Effects Of Vegetation Rehabilitation And Abiotic CO₂Exchange Between Soil And Atmosphere On Soil Carbon In A Desert Ecosystem

Soil is the largest organic carbon pool in the biosphere, and its subtle fluctuations of such a huge carbon pool may potentially alter the atmospheric CO2concentration and the global climate. So it is very important to increase the current soil carbon pool. Part of CO2captured by photosynthesis can be stored in the soil by plant, which is a significant pattern for compensating the soil carbon pool. Vegetation rehabilitation was being widely established in semi-arid area in China, its effect on storage and stability of soil carbon is still unknown. In addition, soil in desert can absorb CO2from atmosphere, excluding photosynthesis, may be the other pattern for increasing the soil carbon pool. However, it has not been accepted universally, because the related studies locate in the initial stage.We compared the soil organic carbon and its density fractions at the depth of0-100cm after planting Artemisia ordosica, Astragalus mongolicum, Salix psammophila on shifting sand land and planting Caragana microphylla on degraded pasture in Mu Us Desert. After eliminating soil respiration by sterilization, we continuously measured the abiotic soil CO2flux in a long term, and analysised the effect of soil temperature and the rate of change in soil temperature on abiotic soil CO2flux. We quantified the CO2in soil solid phase, vapor phase and escapation, and traced the whereablout of the ’CO2, following adding CO2on natural soil. The conclusions are as follows:In Mu Us Desert, after planting shrubs on sand land, the accumulation rates under A. ordosica, A. mongolicum and S. psammophila were59.7g m·2y-1,56.5g m-2y-1and27.6g m-2y-1, respectively. The proportions of light fraction of soil organic carbon with the fast turnover rate in the increased carbon were42.7%、80.6%and59.8%. The high proportions of light fraction of soil organic carbon in increased soil organic carbon resulted in restricting soil organic carbon sequestration. After planting C. microphylla on degraded pasture, there was no significant difference in soil organic carbon at the depth of0-60cm between shrubland and degraded pasture. The induce of heavy fraction of soil organic carbon with slow turnover rate in the deep layer (60-100cm) results in the9.3%of erosion for the total soil organic carbon within0-100cm depth.In Mu Us Desert, soil in semi-arid area can absorb CO2from atmosphere is an important pattern for increasing the soil carbon pool. There is an abiotic CO2exchange process between soil and atmosphere. During this process, soil from our study site sequesteres atmospheric CO2at a rate of0.10μmol m-2s-1continuously and most of the absorbed CO2is stored in the solid phase. The added13CO2stored in the soil solid phase, in vapor phase and emitted form soil accountted for72.9%,<0.1%and7.1%of the absorbed13CO2, respectively. The abiotic CO2exchange between soil and atmosphere respond to temperature sensitively. The abiotic soil carbon flux was strongly positively correlated with the rate of change in soil temperature (γ>0.90, p<0.01). CO2absorption and emission by soil is driven by the rate of falling and rising of soil temperature, respectively.In Mu Us Desert, vegetation rehabilitation is not the sole way to sequestrate the soil carbon, and abiotic pattern plays an important role in the soil carbon sink. This study may change the traditional standpoint which soil only conserve organic carbon to realize carbon sequestration, while the detailed mechanism of abiotic pattern should be studied in the future study. Our results areis also benefit to assess the capacity of soil carbon sink and identify the carbon cycle in desert ecosystem.