Since substrates for respiration are supplied mainly by recent photo-assimilates, there is a strong but time-lagged link between short-term above- and belowground carbon (C) cycling. However, regulation of this coupling by environmental variables is poorly understood. Whereas recent studies focussed on the effect of drought and shading on the link between above- and belowground short-term C cycling, the effect of temperature remains unclear.

We used a ¹³CO₂ pulse-chase labelling experiment to investigate the effect of a sudden temperature change from 25 to 10 °C on the short-term coupling between assimilatory C uptake and respiratory loss. The study was done in the laboratory using two-month-old perennial rye-grass plants (Lolium perenne L.). After label application, the δ¹³C signal of respired shoot and root samples was analysed at regular time intervals using laser spectroscopy. In addition, δ¹³C was analysed in bulk root and shoot samples.

Cold temperature (10 °C) reduced the short-term coupling between shoot and roots by delaying belowground transfer of recent assimilates and its subsequent respiratory use, as indicated by the δ¹³C signal of root respiration (δ¹³C_RR). That is, the time lag from the actual shoot labelling to the first appearance of the label in ¹³C_RR was about 1.5 times longer under cold temperature. Moreover, analysis of bulk shoot and root material revealed that plants at cold temperature invest relatively more carbon into respiration compared to growth or storage. While the whole plant C turnover increased under cold temperature, the turnover time of the labile C pool decreased, probably because less ¹³C is used for growth and/or storage. That is, (almost) all recent C remained in the labile pool serving respiration under these conditions. Overall, our results highlight the importance of temperature as a driver of C transport and relative C allocation within the plant–soil system.