湿润持续时间对生物土壤结皮固氮活性的影响

土壤可利用氮是干旱半干旱区生态系统中除水分之外的关键限制因子,研究湿润持续时间和温度变化对温性荒漠藻类结皮和藓类结皮固氮活性的影响,对于深入认识和准确评价全球变化大背景下生物土壤结皮对区域生态系统的氮贡献至关重要。通过野外调查采样,在一次较大降水事件发生后,利用开顶式生长室,采用乙炔还原法连续测定了沙坡头地区人工植被区和天然植被区两类典型生物土壤结皮固氮活性的变化,分析了湿润持续时间和模拟增温对其固氮活性的影响。研究结果表明:在经历31d持续干旱,降水发生后第4天两类结皮的固氮活性达到最大,此后随样品水分含量下降,至第10天其固氮活性将至最低;结皮固氮活性与水分含量之间呈显著的二次函数关系,其固氮活性随水分含量的增加呈先上升后下降的趋势,藻类结皮的固氮活性显著高于藓类结皮;短期模拟增温并不能显著提高其固氮活性,增温主要通过加速结皮水分散失来影响其固氮活性。上述结果反映了水分是控制生物土壤结皮固氮活性的关键因子,而实验前样品所经历的环境条件则决定了降水发生后其到达最大固氮速率的时间,野外长期观测结合控制严格的室内实验才能准确评价生物土壤结皮对区域生态系统的氮贡献。

Effect of wetting duration on nitrogen fixation of biological soil crusts in Shapotou, Northern China

In addition to moisture, availability of soil nitrogen is generally the key limiting factor in arid and semiarid ecosystems. Thus, research on the potential nitrogen-fixing activity (NA) of algae and moss crusts for different wetting durations and temperatures in temperate desert was conducted. The present study is especially important for the understanding and accurate evaluation of nitrogen contribution ability of biological soil crust (BSC) to region ecosystems in the context of global change. Algae and moss crusts were collected at artificial revegetation desert area and at the nearly natural vegetation area in the Shapotou area of Tengger Desert. Samples were incubated in OTC. The NA of two BSC types was continuously measured using the method of acetylene reduction assay (ARA) after a large precipitation event. Subsequently the influence of wetting duration and temperature on NA of BSC was analyzed. The results demonstrated that the NA of two crusts reached the maximum level 4 days after precipitation at drought condition (i.e., without precipitation for 31 days). The NA achieved the lowest value on the 10th day with the decrease of moisture content. Significant quadratic function relationship was observed between NA and water content. The NA of algae crust was significantly higher than that of moss crust; the average values were 5.3×10⁴ and 0.9×10⁴ nmol C₂H₄ · m^-2 · h^-1 for algae crust and most crust, respectively. The NA of BSC increased and subsequently decreased with the increase of moisture content. Short-term simulated warming did not increase the nitrogen-fixation activity of BSC significantly, which influenced the NA of BSC by accelerating water loss of the crusts. These results showed that moisture was the key factor controlling the NA of BSC. The environmental conditions prior to the experiment determined the maximum time required to reach nitrogen fixation rate after rainfall. In the future, the combination of the long-term field observations and strict laboratory work will be necessary to achieve accurate evaluation of the nitrogen contribution of BSC to the region ecosystems.