World’s Soil Resources

6.2.7 | Future loss of SOC under climate change
Projected changes in climate (temperature and precipitation) are likely to affect the SOC stock both directly
and indirectly. Directly, the rate of decomposition by microbial processes is affected by both soil temperature
and moisture regimes. Indirectly, changes in climate affect plant growth, net primary productivity, above
and below-ground biomass, and the type and amount of residues with differential amounts of materials
recalcitrance. Further, the rate and susceptibility to accelerated erosion, salinization and other degradation
processes may be exacerbated by an increase in frequency of extreme events. Indeed, climate change can
impact several soil forming factors, including rainfall, temperature, micro-organisms/biota and vegetation,
thus affecting the rate of SOC accumulation (Jenny, 1930). Climate change may also alter species composition,
and the rate of litter fall. However, disagreement exists regarding the effect of warming on SOC stock.
The annual rate of litter return, on which the rate of SOC accretion depends, varies among biomes (White,
1987; Grunwald, 1999). The rate of litterfall (Mg ha -1 yr -1 ) is estimated at 0.1 to 0.4 for alpine and arctic regions,
2 - 4 for temperate grassland, 1.5 - 3 for coniferous forest, 1.5 - 4 for deciduous forest, 5 - 10 for tropical rainforest,
and 1 to 2 for arable land (White, 1987). Increase in soil temperature may exponentially increase the rate of soil
respiration (Tóth et al., 2007; Lenton and Huntingford, 2003). However, because of increase in the number and
activity of soil fungi in the warmer soil, there may also be increase in the relative amount of lignin and other
recalcitrant compounds (Simpson et al., 2007). The SOM decomposition is also more temperature-sensitive at
low than at high temperature (Kirschbaum, 1995, 2000, 2006).
Thus, knowledge about the temperature–sensitivity of diverse SOC fractions, and their change in the soil
under climate change, is important. Change in temperature by 1º Celsius may decrease the turnover times of 4 -1 1
percent and 8 -1 6 percent for the intermediate and stabilized fractions, respectively (Hakkenberg et al., 2008).
The decomposition rate is also influenced by the presence of physicochemical protection mechanisms (Conant
et al., 2011), especially occlusion within aggregates and by association with mineral surfaces (Freedman,
2014). It is argued that CO 2
emissions from soil response to climate warming are over-estimated, because the
decomposition of old SOM is tolerant to temperature (Liski et al., 1999). Thus, the effects of warming on SOM
decomposition are governed by complex and interactive factors, and are difficult to predict. Despite much
research, no consensus has yet emerged on the temperature sensitivity of SOM decomposition (Davidson and
Janssens, 2006).
6.2.8 | Conclusions
Global SOC stocks have been estimated at about 1 500 Pg C for the topmost 1 m. However, a large
uncertainty attaches to this estimate, which cannot easily be assigned to a specific period in time. Local
variations may also be high, for example for SOC stocks in arctic regions and peatlands. Estimates of SOC
stocks below 1 m depth are still evolving, with a tendency for more recent estimates to be higher than previous
values. Estimates of the historic loss of SOC pools are also highly variable, ranging from 40 to 537 Pg. The
global loss of SOC pool since 1850 is estimated at about 66 ±12 Pg. The projected response of SOC stock to
climate change is a debatable issue. While an increase in temperature may increase the rate of respiration at
low soil temperature, it may also shift microbial populations to fungi, increase relative proportions of lignin
and other recalcitrant fractions, and increase protective mechanisms such as aggregation and reaction with
mineral surfaces.
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