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'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’) plantation using the trenching method with a LI-8100 automated CO₂ flux system (LI-COR, Inc., Lincoln, NE, USA). The study was conducted at a 15-year-old poplar plantation located at Siyang Farm, Siyang County, Jiangsu Province, China (33°42'N, 118°09'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. The three treatments were designed as litter removal with trenching (treatment A), litter removal without trenching (treatment B), and a control(treatment C). Based on the trenching method theory, soil microbial respiration equaled soil respiration from treatment A, while root respiration and litter respiration were obtained based on the difference in soil respiration between treatments B and A and between treatments C and B, respectively. Soil respiration field observations were conducted during the middle of January, March, May, June, July, August, September, October, and December, in 2009, using a LI-8100 automated CO₂ flux system. Soil temperature at 10 cm depth was recorded hourly at a site using a thermo recorder (TR-71U, T & D Corporation, Japan). Root activity of four different root diameter classes was also analyzed after cutting by TTC (2, 3, 5-triphenyltetrazolium chloride) reaction.
The results indicated soil respiration seasonally fluctuated, reaching a maximum in summer and a minimum in winter. Treatment C obviously had the highest soil respiration of the three treatments, while treatments A and B were not distinctly different during the first 9 months. Starting in the tenth month, treatment A had obviously lower soil respiration than treatment B. These results show root and microbial respiration are difficult to separate in the first ten months after trenching and the contribution of each could not be evaluated because the truncated roots still respired as they decomposed. Also, the annual carbon dioxide efflux from the soil was 9.74 tC·hm^-2·a^-1 in the 15-year-old poplar plantation, divided as follows: 27.0% for decomposition of the litter layer (2.63 tC·hm^-2·a^-1) and 73.0% for root-microbial respiration (7.11 tC·hm^-2·a^-1). Soil respiration of each treatment was exponentially and significantly correlated to soil temperature at 10 cm deep. The correlation coefficients of the regression equation were in the order of treatment A > treatment B > treatment C. The Q₁₀ values of each treatment were calculated using the regression equations, and showed a similar order: 2.94, 2.69, and 2.59 for treatments A, B, and C, respectively. The largest Q₁₀ value of treatment A revealed microbial respiration might be more sensitive to temperature than root respiration. The truncated roots had different survival times depending on root diameter. After cutting, the activity of fine roots (D<2 mm) diminished to 0 during the first month. Small (2 mm≤D<5 mm), medium-sized (5 mm≤D<10 mm), and coarse roots (D>10 mm) showed the highest activity during the 2nd, 3rd, and 4th month after cutting. As the diameter increased, roots obviously survived for longer periods.