We report on the effects of forest management practices of understory removal and N-fixing species（Cassia alata） addition on soil CO2 fluxes in an Eucalyptus urophylla plantation（EUp）,Acacia crassicarpa plantation（ACp）,10-species-mixed plantation（Tp）,and 30-species-mixed plantation（THp） using the static chamber method in southern China.Four forest management treatments,including（1） understory removal（UR）;（2） C.alata addition（CA）;（3） understory removal and replacement with C.alata（UR＋CA）;and（4） control without any disturbances（CK）,were applied in the above four forest plantations with three replications for each treatment.The results showed that soil CO2 fluxes rates remained at a high level during the rainy season（from April to September）,followed by a rapid decrease after October reaching a minimum in February.Soil CO2 fluxes were significantly higher（P 〈 0.01） in EUp（132.6 mg/（m2.hr）） and ACp（139.8 mg/（m2.hr）） than in Tp（94.0 mg/（m2.hr）） and THp（102.9 mg/（m2.hr））.Soil CO2 fluxes in UR and CA were significantly higher（P 〈 0.01） among the four treatments,with values of 105.7,120.4,133.6 and 112.2 mg/（m2.hr） for UR＋CA,UR,CA and CK,respectively.Soil CO2 fluxes were positively correlated with soil temperature（P 〈 0.01）,soil moisture（P 〈 0.01）,NO3？-N（P 〈 0.05）,and litterfall（P 〈 0.01）,indicating that all these factors might be important controlling variables for soil CO2 fluxes.This study sheds some light on our understanding of soil CO2 flux dynamics in forest plantations under various management practices.
A pot experiment was conducted in artificially Cd-contaminated （5 mg Cd kg 1） soils to investigate the feasibility of using lime （3 g kg-1） or phosphate （80 mg P kg-1） to mitigate uptake of Cd by vegetables. Five common vegetables in South China, including lettuce （Lactuca sativa L. var. ramosa Hort.）, Chinese cabbage [Brassica rapa L. subsp. Chinensis （L.） var. parachinensis （L. H. Bailey） Hanect], Chinese broccoli （Brassica oleracea L. vat. albiflora Kuntze）, white amaranth （Amaranthus tricolor L.） and purslane （Amaranthus viridis L.）, were grown in the soils and harvested after 60 d. The results showed that liming significantly reduced Cd uptake by most vegetables by 40%-50% （or a maximum of 70%）, mainly due to immobilization of soil Cd. Increased availability of Ca in the soil might also contribute to the Cd uptake reduction as a result of absorption competition between Ca and Cd. Liming caused biomass reduction in white amaranth and purslane, but did not influence growth of the other vegetables. Phosphate decreased Cd uptake by vegetables by 12% 23%. Compared with lime, phosphate decreased, to a smaller extent, the bioavailability of Cd in most cases. Phosphate markedly promoted growth of vegetables. Changes in soil chemistry by adding lime or phosphate did not markedly influence nutrient uptake of vegetables except that lime increased Ca content and phosphate increased P content in shoots of the vegetables. The results suggested that a proper application of lime could be effective in reducing Cd uptake of vegetables, and phosphate could promote growth of the vegetables as well as alleviate the toxicity of Cd.
TAN Wan-Neng LI Zhi-An QIU Jing ZOU Bi LI Ning-Yu ZHUANG Ping WANG Gang
