Spatial variation in forest soil respiration: A systematic review of field observations at the global scale

As it is responsible for the second largest CO₂ flux in the terrestrial ecosystem, the accurate estimation and prediction of soil respiration (SR) are necessary, especially for forest ecosystems, which are a major contributor to the total terrestrial SR. Spatial variation is one of the challenges affecting the accurate estimation and prediction of forest SR in ecosystems. Although a number of studies have examined spatial variation in SR within individual forests, the magnitude and patterns of spatial variation in SR within forest ecosystems (CV of SR [%]) remain unexplored at the global scale. In this study, we collected 94 field observation studies to demonstrate the range and pattern of the CV of SR, and to clarify the controlling factors. Through our analysis, the CV of SR was found to range from 1.8 % to 89.3 % on the global scale; it was highest in the equatorial zone (39.0 % ± 13.8 %) and followed by the warm temperate zone (32.6 ± 14.5 %) and the snow zone (30.0 % ± 16.3 %). There was a significant negative correlation between the CV of SR and soil water content, bulk density, fine root biomass, and elevation at both the global scale and in each climatic zone (P < 0.01). Other factors such as total nitrogen content, mean of diameter at breast height, slope, etc., were also significantly correlated with the CV of SR, but the correlation was different among climatic zones. This study provides an overall perspective of the CV of SR by clarifying the range, patterns, and controlling factors at both the global scale and in each climatic zone. However, further research is needed, especially regarding the mechanisms between the CV of SR and its controlling factors.