Carbon sequestration determined using farm scale carbon balance and eddy covariance
Introduction
Grasslands are a major land use in Europe, occupying some 62.7 Mha in the EU 25 plus Norway and Switzerland (Janssens et al., 2005). These areas are an important component of the European carbon (C) budget (Janssens et al., 2003) and there is a need to understand their C balance. Furthermore, article 3.4 of the Kyoto protocol makes provision for the use of soil C stock changes in grazing lands to offset greenhouse gas (GHG) emissions and facilitate the achievement of emissions reduction targets. Therefore, there is a need to assess the viability of a range of strategies to reduce greenhouse gas emissions. Such strategies should have a whole farm approach (Oenema et al., 2001) and be capable of reducing or offsetting greenhouse gas emissions or promoting C sequestration. If mitigation strategies are tailored to specific farming systems they are more likely to be accepted by farmers. An essential prerequisite to analysing the effectiveness of GHG reduction strategies is an understanding of the C balance at farm level. The farm scale balance methodology has been used widely and successfully to quantify losses of farm nutrients (Oenema et al., 2003). It is desirable to apply techniques successfully used in other areas of research (such as nutrient losses from soils to water, e.g. Tunney et al. (2003)) to help quantify the C source/sink status of complex ecosystems such as grasslands.
The net ecosystem exchange (NEE) in grasslands is determined by the difference between carbon dioxide (CO2) uptake through photosynthesis and CO2 loss through respiration (Byrne et al., 2005). The technique most widely applied to measure this at ecosystem level is eddy covariance and over 180 systems are operating globally on a long term and continuous basis (Baldocchi, 2003). By measuring the covariance between the fluctuations in vertical wind velocity and the CO2 mixing ratio, the EC technique determines the exchange rate of CO2 across the biosphere/atmosphere interface. The area sampled, called the flux footprint, has longitudinal dimensions varying between hundreds of meters and several kilometers (Schmid, 1994). The EC technique has been applied in a range of ecosystems and numerous studies have shown grasslands to be both sinks and sources of CO2 (e.g. Flanagan et al., 2002, Hunt et al., 2004, Jaksic et al., 2006, Novick et al., 2004). When deployed in ecosystems where there is no significant lateral C movement into or out of the study site (such as in a forest when no harvesting is occurring) EC measurements on their own provide an estimate of the C sink or source status of the ecosystem. However, EC studies do not adequately represent the farm level C balance given that they do not capture farm outputs such as milk and meat production and C inputs such as concentrate feed. Recent studies by Nieveen et al. (2005) and Lloyd (2006) have combined EC derived NEE measurements with estimates of farm C exports to estimate the sink/source status of soil C. However, these studies do not address all the pathways of C inputs and outputs.
In addition, when considering the contribution of farming systems to GHG emissions there is a need to consider N2O in addition to CO2 and CH4. By considering all biogenic GHGs the net radiative forcing of the system can be assessed. This is done using global warming potential (GWP) (Houghton et al., 2001).
In this paper, we have the following objectives: (1) quantify the farm scale C balance during 2004 for two dairy farms in South West Ireland by combining results of on-site EC studies with farm management data and emission factors derived from published literature; (2) estimate the sink/source status of C as the difference between C inputs and outputs; (3) estimate the uncertainty ranges associated with the major components of the C balance; (4) calculate the net GWP of both farms.
Section snippets
Site description
This study was located in an area of intensively managed grassland 200 m above sea level in County Cork, southern Ireland (Latitude: 51°59′N, Longitude 8°45′W). The climate is temperate maritime with an average rainfall of 1470 mm year−1 and an annual daily mean temperature of 6.2 °C in January and 13.7 °C in July. Photosynthetic photon flux density (PPFD) has a clear seasonal trend with the highest values occurring during the summer months (Fig. 1a). The average air temperature was above 5 °C on 320
Results
Carbon inputs are similar in both Farms A and B. This is due to similarity in the management of both farms (Table 1). The EC measurements show that the non-farmyard area of both farms (i.e. fields) are net sinks for atmospheric C with NEE (the difference between gross primary productivity and respiration) being 2.9 ± 0.5 t C ha−1. NEE accounts for 84 and 79% of carbon inputs in Farms A and B, respectively. Carbon in concentrate feed accounts for 12 and 19% of C inputs in Farms A and B, respectively.
Discussion
By quantifying the farm level C balance this study suggests that these grass based dairy farms are C sinks. The dominant pathway of C input is photosynthesis and the measured NEE is similar to the value of 3 t C ha−1 year−1 reported for L. perenne grassland in the Netherlands (Schapendonk et al., 1997). NEE is affected by both climate and management and therefore varies between years. The EC measured NEE was 1.9 t C ha−1 year−1 in 2002 and 2.6 t C ha−1 year−1 in 2003 (Jaksic et al., 2006). Given that NEE
Conclusions
This study finds that grassland in this temperate maritime climate zone with grazing and harvesting of grass is a sink of C to an amount of ∼2 t C ha−1 year−1. This sink was not partitioned between the amounts sequestered in the soil and the vegetation. The approach described here identifies the most significant factors in the Farm C balance and radiative forcing. The estimated C sequestration would need to be verified with soil C measurements on a range of representative soil types before the
Acknowledgements
This study was funded by the Environmental ERTDI Programme 2000–2006, financed by the Irish Government under the National Development Plan and administered on behalf of the Department of Environment and Local Government by the Environmental Protection Agency (CELTICFLUX 2001-CC-C2-M1). KA Byrne was funded by an Environmental Protection Agency Postdoctoral Fellowship (2003-FS-CD-LS-17). Adrian Birkby maintained the eddy covariance system.
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