The influence of temperature and labile C substrates on heterotrophic respiration in response to elevated CO2 and nitrogen fertilization

Important effects of elevated [CO2] on SOM are expected as a consequence of increased labile organic substrates derived from plants. The present study tests the hypotheses that, under elevated [CO2]: 1) soil heterotrophic respiration will increase due to roots-microbes-soil interactions; 2) the increased labile C will boost soil heterotrophic respiration, depending on N availability; 3) the temperature sensitivity of soil respiration will change, depending on nitrogen inputs and plant activity. To test these hypotheses, we measured the heterotrophic respiration of intact soil cores collected in a poplar plantation exposed to elevated [CO2] and two nitrogen inputs, at different temperatures. Additional physical (water content, root biomass) and biochemical parameters (microbial biomass, labile C) were determined on the same samples. The soil samples were collected at the POP-EuroFACE experimental site (Italy), where a Populus x euramericana plantation was exposed for 6 years to 550 ppm [CO2] (Free Air CO2 Enrichment) at two different nitrogen inputs (none or 290 kg ha(-1)). The higher heterotrophic respiration under elevated [CO2] (+30% on average) was driven by the larger pool of soil labile C (+57% on average). The temperature sensitivity of soil respiration was unaffected by elevated [CO2], but was positively affected by N fertilization. Our results indicate that only a fraction of the extra carbon fixed by photosynthesis in elevated [CO2] will contribute to enhanced carbon storage into the soil because of the contemporary stimulation of soil heterotrophic respiration. At the same time, the fraction remaining in the soil will enhance the pool of soil labile C.