| The experiment was conducted in the rain-fed wheat field at the loess plateau during2011-2013three consecutive seasons. Through combined eddy covariance system continuousobservation with soil respiration measurement system fix observation, systematic analysisdynamic variation characteristics of CO2flux and it,s environment responding mechanism inrain-fed wheat field ecosystem at the Loess Plateau. This research mainly achieved thefollowing conclusion:1. Variation of soil respiration rate in a wheat field(1) From reviving to ripening period, the seasonal variation of soil respiration rate inwheat field soil was relatively large, and the changes in different years were big too. In May16,2012(booting stage), soil respiration rate reached the maximum value (299.68mg m-2h-1),soil respiration rate reached the minimum value (133.67mg m-2h-1) in March27(revivingstage); In2013, soil respiration rate reached the maximum value (363.18mg m-2h-1) in May20(heading stage), the minimum value (132.59mg m-2h-1) appeared in March24(revivingstage). According to the analysis results in the two years, the maximum value of daily soilrespiration (2012,0.017Mg ha-1;2013,0.021Mg ha-1) appeared in May, while the minimumvalue of daily soil respiration (2012,0.010Mg ha-1;2013,0.009Mg ha-1) appeared in March.(2) Through the study in2012and2013, the results showed that wheat field soilrespiration rate was significant correlation (P<0.01) with soil microbial biomass carbon andthe ratio of bacteria/fungus, and it was significant correlation (0.01<P<0.05) with hydrogenperoxide activity and living root biomass. The correlation between soil respiration rate andurease, protease, sucrase activity not reached a significant level (P>0.05), but soil respirationrate was significantly negatively correlated (0.01<P<0.05) with soil microbial biomassnitrogen.(3) Wheat field soil respiration rate had obvious diurnal variation pattern, the diurnalvariation showed a single peak curve, the peak usually appeared at11:00(flowering, fillingand harvest stages) or13:00(reviving and jointing stages), the lowest value appeared ataround7:00in the morning. Soil respiration rate showed a significant correlation with soiltemperature in0-30cm soil layer, and the relationship fitted the quadratic equation model (Therange of R is0.52to0.56, P <0.05), and0-15cm soil temperature was the more relevant (R=0.56*). Soil respiration rate was the most relevant with0-15cm soil moisture (R=0.60*). Inthe same soil layer, coefficient of determination obtained from a bivariate model whichcontain soil temperature and soil moisture was significantly higher than single factor model.(4) During the whole observation period, soil respiration rate in wheat field soil washigher than bare soil. Through study in0-15cm and15-30cm soil layers, we found that in thesame soil depth, the correlation between soil respiration rate and soil water content wassmaller in bare land than in wheat field, and with the increased of soil depth, the correlationcoefficient reduced gradually. In the same soil depth, correlation between soil respiration rateand soil temperature in wheat field was smaller than in bare land, and with the increased ofsoil depth, the correlation coefficient of the two treatments was gradually decreased.2. Variation characteristics of wheat field ecosystem carbon flux(1) Diurnal variation of CO2flux in wheat field ecosystem was a single peakcurve. In the seeding and tillering stage, the CO2flux value was very small, with an order ofmagnitude of10-3. From reviving to filling stage, during7:30~8:00am in a day, wheat fieldecosystem changes from releasing CO2into absorbing CO2, namely from carbon sourceinto carbon sink; At about6:00pm, it changes from carbon sink into carbon source. Inripening stage, wheat field ecosystem performed CO2release each day. The maximum dailyCO2flux value occurred in jointing and booting stage. Seeding and tillering stage, winteringstage, filling stage and ripening stage were carbon sources, other stages were carbon sinks.In seeding and tillering stages, CO2release per unit area was the most, which was69.2423gCO2m-2; In mature stage, CO2release per unit area was the least, which was16.7605gCO2m-2; In jointing and booting stages, CO2uptake per unit area was the most, which was254.6388gCO2m-2; In heading stage, CO2uptake per unit area was the least, which was65.1799gCO2m-2. The wheat field ecosystem absorb CO2per unit area was about400gCO2m-2during the whole wheat growing season.(2) In2012and2013, the average daily CO2flux range was-18.32to2.11g m-2d-1and-16.36to2.35g m-2d-1. In2013, wheat field ecosystem changed into obvious carbon sink(After sowing177d) was later than2012(After sowing157d). In2013, from reviving toheading stage, CO2sink function in wheat field ecosystem was lower than the same stages in2012, but from filling to ripening stage, CO2source function in wheat field ecosystem washigher than the same period in2012. In wheat growing season, the net CO2absorption inwheat field ecosystem at the Loess plateau was greater than the other wheat field ecosystems,but lower than maize field ecosystem.(3) CO2flux showed a significant linear relationship with5cm soil temperature(P<0.01). Correlation between5cm soil temperature and CO2flux in2012was higher than in 2013. The relationship between nighttime CO2flux and air temperature wassignificant (P<0.01) and fitted the exponent model. The relationship between daytime CO2flux and photosynthetically active radiation was significant (P<0.01) and fitted quadraticmodel. When photosynthetic active radiation was low, with the increased of photosyntheticactive radiation, CO2uptake in wheat field ecosystem increased; when photosynthetic activeradiation exceed a certain threshold, with the increased of photosynthetic active radiation,CO2uptake in wheat field ecosystem decreased.(4) The relationship between CO2flux and air temperature reached extremelysignificant level (P<0.01) in erecting and heading stages, the correlation coefficient reachedthe maximum value in jointing and booting stages (P<0.01); The relationship betweendaytime CO2flux and photosynthetic active radiation reached extremely significant level(P<0.01) in erecting stage, jointing and booting stages and heading stage; The relationshipbetween nighttime CO2flux and5cm soil temperature reached extremely significantlevel (P<0.01) in jointing and booting stage and heading stage. Given the influence of rainfall,the degree of correlation between CO2flux and air temperature, soil temperature,photosynthetic active radiation weakened at different growth stages.(5) The relationship between under-ground biomass, SPAD value, LAI and the CO2flux value showed a negative correlation level, and the relationship between LAI and CO2fluxshowed a significant negative correlation level (P<0.01); Above-ground biomass was positivecorrelated with CO2flux, but not reached significant level (P>0.05). Through multiple linearstepwise regression analysis between CO2flux and soil factors, the results showed that in soilimpact factors hydrogen peroxidase activity accounted for79%of CO2flux change, alongwith the change of hydrogen peroxidase activity, the change direction of CO2flux was in theopposite.(6) In different growth stages, the contribution of CO2released by wheat field soilrespiration to wheat ecosystem CO2flux was different. In late-filling and ripening stages, CO2released by wheat field soil respiration had a decisive influence on wheat ecosystem CO2flux. |
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