高氮和NO_2^-对中亚热带森林土壤N_2O和NO产生的影响

GENERATION OF N_2O AND NO IN MID-SUBTROPICAL FOREST SOIL AS AFFECTED BY HIGH N AND NO_2^- CONTENTS

利用15N同位素标记方法,研究在两种水分条件即60%和90%WHC下,添加硝酸盐(NH4NO3,N 300 mg kg-1)和亚硝酸盐(NaNO2,N 1 mg kg-1)对中亚热带天然森林土壤N2O和NO产生过程及途径的影响。结果表明,在含水量为60%WHC的情况下,高氮输入显著抑制了N2O和NO的产生(p<0.01);但当含水量增为90%WHC后,实验9h内抑制N2O产生,之后转为促进。所有未灭菌处理在添加NO-2后高氮抑制均立即解除并大量产生N2O和NO,与对照成显著差异(p<0.01),在60%WHC条件下,这种情况维持时间较短(21 h),但如果含水量高(90%WHC)这种情况会持续很长时间(2周以上),说明水分有效性的提高和外源NO2-在高氮抑制解除中起到重要作用。本实验中N2O主要来源于土壤反硝化过程,而且加入未标记NO2-后导致杂合的N2O(14N15NO)分子在实验21 h内迅速增加,表明这种森林土壤的反硝化过程可能主要是通过真菌的"共脱氮"来实现,其贡献率可多达80%以上。Spearman秩相关分析表明未灭菌土壤NO的产生速率与N2O产生速率成显著正相关性(p<0.05),土壤含水量越低二者相关性越高。灭菌土壤添加NO-2能较未灭菌土壤产生更多的NO,但却几乎不产生N2O,表明酸性土壤的化学反硝化对NO的贡献要大于N2O。

An experiment using the 15N_labeling method was carried out to investigate effects of application of ammonium nitrate （NH4NO3, N 300mg kg-1） and nitrite（NaNO2, N lmg kg-1） on generation of N2O and NO in mid-subtropical forest soils, different in water regime （60% and 90% of WHC） , in Southeast China. Results show that in soils 60% of WHC in soil water content, generation of N2O and NO was significantly inhibited by high nitrogen input （ p 〈 0. 01 ） ; while in soils 90% WHC in soil water content, a similar phenomenon was observed with the generation of N2 0 within the first 9 hours of incubation, but afterwards the trend turned reversely; When NO2 added the inhibitions of high nitrogen input were lifted for all unsterilized soils, addition of NO2- immediately offset the inhibitive effect of high soil N content and trig- gered off generation of a large amount of N2O and NO, forming a sharp contrast to CK （ p 〈0. 01 ）. Such a phenomenon did not last long （for 21 h only） in soils 60% of WHC; but it did quite a long time （over 2 weeks） in soils 90% of WHC, which suggests that high water availability and exogenous NO2- played an important role in offsetting the inhibitive effect of high soil N content. In this experiment, N20 was generated mainly in denitrification process of the soil, and what is more, the addition of unlabeled NO2- led to rapid increase of hybrid N2O（14N15NO） molecules within the first 21 hours of the experiment, demonstrating that denitrification in the forest soil may probably proceed mainly through codenitrification by fungi, which may explain over 80% of the denitrification. Spearman rank correlation analysis indicates that there is a significantly positive correlation between the NO and N20 production rates （ p 〈 0. 05 ） in non-sterilized soils, and the lower the soil water content, the higher the correlation between the two. After addition of NO2 , sterilized soil produced more NO than unsterilized soil, but almost no N20, which indicates that in acid soils chemical denitrification contributes more to NO than to N2O.