增氮对青藏高原东缘高寒草甸土壤甲烷吸收的早期影响

研究大气氮沉降对青藏高原高寒草甸土壤CH₄吸收的影响,对于揭示氮素调节土壤CH₄吸收的机制和评价氮沉降增加背景下大气CH₄收支平衡至关重要。通过构建多形态、低剂量的增氮控制试验,测定土壤CH₄净交换通量和相关土壤理化性质,分析高寒草甸土壤CH₄通量变化特征及其主要驱动因子。研究结果表明:自然状态下高寒草甸土壤是大气CH₄汇,CH₄平均吸收量为(35.40±1.92)μg·m^-2·h^-1。土壤CH₄吸收主要受水分驱动,其次为土壤NH₄⁺-N和NO₃^--N含量。NH₄⁺-N抑制CH₄吸收,NO₃^--N促进CH₄吸收;不同剂量氮素输入对土壤CH₄吸收影响也不尽相同,低氮处理促进土壤CH₄吸收,而中氮和高氮处理抑制土壤CH₄吸收。结果显示青藏高原高寒草甸土壤是重要的大气CH₄汇,在未来大气氮沉降加倍的情景下CH₄汇功能增强,但当氮沉降量增加两倍以上时CH₄汇功能将会减弱。

The early effects of nitrogen addition on CH4 uptake in an alpine meadow soil on the Eastern Qinghai-Tibetan Plateau

The increase in atmospheric nitrogen (N) deposition caused by human activities significantly changes carbon cycling and the carbon budget in terrestrial ecosystems. Unsaturated soil is a unique biological methane (CH₄) sink, accounting for about 5% of all atmospheric CH₄ absorption. Alpine meadow is a typical N-limited ecosystem in which the soil microorganisms and plants have adapted to the N poor environment so that long-term slow N input will change the carbon cycle process, structure and function of this fragile ecosystem. Therefore, it is crucial and helpful to explore the mechanisms responsible for the responses of soil CH₄ uptake to exogenous N input. In 2007, a multi-form, low-level N addition experiment was conducted at the Haibei Alpine Meadow Ecosystem Research Station on the Qinghai-Tibetan Plateau. Three N fertilizers; NH₄Cl, (NH₄)₂SO₄, and KNO₃ were added at four rates; control (0 kgN·hm^-2·a^-1), low N (10 kgN·hm^-2·a^-1), medium N (20 kgN·hm^-2·a^-1), and high N (40 kgN·hm^-2·a^-1). Each N treatment contained three replicates, making a total of 36 plots. During the growing season (May to October), soil CH₄ fluxes were monitored weekly by the static chamber-gas chromatograph technique. Simultaneously to the soil CH₄ flux measurements, soil temperature and soil moisture were also recorded at 10 cm depth. In addition, the soil ammonium, nitrate and dissolved organic carbon contents as well as the pH were measured monthly to examine the key driving factors of soil CH₄ uptake. The results showed that: (1) N addition significantly changed the soil moisture content. Both low and high levels of N addition tended to consume soil moisture, whereas a medium level of N input was favorable to maintaining soil moisture, which mainly depended on the soil moisture balance of precipitation, soil evaporation and plant transpiration. (2) N addition significantly changed soil NH₄⁺-N, but did not affect the soil NO₃^--N pool. As the most vulnerable part of the ecosystem to a change in N forms, plants preferentially absorbed NO₃^--N rather than NH₄⁺-N and added NO₃^--N was absorbed more strongly by plants than NH₄⁺-N. Therefore, accumulation of soil NH₄⁺-N caused by N addition was more significant than that of soil NO₃^--N. Nitrogen addition did not significantly decrease the soil pH values. (3) Soil-dissolved organic carbon strongly fluctuated within the growing season. After accumulating over the non-growing season, dissolved organic carbon content was at a maximum in May, and then continued to decrease until October because of microbial consumption. (4) Soil CH₄ uptake was mainly driven by soil moisture, followed by soil NH₄⁺-N and NO₃^--N contents. Ammonium fertilizer addition inhibited soil CH₄ uptake, where as nitrate fertilizer promoted it. This research suggests that alpine meadow soils are an atmospheric CH₄ sink and that N addition can significantly affect CH₄ uptake in the short term. Low N addition can promote CH₄ uptake from alpine meadow soil, while medium and high N deposition inhibits CH₄ uptake and decreases the atmospheric CH₄ sink. The critical level of atmospheric N deposition is about 20 kgN/hm²·a if the ambient N deposition rate is considered.