World’s Soil Resources

capacity declines as N loads increase above a point where N can be taken up by plants and soil processes
(Aber et al., 1989). While studies illustrate that the rate of N removal does generally decline with increasing N
inputs (e.g. Perakis, Compton and Hedin, 2005), there are still questions about the ability of soils to retain N
over time. The saturation point may vary by ecosystem and soil type. For example, wetland ecosystems have
a tremendous capacity to retain N – a recent meta-analysis indicates that wetland N removal is linear with N
loading, removing about 47 percent of N inputs even at very high loads (Jordan, Stoffer and Nestlerode, 2011).
However, recent work on agricultural soils found that N 2
O production increases with increased N loading
(Shcherbak, Millar and Robertson, 2014). This reinforces the pattern of decline in capacity of soils to serve as a
stable N sink under high N inputs, and suggests that efforts to reduce N 2
O production should target areas of
high N loads where larger benefits will be seen per unit N.
The connection between ecosystem services and soil processes is sometimes distant. The benefit of N
uptake in a riparian soil in Iowa might be most appreciated in distant coastal fisheries. In addition, ecosystem
services do not turn on or off with the flick of a switch; for example, it may take decades to recover water
quality after a widespread land use change (Hart, 2003; Howden et al., 2010). Our perspectives about soils and
ecosystem services should include these distant connections and time lags.
Removal of N from the cascade has implications for many aspects of human health and well-being (Figure 1;
Brauman et al., 2007; Compton et al., 2011), and an increasing number of studies are including soil processes in
ecosystem service assessments and valuation frameworks (De Groot, Wilson and Boumans, 2002; Robinson
et al., 2013). Soil N and P removal is generally seen as an intermediate service or a supporting or regulating
service in current ecosystem services classification schemes, as it affects a number of final ecosystem goods
and services (Boyd and Banzhaf, 2007).
Impacts of nitrogen on ecosystem services (ES), on the economy and on human well-being have been
examined in a number of studies (Birch et al., 2010; Compton et al., 2011; van Grinsven et al., 2013). Soil N and
P storage could have implications for many benefits, including the following: (i) avoidance of consequences
to ecosystem services provided by freshwater, groundwater and coastal waters from reduced quality for
swimming, drinking, recreation or fishing; (ii) avoidance of air quality problems associated with N such as those
affecting human respiratory health or visibility (NOx, NHy); (iii) avoidance of damage from climate change and
stratospheric ozone depletion (N 2
O); and (iv) maintenance of soil fertility and ecosystem production (both N
and P). Eutrophication of coastal areas and associated hypoxia can result in physiological and behavioural
impacts on important coastal organisms, populations and ecosystems that result in lowered fitness and
productivity. However, there is a good deal of uncertainty about the economic damages associated with
coastal eutrophication in many areas (Rabotyagov et al., 2014). Efforts to inform policy should bring together
ecologists and economists to study the impacts of N and P on ecosystem services all along the cascade.
7.5.2 | Acidification buffering
Soil acidity is controlled by both biota (plant roots and microorganisms) and particles (soil minerals and
organic matter). Production of carbon dioxide, organic matter decomposition, and the excretion of acidic
compounds by biota increase soil acidity, while binding of acidic compounds to root and particle surfaces,
as well as mineral weathering, decrease it (Sposito, 2008). Over periods ranging from centuries to millennia,
while most of the less resistant minerals become depleted through weathering reactions with rainwater and
subsequent leaching, highly acidic soils are produced naturally. They now occupy about one-third of the icefree
land area on Earth (Guo et al., 2010), mainly in the humid tropics and in the forested regions of temperate
zones.
Status of the World’s Soil Resources | Main Report The impact of soil change on ecosystem services
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