Responses Of Soil Respiration To Rain Additions In A Nitraria Tangutorum Desert Ecosystem

Climate models often predict that more extreme precipitation events will occur in the arid and semiarid regions, where C cycling is particularly sensitive to the change of precipitation patterns. To know how soil respiration will respond to precipitation in these regions a field manipulative experiment was conducted with five simulated rain addition treatments (natural rains + 0%, 25%, 50%, 75%, 100% of annual mean precipitation) in a desert ecosystem in Inner Mongolia province, China. The rain addition treatments were applied with 16 field rain enrichment systems on the 13^th day of each month from May to September, 2010. Soil water content, surface temperature and soil CO₂ efflux rates were measured in both bare and vegetated soils before and after the rain addition during a 3-week period for each rain treatment. The major conclusions are as follows:(1) In most cases, positive responses of soil CO₂ efflux and carbon emission to rain addition were observed in both bare and vegetated soils of a semiarid desert ecosystem dominated by N. tangutorum, suggesting that soil water availability determines the carbon balance in this desert region. We also found that rain addition with 25%-100% of the mean annual precipitation (36.5mm-145mm) during a growing season from May to September caused about 3%-56% increase in total CO₂ emission in the bare soils, and about 33%-105% increase in the vegetated soils. The aboveground biomass of Nitraria tangutorum were significantly increased by the rain addition treatments in the vegetated soils.The increase in total carbon emission in the vegetated soils was likely mediated, at least in part, by plant responses to the rain addition treatments including an increase in downward carbon transportation and potential increase in root-associated respiration.(2) The seasonal variation of soil respiration in bare soils didn't present a single peak curve under all rain addition treatments. There was a week relationship between soil temperature and soil respiration on seasonal scale regardless of soil water content. Fluctuation of soil respiration in bare soils was influenced by rainfall. On the contrary soil respiration in vegetated soils under rain addition treatments reached their peaks in June. This was due to not only physiological activity of Nitraria tangutorum but also good soil temperature and soil water content conditions.(3) The response magnitude and duration of soil CO₂ efflux to rain addition depended not only on the rain amount but also on the type of vegetation covers and the timing of rain addition treatments. Soil CO₂ efflux increased to rain addition only in June but less in August, not in September in bare soils. Soil CO₂ efflux increased to rain addition in June and September but less in August in vegetated soils. The differences in the responses of soil CO₂ efflux to rain addition between the bare and vegetated soils could be explained by the root activities stimulated by added rain water, while the difference in soil CO₂ efflux response to rain addition among treatment times could be attributed to soil water condition prior to rain addition and physiological activity of Nitraria tangutorum. Thus, both vegetation cover and rain timing can regulate responses of soil CO₂ efflux to future precipitation change in arid desert ecosystems, which should be considered when predicting future carbon balance of desert ecosystems in arid region.(4) Rain addition treatments could enhance the soil respiration sensitivity on both bare and vegetated soils. Q₁₀ of vegetated soil was more sensitive than that of bare soil. The fluctuation of Q₁₀ on bare soil was more violent than that on vegetated soil. Hysteresis effect on bare and vegetated soil was more obvious with the increase of soil water content. The degree of hysteresis effect was more increasing on vegetated soil than that on bare soil.(5) Soil respiration was highly significantly correlated to SLA, aboveground biomass and net photosynthesis rate of Nitraria tangutorum. Thus, soil respiration could be described as SR=1.14e0.04TW0.45P0.13 (R²=0.58) during the growing seasons, integrating soil temperature(T), soil water content(W)and net photosynthesis rate(P) into experimental soil respiration equation. It implied that accurately evaluating annual soil respiration should include the effects of plant biomass production and other abiotic factors.