Soil depth- and root diameter-related variations affect root decomposition in temperate pine and oak forests

Aims Assessment of factors regulating root decomposition is needed to understand carbon and nutrient cycling in forest ecosystems. The objective of this study is to examine the effects of soil depth and root diameter on root decomposition and to analyze the relationship of root decomposition with factors such as soil environmental conditions and initial litter quality. Methods Two decomposition experiments were conducted in natural pine (Pinus densiflora) and oak (Quercus serrata) forests over a 2-year period using the litterbag technique. For the soil depth experiment, 216 litterbags containing fine roots (empty set = 0-2 mm) were buried at 0-10-, 10-20- and 20-30-cm soil depths. Soil properties and soil enzyme activities and microbial biomass at each soil depth were analyzed. For the root diameter experiment, 216 litterbags containing roots 0-1-, 1-2- and 2-3-mm in diameter were buried at 10-cm soil depth. The initial litter qualities (carbon (C), nitrogen (N), calcium (Ca) and phosphorus (P) concentrations) for each of the root diameter classes were analyzed. Litterbags were retrieved after 3, 6, 12 and 24 months in each forest type. Important Findings The root decomposition rate was significantly altered by soil depth and root diameter. After 2 years, the root decay constant at 0-10-cm depth (pine: 0.35 and oak: 0.41) was significantly higher than that at 10-20-cm (0.31 and 0.37) and 20-30-cm (0.32 and 0.33) depths in the P. densiflora and Q. serrata forests. Enzyme activities and microbial biomass declined with soil depth, which may be associated with decreasing soil moisture and organic matter. The decay constant for the 0-1-mm roots (pine: 0.32 and oak: 0.37) was higher than that of 1-2-mm (0.29 and 0.33) and 2-3-mm roots (0.26 and 0.33) for the P. densiflora and Q. serrata forests. Difference in initial P concentration and C/N ratio among the different diameter roots were linearly related with root decomposition. In particular, the increasing C/N ratio with root diameter resulted in decreases in the decomposition rate. These results indicate the surface soil microbial activities and initial C/N ratio of root litter as important drivers of C dynamics in temperate pine and oak forests.