Abstract. The magnitude of daily minimum temperature increase is greater than that of daily maximum temperature increase under climate warming. This study was conducted to examine whether changes in soil respiration under diurnal warming are equal to the summed changes under day and night warming in a temperate steppe in northern China. A full factorial design with day and night warming was used in this study, including control, day (06:00 a.m.-06:00 p.m., local time) warming, night (06:00 p.m.-06:00 a.m.) warming, and diurnal warming. Day warming showed no effect on soil respiration, whereas night warming significantly increased soil respiration by 7.1% over the 3 growing seasons in 2006-2008. The insignificant effect of day warming on soil respiration could be attributable to the offset of the direct positive effects of increased temperature by the indirect negative effects via aggravating water limitation and suppressing ecosystem C assimilation. The positive effects of night warming on soil respiration were largely due to the stimulation of ecosystem C uptake and substrate supply via overcompensation of plant photosynthesis. Changes in both soil respiration (+20.7 g C m −2 y −1 ) and GEP (−2.8 g C m −2 y −1 ) under diurnal warming are smaller than their summed changes (+40.0 and +24.6 g C m −2 y −1 , respectively) under day and night warming. Our findings that the effects of diurnal warming on soil respiration and gross ecosystem productivity are not equal to the summed effects of day and night warming are critical for model simulation and projection of climatecarbon feedback.

Abstract. This study was conducted to examine potentially differential effects of day and night warming on soil respiration in a temperate steppe in northern China. A full factorial design with day and night warming was used in this study, including control (C), day (6 a.m.–6 p.m., local time; D) warming, night (6 p.m.–6 a.m.; N) warming, and diurnal warming (W). Day warming showed neutral effect on soil respiration, whereas night warming significantly increased soil respiration by 7.1% over the first 3 growing seasons. The insignificant effect of day warming on soil respiration could be attributable to the offset of the direct positive effects by the indirect negative effects via aggravating water limitation and suppressing ecosystem C assimilation. The positive effects of night warming on soil respiration were largely due to the stimulation of ecosystem C uptake and substrate supply via over-compensation of plant photosynthesis. In addition, day and night warming showed antagonistic effects on soil respiration, which could be ascribed to their contrasting effects on ecosystem C assimilation. The results suggest differential and non-additive effects of day and night warming on soil respiration, which was driven by the treatment-induced changes in substrate supply.