Reviewers: Vicca S2, Weigel R3
Measurement unit: g C m-2 y-1, Mg C ha-2 y-1; Measurement scale: plot; Installation costs: €€€; Running costs: €€€; Installation effort: high; Maintenance effort: medium; Knowledge need: high; Measurement mode: manual or data logger
The total belowground carbon flux (TBCF) describes the flow of carbon from photosynthetic organisms through the phloem to the belowground system. It is critical to recognise that aboveground litterfall is not included in TBCF (Davidson et al., 2002). As such, it provides an estimate of belowground “allocation” or, if divided by gross primary production (GPP), of belowground partitioning (Litton et al., 2007). This carbon flow supports root biomass production, root respiration, root exudation, and the many heterotrophic organisms that rely on recent photosynthate for their respiratory substrate. Although it has been quantified in rather few studies so far, it is likely to be influenced by climate change and other global-change drivers, because temperature (Litton & Giardina, 2008), CO2 (Palmroth et al., 2006), and nitrogen deposition (Liu & Greaver, 2010) all have an effect on aboveground sources and belowground sinks of carbon. It is critical to recognise that this is an estimation of the flux, not the accumulation of biomass belowground: the two may be completely uncoupled (Litton et al., 2007). The method has so far been applied mostly to forests, but it has also been applied to CO2 effects on grasslands (Adair et al., 2009) and grass species differ in belowground flux (Sumiyoshi et al., 2017). The measurement is based on a soil carbon budget using component processes described in protocols 2.1.2 Belowground biomass, 2.2.3 Soil CO2 (and other trace gas) fluxes. Specifically,
TBCF = Rs + DSOM + DCR – LF
where Rs is cumulative annual soil respiration, DSOM is the annual increase in soil organic matter, DCR is the annual increase in coarse root biomass, and LF is annual aboveground litterfall and subtracted in the equiation (see 2.1.1 Aboveground plant biomass for how to measure litterfall). Although estimates of these parameters are not rare, they have seldom been combined to estimate TBCF, which has limited the application of this concept to date.
188.8.131.52 What to measure?
Some of the components in the equation above have been discussed elsewhere in this chapter (see above), except perhaps coarse-root biomass increase in trees. This can be estimated using allometric equations based on stem diameter (McDowell et al., 2001). Note that DCR is positive when stem diameter increases.
It is often true that ΔSOM is stable relative to the other carbon fluxes, at least in the absence of disturbance or nitrogen fertilisation. Under such conditions, it may be justified to set this term to zero (Litton et al., 2007) or to some low value (e.g. 5 g C m-2 yr-1; Peltoniemiet al., 2004). This assumption is likely invalid in manipulation experiments. This measurement relies on measurements made using other techniques and described elsewhere in this manual (see above).
Theory, significance, and large datasets
Raich & Nadelhoffer (1989), Davidson et al. (2002), and Giardina et al. (2014) provide good overviews of the method and global summaries of TBCF estimates from forests. Gill & Finzi ( 2016) have presented a meta-analysis of data by biome.
More on methods and existing protocols
Litton et al. (2007) provide a careful discussion of terminology and a comparison among intensively monitored sites. Where necessary, one should also account for the effects of disturbance (Sumiyoshi et al., 2017) and nitrogen fertilisation on DSOM (Janssens et al., 2010). Finally, because this method is based on a difference between sums rather than a direct measurement, error propagation must be considered.
Adair, E. C., Reich, P. B., Hobbie, S. E., & Knops, J. M. H. (2009). Interactive effects of time, CO2 , N, and diversity on total belowground carbon allocation and ecosystem carbon storage in a grassland community. Ecosystems, 12(6), 1037-1052.
Davidson, E. A., Savage, K., Bolstad, P., Clark, D. A., Curtis, P. S., Ellsworth, D. S., … Randolph, J. C. (2002). Belowground carbon allocation in forests estimated from litterfall and IRGA-based soil respiration measurements. Agricultural and Forest Meteorology, 113(1), 39-51.
Giardina, C. P., Litton, C. M., Crow, S. E., & Asner, G. P. (2014). Warming-related increases in soil CO2 efflux are explained by increased below-ground carbon flux. Nature Climate Change, 4(9), 822-827.
Gill, A. L., & Finzi, A. C. (2016). Belowground carbon flux links biogeochemical cycles and resource-use efficiency at the global scale. Ecology Letters, 19(12), 1419-1428.
Janssens, I. A., Dieleman, W., Luyssaert, S., Subke, J.-A., Reichstein, M., Ceulemans, R., … Law, B. E. (2010). Reduction of forest soil respiration in response to nitrogen deposition. Nature Geoscience, 3(5), 315–322.
Litton, C. M., & Giardina, C. (2008). Below-ground carbon flux and partitioning: global patterns and response to temperature. Functional Ecology, 22(6), 941-954.
Litton, C. M., Raich, J. W., & Ryan, M. G. (2007). Carbon allocation in forest ecosystems. Global Change Biology, 13(10), 2089-2109.
Liu, L., & Greaver, T. L. (2010). A global perspective on belowground carbon dynamics under nitrogen enrichment. Ecology Letters, 13(7), 819-828.
McDowell, N. G., Balster, N. J., & Marshall, J. D. (2001). Belowground carbon allocation of Rocky Mountain Douglas-fir. Canadian Journal of Forest Research, 31(8), 1425-1436.
Palmroth, S., Oren, R., McCarthy, H. R., Johnsen, K. H., Finzi, A. C., Butnor, J. R., … Schlesinger, W. H. (2006). Aboveground sink strength in forests controls the allocation of carbon below ground and its CO2-induced enhancement. Proceedings of the National Academy of Sciences USA, 103(51), 19362-19367.
Peltoniemi, M., Mäkipää, R., Liski, J., & Tamminen, P. (2004). Changes in soil carbon with stand age – an evaluation of a modelling method with empirical data. Global Change Biology, 10(12), 2078-2091.
Raich, J. W., & Nadelhoffer, K. J. (1989). Belowground carbon allocation in forest ecosystems: global trends. Ecology, 70(5), 1346-1354.
Sumiyoshi, Y., Crow, S. E., Litton, C. M., Deenik, J. L., Taylor, A. D., Turano, B., & Ogoshi, R. (2017). Belowground impacts of perennial grass cultivation for sustainable biofuel feedstock production in the tropics. GCB Bioenergy, 9(4), 694-709.
Author: Marshall J1
Reviewers: Vicca S2, Weigel R3
1 Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden
2 Centre of Excellence PLECO (Plants and Ecosystems), Biology Department, University of Antwerp, Wilrijk, Belgium
3 Plant Ecology, Albrecht-von-Haller Institute for Plant Sciences, University of Goettingen, Goettingen, Germany