干旱沙区生物土壤结皮覆盖土壤CO_2通量对脉冲式降雨的响应

Responses of Carbon Dioxide Fluxes from Biological Soil Crusted Soils to Pulse Rain in Arid Desert Ecosystem

水分是干旱区生态过程中主要限制因子,降水可通过改变土壤的干湿状况直接影响土壤的生态过程,继而引起土壤碳库的变化。生物土壤结皮作为干旱区主要的地表覆盖物,其自身不但可以进行呼吸作用,还能充分利用有限的水分通过光合作用固碳,改变土壤圈与大气圈之间的碳交换通量。通过模拟0、2、5、8、15mm降雨,利用红外气体分析仪,对腾格里沙漠东南缘人工固沙植被区主要的生物土壤结皮覆盖土壤净CO2通量进行了原位测定,探讨生物土壤结皮覆盖土壤CO2释放和光合固定CO2(吸收)共同作用下的土壤净CO2通量对模降雨的响应特征。结果表明:(1)降雨会迅速激发生物土壤结皮覆盖土壤CO2释放,降雨激发CO2释放速率和有效时间取决于降雨量,降雨量越高,激发程度越低,激增的生物土壤结皮覆盖土壤CO2释放(源)效应有效时间随降雨量的增加而延长;降雨激发的土壤碳释放总量随着降雨量的增加显著增加,且藓类结皮覆盖土壤碳释放总量显著高于藻类结皮(P<0.05)。(2)降雨引起生物土壤结皮覆盖土壤CO2吸收速率在初期呈单峰变化,后逐渐回归到降雨前的水平,随降雨量的增加,CO2吸收的效应的时间越长,峰值越高;降雨量越高,生物土壤结皮光合碳固定量越多,当降雨量增加到15mm时,藻类结皮光合碳固定量显著低于8mm时的碳固定量;降雨量<5mm时,藓类结皮光合碳固定量显著低于藻类结皮(P<0.05),≥5mm时,藓类结皮光合碳固定量显著高于藻类结皮(P<0.05)。(3)干旱荒漠地区生物土壤结皮覆盖土壤,在无降雨的干旱期表现为较低水平的净碳排放效应,不同程度降雨的初期阶段都有短暂的增加土壤碳的汇效应,且碳汇效应的时间随降雨量的增加而延长;适度的降雨会降低长期干旱藻类结皮覆盖土壤向大气的碳排放量,而过高或过低的降雨都会不同程度地增加藻类结皮覆盖土壤向大气的碳排放,降低土壤碳的储量。不论降雨量大小,降雨都会增加藓类结皮覆盖土壤更多碳向大气排放,但随着降雨量的增加,源效应逐渐减弱。降雨量≤8mm时,藓类结皮覆盖土壤净碳排放总量显著高于藻类结皮(P<0.05),当降雨量>8mm时,藓类结皮覆盖土壤净碳排放量显著低于藻类结皮覆盖土壤(P<0.05)。因此,干旱区在估算生物土壤结皮覆盖土壤与大气碳交换对降雨的响应规律时,应该充分考虑降雨量大小对生物土壤结皮碳固定量和土壤碳释放组分的效应,明确降雨事件大小对不同类型生物土壤结皮覆盖土壤与大气之间碳交换的作用。

Biological soil crusts（BSC）are an integral part of the soil system in arid regions worldwide,and make an important role in carbon cycle for its photosynthesis and respiration.Rainfall can affect the change of soil carbon pool by altering the soil moisture.BSC in photosynthesis and respiration may change the carbon fluxes between soil and atmosphere by utilizing the limited moisture.The release fluxes and net fluxes of carbon dioxide from biological soil crusted soils were tested under different simulated rainfall（0,2,5,8,15 mm）in arid desert ecosystem.The resulted showed that rainfall was able to stimulate the release fluxes of carbon dioxide from biological soil crusted soils.The degree and available time of release flux depended on size of rainfall events.Small rainfall motivated higher velocity and shorter release flux of carbon dioxide,but oppositely for large rainfall.The total carbon productions of biological soil crusted soil from release flux increased with the increasing rainfall.Moss dominated crusted soil has higher total carbon production than cyanobacterial-algae dominated crusted soil（P〈0.05）in all level of rainfall events.Fluxes of carbon sink for photosynthesis of BSC by reason of rainfall dropped to the level of that without rain after a single peak.Stimulated available time and the peak value of carbon sink velocity were longer and higher for large rainfall.The total carbon production from sequestration increased with augment rainfall in moss dominated crusts.But,when rainfall reaches to 15 mm,cyanobacterial-algae dominated crusts lowered in the total carbon production of sequestration.When the rainfall was lower（〈5 mm）,the total carbon production of carbon sink from moss dominated crusts was lower than that of cyanobacterial-algae dominated crusts（P〈0.05）.When the rainfall was higher（≥5 mm）,the effect was contrary.Net carbon flux for most of the year in arid region showed a carbon source during the drought,but was transitory carbon uptake at the beginning of rainfall from crusted soil towards atmosphere.The effect of carbon source for cyanobacterial-algae dominated crusted soil can be enhanced in both large and small rainfall events,and moderate rainfall could reduce this kind of effect.Rainfall events no matter size were capable to increase the effect of carbon source from moss dominated crusted soil.But,the enhanced effect of carbon source for moss dominated crusted soil was reduced with increased rainfall.The carbon productions from net carbon release in cyanobacterial-algae crusted soil under less than or equal 8mm rainfall were lower than those of moss crusted soil,adversely under greater than8 mm rainfall.The results suggested that the effect from size of rainfall on carbon release should be taken into account when carbon budget were evaluated between soil and atmosphere in arid region.The role of rainfall size in carbon exchange from crusted soil towards atmosphere should be differentiated in different biological soil crusted soils.