Vegetation type and climate determine temperature thresholds of soil respiration across drylands

Abstract

Soil respiration (SR) is a key component of terrestrial carbon-climate feedbacks, yet its seasonal dynamics in drylands remain poorly understood. In mesic ecosystems, SR is primarily temperature driven, whereas in drylands it shifts seasonally from temperature to moisture control as autotrophic and heterotrophic respiration become water limited during dry periods. Identifying the soil temperature at which SR transitions from temperature to moisture limitation is therefore essential for predicting SR under climate change. We examined temperature and moisture response functions of SR across forests, shrublands, and grasslands in arid and semi-arid regions to determine the soil temperature threshold of SR (STT_SR) and its drivers. Across sites, SR was positively correlated with mean annual precipitation, soil moisture, and soil organic carbon, while negatively correlated with soil temperature. The significant variability in the temperature thresholds of SR (STT_SR) that was observed between sites (17.9 °C ± 5.3 °C; mean ± SD) was best explained by the mean annual temperature (MAT) at the site. Sites with higher air temperatures exhibited higher STT_SR, suggesting that the compartments and metabolic processes involved in SR are adapted to local temperatures. This observed SR adaptation occurred at two different scales. Besides STT_SR were positively correlated with MAT within each vegetation type, STT_SR were systematically higher under short-stature vegetation types (grasslands and shrublands) compared to high-stature vegetation types (forests), suggesting that grasses and shrubs have developed the evolutionary capacity to push the STT_SR to warmer temperatures and hence withstand better drought stress than trees. Our findings suggest that: (1) process-based models assuming simple linear or exponential SR-temperature relationships overestimate SR in water-limited ecosystems; and (2) projected warming and increasing water scarcity, together with shifts in vegetation dominance, may strongly modify the temperature sensitivity of SR.