碳源和氧对设施菜田土壤N_2O排放的影响

Effects of Oxygen Levels and Carbon Inputes on N_2O Emission in Greenhouse Vegetable Soil

利用在线自动监测培养系统(Robot系统),研究不同氧分压、碳源投入以及不同氧分压和碳源投入组合下,添加硝化抑制剂双氰胺(DCD)对设施菜田土壤N_2O排放的影响。结果表明:随着土壤氧分压的升高,N_2O排放量呈指数下降(P<0.001),土壤氧分压大于等于3%O_2后,N_2O排放量不足于无氧和微量氧(1%氧)处理的30%。添加碳源降低了有氧条件下土壤N_2O和N_2产生量,显著增加了微量氧下异养反硝化途径对N_2O的贡献量(P<0.01)。在微量氧和3%O_2下,与未添加DCD的处理相比,无碳源添加且施用DCD后,N_2O的排放分别降低了64.4%和88.8%,同时N_2排放分别降低了23.4%和18.6%。从微量氧至3%O_2,虽然无碳源添加的处理硝化细菌反硝化作用对N_2O排放的贡献从17.2%增加至42.6%,但由于排放总量的急剧下降,硝化细菌反硝化作用对设施菜田土壤N_2O排放的贡献较小。本研究所用土壤pH较高,且添加DCD的处理培养前后硝酸盐基本平衡,异养的同步硝化-反硝化过程可能很弱。总之,设施菜田土壤N_2O排放主要发生在无氧和微量氧条件下。异养反硝化菌对土壤N_2O排放的直接贡献最大,尤其是在碳源较为充足的条件下。

【Objective】 In order to explore effects of oxygen level(0%, 1%, 3%, 10% and 21%)and carbon input on major sources of N2 O emission in greenhouse vegetable soil, an on-line robotized monitoring and incubation system was used to automatically monitor real-time dynamics of O2, N2 O and N2 in greenhouse vegetable soil after a crop of tomato was harvested. In addition, dicyandiamide(DCD), a kind of nitrification inhibitor, was added to investigate contribution of nitrification to N2 O emission from the soil relative to carbon source and oxygen level.【Method】 A certain amount of soil sample collected from a vegetable field under greenhouse after a crop of tomato was harvested, was washed with deionized water and divided into two groups, labeled as T1 and T2, separately. T2 was spiked with glutamate, whereas T1 was not. NH4 NO3 was added as nitrogen source for both groups. Then the pretreated soil samples were put into 120 mL vials, 10.0 g in dry soil weight in each. Deionized water or solution containing NH4 NO3,C.H8 NO.Na·H2 O or DCD was sprayed onto the soil surface to adjust moisture content of the soil up to 250 g·kg-1 in line with the requirement of the treatment, T1 or T2. Then the soils in the sealed vials were placed in a thermostatic(20℃) waterbath trough for incubation. Concentrations of O2, N2 O, N2 and CO2 in the headspace of a vial were monitored online at intervals of 6 h, and pure O2 was supplemented with a hermetic syringe in the light of the monitoring results of the gases in the headspace to maintain an approximately constant oxygen concentration(1%, 3%, 10% or 21%(v/v)) in the sealed vials. 【Result】Results show that N2 O emission declined exponentially with rising soil oxygen partial pressure(OPP)(R2=0.82, P<0.001).It peaked when OPP was 0% or 1%, and fell below 30% of the peak when OPP got equal to or higher than3%. Addition of available carbon into the vial reduced N2 O and N2 production in the soil under aerobic conditions, while significantly increasing the contribution of the process of heterotrophic denitrification in the soil to N2 O emission under near-anaerobic conditions(P<0.01), which suggests that this kind of soil is highly capable of triggering heterotrophic denitrification, and anaerobicity and near-anaerobicity is more favorable to N2 O emission. Compared with Group T1, Group T2 was 64.4% and 88.8% lower in N2 O emission, 23.4% and 18.6% lower in N2 emission, and 14.5% and 62.3% lower in N2 O/(N2 O+N2)index(IN2 O), respectively, when OPP was 1% and 3% and no carbon supplemented. However, when carbon was supplemented, the two groups did not vary much in N2 O and N2 emissions and IN2 O, which suggests that strong nitrification occurs in the soil with no carbon supplemented and N2 O comes mainly from heterotrophic denitrification process(HD) under aerobic conditions in the soil with sufficint carbon supply.DCD would lower the accumulation of NO2-, a substrate of nitrification-coupled denitrification(NCD),which is considered to be an important contributor to N2 O emission under aerobic conditions;nevertheless,the process of NCD is still a heterotrophic one in nature. That is to say, DCD would reduce N2 O emission from nitrification processes, and from heterotrophic denitrification processes, too. With OPP rising from1% to 3%, N2 O emission from nitrifier-induced denitrification(ND) increased from 17.2% to 42.6%,however, the contribution of ND to N2 O emission in the vegetable soil was still quite low due to the drastic reduction in total N2 O emission. Moreover, the commonly used dual isotopic labeling method would lead to overestimation of the contribution of ND to N2 O emission because this method may rule partial NCD or HD processes into ND. As the soil in this study was quite high in pH and remained almost unchanged in nitrate content after the treatment with DCD, simultaneous heterotrophic nitrification-denitrification processes might be very weak. 【Conclusion】Soil N2 O emission mainly occurs under anaerobic and near-anaerobic conditions(OPP=1%). Heterotrophic denitrifiers make the biggest direct contribution to soil N2 O emissions,especially when carbon sources are abundant. NO2-is an important substrate of the process of nitrificationcoupled denitrification.