Scottish Government

Chapter 2 Loss of soil organic matter

This chapter describes the importance of soil organic matter in Scottish soils, how Scottish soils differ in this respect compared with some other parts of the UK and a discussion of the status of organic matter and organic carbon in Scottish soils.

2.1 Summary

• Scotland's soils are relatively rich in soil organic matter, particularly in the hills and uplands but also in arable and grassland soils compared with some other parts of the UK. Scotland contains a much higher proportion of organic soils than the rest of the UK.
• Soil organic matter is a key property of soil and helps soils fulfil a large number of functions including mediating climatic conditions through carbon storage ( Chapter 3).
• There is some evidence that losses of dissolved organic carbon in streams draining peaty soils have increased in the last two decades implying a possible increase in losses of organic matter from these soils.
• Soil organic matter is being lost through peat erosion and better information on peat erosion processes, how to monitor it, and if possible how to mitigate it are required (Chapter 6).
• Large decreases in organic carbon concentrations of soils in England and Wales, particularly in soils of greater initial organic carbon concentration, have been reported recently; this trend, if also occurring in Scotland, would have serious implications for Scottish soils as a carbon store and in the climate change debate ( Chapter 3).
• Maintenance of soil organic matter in soils is a GAEC requirement within the Single Farm Payment.

2.2 Introduction and Description of Threat

Soil organic matter ( SOM) is a vital component in soils and helps soils fulfil a large number of functions. Brief outlines of its contributions to the range of soil functions are given below. In addition to these, SOM also has a major role to play in mediating climatic conditions, at a global level, as it is a significant carbon store; any loss of soil organic matter from soils would effectively increase carbon dioxide emissions to the atmosphere.

Organic carbon is measured as a percentage of dried soil or calculated from loss on ignition ( LOI) assuming a standard % C in organic matter. Although there is some doubt about the use of the conversion factor of 1.72 between soil organic carbon and soil organic matter, we have taken the view that they can be viewed as synonymous for the purposes of this study. There are known limitations about the LOI method in that soil carbonates can also be burnt off in addition to the soil organic matter and is particularly relevant to some soils in Central Scotland where coal fragments are found. Concern has been expressed about its use in policy formulation, but it does have the distinct advantage that large sample numbers can be handled quickly and it is relatively inexpensive. Other methods do exist but they tend to be slower and more expensive. There is also a need for measurements of bulk density and horizon depths if the absolute amount of C is to be determined as g m ^-2.

There is growing concern that soil organic matter levels, even within temperate regions such as Scotland, may be at risk due to soil management and global environmental change. Cultivation has been shown to increase accessibility of organic matter stored within natural soil aggregates to decomposer organisms and thereby decreases organic matter concentrations. In general terms, soil organic matter will decompose more rapidly under warmer conditions and if the Scottish climate is on a slowly warming trend (Scottish Executive 2006) then there may be implications for soil organic matter levels. Periodic drought conditions, which themselves are a function of temperature/rainfall interactions, may also be a cause of carbon loss from peaty soils and could be more serious than a temperature increase alone.

Biomass, food and fibre production

There is little hard evidence that a decline in soil organic matter content will adversely affect crop yield in mineral soils (Loveland and Webb, 2003) and there does not appear to be a critical threshold of organic matter in the agricultural soils of temperate regions. However, how much of this is down to continued high fertiliser inputs is unclear. Soil organic matter does increase aggregate stability (Chapter 5) and the water storage capacity of the soil, hence directly influencing plant growth. There is a gradual increase in the uptake of organic farming in Scotland and this will have a beneficial effect on the organic matter status on these soils.

Environmental Interactions

Organic matter contributes to a number of the environmental interaction functions of soil:

• As a carbon store. If soil organic matter levels decline either by emission of carbon dioxide or methane to the air or by physical movement to water courses, this function has clearly diminished. This has clear implications for issues covered in Chapter 3 (Climate change)
• It contributes to aggregate stability with clear evidence that as organic matter levels increase, the mean weight diameter of water-stable aggregates decrease. This reduces the potential for soil compaction and erosion and this effect is particularly marked on sandy soils. Clear differences between arable and pasture soils of the same soil series in organic matter content and aggregate stability have been shown to occur (Chaney and Swift, 1984). These issues are covered in Chapters 5 and 6.
• Contributes to the water holding capacity of the soil and therefore contributes to the potential amelioration or enhancement of flood risk ( Chapter 6)
• Contributes to the ability of soil to adsorb and degrade pollutants ( Chapter 7).

Support of ecosystems, habitats and biodiversity

The absolute content, type and age of soil organic matter is inextricably linked to soil macro- and mesofauna as well as microrganisms. However other factors such as soil acidity, soil moisture content and texture also influence biodiversity. Changes in soil organic matter status will change both the soil organism community as well as the ability of the soil to act as the base for semi-natural habitats. There is not a simple relationship between SOM content and biodiversity, but there is some emerging evidence from the Netherlands suggesting that diversity is reduced in soils with low organic matter content which themselves are related to intensity of management ( Chapter 4).

