| Global warming is one of the most severe environmental problems that threatening the survival of human being. To meet the European target of reducing emissions of greenhouse gases, conversions between arable land and grassland have been increasing in northern German temperate maritime climate zone. It is crucial to local governments to understand the effect of land use changes on the emission of CO2 in northern Germanagricultural area. Soil respiration varies temporally, and it plays important roles in increasing atmospheric greenhouse gases and aggravating global warming. The research of agricultural soil respiration dynamics and its driving factors in northern German temperate maritime climate zone provides scientific tools in thereduction and accurate prediction of emissions of CO2. Moreover, it improves the theories of carbon cycle while further revealing the process of soil respiration and its driving forces.The objectives of this research are comparing temporal variations in soil respiration and its temperature sensitivity in grassland and arable land from October 2010 to July 2011 and November 2011 as well as from April 2012 to July 2012, and illustrating soil respiration dynamics in agricultural soils and revealing its driving factors in northern German temperate maritime climate zone. It was applied with experiments of timely field monitoring and laboratory incubation of soil respiration, and statistical methods of principal component analysis and hierarchical cluster analysis in this study. The conclusions are as following:(1) Temporal variations in arable soil respiration based on field monitoring were analyzed in the research area. At the diurnal scale, during the whole research period from October 2010 to July 2011, mean diurnal soil respiration rates changed as a single peak curve with values were lower before the peak and higher after the peak. Diurnal soil respiration rate of each month showed that the range was 3-236 mg CO2 m-2 h-1and mean values was 99 mg CO2 m-2 h-1of the maximum values of each month, and the range was 2-154 mg CO2 m-2 h-1and mean values was 67 mg CO2 m-2 h-1of the minimum values of each month, moreover, the range was 2-186 mg CO2 m-2 h-1and mean values was 78 mg CO2 m-2 h-1of the mean values of each month. The highest and lowest diurnal values mainly occurred at 12:00-16:00 and 04:00-08:00, respectively. At the seasonal scale, soil respiration rate was highest in summer and was lowest in winter, while it maintained in the middle in spring and autumn with higher values occurred in spring than autumn. Monthly mean soil respiration rates in arable land ranged between 59-4473 mg CO2 m-2 day-1, while the highest and lowest values occurred in January 2011 and May 2011, respectively. The mean values of soil respiration in winter, spring, summer and autumn were 412, 2519, 3720, and 1845 mg CO2 m-2 day-1, respectively.(2) Temporal variations in arable soil respiration based on laboratory incubation were studied in the research area.Laboratory incubation was applied to arable soils taken during November 2011 and from April to July 2012 to study the seasonal variation of soil respiration. The median values of each month of arable soil respiration rate under incubating temperature of 5-40 oC and soil moisture of 30%, 50%, and 70% of the full water holding capacity changed in similar seasonal patterns and ranged at 0.00-8.84 μg CO2 g-1 h-1. When incubating temperature was 5-15 oC, it showed higher values in winter than in the next spring and summer. When incubating temperature was 20-40 oC, it showed lower values in winter than in the next spring and summer. Under all incubating moisture in this study, when incubating temperature was 5-15 oC, the highest median arable soil respiration rates of each month all occurred in November 2011 between 4.56 and 5.18 μg CO2 g-1 h-1, and the lowest median values of each month occurred during from April to July 2012 at 0.00 μg CO2 g-1 h-1. When incubating temperature was 20-40 oC, the highest median values of each month all occurred during from April to July 2012 between 4.56 and 5.18 μg CO2 g-1 h-1, and the lowest median values of each month mainly occurred in November 2011 at 0.00 μg CO2 g-1 h-1.