| Precipitation controls ecosystem processes in the arid and semi-arid regions and affects the greenhouse gas (CO2, CH4 and N2O) emissions and global warming potential. A set of precipitation simulation experiments were carried out in a dryland winter wheat filed on the Loess Plateau and the fluxes of greenhouse gas (CO2, CH4 and N2O) were monitored with a static closed chamber-gas chromatograph method in the present study. The main results are as follows:1. Influence of simulated precipitation on dryland soil CO2 and CH4 emissions in the Loess Plateau 1. We conducted a precipitation simulation experiment in the field to measure the short-term response of precipitation levels (1-32 mm) on soil CO2 and CH4 flux from 0 to 72 h under winter wheat jointing stage and summer fallow. Soil CO2 flux increased with increased precipitation levels. Soil CO2 flux peaked in 4 h with 1 to 16 mm of precipitation, but the peak delayed by another 4 h with 32 mm. The pulse intensity of soil CO2 flux rate (Rpi) increased exponentially with increased precipitation (P) (growth stage:Rpi=0.97P0.09, R2=0.5, P<0.05, fallow:Rpi=1.07P0.09, R2=0.98, P<0.01). Cumulative soil CO2 flux after 72 h (CO2-P) increased linearly with the precipitation level (grow:CO2-P=0.03P+5.99, R2=0.58, P<0.05; fallow:CO2-P=0.11P+6.04, R2=0.86, P<0.01)). The temperature sensitivity of soil CO2 flux (Q10) was correlated binomially with the precipitation level (grow:Q10=-0.005P2+0.18P+1.47, R2=0.37 P0.05, fallow:Q10=-0.0065P2+0.21P+1.18, R2=0.95, P0.01). Higher precipitation levels delayed but increased soil CO2 flux compared to lower precipitation levels, probably by influencing soil microbial activity.Soil CH4 flux during 0 to 72 h fluctuated at 1 to 8 mm of precipitation application and peaked at 16 and 32 mm. Cumulative CH4 flux after 72 h of precipitation (CH4-C) increased linearly with the precipitation amount (P) (wheat jointing stage:CH4-C=2.45P-6.09, i?2=0.92, P<0.01; summer fallow:CH4-C=2.43P-4.73, R2=0.91, P<0.01), indicating that a greater precipitation amount would cause a greater CH4 emission. Statistical analysis showed that CH4 flux was also correlated with soil water content and microbial biomass carbon but not with soil temperature. In the long run, small precipitation event (1-8 mm) could enhance the intensity of soil CH4 sink and such promoting effect would weaken with increased precipitation amount. However, large precipitation event (>16 mm) could change soil function on CH4 flux from sink to source by stimulating the activity of soil methanogens during a short term period.2. Responses of soil CO2 and CH4 emissions to precipitation under different frequencies in the Loess Plateau. We manipulated three frequencies precipitation during 60 days at winter wheat field in the Loess Plateau to determine how frequency influences the response of soil CO2 and CH4 efflux to precipitation event. Result shows that the CO2 flux increased immediately following precipitation events, with peak fluxes of 7.9,8.2, and 7.7 μmol CO2 m-2 s-1 for 15,I10, and I20, respectively. Cumulative CO2 flux from 55 to 60 d was greater for 15 and 110 than 120. Greater precipitation frequency at shorter precipitation intervals increased CO2 emissions than longer intervals due to increased soil water content. Soil CO2 flux related nonlinearly with soil water content and temperature (Fig.6). Soil water content explained 43 to 76% of variability in CO2 flux depending on the precipitation interval and the degree of relationship increased with increased precipitation interval. The relationship between CO2 flux and soil temperature was not significant. The Q10 values decreased with increased precipitation interval, suggesting that high frequency precipitation showed greater temperature sensitivity than low frequency precipitation. High frequency precipitation increased soil oxide absorption CH4 in the soil and strengthened the characteristics of soil as the sink of CO2 and CH4. Low frequency precipitation could change soil function on CH4 flux from sink to source in the short-time。 soil CH4 flux intensity in dry farmland influenced by temperature and moisture, Temperature is the dominant factor affecting the CH4 flux for high frequency precipitation, moisture is the dominant factor affecting the CH4 flux for low frequency precipitation.Precipitation frequency could significantly influence the soil oxidation absorb and generate CH4 by change the soil temperature and moisture.3. Effect of increasing precipitation in winter wheat growing season on greenhouse gas emission and net global warming potential. We evaluated the effect of five precipitation intensities (0,8,16,32 and 64 mm) on greenhouse gas emissions (GHG), net global warming potential (GWP), and greenhouse gas intensity (GHGI) under dryland winter wheat in the Loess Plateau of China. Soil N2O, CO2, and CH4 fluxes were monitored at 3 to 30-d intervals by using the static chamber method during a winter wheat growing season. Although CH4 emissions was not affected by precipitation intensity, CO2 emissions increased from 2.5% to 9.1% and N2O emissions from 4.3% to～11.7% as the intensity of the precipitation increased (p<0.05).. Compared with no precipitation simulation, wheat grain yield increased from 24.5 to 34.0% and biomass from 13.7 to and 25.1%, as precipitation intensity increased from 8 to 64 mm (p<0.05). As a result, increasing precipitation simulations shifted from 486 kg C hm-2 to-319～-1287 kg C hm-2, however, decreased GHGI by 16.1%～22.7%(p<0.05). The GHG emissions, GWP, yield and GHGI were well related with precipitation intensity (R2= 0.51～0.84,p<0.01). Increased precipitation intensity increased greenhouse gas emissions and global warming potential, but reduced yield-scaled global warming potential due to increased wheat yield. |
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