Local-scale fluxes of carbon dioxide in a poplar short rotation coppice

Abstract

Short rotation crops can provide biomass fuel for a wide range of users in co-firing plants and to replace other conventional energy resources used for heating and/or power generation. These biomass systems have also a considerable potential for carbon sequestration, which may constitute a good option to offset greenhouse gas emissions. With the purpose of evaluating the contribution of these intensively managed crop systems to carbon sequestration, we measured daily and seasonal patterns of net exchange rates of CO2 and soil effluxes in a poplar short rotation plot installed in the vicinity of the city of Bragança, Portugal. Open top chamber based-techniques were designed to measure both CO2 soil effluxes and exchange rates between the crop canopy and the atmosphere. Chambers inflow and outflow CO2 concentrations were measured by means of an ADC 2250 differential infrared gas analyzer coupled to an automatic sampling and acquisition system. This study started in the summer of 2010 and the data collected so far covers only part of the annual growth cycle of the poplar coppice. Poplar trees were already in an advanced stage of their growth cycle and from then on they have been subjected to stress environmental conditions, especially heat and high soil moisture deficit, which were observed until the end of September. The main results showed a significant variation of CO2 fluxes over the evaluation period. Day/night variation pattern was a common characteristic, but with shapes and magnitudes clearly influenced by the physiological activity of the poplar trees. The pronounced diurnal cycle with CO2 uptake during daytime and an upward flux at night registered in late September gave gradually rise to a CO2 emission pattern in the second half of November, slightly before the falling of the leaves. These preliminary results showed the potential of this energy crops to absorb large amounts of carbon dioxide during the growing season, but in the senescent period and/or under very stressful environmental conditions these biomass systems become predominantly a source of CO2