降雨对草地土壤呼吸季节变异性的影响

利用土壤碳通量自动观测系统(LI-8150)对呼伦贝尔草原在自然降雨条件下的土壤呼吸作用进行了野外定位连续观测,研究结果表明:降雨对土壤呼吸作用存在激发效应和抑制效应,降雨发生后1-2 h内土壤呼吸速率可增加约1倍,当单次或者连续降雨累积量大于7-8 mm,或土壤含水量大于29%-30%时,降雨对土壤呼吸会产生明显的抑制作用。土壤呼吸的激发效应往往体现在次日,表现为次日平均土壤呼吸速率的显著升高;而抑制效应则在当日即可体现出来,表现为观测当日平均土壤呼吸速率的明显下降。土壤呼吸季节变异性与降雨频率和降雨强度密切相关,在降雨量一定的情况下,较低的降雨频率和较高的降雨强度会增加土壤呼吸的变异性。呼伦贝尔草甸草原而言,在生长季土壤平均含水量为16.5%时,土壤呼吸的温度敏感性值(Q₁₀)为2.12;而平均土壤含水量为26%时,Q₁₀值为2.82,明显高于前者,土壤含水量与Q₁₀之间存在正相关关系。降雨导致土壤呼吸的激发效应和抑制效应交替发生,使草地土壤呼吸的季节变异性增加,降雨格局变化必然会对草地碳循环和碳通量特征产生深刻影响。

Effect of rainfall on the seasonal variation of soil respiration in Hulunber Meadow Steppe

As the main determinant of carbon balance, soil respiration is one of the major pathways for carbon loss from the soil to atmosphere and is largely influenced by soil temperature and soil moisture.During the growing season, discrete rainfall events create an important disturbance to soil respiration and lead to large uncertainties in estimating carbon exchange, especially in arid and semiarid regions.In order to examine the effect of rainfall events on soil respiration, measurements were conducted in natural rainfall conditions at a field site in the Hulunber meadow steppe.Soil respiration was continuously monitored by an automated chamber system (LI-8150) during the growing seasons from 2009 to 2012.Five chambers (diameter 20 cm) were set in the field with at least 10 m separation between them.The results showed that the response of soil respiration to rainfall displayed different patterns, including the "birch effect" and "inhibiting effect".Soil respiration doubled in the 1-2 hours during or after rainfall.Once rainfall reached 7-8 mm or the soil water content reached 29%-30%, which was almost equal to the value of field moisture capacity, soil respiration would be inhibited and noticeably decreased.It was suggested that moderate rainfall may stimulate the roots and microbial activities, thus enhancing soil respiration.However, heavy rainfall or an excess of moisture could reduce the soil air-filled pore space and increase anaerobism, suppressing soil CO₂ flux.The increase in diurnal average soil respiration was often expressed on the day following rainfall when the "birch effect" occurred, but decreased on the day of rainfall when the "inhibiting effect" was observed.The response of soil respiration to the "inhibiting effect" seemed quicker than the response for the "birch effect".It was explained that the "inhibiting effect" is mainly a result of a rapid physical process and the "birch effect" was controlled by a slow biological process.Compared to the amount of rainfall, the frequency and density of rainfall were much more important factors in determining the seasonal variation of soil respiration.Over a certain amount of rainfall, a lower frequency and higher density of rainfall could strongly enhance the variation of soil respiration.During the growing seasons from May to September, the temperature sensitivity of soil respiration (Q₁₀) in the Hulunber grassland was 2.82, when the average soil water content was 26%.This was higher than that for a soil water content of 16.5% where Q₁₀=2.12.The Q₁₀ had a positive relationship with soil water content.These results demonstrated that rainfall events had a significant influence on soil respiration and strongly enhanced its seasonal variation, which depended not only on the amount of rainfall, but also on its frequency and density.The seasonal distribution of rainfall may have important consequences for the estimation of carbon flux and C balance.