation experiment. This study will interpret thecharacteristics and pathway of N2O emissions and thereafter provide basic data support on mitigationN2O emissions from vegetable fields.The following results were concluded through this study:N2O emissions from vegetable fields showed the following characteristics: The magnitude ofemission peaks showed lower and lasted longer (ie. two weeks) when the air temperature was lower;while the emission peaks emerged earlier with higher magnitude and lasted shorter during the periodbetween June and August when the air temperature was higher (>21℃) than in May when airtemperature was relatively lower. The cumulative N2O emissions during the period between June23rdand September11thaccounted for82.2%of annual N2O emissions.It is necessary to distinguish the emission factors by applied N type in order to estimate theregional N2O emissions. The calculation showed that calculating the N2O emission factors withoutdistribution the chemical and manure N (0.27-0.41%0.49-0.64%in the spring cucumber and autumncabbage period respectively) would decrease the emission factors of chemical N (0.55-1.30%and0.80-1.59%in the spring cucumber and autumn cabbage period respectively) and overestimate theemission factors of manure N (0.20%and0.42%in the spring cucumber and autumn cabbage periodrespectively). NN, contributing54%of N2O emissions within the twelve days of application,is the main N2Oemission processesafter fertilization with ammonium N in the calcareous vegetable soil. ND, is thesecondary important N2O emission process(ie. contributed36%of N2O emissions) within the first twodays of ammonium application. The contribution rate of DD (ie. about30%of N2O emissions after2weeks later) on N2O emissions was lower after ammonium N was applied.The magnitude of N2O fluxes emitted from the acidified vegetable soil after ammonium N wasapplied was much lower smaller than from the calcareous vegetable soil. Other processes (eg.heterogeneous processes and so on) emitting20%of N2O emission fluxes contributed obvious share(20%) to the N2O emissions in the acidified vegetable soil during three weeks incubation afterammonium N was applied. Denitrifier denitrification was the main N2O emission process within sixdays after fertilization (ie. contributing over44%N2O emission fluxes), and its contribution on N2Oemission fluxes decreased to about17%at the20th day after N was applied. Contribution rate of ND onN2O emissions increased after9th days later and shifted to be the important N2O emission process withthe relative contribution rate over36%share.Autotrophic nitrification, contributing73%share of N2O emission during three weeks incubationperiod after ammonium N was applied, was still the main N2O emission process in the salinizedvegetable soil. The contribution of ND on N2O emission fluxes is relatively higher (over30%)between6thand18thday, and it totally contributed19%N2O emissions during three weeks incubationperiod. DD shifted to be the secondary important N2O emission processes after twelve days later withover22%N2O emission share when soil ammonium was converted to be nitrate.N2O emission fluxes were associated with the dynamic content of soil mineral N (ammonium,nitrate, and nitrite) and can be reflected by soil pH. Strong nitrification caused by ammoniumapplication can resulted in the trend of soil acidification in the short term. Control the autotrophicnitrification was suggested to be an effective N2O mitigation practice in the calcareous vegetable soil.