青藏高原高寒草甸土壤CO2排放对模拟氮沉降的早期响应

研究大气氮沉降输入对青藏高原高寒草甸土壤-大气界面CO₂交换通量的影响,对于准确评价全球变化背景下区域碳平衡至关重要。通过构建多形态、低剂量的增氮控制试验,利用静态箱-气相色谱法测定土壤CO₂排放通量,同时测定相关土壤变量和地上生物量,分析高寒草甸土壤CO₂排放特征及其主要驱动因子。研究结果表明:低、高剂量氮输入倾向于消耗土壤水分,而中剂量氮输入有利于土壤水分的保持;施氮初期总体上增加了土壤无机氮含量,铵态氮累积效应更为显著;施氮显著增加地上生物量和土壤CO₂排放通量,铵态氮的促进效应显著高于硝态氮。另外,土壤CO₂排放通量主要受土壤温度驱动,其次为地上生物量和铵态氮储量。上述结果反映了氮沉降输入短期内可能刺激了植物生长和土壤微生物活性,加剧了土壤-大气界面CO₂排放。

Early responses of soil CO2 emission to simulating atmospheric nitrogen deposition in an alpine meadow on the Qinghai Tibetan Plateau

Soil-atmosphere carbon dioxide (CO₂) exchange is a key carbon cycling process in terrestrial ecosystems. To assess the effects of atmospheric N deposition on the C budget of an alpine meadow ecosystem on the Qinghai-Tibetan Plateau, it is necessary to explore the responses of soil-atmosphere CO₂ exchange to N addition. Since 2007, a multi-form, low-level N addition experiment has been 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 kg N hm^-2a^-1), low N (10 kg N hm^-2a^-1), medium N (20 kg N hm^-2a^-1), and high N (40 kg N hm^-2a^-1). Each N treatment has three replicates. Each plot has an area of 9 m² (3 m × 3 m) and a 2 m isolation band is set between adjacent plots. During the growing season (May to September), soil CO₂ effluxes were monitored weekly by static chamber and gas chromatograph techniques. Parallel to the flux measurements, soil temperature at the soil surface and at 5 cm and 10 cm depth and soil moisture at 10 cm depth were recorded. Soil ammonium and nitrate contents and aboveground biomass were measured monthly to examine the key factors driving soil CO₂ efflux. N addition did not alter soil temperature, but significantly changed soil moisture content. Both low and high levels of N addition tended to reduce soil moisture, whereas a medium level of N input maintained soil moisture. This mainly depended on the soil moisture balance of precipitation, soil evaporation and plant transpiration. N addition slightly increased the soil NH⁺₄-N pool but did not significantly change the NO^-₃-N pool. Competition for soil available N between plants and soil microorganisms, priority use of nitrate by plants, and removal by livestock grazing are responsible for this lack of significant accumulation in the soil nitrate pool. In control plots, soil CO₂ efflux from alpine meadow soils ranged from 120.9 to 1000.4 mg CO₂ m^-2 h^-1, with an average of 544.7±40.0 mg CO₂ m^-2 h^-1. N addition significantly increased aboveground biomass and soil CO₂ efflux. Ammonium-N fertilizer promoted soil CO₂ efflux more significantly than did nitrate-N fertilizer, which was mainly attributed to competition and cooperation in the use of multi-form nitrogen between plants and soil microorganisms. Soil CO₂ efflux was mainly driven by soil temperature, followed by aboveground biomass and the NH⁺₄-N pool. This indicates that the contribution of heterotrophic respiration to CO₂ efflux from the alpine meadow soil is greatest, followed by autotrophic respiration from plant roots. Soil NH⁺₄-N accumulation can increase the contribution of root autotrophic respiration and soil microbial heterotrophic respiration, suggesting that CO₂ emissions from alpine meadow soil are sensitive to exogenous N input. Chronic atmospheric N deposition will stimulate CO₂ emission from alpine meadow soils on the Qinghai-Tibetan Plateau in the short term. We can also deduce that chronic N deposition may accelerate degradation of the grazing alpine meadow ecosystem.