World’s Soil Resources

SOM feedbacks to climate change (Figure 7.6) include direct responses such as: (i) the alteration of microbial
activity with temperature (Conant et al., 2011); and (ii) moisture-related changes in the supply of O 2
relative to
other electron acceptors that reflect precipitation change. In this context, probably the biggest concern is the
thawing of large stores of C in permafrost at high northern latitudes, which will make organic C that has been
frozen for millennia available for decomposition. This response is predicted to create a significant positive
feedback to climate change (Schuur et al., 2008).
Another direct response of soil organic carbon pools is predicted in response to elevated CO 2
and its effect
on ecosystem productivity. Free Air Carbon Dioxide Enrichment (FACE) studies have shown that belowground
productivity can be strongly affected, with cascading and mixed consequences for SOM storage. However
indirect effects are such as altered stabilization of older C associated with the increased inputs of fresh
plant inputs (‘priming’) add uncertainty to the prediction of future soil C responses. As with CO 2
fertilization,
increased deposition of reactive N associated with regional air pollution affects production, quality and spatial
distribution of plant inputs (e.g. above- versus belowground) and can alter the decomposition rates through
changes in the soil microbial community (Berg and Matzner, 1997). Hence the net effects on soil C storage are
difficult to predict, though the combined effects of climate change and fertilization are expected to result in
net losses of soil C overall from temperate forest soils (Hopkins, Torn and Trumbore, 2012)
Many of the processes affecting SOM over the past century have been dominated by human management
of vegetation, which in turn affects the inputs and status of SOM. Changes in vegetation cover, including
those occurring in response to climate as well as to land use or management, influence soil organic matter by
altering the rates, quality and location of plant litter inputs to soils. In turn, litter inputs influence the amount
and composition of the decomposer organisms, including soil fauna, as well as the soil microbial community.
Studies of a number of vegetation transitions – for example the replacement of forests with agriculture or
pasture – have shown that these transitions have led to a loss of soil C to the atmosphere. However, the
trajectory of vegetation change in response to climate, and the consequences for atmospheric CO 2
, are not
well known, as soils will in turn determine what kind of vegetation will take over. For example, C from thawing
permafrost soils may eventually be sequestered in the biomass of forests that can grow in the warmer climate.
Evidently the time lags required for these transitions are an important part of understanding the net effect of
soil C on the carbon cycle.
In addition to direct effects of changes in plant litter addition to soils, management or vegetation change
also alters the chemical and physical framework of soil and thereby the organisms inhabiting it. For example,
ploughing can break up soil aggregates and make organic matter that was previously protected available
to decomposers. Changes in evapotranspiration can change local and regional water resources. Addition of
fertilizers increases plant productivity but also alters soil microbial communities and can stimulate production
of reactive N gases and N 2
O.
Large-scale soil erosion is thought to slow decomposition of buried, eroded organic matter, while growth of
vegetation on the remaining soil will tend to increase soil C storage. However, these effects have been shown
to be relatively small (Van Oost et al., 2007). By removing topsoil that is generally high in organic matter,
erosion can have profound effects on physical and chemical soil properties such as water retention and cation
exchange capacity.
Global increases in carbon stocks have a large, cost-competitive potential for climate change mitigation
(Smith et al., 2008). Mechanisms include reduced soil disturbance, improved rotations and residue/organic
input management, and restoration of degraded soils. Nevertheless, limitations on soil C sequestration
include time limitation, non-permanence, displacement and difficulties in verification (Smith, 2012). Despite
these limitations, soil C sequestration can be useful to meet short- to medium-term targets. In addition, soil C
sequestration confers a number of co-benefits on soils. It is thus a viable option for reducing the atmospheric
Status of the World’s Soil Resources | Main Report The impact of soil change on ecosystem services
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