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Chen Z.-G.,Xiamen University | Yin X.-J.,State Oceanic Administration | Zhou Y.,Institute for Landscape Biogeochemistry | Zhou Y.,Leibniz Institute of Freshwater Ecology and Inland Fisheries
Journal of Mass Spectrometry

Although deemed important to δ18O measurement by on-line high-temperature conversion techniques, how the GC conditions affect δ18O measurement is rarely examined adequately. We therefore directly injected different volumes of CO or CO-N2 mix onto the GC column by a six-port valve and examined the CO yield, CO peak shape, CO-N2 separation, and δ18O value under different GC temperatures and carrier gas flow rates. The results show the CO peak area decreases when the carrier gas flow rate increases. The GC temperature has no effect on peak area. The peak width increases with the increase of CO injection volume but decreases with the increase of GC temperature and carrier gas flow rate. The peak intensity increases with the increase of GC temperature and CO injection volume but decreases with the increase of carrier gas flow rate. The peak separation time between N2 and CO decreases with an increase of GC temperature and carrier gas flow rate. δ18O value decreases with the increase of CO injection volume (when half m/z 28 intensity is <3 V) and GC temperature but is insensitive to carrier gas flow rate. On average, the δ18O value of the injected CO is about 1‰ higher than that of identical reference CO. The δ18O distribution pattern of the injected CO is probably a combined result of ion source nonlinearity and preferential loss of C16O or oxygen isotopic exchange between zeolite and CO. For practical application, a lower carrier gas flow rate is therefore recommended as it has the combined advantages of higher CO yield, better N2-CO separation, lower He consumption, and insignificant effect on δ18O value, while a higher-than-60 C GC temperature and a larger-than-100 μl CO volume is also recommended. When no N2 peak is expected, a higher GC temperature is recommended, and vice versa. Copyright © 2015 John Wiley & Sons, Ltd. Source

Zhou Y.,Institute for Landscape Biogeochemistry | Zhou Y.,Curtin University Australia | Zhou Y.,Chinese Academy of Sciences | Zhou Y.,Leibniz Institute of Freshwater Ecology and Inland Fisheries | And 9 more authors.

It has long been theorized that carbon allocation, in addition to the carbon source and to kinetic isotopic effects associated with a particular lipid biosynthetic pathway, plays an important role in shaping the carbon isotopic composition (13C/12C) of lipids (Park and Epstein, 1961). If the latter two factors are properly constrained, valuable information about carbon allocation during lipid biosynthesis can be obtained from carbon isotope measurements. Published work of Chikaraishi et al. (2004) showed that leaf lipids isotopic shifts from bulk leaf tissue δδ 13C bk - lp (defined as δ 13Cbulkleaftissue - δ 13Clipid) are pathway dependent: the acetogenic (ACT) pathway synthesizing fatty lipids has the largest isotopic shift, the mevalonic acid (MVA) pathway synthesizing sterols the lowest and the phytol synthesizing 1-deoxy-d-xylulose 5-phosphate (DXP) pathway gives intermediate values. The differences in δδ 13C bk - lp between C3 and C4 plants δδ13Cbk-lp,C4-C3 are also pathway-dependent: δδ13Cbk-lp,C4-C3ACT > δδ13Cbk-lp,C4-C3DXP > δδ13Cbk-lp,C4-C3MVA. These pathway-dependent differences have been interpreted as resulting from kinetic isotopic effect differences of key but unspecified biochemical reactions involved in lipids biosynthesis between C3 and C4 plants. After quantitatively considering isotopic shifts caused by (dark) respiration, export-of-carbon (to sink tissues) and photorespiration, we propose that the pathway-specific differences δδ13Cbk-lp,C3-C4 can be successfully explained by C4 -C3 carbon allocation (flux) differences with greatest flux into the ACT pathway and lowest into the MVA pathways (when flux is higher, isotopic shift relative to source is smaller). Highest carbon allocation to the ACT pathway appears to be tied to the most stringent role of water-loss-minimization by leaf waxes (composed mainly of fatty lipids) while the lowest carbon allocation to the MVA pathway can be largely explained by the fact that sterols act as regulatory hormones and membrane fluidity modulators in rather low concentrations. © 2014 Elsevier Ltd. Source

Biernath C.,Helmholtz Center for Environmental Research | Bittner S.,Helmholtz Center for Environmental Research | Klein C.,Helmholtz Center for Environmental Research | Gayler S.,University of Tubingen | And 6 more authors.
European Journal of Agronomy

In this study, we developed and analyzed a new model for the simulation of photosynthetic active nitrogen (NP) turnover dynamics in crops and assessed its impact on the acclimation of canopy photosynthesis to atmospheric CO2 enrichment. Typical canopy models assume a vertical exponential decline of light interception following the Beer-Lambert law and vertical distributions of leaf NP contents directly proportional to the light distribution. This assumption is often inconsistent with experimental observations. We therefore modified and extended the photosynthesis model of the GECROS crop model to consider the trade-off that occurs between the use of degraded NP for plant growth and the synthesis of new NP. This model extension thus enabled the examination of the CO2-induced down-regulation of photosynthesis hypothesis using a crop model. The simulation results of the original and modified GECROS model were compared and evaluated based upon measurements of field-grown spring wheat. The modified GECROS model better simulated the dynamics of crop growth under varying atmospheric CO2 concentrations. Furthermore, the application of different temperature functions to NP degradation strongly influenced the simulation results, revealing the necessity for improving the understanding of the temperature dependence of NP turnover for different crop species and varieties. In conclusion, the redistribution of nitrogen within the plant and its alternative use either for growth or the optimization of the photosynthetic apparatus is an important mechanism for crop growth acclimation to regionally changing climatic conditions and in particular, atmospheric CO2 enrichment. © 2013 Elsevier B.V. Source

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