Soil respiration driven by biomass accumulation under warming but not elevated [CO2] in tropical agroecosystems

Background/Question/Methods

Moderate scenarios of global change predict 2°C warming in tropical latitudes by 2050, when atmospheric CO₂ concentration ([CO₂]) is expected to reach 600 µL L^-1. These are two important environmental variables that directly or indirectly influence ecosystem structure and function. Foraging pastures are a dominant and fast growing land use in the Neotropics: they represent roughly 60% of the total tropical agroecosystems in Brazil. Tropical pastures contain a dominant perennial C₄ species increasing productivity and the transfer of atmospheric carbon belowground. A FACE (Free-Air Carbon Enrichment) and T-FACE (Temperature Free-Air Controlled Enhancement) system were installed on a field station at The University of São Paulo, Brazil. Using infrared reflectors, canopy temperature had a 2°C warming above ambient temperatures 24 hours a day. With a fumigation system, [CO₂] was enhanced to 600 µL L^-1 from 6:00am to 6:00pm. We used a C4 grass (Panicum maximum), highly productive, and widely used as a foraging pasture in Brazil, under irrigated plots. Our hypothesis was that warming and elevated [CO₂] would increase soil respiration.

Results/Conclusions

Total biomass was increased under warming by 22% on average, regardless of [CO₂]. There was a significant correlation of soil carbon efflux with soil temperature but not with soil moisture. Under elevated [CO₂] total soil respiration was reduced by 5.2% despite 61% increase in root biomass, but under warming and under warming + elevated [CO₂] soil respiration increased by 20.6% and 27.1% respectively. Warming alone also increased soil respiration with time by 35% while elevated [CO₂] alone or elevated [CO₂] + warming had no effects along the experimental period. These findings highlight that soil carbon respiration in tropical ecosystems respond to warming but not to elevated [CO₂]. This is particularly true for ecosystems with majority of C4 plants, which are adapted to high temperatures and not responsive to elevated [CO₂]. The findings suggest that soil respiration in tropical ecosystems are driven by accumulation of biomass not by heterotrophic microorganisms activities. Regardless of the [CO₂], soil warming supposedly increased carbon efflux to the atmosphere, suggesting that these agroecosystems may have reduced carbon sink capacity.