Soil warming effect on net ecosystem exchange of carbon dioxide during the transition from winter carbon source to spring carbon sink in a temperate urban lawn

The significant warming in urban environment caused by the combined effects of global warming and heat island has stimulated widely development of urban vegetations. However, it is less known of the climate feedback of urban lawn in warmed environment. Soil warming effect on net ecosystem exchange (NEE) of carbon dioxide during the transition period from winter to spring was investigated in a temperate urban lawn in Beijing, China. The NEE (negative for uptake) under soil warming treatment (temperature was about 5°C higher than the ambient treatment as a control) was ?0.71 μmol/(m²·sec), the ecosytem was a CO₂ sink under soil warming treatment, the lawn ecosystem under the control was a CO₂ source (0.13 μmol/(m²·sec)), indicating that the lawn ecosystem would provide a negative feedback to global warming. There was no significant effect of soil warming on nocturnal NEE (i.e., ecosystem respiration), although the soil temperature sensitivity (Q₁₀) of ecosystem respiration under soil warming treatment was 3.86, much lower than that in the control (7.03). The CO₂ uptake was significantly increased by soil warming treatment that was attributed to about 100% increase of a (apparent quantum yield) and A_max (maximum rate of photosynthesis). Our results indicated that the response of photosynthesis in urban lawn is much more sensitive to global warming than respiration in the transition period.