Spatial variation in soil respiration rate is controlled by the content of particulate organic materials in the volcanic ash soil under a Cryptomeria japonica plantation

Soil respiration is one of the major C fluxes in the global C cycle and is a key factor in understanding the global C balance associated with climate change, but the factors controlling its spatial variability have not been well explored. This study aimed to clarify the causes of spatial variation in soil respiration rate on volcanic ash soil. We established a standing-tree (ST) plot and a clear-cutting (CC) plot in December 2012 at a 35-year-old Cryptomeria japonica plantation in Tokyo, Japan. CC plot was logged in March 2013, and new organic matter supply was halted after clear-cutting. From January 2013 to August 2019, soil respiration rates were measured periodically at 21 and 19 measuring points in ST and CC plots, respectively. The measuring points were randomly distributed with varying distances in ST plot (0.24 ha) and CC plot (0.23 ha). In August 2019, the carbon content of the litter (Ao) layer, total carbon content of soil, carbon content of the low-density fraction (LF-C; < 1.6 g cm−3) of soil, fine root biomass, and bulk density of soil for 0–5, 5–15, and 15–30 cm mineral soil layers were measured at all measuring points. The spatial pattern of the variation in soil respiration rates remained stable in the ST plot throughout the 7-year study period. Results of the multiple regression analysis in the ST plot showed that the model with only the LF-C as an explanatory variable had the highest capability for predicting the respiration rate at a soil temperature of 20 °C (R20); the addition of other factors as explanatory variables did not increase the predictive capability. The organic carbon content in soil did not correlate with R20. The R20 in the CC plot significantly decreased six years after clear-cutting compared with the first year after clear-cutting, with values of 3.03 and 1.86 μmol CO2 m−2 s−1, respectively. This decrease tended to be greater in the measuring points where R20 was high in 2013. The LF-C stock for 0–30 cm soil layer in CC plot in the 7th year after clear-cutting was 0.33 kgC m−2 which was much lower than 1.60 kgC m−2 in ST plot. These results further support the conclusion that LF-C was the main factor responsible for the spatial variation in soil respiration rate in the ST plot, and suggest that LF-C is more suitable than SOC as an explanatory variable for the spatial variation in soil respiration rate in volcanic ash soil.