为了解农田土壤一氧化氮(NO)周年排放特征,对高原气候区青藏高原东缘种植饲草(燕麦)农田NO排放通量和主要环境因子进行了周年连续定量研究,施肥(F)和不施肥处理(UF)周年累积排放量分别为0.80 0.06 kg N·hm~(-2)·a~(-1)和0.18 0.04 kg N· hm~(-2)·a~(-1),翻耕施肥期和冻融期NO排放显著贡献了全年累积排放量(F和UF分别为85%和65%)。土壤温度、湿度、无机氮和水浸提有机碳含量显著影响NO排放通量的季节变化,采用土壤湿度、铵态氮、硝态氮和水浸提有机碳含量作为指前因子的指数方程拟合这些土壤变量对NO通量的影响,决定系数高达92%,能够很好地表征碳氮底物有效性、微生物活性和氧气有效性(土壤温度和湿度)对NO排放的协同效应,由此得到NO排放通量对表层(5 cm)土壤温度的敏感性指数(Q 10)值为2.4(F)和2.5(UF),这意味着全球增温对NO排放的促进效应将远低于施肥量增加对NO排放的促进作用。该燕麦农田NO直接排放因子为0.93%0.10%,有别于其他区域、全国和全球的平均状况,因此不宜采用缺省排放因子计算高原气候区粗放管理方式下农田NO排放清单。考虑到青藏高原地区降水量存在巨大的年际变幅,未来应加强典型农田NO排放的多年际连续观测研究。
英文摘要:
Fertilized soils are major sources of atmospheric nitric oxide(NO). However, year-round measurements of NO emissions from croplands are still scarce and exclusively concentrated in the temperate and subtropical monsoon regions of China. In this study, yearround measurements of NO fluxes and environmental factors were conducted in a typical forage grass(oat)cropland in the Qinghai-Tibetan Plateau, which has a plateau climate. The annual NO emissions from the fertilized(F)and unfertilized(UF)treatments were 0.800.06 kg N·hm~(-2)·a~(-1) and 0.18 0.04 kg N·hm~(-2)·a~(-1), respectively. The cumulative emissions from the tillage-fertilization and freeze-thaw periods dominated the annual totals. The fitting equations between environmental factors and NO fluxes well characterized the integrated effects of substrate availability, oxidation-reduction condition, and microbial activity on NO emissions(r 2 =0.92). The sensitivity coefficients of NO emissions on soil temperatures(Q 10)were 2.4(F)and 2.5(UF). The lowQ 10 values indicated that the enhanced effects of global warming on NO emissions were much lower than the promoted effects of fertilization on the emissions. The direct emission factor(EFd)was calculated as 0.93%0.10% in the alpine oat field, which was higher than that of the regional, national, and global averages. Thus, the default EFd provided by other studies should not be applied to estimate NO emissions from croplands in a plateau climate. Considering the huge variation of precipitation across the plateau, multi-year continuous measurements are essential for quantifying the interannual variations of cumulative emissions and EFd.