Ecology, 90(10), 2009, pp. 2700–2710Ó 2009 by the Ecological Society of AmericaPhotosynthetic overcompensation under nocturnal warmingenhances grassland carbon sequestrationSHIQIANG WAN,1,3JIANYANG XIA,1,2WEIXING LIU,1,2AND SHULI NIU11State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences,Xiangshan, Beijing 100093 China2Graduate School of Chinese Academy of Sciences, Yuquanlu, Beijing 100049 ChinaAbstract. A mechanistic understanding of the carbon (C) cycle–climate change feedbackis essential for projecting future states of climate and ecosystems. Here we report a novel fieldmechanism and evidence supporting the hypothesis that nocturnal warming in a temperatesteppe ecosystem in northern China can result in a minor C sink instead of a C source asmodels have predicted. Nocturnal warming increased leaf respiration of two dominant grassspecies by 36.3%, enhanced consumption of carbohydrates in the leaves (72.2% and 60.5% forsugar and starch, respectively), and consequently stimulated plant photosynthesis by 19.8% inthe subsequent days. Our experimental findings confirm previous observations of nocturnalwarming stimulating plant photosynthesis through increased draw-down of leaf carbohydratesat night. The enhancement of plant photosynthesis overcompensated the increased C loss viaplant respiration under nocturnal warming and shifted the steppe ecosystem from a minor Csource (1.87 g Cm 2yr 1) to a C sink (21.72 g Cm 2yr 1) across the three growing seasonsfrom 2006 to 2008. Given greater increases in daily minimum than maximum temperature inmany regions, plant photosynthetic overcompensation may partially serve as a negativefeedback mechanism for terrestrial biosphere to climate warming.Key words: carbohydrate; carbon; China; climate warming; ecosystem; photosynthesis; respiration;temperate steppe; temperature.INTRODUCTIONGlobal mean temperature has increased by ;0.768Csince 1850 and is predicted to rise an additional 1.88–4.08C by the end of this century (IPCC 2007). There issubstantial spatial and temporal vari ability in themagnitudes of temperature increases, which can pro-foundly impact ecosystem carbon ( C) cycling withconsequent feedbacks to climate change. For example,it has been recently revealed that both extreme warmingevents (Ciais et al. 2005), which are predicted to increasein frequency, and autumn warming (Piao et al. 2008)have the potential to reverse terrestrial ecosystems fromnet C sinks to net C sources. In addition to the seasonaland interannual variability in the warming trend,historical meteorological records and climate modelprojections have shown greater increases in dailyminimum than maximum temperature and subsequentdeclining diurnal temperature ranges (Karl et al. 1991,Eastering et al. 1997, Stone and Weaver 2002, Vose et al.2005, Lobell et al. 2007, Zhou et al. 2007). A growingbody of evidence from long-term observations (Stooks-bury and Michaels 1994, Nicholls 1997, Alward et al.1999, Peng et al. 2004, Lobell et al. 2005, Schlenker andRoberts 2006, Lobell 2007), manipulative experiments(Ziska and Manalo 1996, Volder et al. 2007), and modelsimulations (Rosenzweig and Tubiello 1996, Dhakhwaand Campbell 1998) has demonstrated differentialimpacts of increasing daily minimum vs. maximumtemperatures on biomass production and yield ofgrassland plant and crop species. However, underlyingmechanisms for the differential responses of terrestrialplants to asymmetrical vs. symmetrical diurnal warmingand their consequent influence on terrestrial ecosystem Ccycling remain elusive.To examine possible differences in the role of day vs.night warming on C cycling in terrestrial biomes, wehave c onducte d a field warming experiment usinginfrared radiators in a temperate steppe in northernChina since 23 April 2006 (see Plate 1). The temperatesteppe is an expansive arid and semiarid biome thatstretches across the Eurasian continent and is sensitiveto climate change (Christensen et al. 2004, Niu et al.2008). Twenty-four 3 3 4 m plots were randomlyassigned to one of the four treatments: (1) control, (2)day (06:00–18:00, local time) warming, (3) night (18:00–06:00) warming, and (4) diurnal (24-h) warming.Because most plant photosynthetic processes occurduring daytime and there is only plant respiration atnight, we specifically tested (1) whether the asymmetricaldiurnal warming regimes differentially influence plantphotosynthesis and nighttime respiration, thus leaf Cbalance and (2) whether leaf-level physiological respons-es of plants are manifested at an ecosystem scale.Manuscript received 2 November 2008; accepted 13 January2009. Corresponding Editor: T. E. Huxman.3E-mail: [email protected] AND METHODSSite description and experimental designThe research site (428020N, 1168170E, 1324 m abovesea level) is located in Duolun County, Inner Mongolia,China. Mean annual precipitation is 385.5 mm, with;86% occurring from May to September. Mean annualtemperature is 2.18C, with the minimum and maximumtemperatures ranging from 17.58C in January to 18.98Cin July. The sandy soil in the study site is classified aschestnut according to the Chinese classification orHaplic Calcisols according to the Food and AgriculturalOrganization of the United Nations (FAO) classifica-tion, with 62.75% 6 0.04% sand (mean 6 SE), 20.30% 60.01% silt, and 16.95% 6 0.01% clay. Soil bulk densityand pH are 1.31 g/cm3and 6.84 6 0.07, respectively. Soilorganic C and total N contents are 16.10 6 0.89 g/kgand 1.48 6 0.10 g/kg, respectively. The plant communityat our experimental site is dominated by Stipa krylovii,Artemisia frigida, Potentilla acaulis, Cleistogenes squar-rosa, Allium bidentatum, and Agropyron cristatum. Thisresearch site was overgrazed by cattle from the early1980s to 2001, resulting in severe degradation. In 2001, itwas fenced to exclude grazing for ecological restoration.We used a complete random block design with sixtreatments replicated six times. Thirty-six 3 3 4m2plotswere arranged in a 6 3 6 matrix. The distance betweenany two adjacent plots was 3 m. One of the six plots ineach row (i.e., a replication) was randomly assigned toone of the six treatments, including (1) control, (2) day(06:00–18:00, local time) warming, (3) night (18:00–06:00) warming, (4) diurnal (24-h) warming, (5) nitrogen(N) fertilization, and (6) diurnal warming plus Nfertilization. The effects of N fertilization and its