外源铵态氮对典型耕作土壤冻结过程中N_2O排放的影响

Effects of ammonium-N application on N_2O emission from three types of soils in freezing process

【目的】农业土壤是N2O的主要排放源,国内以往研究多集中在外源养分对作物生长季N2O排放的影响,而对冬季N2O排放特征和影响因素缺少系统研究。为明确添加外源铵态氮对典型耕作土壤冻结过程中N2O排放特征的影响,本文应用冰柜模拟冬季土壤冻结过程,研究室温—冻结过程不同铵态氮浓度对3种典型地带性耕作土壤N2O排放的影响,以期为调控农田氮肥管理控制土壤N2O排放提供理论依据。【方法】试验设12个处理,包括3种土壤类型(黑土、潮土、黄土),4个外源NH+4-N浓度梯度(0、80、200、500 mg/kg土,分别以N0、N80、N200、N500表示)。具体方法是将3种土壤的风干土样150 g分别装入广口瓶中,加入NH+4-N溶液,使土壤湿度达到田间持水量,置于25℃恒温环境中培养,24 h后分别于0、10、20、30 min时采集气体,再放到-10℃的冰柜中,分别在冷冻0.5 h、2.5 h、6.5 h、13.5 h、23.5 h、43.5 h时采集气体,用气相色谱检测样品的N2O气体浓度。【结果】室温条件下在一定范围内增加外源铵态氮施用量能够促进黑土和潮土N2O的排放,添加80 mg/kg铵态氮的黑土和潮土的N2O排放通量分别比各自对照增加2854.7%和192.1%,均达5%显著水平,但铵态氮浓度过高会抑制黑土和潮土的N2O排放;黄土在室温培养条件下N2O排放通量接近零。随冻结时间的延长,黑土和潮土的N2O排放通量逐渐降低,其降低速度均呈现N80>N200、N500>N0的趋势,且两种土壤的N80处理分别与各自其他处理的差异达5%显著性水平;冻结0.5 h的黑土添加外源NH+4-N处理的N2O排放通量比对应初始值(冻结0 h)降低了64.95%72.46%,冻结2.5 h后比初始值降低79.1%89.29%,在冻结6.5 h时接近零排放,黑土的N0处理在冻结过程中N2O排放通量基本无变化,数值始终较小;潮土各处理在冻结0 6.5 h内N2O排放通量逐渐降低,且处理间差异减少,冻结0.5 h时潮土4个处理的N2O排放通量比对应初始值(0 h)降低了47.25%58.34%,冻结2.5 h降低了84.35%94.99%,其中N0处理的N2O排放通量在冻结2.5 h后达到稳定的零排放状态,而3个添加外源NH+4-N的处理在冻结6.5 h后达到稳定的零排放状态;黄土各处理在室温和冻结过程中N2O排放通量始终处于较低的水平,且变化范围较小,处理间无显著差异。室温—冻结全过程黑土和潮土的N2O累计排放量均呈N80>N200、N500>N0,且黑土的N80处理与N0处理间均达到5%显著水平;潮土不同铵态氮浓度处理间无显著性差异;黄土N2O累计排放量处于较低水平或呈负排放状态,其中N500处理N2O的累计负排放量最大。方差分析结果表明,外源铵态氮对3种土壤N2O累计排放量均有显著影响。【结论】室温条件下,适量的外源铵态氮可促进黑土和潮土的N2O排放,但外源铵态氮浓度过高则可抑制N2O的排放;冻结过程中添加外源铵态氮黑土和潮土的N2O排放通量逐渐降低,且降低速度逐渐变缓最终接近零排放;室温—冻结过程添加外源铵态氮黄土的N2O排放通量始终处于极低水平,甚至出现负排放现象;添加外源铵态氮对室温—冻结过程不同土壤类型N2O累计排放量有显著影响。建议在潮土和黑土上降低冻前土壤的铵态氮含量从而减少N2O的排放。

