| Under the background of global climate change, the research of soil CO2 fluxes is a widespread focus, particularly in tropical areas where land type changes have become more active. Howerver the research data of the CO2 flux of tropical soil are relative lack and weak in China. Based on this, the soil respiration rates of the green land and Eucalyptus plantation were measured by using the LI-8100 A Soil CO2 Flux System to explore the relationship between the soil CO2 flux and environmental factors of different type lands, to reveal the environmental impact mechanism of soil respiration, and compare the differences of soil respiration in two different land-use types, which will provide a scientific basis for accurately estimating the amount of CO2 released from different types of soil. The main conclusions are as follows:(1) Diurnal variation of air temperature showed a single peak curve near the surface of grassland and forestland, but the temperature of grassland was higher than forest land. The change of soil temperature in forest land lagged behind the grassland. The maximum temperature of the grassland soil in the depth of 5cm appeared at about 14:30 and the minimum temperature at 7:30, while the maximum and minimum of forestland soil appeared at 16:00, 8:00, respectively. The monthly mean temperatures of air near ground and soil in the depth of 5cm of grassland were generally higher than that of Eucalyptus forestland. The content of organic matter was significantly higher in grassland than that in forest land, and the microbial functional diversity was slightly higher, the microbial activity was stronger in grassland soil than that in Eucalyptus forest land.(2) Diurnal variation of soil respiration showed a single peak curve in grassland, and peak value of soil respiration occured mainly between 9:30 a.m to 3:00 p.m, the valley value between 6:00~ 8:30 a.m. The daily variation of the soil respiration ranged from 0.26 to 3.09 ?mol·m-2·s-1. During the rainy season, the soil respiration rate was about 2 times of the dry season, and the diurnal variation was more significant than that in the dry season. Seasonal variation of soil CO2 flux showed a single peak curve. The peak value appeared between June to Auguest, and the valley value, December to February.It was 1.0~ 9.8% that the absolute deviation between the measured value of 8:00 to 10:00 a.m and the daily average value of soil CO2 flux in different months, 9:00 to 11:00 a.m, 0.0~ 6.7%, and 10:00 to 12:00 a.m, 1.3~ 6.9%. By chosing the representative period of soil respiration in different months, it can be calculated that the monthly average was 101.35~ 503.22 g·m-2·m-1, the annual total was 3564.68 g·m-2·a-1 of soil CO2 emission in 2014, and the monthly mean 103.25~ 414.83 g·m-2·m-1, the annual total 3147.27 g·m-2·a-1in 2015. The average total respiration was 3355.98 g·m-2·a-1 in the years of 2014 and 2015.(3) The diurnal variation of soil respiration showed a fluctuating bimodal curve during the dry season in Eucalyptus forest land, but mostly unimodal curve during the rainy season. The peak value of soil respiration occured mainly between 13:30 p.m to 16:30 p.m, and the valley value between 7:30 to10:00 a.m. The diurnal range of soil respiration was 0.27~ 1.83 ?mol·m-2·s-1, and the maximum in June, the minimum in November. Seasonal variation of soil CO2 flux showed a single peak curve. The peak value appeared in June, 3.72 ?mol·m-2·s-1, and the valley value in January, 1.82 ?mol·m-2·s-1. Change trends of soil respiration were basically consistent with soil temperature in the depth of 5cm in Eucalyptus forest, but the maximum value of soil respiration was delayed by the maximum of soil temperature.The absolute deviation was 0.14~ 10.14% between the measured value of 8:00 to 10:00 a.m and the daily average value of soil CO2 flux in different months, 9:00 to 11:00 a.m, 1.61~ 7.11%, and 10:00 to 12:00 a.m, 1.28~ 6.27%. By choosing the representative period of soil respiration, the soil CO2 flux of Eucalyptus forest land was calculated in different months. The highest soil CO2 flux was 411.71g·m-2·m-1 in June, the lowest 189.47 g·m-2·m-1 in February, and the annual total was 3564.65 g·m-2·a-1 in 2015.(4) There was a significantly exponential correlation between soil respiration and temperature. 50.3~ 63.4% variation of soil respiration could be interpreted by soil temperature in the depth of 5cm, and 38.3~ 53.2% by air temperature. The Q10 value(the temperature sensitivity) was 1.169~ 2.208 in Eucalyptus forest land, 0.852~2.032 in grassland. The maximum Q10 appeared in the dry season, the minimum in the rainy season in grassland, while the maximum and minimum Q10 were found in the rainy season in forest land. Q10 was generally higher in dry season than that in rainy season, and the effect of soil temperature was more significant on soil respiration in dry season.There were highly significant correlations between soil respiration and soil water, air humidity and vapor pressure deficit. The correlations between soil respiration and air humidity, vapor pressure deficit were higher, and the correlation between soil respiration and soil water at 0-10 cm depth was lower in grassland than that in Eucalyptus forestland. The response of soil respiration to soil water was more obvious in Eucalyptus forest, and the effect was greater of grassland temperature on soil respiration.(5) Compared with the sunny day, changes of soil respiration were more complex in rainy day. Rain infiltration usually increased soil respiration, however, when the rainfall exceeded a certain threshold, the soil water would have an inhibitory effect on respiration. A general intensity rainfall could increase soil respiration, and the increasing range was associated with rainfall and duration of precipitation. Rainfall could increase soil respiration by 4~ 6 times in a short period. However, heavy rainfall inhibited soil respiration, and respiration decreased by 86~ 93% in a short period. |
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