The contribution of root respiration to total soil respiration is one of the most interesting, important, and methodologically complicated problems in the study of the carbon budget in soils. A trenching method and a root biomass regression method were used to determine the contribution of root respiration to total soil respiration in a subtropical forest ecosystem. The average root respiration contributions were 37.15% with the range of 13.04-51.23% by trenching method and 31.80% with the range of 10.64-56.10% by root biomass regression method from August 2005 to July 2006. In growing season, the contributions of root respiration were 47.25 and 43.53% estimated by trenching method and root biomass regression method, respectively. There was no significant difference between the monthly attributions of root respiration to soil respiration estimated by two methods. Our results showed that trenching and root biomass regression methods were both suitable to estimate the root respiration in this forest.

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CO 2 emissions from soils were measured under 3 land-use types at the adjacent plots of forest plantation, grassland, and cropland from January 2005 to December 2006. Mean soil CO 2 efflux rates measured during the 2-year study varied from 59 to 527 mg CO 2 /m 2 .h in forest plantation, 37 to 498 mg CO 2 /m 2 .h in grassland, and 32 to 397 mg CO 2 /m 2 .h in cropland. Soil respiration in the 3 types of land-use showed a similar seasonal pattern in variation during both years, in which the single-peaked curve occurred in early summer and the minimum in winter. In particular, the date of maximum soil CO 2 efflux rate in cropland occurred about 30 days earlier than in forest and grassland in both 2005 and 2006. The relationship of soil respiration rate (R) with soil temperature (T ) and soil moisture (W ) fitted well to the equation R = b 0 e b 1 T W b 2 (a, b, c were constants) than other univariate models which consider soil water content or soil temperature alone. Soil temperature and soil moisture together explained 69-92% of the temporal variation in soil respiration in the 3 land-use types. Temperature sensitivity of soil respiration (Q 10 ) was affected positively by soil moisture of top 0.1 m layer and negatively by soil temperature at 0.05 m depth. The relationship between Q 10 values and soil temperature (T ) or soil moisture (W ) indicated that a 18C increase in soil temperature at 0.05 m depth will reduce the Q 10 value by 0.07, 0.05, and 0.06 in forest, grassland, and cropland, respectively. Similarly, a 1% decrease in soil moisture of the top 0.1 m layer will reduce the Q 10 value by 0.10, 0.09, and 0.11 in forest, grassland, and cropland.