Responses of temporal and spatial variation in soil respiration to biotic and abiotic factors in a desert shrubland in northwest China

The aim of this work was to analyze the responses of temporal and spatial variation in soil respiration to biotic and abiotic factors in a desert shrubland in Ningxia, northwest China. For this purpose, Rs together with abiotic (soil temperature (Ts), soil water content (SWC), precipitation (PPT)) and biotic (root biomass, litter fall, leaf area index, soil nitrogen) factors were measured, and plant phenophases were recorded over a typical sand dune in 2012-2014. The specific aims of this study were to: (1) quantify the diurnal and seasonal variation of Rs and its controlling factors, and to understand the influences of SWC on the temperature sensitivity of Rs (Paper I); (2) explore the mechanisms controlling the spatial heterogeneity in Rs and the plant effects on spatial variation of Rs in different phenophases (Paper II); (3) examine the seasonal variation of diel hysteresis in Rs-Ts relationship and its controlling factors (SWC and photosynthesis) (Papers I and III); and (4) explore the influences of biological soil crusts on Rs and its climatic (Ts, SWC, PPT) responses (Paper IV). As a result, both diurnal and seasonal variation in Rs were controlled dominantly by Ts, but the diurnal and seasonal response of Rs to Ts was modified by SWC and biological soil crusts (Papers I, III and IV). At diel scale, Rs was strongly regulated by Ts at moderate and high SWC, but decoupled from Ts under low SWC, due to significant diel hysteresis between Rs and Ts (Papers I and III). This diel hysteresis varied seasonally with SWC, showing increasing lag time with decreasing SWC (Papers I and III). Variation in the diel hysteresis with changing SWC was regulated by photosynthesis of the dominant shrub species (Paper III). At seasonal scale, Rs significantly correlated with Ts at SWC > 0.08 m 3 m (Paper I). In addition, the temperature sensitivity of Rs increased with increasing of SWC (Paper I). The Rs at both non-crusted (NCS) and lichen-crusted (LCS) soils increased with increasing Ts, opposite to that on moss-crusted soil (MCS), where Rs declined with increasing Ts as Ts > ~ 20 C (Paper IV). Root biomass of shrubs and grasses, litter fall and soil nitrogen affected the topographic variation in Rs (Paper II). During the flowering-bearing phase of the dominant shrub, root biomass affected Rs the most, whereas during the leaf coloration-defoliation phase, soil nitrogen content affected Rs the most, explaining 72 and 56% of the total variation, respectively (Paper II). To conclude, SWC, biological soil crusts and shrubs exert strong influences on the temporal and spatial responses of Rs to Ts in a desert shrubland. These results highlight the necessity to account their interactive effects in estimation of carbon balance for desert ecosystems and in modelling of global carbon cycle in order to increase the accuracy of model predictions.