Soil respiration in forests is an important part of the carbon cycle in terrestrial ecosystems. It primarily includes respiration occurring in plant roots (autotrophic respiration) and in soil microbes (heterotrophic respiration). The carbon sources for autotrophic and heterotrophic respiration are different, and their contributions to total soil respiration may also be different. Understanding the mechanisms of soil respiration require separating its component. Many studies have focused on developing methods to separate the components of soil respiration, such as the root exclusion method, isotopic method, and in situ root techniques although each method has some defects. The trenching method is generally considered effective and can separate root and microbial contributions from total soil respiration in forested ecosystems. Poplar, one of the most important timber species, also provides important ecological services. Currently, China has about seven million hectares of poplar plantations, and it plays an important role in a supplying the country忆s timber demand. However, few published reports can be found related to the characteristics of soil respiration. The contributions of root and microbial respiration to the total soil respiration in poplar plantations remain unclear. In this paper, the components of soil respiration were partitioned in a poplar (Populus deltoides Bartr. cv. ' Lux 爷) plantation using the trenching method with a LI鄄8100 automated CO 2 flux system (LI鄄COR, Inc. , Lincoln, NE, USA). The study was conducted at a 15鄄year鄄old poplar plantation located at Siyang Farm, Siyang County, Jiangsu Province, China (33毅42忆N, 118毅09忆E). The trees averaged 30 cm diameter at breast height and 26 m tall. In early January, 2009, experiments with three treatments and four repetitions were set up in this poplar plantation.

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