Low soil moisture during hot periods drives apparent negative temperature sensitivity of soil respiration in a dryland ecosystem: A multi-model comparison

Arid and semiarid ecosystems (drylands) may dominate the trajectory of biosphere-to-atmosphere carbon (C) exchange, and understanding dryland CO₂ efflux is important for C cycling at the global-scale. However, unknowns remain regarding how temperature and moisture interact to regulate dryland soil respiration (R _s ), while ‘islands of fertility’ in drylands create spatially heterogeneous R _s . At a site in southeastern Utah, USA we added or removed litter (0–650 % of control) in plots associated with either shrubs or biological soil crust-dominated interspaces between vascular plants. We measured R _s , soil temperature (T_s), and water content (θ) repeatedly from October 2013 to November 2014. R _s was highest following rain in late summer at T_s ~30 °C, and lowest mid-summer at T_s > 40 °C, resulting in apparent negative temperature sensitivity of R _s at high temperatures, and positive temperature sensitivity at low-moderate temperatures. We used Bayesian statistical methods to compare models capturing a range of hypothesized relationships between T_s, θ, and R _s . The best model indicates that apparent negative temperature sensitivity of R _s at high T_s reflects the control of water content, not high temperatures. Modeled Q₁₀ ranged from 2.7 to 1.4 between 5 and 45 °C. Litter addition had no effect on Q₁₀ or reference respiration (R _ref  = R _s at 20 °C and optimum θ) beneath shrubs, and little effect on R _ref in interspaces, yet R _ref was 1.5 times higher beneath shrubs than in interspaces. Altogether, these results suggest reduced R _s often observed at high T_s in drylands is dominated by the control of θ, and, on shorter-timescales, variable litter inputs exert minimal control over R _s .