氮和水分添加对内蒙古荒漠草原放牧生态系统土壤呼吸的影响

土壤呼吸是生态系统碳循环的重要组成部分, 同时也是评价生态系统健康状况的重要指标, 对于评估退化草地恢复过程中生态系统功能具有重要意义。该研究在内蒙古四子王旗短花针茅(Stipa breviflora)荒漠草原长期放牧实验平台上进行, 该平台设置对照(CK)、轻度(LG)、中度(MG)和重度(HG) 4个放牧强度。通过在4个放牧处理区设置氮、水添加实验处理, 探讨不同放牧强度背景下, 氮、水补充对荒漠草原土壤呼吸过程的影响。结果表明: (1)历史放牧强度除2015年对土壤呼吸无显著影响, 2016和2017年都有显著影响, 放牧区3年平均土壤呼吸速率基本都高于对照区。此外, 氮和水分添加显著增加了MG区土壤呼吸速率, HG区氮、水同时添加对土壤呼吸速率有显著增加作用; (2)无论是历史放牧强度, 还是氮、水添加处理, 都没有改变荒漠草原生长季土壤呼吸速率的季节动态变化趋势, 土壤呼吸速率基本表现为单峰曲线模式, 峰值出现在水热同期的7月份; (3)不同年份生长季土壤呼吸速率对氮、水处理的响应并不相同, 氮添加至第3年产生显著影响。水分添加在平水年份(2015和2017年)对土壤呼吸产生显著影响, 但在丰水年份(2016年)无显著影响。氮、水共同添加分别在CK、LG和HG区3年平均土壤呼吸速率显著高于单独加水处理, 说明氮添加的有效性依赖于水分条件, 两者表现为协同作用; (4)不同处理下荒漠草原土壤呼吸的温度敏感性(Q₁₀)值介于1.13-2.41之间, 平均值为1.71。在无氮、水添加时, 放牧区的Q₁₀值都小于CK区, 总体表现为CK 大于 MG 大于 LG 大于 HG; 加水和氮水共同添加处理后, Q₁₀值都有明显增加, 其中NW处理下Q₁₀值都增加到2.0以上。上述结果说明在过去受不同放牧强度影响的荒漠草原在停止放牧后的恢复过程中, 土壤水分仍是影响土壤呼吸的主导环境因子, 外源氮添加只有在满足一定水分供给的基础上才起作用, 尤其是过去的重度放牧区土壤呼吸速率对氮、水补充的响应最为强烈。该研究结果可以为评估荒漠草原恢复过程中土壤呼吸速率受养分和水分影响提供基础资料和依据。

Effects of nitrogen and water addition on soil respiration in a Nei Mongol desert steppe with different intensities of grazing history

Aims Soil respiration is an important indicator for evaluation of ecosystem health in the grazing grasslands of arid regions, and thus can be used to assess dynamics of ecosystem functioning during the restoration of degraded grasslands from enduring intensive grazing.
Methods This study was carried out in a Nei Mongol desert grassland with four grazing intensity treatments, i.e., control, light, moderate, and heavy grazing intensity designated as CK, LG, MG, and HG, respectively. Our objectives of this study were to explore the responses of soil respiration in these treatments with additional nitrogen (N) and water (W) addition. The plant community was dominated by a grass species, Stipa breviflora.
Important findings Our results showed that: (1) previous grazing intensity had significant impacts on soil respiration in 2016 and 2017, but not in 2015. Grazing increased soil respiration. Moreover, both nitrogen and water addition significantly enhanced soil respiration in MG plots, while only combined addition of nitrogen and water significantly increased soil respiration in HG plots. (2) Neither grazing intensity nor addition of nitrogen and water changed the seasonal dynamics of growing season soil respiration in this desert grassland. Soil respiration showed a single-peak curve model, and the peak occurred in July with both rain and heat. (3) The effects of nitrogen and water addition varied in different growing seasons. Nitrogen addition had no significant effects in the first two years (2015 and 2016), while showed significant effects in the third year (2017). Water addition had significant effects in years with normal precipitation (2015 and 2017), while had insignificant effect in the year with high precipitation (2016). Combined addition of nitrogen and water showed stronger effects than only addition of water in CK, LG, and HG plots, indicating that the synergistic effects of nitrogen and water addition on soil respiration. (4) The sensitivity of soil respiration to soil temperature at 10 cm depth (i.e., the Q₁₀ value) ranged between 1.13 and 2.41, with an average value of 1.71. Without addition of nitrogen and water, Q₁₀ values in grazing plots were all lower than in CK plots, with the lowest value occurring in HG plots. With the addition of water and combined addition of water and nitrogen, the Q₁₀ value increased significantly by 100%. Taken together, our results indicated that soil moisture was the leading environmental factor affecting soil respiration in this desert grassland, while nitrogen played an effective role only after the minimum requirement of water availability was met. Results from this study will provide important helpful information for the restoration and rational utilization of the degraded desert steppe.