Search

Carbon sequestration and greenhouse gas fluxes in grassland. A review. Grassland carbon (C) sequestration can play an important role in mitigating total greenhouse gas (GHG) emissions of livestock production systems. An accurate inventory of livestock production system contribution to GHG emissions requires to think in terms of global budget, by considering both the GHG sources and the mitigation potential trough grassland soil carbon sequestration. There is a huge variability in C and GHG balances of grasslands that is mainly due to management practices and climatic conditions. The present article shows that, to reduce the uncertainties of the results, long term measurements at the field scale are necessary. Also, it shows the importance of taking into account the fluxes of the three main GHGs (carbon dioxide, nitrous oxide, methane) into account when calculating the GHG budget. This article also highlights the need for a better understanding of the mechanisms driving the fluxes, in relation to environmental factors and management practices, in order to propose mitigation strategies able to enhance soil carbon sequestration in soils and to reduce methane and nitrous oxide emissions.

Abstract: Data sets of biometeorological measurements of ecosystem CO2 flux, combined with harvest and manure data from several European cropland were integrated to provide an assessment of the carbon budget. Sites encompassed different climatic regions and contrasting crop rotations. The influence of different crops and management practices was also assessed to identify some of the major factors contributing to the cropland carbon balance. Since crops are rotated and cropping periods do not always follow the calendar year, net ecosystem production (NEP) as well as net biome production (NBP) sums of full crop rotations or of at least 4 years of longer-term crop rotations and of monocultures were used. In a second analysis NBP sums were correlated to soil properties. Finally, the data were combined with additional data to derive a mean annual GHG balance for the European cropland sites under consideration. Five crop rotations and two monocultures were integrated over 4 years. During 4 years the average annual NEP was −240±113gCm−2 y−1. On average, 382±117gCm−2 y−1 were harvested, where as average carbon inputs by manure and seeding was 47±51gCm−2 y−1. The average NBP of the seven sites under consideration was estimated to be a carbon loss of 95±87gCm−2 y−1. The full GHG balance of the considered sites was estimated to be 160gCm−2 y−1 in CO2-equivalents. These results challenge current good practice guidelines that predict neutral carbon budgets for systems where the inputs of manure and crop residues are of comparable magnitude to those associated with the sites examined in this study. Ongoing humus loss in spite of good practice is mainly related to soils with high carbon concentrations which are not in equilibrium but may also be a result of already ongoing climate change. A modification in the good practice guidelines to increase carbon inputs may be required. Results from a representativeness analysis suggest that more than 50 sites are necessary for a European cropland flux network to adequately represent the variability of climate, soil and management within the European continent. Thus, the uncertainties due to the network design are currently bigger than the uncertainty intrinsic in the measurement method. [Copyright &y& Elsevier]