青藏高原海北高寒湿地土壤呼吸对水位降低和氮添加的响应

近20年来, 青藏高原高寒湿地经历了明显的气候变化, 从而导致多数湿地水位下降和氮沉降的增加。对于湿地生态系统来说, 水位下降意味着土壤通气性能的改善, 可能会导致土壤呼吸的增加; 而氮沉降的增加可能会降低土壤微生物生物量和pH值, 从而可能抑制土壤呼吸。为此, 在青海海北高寒草地生态系统国家野外科学观测研究站利用中宇宙(Mesocosm)实验方法, 探讨了青藏高原高寒泥炭型湿地土壤呼吸对水位降低和氮添加的响应。结果表明: (1)水位降低显著增强了土壤呼吸, 而氮添加对土壤呼吸的影响依赖于水位的变化: 对照水位下, 氮添加显著抑制土壤呼吸; 而水位降低时, 氮添加对土壤呼吸速率无显著影响。(2)土壤呼吸速率与地上生物量、枯落物累积量之间呈显著正相关关系, 而与根系生物量无显著相关关系。(3)水位降低显著提高了土壤呼吸的温度敏感性, 而氮添加对其无显著的影响。因此预测: 随着氮沉降的升高, 高寒泥炭湿地土壤CO₂的排放量将会减少; 然而随着暖干化背景下水位的降低, 青藏高原高寒湿地会排放更多的CO₂。

Responses of soil respiration to reduced water table and nitrogen addition in an alpine wetland on the Qinghai-Xizang Plateau

Aims Over the past 20 years, alpine wetlands have been subjected to a rapid change in climate, resulting in water table drawdown and increased nitrogen deposition. In wetland ecosystems, the water table drawdown can improve soil aeration, hence leading to a higher soil respiration rate; whereas an increased nitrogen deposition could reduce the microbial biomass and pH value, suppressing soil respiration. Understanding the responses of soil respiration to reduced water table and increased nitrogen deposition in alpine wetlands is thus critical to predicting the carbon cycle of wetland ecosystems and its feedbacks to ongoing climate changes. This study tests the effects of water table reduction and nitrogen addition on soil respiration in the Luanhaizi wetland on the Qinghai-Xizang Plateau.Methods We imposed four treatments, including control (WT⁰N⁰), reduced water table (WT^-N⁰), nitrogen addition (WT⁰N⁺), and a combination of reduced water table and nitrogen addition (WT^-N⁺), on 20 peat monoliths collected from the Luanhaizi wetland at the Haibei station. Soil respiration was measured from late July through mid-September under all treatments.Important findings A reduction in water table significantly increased the rate of soil respiration. In contrast, nitrogen addition suppressed soil respiration only when water table was not reduced. A positive correlation was found between the aboveground biomass and soil respiration, while no correlation was detected between root biomass and soil respiration. The temperature sensitivity of soil respiration was increased by reduced water table, but was not affected by nitrogen addition. Our results suggest that nitrogen deposition is likely to reduce soil CO₂ emission in alpine wetlands where water level remains high. However, future warmer and drier conditions could result in reduced water table, and consequently alpine wetlands would be predicted to release substantially more CO₂ than previously estimated.