Risk/opportunity:(from the Climate Change Risk Assessment for Scotland 2012):
AG19 Risk of soil erosion and leaching

Narratives: Condition of agricultural soils

SCCAP theme: Natural environment

SCCAP objectives:
N3: Sustain and enhance the benefits, goods and services that the natural environment provides

Latest figures

Mean carbon concentration of arable and horticultural soils (0-15 cm): 32.3 g/kg (2007)

Trend
At a glance
  • Soils play a vital role in maintaining ecosystem function and supporting delivery of ecosystem services
  • Carbon is an important indicator of soil quality
  • Projected climate change may increase the loss of soil carbon and also lead to a significant increase in the land area suitable for arable farming
  • It is therefore important to monitor and understand changes in soil carbon in arable soils
  • This can also help us assess the efficacy of policies designed to limit soil degradation and maintain soil quality on arable land

Soils play a vital role in maintaining ecosystem function and supporting delivery of ecosystem services including water regulation and purification, crop production and carbon sequestration (Brown et al, 2012). Soils contain the largest terrestrial pool of carbon, and in turn, carbon is an important indicator of soil quality. It is the primary source of energy for soil organisms and plays an important role in maintenance of structural condition, water retention and resilience (Emmett et al, 2010).

Rates of soil erosion may increase with increasing rainfall intensity. Climate projections for Scotland indicate more heavy rainfall days (i.e. days with more than 25mm of rain) in both summer and winter, and an increase in winter rainfall. Projected climate change is also likely to lead to a significant increase in the land area suitable for arable farming (Defra, 2012).

Soil erosion can lead to the loss of soil organic material, resulting in reduced soil fertility and the loss of soil carbon, releasing stored carbon to the atmosphere thereby increasing greenhouse gas emissions (Defra, 2012). It is also detrimental to watercourses, leading to increased sedimentation and, in the case of soil erosion on arable land, increased pollution by fertilisers and pesticides.

These impacts could be exacerbated as projected climate change is also likely to lead to a significant increase in the land area suitable for arable farming, and an associated increase in pesticide and fertiliser use (ibid).  There is therefore a need to monitor and understand changes in soil carbon in arable soils.

‘There are great uncertainties in climate impacts on soil carbon cycling. Countryside Survey soil carbon data will contribute to knowledge of how soil carbon is changing, how this relates to vegetation change and land use and management and provide evidence of the effectiveness of soil protection legislation.’ (Emmett et al, 2010)

The figures reported in this document are for soils in the ‘Arable and Horticultural’ Broad Habitat type drawn from the Countryside Survey.  They refer to top soil (0-15cm) carbon concentrations.

Related Indicators:

NA10 Soil erosion risk

NA12 Agricultural production methods which reduce erosion risk

The mean carbon concentration in Arable and Horticultural soils is 32.3 g/kg (2007). This is a statistically significant reduction of 9.3% from 1998.

Between 1998 and 2007 there was a statistically significant decline in the mean soil carbon in Arable and Horticultural soils.  This corresponds with a statistically significant decline in soil carbon in all habitat types in Scotland.

 Table 1. Carbon concentration of soils (Arable and Horticulture): 1978-2007

Year

Mean carbon concentration (g/kg)

2007

32.3

1998

35.6

1978

35.8

Table 2. Change in carbon concentration of soils (Arable and Horticulture): 1978-2007

Year

Change in carbon concentration (%)

Statistically significant change

1978 - 1998

-0.6

N

1998 - 2007

-9.3

Y

1978 - 2007

-9.8

N

Source: SESO (from Countryside Survey, 2007)

Emmet et al (2010) advise that further analysis of data from the Countryside Survey 2007, along with data from other surveys and from modelling, is needed before it is possible to comment on the likely future trends of soil carbon concentrations.

Changes in soil organic carbon have been attributed to multiple drivers with climate change playing a relatively minor role; accounting for not more than 10% of losses (Kirk et al, 2010; Smith et al, 2007a; cited in Brown et al, 2012). Historically, land use change and management are of more significance in driving change; however it is expected that climate change will increase in significance over time (ibid).

‘The paleo-environmental record clearly shows that soil organic matter is sensitive to changes in climate’ (Brown et al, 2012, p93) but the relationship between soil carbon and climate is complex (ibid).

