Response of soil CO₂ efflux to forest conversion in subtropical zone of China

In many areas of South China, native broad-leafed forests have been cleared for the last several decades, and subsequent development has involved the plantation of more productive forest species. Currently, there is little known about the effects of forest conversion to tree plantations on C sequestration in the subtropical China. Soil respiration in forest ecosystems is one of the major pathways of C flux in the global C cycle, second only to the gross primary productivity, and is markablely affected by forest managements. The present study, as a part of the programe examining what changes in ecosystem carbon dynamics during conversing native forest into tree plantations, investigated changes in soil CO₂ efflux between a native forest of Castanopsis kawakamii (NF) and two plantations of Castanopsis kawakamii (CK) and Cunninghamia lanceolata (Chinese fir, CF) in sanming, Fujian. Mean annual rainfall was 1605.9mm in this area. Total rainfall was 1864.6mm in year 2002, however, year 2003 was characterized by an extremely low total rainfall of 1002.3mm. Soil respiration was measured in situ 5 to 7 times at end of month during 2002-2003 by the alkali sorption technique, and soil temperature and water content at 10 cm depth were determined at each measurement occasion. Over the studies period, soil respiration rate varied from 403.47 to 1001.12 mg CO₂ m^-2 h^-1 in the NF, from 193.89 to 697.86 mg CO₂ m^-2 h^-1 in the CK, and from 75.97 to 368.98 mg CO₂ m^-2 h^-1 in the CF. It showed a similar seasonal pattern among three forests, with the single peak occurred during late spring or early summer and a minimum during dormant period. Neither soil temperature nor soil water content could explain the seasonal variation of soil respiration well, however, the model R = ae^bTW^c (R soil respiration rate, T soil temperature, W soil water content, and a, b, c the constants) indicated that soil temperature and soil water content together could explain 80%-96% of seasonal variation in soil respiration rates. Further, soil water content would become an extreme limiting factor for soil respiration in dry year (2003). Soil respiration rate in the CF had the highest soil temperature and humidity sensitivity, followed by the CK and the NF. Annual soil CO₂ effluxes ranged from 13.742 tC·hm ^-2·a^-1 in the NF to 9.439 tC·hm ^-2·a^-1 in the CK and to 4.543 tC·hm ^-2·a^-1 in the CF. The decline in soil respiration resulted mainly from the many changes associated with forest conversion, including the quantity and quality of organic inputs, root biomass and activity, and the content and component of soil organic carbon.