Soil CO₂ Flux And Its Response To Environmental Factors In Mu Us Desert

Soil carbon exchange in arid and semi-arid lands is significant components of the terrestrial carbon budget. Soil CO2 flux, which is the main way of CO2 exchange between soil and atmosphere, includes abiotic flux (induced by abiotic processes) and biotic flux (or soil respiration). However, for the abiotic soil CO2 flux, our knowledge about its patterns under different scales, responses to environmental factors, mechanisms, and location of the abiotically absorbed carbon are extremely limited. For biotic soil CO2 flux, the previous results on it are debatable because of its inaccurate measurements. To address these, we selected study sites around Yanchi Research Station in Mu Us Desert, employed sterilization method and 13CO2 isotope tracer method, and investigated soil CO2 flux, meteorological factors and soil properties from 2011 to 2014 to analyze soil CO2 flux and its response to environmental factors in the Mu Us Desert. The results suggest that:(1) Abiotic soil CO2 flux and biotic soil CO2 flux both have typical seasonal and diel patterns, yet they (seasonal and diel patterns of abiotic soil CO2 flux vs. seasonal and diel patterns of biotic soil CO2 flux correspondingly) are markedly different. Abiotic soil CO2 flux would decrease with time and then increase under annual scale. While the annual pattern of biotic soil CO2 flux was opposite. At die) scale, under dry conditions, abiotic soil CO2 flux was negative during nighttime and positive during daytime. Under wet condition, it was negative throughout the diel cycle. While biotic soil CO2 flux was positive throughout the diel cycle and increased with time and then decreased under both dry and wet conditions.(2) Abiotic soil CO2 flux has a significant relationship (linear) with rate of change in soil temperature. In addition, precipitation, soil water content, surficial air pressure, surficial turbulance, etc. can also considerably affect abiotic soil CO2 flux. Thus predicting it with soil temperature can be inaccurate. Biotic soil CO2 flux is controlled exponentially by soil temperature over the annual cycle. Moreover, soil water content can affect the response of biotic soil CO2 flux to soil temperature in the growing season. A bivariate exponential-power model can behave well for simulation in the growing season; however, the exponential model behaves better when integrated throughout the year.(3) CO2 outgassing induced by turbulence, expansion of soil air, CO2 effusion from soil water, and carbonate precipitation during daytime could explain the abiotic diurnal CO2 release under dry conditions. Moreover, CO2 pumping from the atmosphere into the soil, caused mainly by carbonate dissolution, can account for nocturnal CO2 absorption under dry conditions. The abiotic soil CO2 flux pattern under wet conditions could be attributed to downward mass flow of soil CO2 and intensified soil air shrinkage, CO2 dissolving in soil water, and carbonate dissolution.(4) Soil of bare sand can absorb large amount of atmospheric CO2 annually in the research areas. And the absorbed CO2 can be mainly fixed (about 65%) in soil liquid phase (as HCO3-) chemically. Accordingly, it can be a carbon sink. In shrub land, the carbon fixed by soil chemically can not offset the carbon release biotically.These results above reveal the varying patterns of abiotic and biotic soil CO2 flux, and the location of the abiotically absorbed carbon in the Mu Us Desert. And provide a basis for recognizing soil CO2 flux and evaluating the role of soil in carbon cycle in semi-arid lands. Moreover, they can partly fill the gaps in knowledge on ecological processes of desert ecosystems, and quantifying the contributions of abiotic processes which can induce abiotic soil CO2 flux and predicting abiotic soil CO2 flux should be studied in the future researches.