The Countryside Survey (Emmett et al, 2010) identifies a number of potential drivers of change including climate change, nutrient deposition, management practices, increasing atmospheric CO2 concentration, land use and land use change. Additionally, multiple drivers can affect the balance of carbon inputs and outputs, further complicating attribution of change in soil carbon concentration. However, the authors rule out land use change and climate change as drivers of the significant changes in topsoil carbon concentration that were observed and conclude that the changes observed in arable soils must be due to other habitat-specific drivers (ibid).

‘The most consistent finding from the Countryside Survey in 2007 is the reduction in soil carbon (0-15 cm) concentration and density in cropland’ (Emmett et al, 2010, p44). A simultaneous reduction in mean total nitrogen concentration suggests that ‘processes such as erosion, deep ploughing and/or increased decomposition may be responsible for this trend’ (ibid). It is thought likely that agricultural intensification is the dominant factor (Stoate et al, 2001; cited in Emmett et al, 2010).  The authors conclude that the loss of soil carbon from the Arable and Horticulture Broad Habitat (0-15cm depth) across Great Britain ‘suggests that current policies in place to limit soil degradation are not maintaining soil quality in cropped land’ Emmett et al, 2010, p44).

The Countryside Survey found no overall change in carbon concentration in soil (0-15cm) in Great Britain between 1978 and 2007. The Arable and Horticulture Broad Habitat was the sole exception to this pattern.

Emmett et al (2010) note that although the Countryside Survey does not take account of soil lost by erosion, a reduction in soil carbon would be expected if soil erosion were widespread.

The Countryside Survey data used here samples soils to a depth of 15cm (0-15cm), the layer which is likely to be most prone to change. However, other studies suggest that a depth of up to 100cm should be used (Chapman et al, 2013). The Intergovernmental Panel on Climate Change (IPCC) recommend sampling to a depth of 30cm (IPCC, 2004; cited in Bradley et al, 2005). For instance, Chapman et al (2013) used samples to a depth of 100cm to monitor change in soil carbon stocks in Scotland between 1978 and 2009. Whereas sampling to 100cm instead of 15cm did not change the findings (a significant decrease in carbon content in arable soils was recorded), it was noted that ‘sampling only to 15cm would have resulted in an incomplete understanding of the changes in carbon stock’ (Chapman et al, 2013).

Bradley, R.I., Milne, R., Bell, J., Lilly, A., Jordan, C. & Higgins, A. (2005) A soil carbon and land use database for the United Kingdom. Soil Use and Management. 21, 363-369.

Brown, I et al (2012) Climate Change Risk Assessment for the biodiversity and ecosystem services sector. UK CCRA https://www.gov.uk/government/policies/adapting-to-climate-change

Chapman, S.J., Bell, J.S., Campbell, C.D., Hudson, G., Lilly, A., Nolan, A.J., Robertson, A.H.J., Potts, J.M. & Towers, W. (2013) Comparison of soil carbon stores in Scottish soils between 1978 and 2009. European Journal of Soil Science. Aug 2013, 64, 455-465.

Defra (2012) A Climate Change Risk Assessment for Scotland. Defra, London.  Available at: http://www.defra.gov.uk/environment/climate/government/  (accessed April 2015)

Emmett, B.A., Frogbrook, Z.L., Chamberlain, P.M., Giffiths, R., Pickup, R., Poskitt, J., Reynolds, B., Rowe, E., Spurgeon, D., Rowland, P., Wilson, J. & Wood, C.M. (2008). Countryside Survey Technical Report No.3/07, Soils Manual. Centre for Ecology and Hydrology (Natural Environment Research Council), Lancaster. Available at: http://www.countrysidesurvey.org.uk/sites/default/files/pdfs/reports2007/CS_UK_2007_TR3.pd

Emmett, B.A., Reynolds, B., Chamberlain, P.M., Rowe, E., Spurgeon, D., Brittain, S.A., Frogbrook, Z., Hughes, S., Lawlor, A.J., Poskitt, J., Potter, E., Robinson, D.A., Scott, A., Wood, C., Woods, C. (2010). Countryside Survey: Soils Report from 2007. Technical Report No. 9/07 NERC/Centre for Ecology & Hydrology 192pp. (CEH Project Number: C03259).  www.countrysidesurvey.org.uk/outputs/soils-report-from-2007

Countryside Survey – measuring change in our countryside: www.countrysidesurvey.org.uk/

Suzanne Martin (RBGE) contributed to this indicator as a co-author.