Temperature dependence of the contribution of soil moisture to soil respiration and the soil respiration temperature threshold in a temperate deciduous forest

Abstract. Soil respiration (Rs) in forest soils is a key flux governing forest carbon balance and the global carbon cycle. Because this flux is expected to respond rapidly to climate warming, understanding the controls on Rs is essential for predicting changes in forest carbon balance induced by warming. In natural field conditions, soil temperature (Ts) and soil moisture content (SMC) often covary seasonally, which tends to limit our ability to isolate and quantify the independent contribution of SMC and to evaluate how its contribution varies with temperature. Although temperature thresholds in Rs have been reported, few studies have quantitatively identified such thresholds from field observations and interpreted potential shifts in the dominant controls based on how moisture responses differ across the threshold. Here, we used two years of continuous automated chamber measurements in a temperate deciduous forest to estimate a Ts threshold for Rs and to assess how the relative contribution of SMC varies with Ts by comparing models across temperature ranges, with particular attention to changes near the threshold. At the annual scale, the explanatory power of SMC alone was limited, but the relationship between SMC and Rs was significant. In contrast, above 15 °C, the relationship between SMC and Rs strengthened consistently, indicating that the contribution of SMC is constrained at low Ts but increases markedly at high Ts. Piecewise regression of the relationship between Rs and Ts identified a Ts threshold near 17 °C, and models including this threshold improved fit relative to an exponential model. These results show that the relative contribution of SMC can change across a specific temperature range, suggesting that changes in the relative influence of SMC on Rs variability across the threshold may reorganize the dominant controls on Rs. Therefore, projections of forest Rs should jointly consider temperature dependent changes in moisture contribution and the presence of Ts thresholds.