【Objectives】Agricultural soil is a main N2 O emission source,which was proposed as one of the six greenhouse gases in the Kyoto Protocol. Previous studies have focused on effects of exogenous available nutrients on soil N2 O emission during crop growing seasons. However,mechanism on N2 O emission in winter is barely reported.In farming fields,lower temperature could block N uptake by plant and leave more N substrate for microbes to produce N2 O. In order to clarify influence of NH+4-N application on N2 O emission in the freezing process,a labscale experiment was conducted to investigate the effects of NH+4-N concentrations on N2 O emission from 3 types of soils,black soil,fluvo-aquic soil and loess soil. 【Methods】The experiment involved 3 types of soils,and each one with 4 NH+4-N concentrations,0,80,200 and 500 mg / kg soil( abbreviated N0,N80,N200 and N500),and three replicates. Each incubator was bottled with 150 g soil sample which was sprayed with the concentration NH+4-N solution into the incubator to reach soil WFC( black soil WFC was 40%,fluvo-aquic soil WFC was 35%,and loess soil WFC was 22%). Put the incubators under 25℃ for microorganisms cultivation lasting for 24 h,then removed the gas samples from the headspace of incubators by the syringes every 10 minutes( 0,10,20 and 30 min)4 times in 30 minutes. After collecting the gas samples,we put the incubators in freezer under- 10℃ temperature condition. Respectively,after 0. 5 h,2. 5 h,6. 5 h,13. 5 h,23. 5 h and 43. 5 h,gas samples were removed from incubators' headspace at 0,10,20 and 30 min and analyzed using a gas chromatograph. 【Results】In this study,the appropriate NH+4-N addition would increase N2 O fluxes of the black soil and fluvo-aquic soil compared with their corresponding controls,and the N2 O fluxes of the 80 mg / kg soil treatment are increased by 2854. 7% and192. 1%( P < 0. 05) compared with the control. While the excessive NH+4-N addition attenuates the promotion,the N2 O emission from loess soil fluctuates around zero in the room temperature incubation. With the freezing proceeding,the N2 O fluxes of two types of soil( black soil and fluvo-aquic soil) added with NH+4-N reduced gradually,and the reduction rates are in the order: N80> N200,N500> N0,in addition,the rates of N80 from the two soils have significant differences( P < 0. 05) with other treatments respectively in the first 0. 5 h. For black soil,the N2 O fluxes of N80,N200 the N500are decreased by 64. 95%- 72. 46% compared with initial values in 0. 5 h,and in2. 5 h,the fluxes decreased by 79. 1%- 89. 29% and are 0 emission by the time of 6. 5 h( consumed over98. 4%). However,the control remains small fluxes and slight changes in the duration. The N2 O fluxes of the fluvo-aquic soil treatments decreased gradually at the freezing stage,the differences among treatments are lower gradually,the soils N2 O emissions are dropped by 47. 25%- 58. 34% in the first 0. 5 hour,and decreased by84. 35%- 94. 99% in 2. 5 h. For the treatments with NH+4-N addition,N2 O emissions are different from that of the control,and achieve stable state( 0 emission) in 2. 5 h. The loess soil N2 O emission shows a low level of fluxes throughout the room temperature and freezing incubation process,and the change range is small. In addition,there are no significant differences among the treatments. The overall process of experiment,rank four treatments ordering to total N2 O emissions of black soil and fluvo-aquic soil: N80> N200,N500> N0,the black soil N80 differs from N0significantly( P < 0. 05),and no significant differences are found among the fluvo-aquic soil treatments.The loess soil treatments have low emission amount,and negative emissions and there are no significant differences among the loess soil treatments. According to the multi-factor analysis of variance,the different levels of NH+4-N application affect the total N2 O emission significantly. 【Conclusions】Under room temperature,the N2 O emissions from black soil and fluvo-aquic soil are promoted by the appropriate concentration of NH+4-N addition,however the N2 O emissions are inhibited by the high concentration addition. In the freeze process,the N2 O fluxes from the two types of soil with the NH+4-N addition are gradually decreased as the time passed,and the N2 O fluxes fall to 0eventually. N2 O flux of loess soil shows low level throughout room-temperature incubation and freezing process. The total N2 O emissions are affected significantly by different concentrations of NH+4-N application. In the respect of agricultural control measures in N2 O emissions,massive NH+4-N fertilizer application should be avoided in black soil and fluvo-aquic soil prior to soil freezing period,however,no significant effect is observed in loess soil N2 O emission during various concentration of NH+4-N application.