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

Abstract

Arid and semiarid ecosystems (drylands) may dominate the trajectory of biosphere-to-atmosphere carbon (C) exchange, and understanding dryland CO 2 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 Q 10 ranged from 2.7 to 1.4 between 5 and 45 °C. Litter addition had no effect on Q 10 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 .