Soil Respiration Of Different Land Use Types On Beach Land In The Middle And Lower Reaches Of The Yangtzi River

Soil CO₂ efflux in terrestrial ecosystems is an important consideration in the studies of the global carbon cycle, its contribution to the global carbon budget has been the focus of wide concern. It has been widely accepted that environmental factors such as temperature, moisture and vegetation are the most important to regulating soil respiration. The relationship between soil CO₂ efflux and environmental factors provides an opportunity to measure the CO₂ efflux to the ecosystem and larger scales, not limited in the chamber level. Beach land is one type of the major terrestrial ecosystems. However, few studies about soil respiration coulde be followd so far. Thus, we measured the soil respiration rate for one year(2005～2006) using the IRGA(LI-8100,LI-COR,Lincom,NE, USA)at the land of Poplar Plantation, Reed, Carex and crop field on the Beach of Yangtzi River to illustrate the characteristics of soil respiration. The eight main results are as follows:(1) The profile of the diurnal variation of soil respiration rate showed a single peak in different types of land use on Beach Land in the Middle and Lower Reaches of the Yangtzi River.Soil respiration rate reached the highest at 11:00～13:00,and the lowest at about 5 o'clock in the early morning.The extent of soil respiration dynamic variation per day in summer and autumn is less than that in spring and winter.The average soil respiration rate a day is 2.6939±0.5335μmol m^-2s^-1(P<0.05,n=32) on Beach Land of Yangtzi River.while the average soil respiration rate a day in four different land use types exhibited:Reed (4.245μmol m^-2 s^-1) > Carex(2.898umol m^-2 s^-1) > Poplar(2.774umol m^-2 s^-1) > crop(0.858umol m^-2 s^-1).Meanwhile, Seasonal variation of soil respiration is obvious. The value of soil respiration rate in " Poplar.Carex and Crop exhibited summer> spring > autumn > winter, while the value of soil respiration rate exhibited spring>summer>autumn>winter in Reed land.(2) The degree of soil respiration changed with different plant communities. The soil respiration rate of four plant communities exhibited Cynodon dactylon+ Hemarthria>Leonurus artemisia>Oenanthe javanica>Cynodon dactylon+Daucus carota>Hemarthria. The variation coefficient in four plant communities is 54.4%.(3) The soil respiration rate of Poplar Plantation on Beach had obvious annual variation laws and its dynamic change showed high correlation with temperature and soil water content(5cm).the latter superior to the former, for the variation of temperature is different with soil respiration rate, appearing lag in time.(4) The correlation of soil respiration rate with the surface soil temperature was better than that with the soil temperature(5cm) in Reed,Carex and crop field. The soil respiration rate of Poplar Plantation correlated with surface soil temperature(R²=0.6225)was close with the soil temperature(5cm) (R²=0.644).The value of Q₁₀ was different according to the different layer soil temperature. The average Q₁₀ on Beach Land of Yangtzi River is 2.275±0.576(P<0.05,n=8).The soil respiration rate of Poplar Plantation,Reed and Carex had good relationship with soil water content(5cm).the value of R² is 0.8062,0.8612,0.8202,respectively. While the soil respiration rate of crop field has bad relationship with soil water content(5cm).Fitting model based on surface soil temperature and soil water content(5cn) would explain soil respiration variation of Poplar Plantation85.5%,Reed58.7%, Carex65.5% and crop field74.9%.The annual emission flux of soil respiration in different land use types was stepped :Reed(6.475 t hm^-2 a^-2)>Carex (3.427 t hm^-2 a^-1)>Poplar Plantation(3.407 t hm^-2 a^-1)>crop field(1.037 t hm^-2 a^-1).(5) Soil nutrients had a great influence on soil respiration. The content of soil nutrients in four different land use types was ranked as follow: Reed > Carex > Poplar Plantation > crop field , the same as soil respiration rank. The variation of soil nutrients is summer>autumn>winter>spring, which is not consistent with soil respiration based on the seasonal variation, in the result of the water and temperature influence. The content of N in soil is just the major factor to affect crop growth and output, not the crucial factor to the soil respiration. But in all, it is convictive to explain the soil respiration by soil N content in some extent.(6) Soil microbial numbers were always varying with time. No matter bacteria,fungus and actinomycete, the seasonal variation of quantity is summer>spring>autumn>winter, which is the same with soil respiration change. Amount of microorganism in four land use types : Poplar Plantation,Reed,Carex,crop field was 66.73×10⁴cfu/g,102.45×10⁴cfu/g,130.95×10⁴cfu/g, 74.65×10⁴cfu/g, respectively. The microbial numbers in Reed is less than Carex, but soil respiration rate is larger than Carex. The main reason is that the extent of roots complex in Reed is much simpler than that in Carex. The other reason is that the microenvironment and soil characteristics in Reed is superior to Carex. The crop field is influenced by human being, though large in microbial number, soil characteristics and water, temperature circumstance are inferior to other land use types, as a result, soil respiration rate is low.(7) The biomass in different land use types was ranked as follow Reed> Carex> Poplar Plantation> crop field, which is the same with soil respiration change. meanwhile the seasonal variation of the biomass may explain the soil respiration change exactly.(8) The soil respiration rate and related influence factor presented obvious variation in different layer of soil profile. Compared with soil temperature, the variation of soil respiration rate had a better correlation with the soil water content(5cm). The plant roots and soil nutrients are also the key influence factors to soil respiration, these factors together played a decisive role in the soil respiration value and variation.