(2) Temporal variations in grassland soil respiration based on laboratory incubation were revealed in the research area.Laboratory incubation was applied to grassland soils taken during November 2011 and from April to July 2012 to study the seasonal variation of soil respiration. The median values of each month of grassland soil respiration rate under incubating temperature of 5-40oC and soil moisture of 30%, 50%, and 70% of the full water holding capacity changed in similar seasonal patterns and ranged at 0.00-30.83μg CO2 g-1 h-1. When incubating temperature was 5-10oC, it showed higher values in winter than in the next spring and summer. When incubating temperature was 15-30oC, it showed a declining trend from April to July 2012. When incubating temperature was 35-40oC, it showed lower values in winter than in the next spring and summer. Under all incubating moisture in this study, when incubating temperature was 5-10oC, the highest median grassland soil respiration rates of each month both occurred during from April to July 2012 between 5.04 and 5.63 μg CO2g-1h-1, and the lowest median values of each month occurred during from April to July 2012 between 0.00 and 1.44 μg CO2 g-1 h-1. When incubating temperature was 15-30oC, the highest median values of each month all occurred in April 2012 between 6.50 and 19.87 μg CO2 g-1 h-1, and the lowest median values of each month mainly occurred in July 2012 between 2.90 and 10.52μg CO2 g-1 h-1.When incubating temperature was 35-40oC, the highest median values of each month both occurred in April 2012 between 21.15 and 26.12 μg CO2 g-1 h-1, and the lowest median values of each month both occurred in November 2011 between 11.00 and 12.57μg CO2 g-1 h-1.(4) The differences of influencing factors of soil respiration between arable land and grassland were compared. For soil taken in November 2011 and from April to July 2012, values of most properties observed in this study including soil bacterial biomass, soil dissolved carbon content, and soil nitrogen content were one to three times higher in grassland than in arable land. There were small differences between grassland and arable land in the values of soil bacterial mean cell volume, ratio of mean bacterial surface to mean cell volume, metabolic quotient, soil temperature and moisture in situ. Only the content of available soil phosphate was higher in arable land than in grassland with values of 1.5 times higher. The seasonal variations in soil microbial, physical and chemical properties in grassland and arable soils were nearly the same. The values of total soil bacterial number, soil bacterial biomass, total bacterial surface, and mean soil bacterial cell volume were higher in winter than in the next spring and summer. While the values of ratio of mean bacterial surface to mean cell volume, metabolic quotient, soil temperature in situ, air temperature, soil nitrate content, and soil nitrite content were lower in winter than in the next spring and summer.(5) The temporal variations and differences in temperature sensitivity of soil respiration in arable land and grassland were explored in this study. Based on field monitoring, the Q10 values of the whole research period from October 2010 to July 2011 changed between 1.76 and 2.38 at diurnal scale. It showed relatively high values during 0:00-04:00 and 20:00-00:00 above 2.3 and relatively low values during 10:00-16:00 below 2.0 ranging from 1.76 to 2.38 at seasonal scale. It showed relatively high values in winter above 4.0 and relatively low values in spring and summer below 1.5. Based on laboratory incubation under various temperature and moisture, the arable Q10 values of the whole research period during November 2011 and from April to July 2012 only varied slightly and were mainly around 1.0 at seasonal scale. However, the Q10 values changed at lager ranges seasonally in grassland than in arable land. In grassland, when incubated under all moisture conditions and 10-20oC, the Q10 values ranged between 1.0 and 3.0 with lowervalues in winter and highervalues in spring and summer while being highest in April 2012 and being lowest in November 2011. The temporal variation of Q10 values generated based on field monitoring was much stronger than that generated based on laboratory incubation. However, the Q10 values generated under both methods significantly decreased as soil temperature increased, while soil moisture only showed limited effects on it.(6) The driving factors of temporal variations in soil respiration and their effects on soil respiration changes were stated in this study. Similar key driving factors was determined by principal component analysis and hierarchical cluster analysis. The most important key driving factors are soil temperature and metabolic quotient in grassland and arable land. Other common key driving factors are the content of soil organic carbon, bulk density, water holding capacity and soil bacterial biomass. Except that, the key driving factors of temporal soil respiration changes still included percentage of silt, soil nitrate content and p H in grassland, while it was percentages of silt and clay, content of available soil phosphate and mean cell volume of soil bacterial in arable land. |
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