Spatial variability of soil respiration (R<inf>s</inf>) and its controls are subjected to strong seasonality in an even-aged European beech (Fagus sylvatica L.) stand

Abstract

Uncertainties arising from the so-far poorly explained spatial variability of soil respiration (Rs) remain large. This is partly due to the limited understanding about how spatially variable Rs actually is, but also on how environmental controls determine Rs's spatial variability and how these controls vary in time (e.g., seasonally). Our study was designed to look more deeply into the complexity of Rs's spatial variability in a European beech even-aged stand, covering both phenologically and climatically contrasting periods (spring, summer, autumn and winter). Although we studied a relatively homogeneous stand, we found a large spatial variability of Rs (coefficients of variation &gt; 30%) characterized by strong seasonality. This large spatial variability of Rs suggests that even in relatively homogeneous stands there is a large potential source of error when estimating Rs. This was also reflected by the sampling effort needed to obtain seasonally robust estimates of Rs, which may actually require a number of samples above that used in Rs studies. We further postulate that the effect of seasonality on the spatial variability and environmental controls of Rs was determined by the seasonal shifts of its microclimatic controls: during winter, low temperatures constrain plant and soil metabolic activities and hence reduce Rs variability (temperature-controlled processes), whereas during summer, water demand by vegetation and changes in water availability due to the microtopography of the terrain (i.e., slope) increase Rs variability (water-controlled processes). This study provides novel information on the spatiotemporal variability of Rs and looks more deeply into the seasonality of its environmental controls and the architecture of their causal-effect relationships controlling Rs's spatial variability. Our study further shows that improving current estimates of Rs at local and regional levels might be necessary in order to reduce uncertainties and improve CO2 estimates at larger spatial scales. Highlights: The spatial variability of soil respiration (Rs) and its environmental controls vary seasonally. Seasonal shifts from temperature- to water-controlled processes determine Rs's spatial variability. Besides microclimate, slope and grass cover explain the spatiotemporal variability of Rs. An intense sampling effort is needed to obtain robust Rs estimates even in homogeneous forests. © 2021 British Society of Soil Science.