What was done
In a temperate steppe grassland located in Duolun County, Inner Mongolia, China (42°02'N, 116°17'E, 1324 meters above sea level), the authors suspended infrared radiators 2.25 meters above the ground over 24 plots divided into four temperature treatments: (1) control, (2) day (06:00-18:00, local time) warming, (3) night (18:00-06:00) warming, and (4) diurnal (24-hour) warming, after which they measured diurnal cycles of net ecosystem gas exchange and daytime ecosystem respiration twice a month over the growing seasons (May-October) of 2006, 2007 and 2008.

What was learned
Wan et al. report that "nocturnal warming increased leaf respiration of two dominant grass species by 36.3%, enhanced consumption [drawdown] of carbohydrates in the leaves (72.2% and 60.5% for sugar and starch, respectively), and consequently stimulated plant photosynthesis by 19.8% in the subsequent days." In addition, they state that "the enhancement of plant photosynthesis overcompensated the increased carbon loss via plant respiration under nocturnal warming and shifted the steppe ecosystem from a minor carbon source (1.87 g C/m²/year) to a carbon sink (21.72 g C/m²/year) across the three growing seasons."

What it means
The four Chinese researchers note that the local climate in their study area "experienced asymmetrical diurnal warming (0.57, 0.45 and 0.30°C increases in daily minimum, mean and maximum temperatures per decade, respectively) over the past half century (1953-2005)," and that "similar diurnal scenarios of climate warming have been widely reported at the regional and global scales," citing the studies of Karl et al. (1991), Easterling et al. (1997), Stone and Weaver (2002), Vose et al. (2005), Lobell et al. (2007) and Zhou et al. (2007). Consequently, in light of the many well-documented "greater increases in daily minimum than maximum temperature," which have been observed throughout the world, Wan et al. conclude that "plant photosynthetic overcompensation may partially serve as a negative feedback mechanism for [the] terrestrial biosphere to climate warming," where "the photosynthetic overcompensation induced by nocturnal warming can ... regulate terrestrial carbon sequestration and negatively feed back to climate change."

References
Easterling, D.R., Horton, B., Jones, P.D., Peterson, T.C., Karl, T.R., Parker, D.E., Salinger, M.J., Razuvayev, V., Plummer, N., Jamason, P. and Folland, C.K. 1997. Maximum and minimum temperature trends for the globe. Science 277: 364-367.

Karl, T.R., Kukla, G., Razuvayev, N., Changery, M.J., Quayle, R.G., Heim, R.R., Easterling, D.R. and Fu, C.B. 1991. Global warming - evidence for asymmetric diurnal temperature-change. Geophysical Research Letters 18: 2253-2256.

Lobell, D.B., Bonfils, C. and Duffy, P.B. 2007. Climate change uncertainty for daily minimum and maximum temperatures: a model inter-comparison. Geophysical Research Letters 34: 10.1029/2006GL028726.

Stone, D.A. and Weaver, A.J. 2002. Daily maximum and minimum temperature trends in a climate model. Geophysical Research Letters 29: 10.1029/2001GL014556.

Vose, R.S., Easterling, D.R. and Gleason, B. 2005. Maximum and minimum temperature trends for the globe: an update through 2004. Geophysical Research Letters 32: 10.1029/2005GL024379.

Zhou, L., Dickinson, R.E., Tian, Y., Vose, R.S. and Dai, Y. 2007. Impact of vegetation removal and soil aridation on diurnal temperature range in a semiarid region: application to the Sahel. Proceedings of the National Academy of Sciences USA 104: 17,937-17,942.