Provision of raw materials

Loss of organic matter from soils does not affect its ability to provide sand, clay or topsoil. Some of the decreases reported by Bellamy et al (2005) would suggest that the nature of highly organic horizons are changing radically and may influence the ability of the soil to be used a peat source.

Protection of cultural heritage.

The main potential impact is the increased erosion risk that might occur should soil organic matter levels decline, particularly on sandy soils, thereby providing less protection to the underlying heritage site of interest.

Provision of a platform

Declining soil organic matter levels do not affect this function.

Soil organic matter influences a number of processes across the range of soil functions and is a key soil property. This is particularly the case in Scotland in relation to climate change (Chapter 3) and greenhouse gas ( GHG) emissions. Reliable evidence that its current status may be under threat should be taken very seriously; firstly to establish the whether the threat exists and secondly, to identify the processes that are contributing to its decline.

2.3 Policy

Soil organic matter is recognised as a vital component of soils and a number of advisory documents such as the PEPFAA code and the Farm Soils Plan provide practical measures to help maintain soil organic matter levels in cultivated agricultural soils. Until the arrival of the GAEC requirements within the cross-compliance measures that form part of the reform of the Common Agricultural Policy, there was no formal or legislative requirement on farmers to manage their soils with the specific objective of maintaining organic carbon levels. This development represents a significant shift in policy; in addition to maintaining soils in good condition for biomass production, farmers are also required to maintain the carbon stocks within the soil for environmental functions.

Woodland soils generally contain higher levels of organic matter than agricultural soils and it is important that forest management recognises this and seeks to minimise damage to soils and losses of organic matter through ground disturbance. Some preparation techniques can be very intrusive. These issues are addressed at a high level within the UK Forestry Standard. In addition, the recently revised Forest and Soils Guidelines suggest a number of management options, for example cultivation, drainage and thinning regimes, that are aimed at maintaining and enhancing soil organic matter levels. The Scottish Forestry Strategy is currently being revised and amongst the many aims is one to establish new woodlands on soils with low current levels of organic matter to enhance carbon sequestration.

Peat is protected under the Habitats Directive and SSSI legislation for its biodiversity benefits whilst some other SSSIs are designated for their geodiversity and protection of key features of the quaternary period.

2.4 Evidence

2.4.1 Current status

Virtually all organic matter in Scottish soils is derived from litter fall, subsurface degradation of plant roots, rhizodeposition and exudation and soil organisms. In agricultural areas, some soil organic matter is derived from farmyard manures and slurries. Very little organic matter in soil originates from non-agricultural wastes such as exempt industrial wastes, composts and sewage sludges. although this proportion may be expected to grow in the future (Scottish Water 2006). Some overdeepened, plaggen soils (Chapter 9) have been enriched in organic matter through the addition of seaweed and other turf translocation techniques.

Compared with many other soil properties, the current status of soil organic matter can be characterised relatively well although there are significant limitations due to heterogeneity of soils and lack of measurements of bulk density needed to calculate absolute masses of C in the soil. Soil organic matter and soil organic carbon are two attributes within the Macaulay Institute ( MI) soil database and they can be used to give a broad picture of the soil organic matter status of Scottish soils. Figure 2.1 illustrates the distribution of SOM content in the uppermost horizon of the 10 km NSIS datapoints. These data refer to the time period between 1978 and 1987 and a subset of these will be revisited to identify whether these values are still current. A bimodal distribution is apparent. The peak at around 5% organic carbon represents the bulk of the agricultural soils in Scotland whilst the peak at around 55% represents many of the blanket peat sites and other organo-mineral soils such as peaty podzols and peaty gleys. Compared with soils in England and Wales, even the mineral soils have higher levels of organic carbon than their counterparts south of the border (Bradley et al., 2005).

Figure 2.1 The frequency distribution of carbon content in the uppermost horizon of the soils sampled at 721 sites on the10 km grid National Soils Inventory Scotland

Additional information in the database has been linked to the 1: 250,000 scale soil map to produce an indicative map of soil organic matter content within the uppermost soil horizon across the country (Figure 2.2). This map provides a summary of data that was captured over a prolonged time period from the 1950s until the 1980s and this must be borne in mind when it is interpreted.

Figure 2.2 Topsoil organic carbon content

There is a clear separation between the highly organic soils throughout much of the Highlands and the Southern Uplands and those on the predominantly cultivated soils of the Central Valley and NE Scotland where much lower levels are present. Most of the cultivated land in Scotland has moderate or high levels (5-10%) of organic matter. High soil organic matter levels exist in the Highlands and Southern Uplands where the cooler and wetter climate conditions inhibit the decomposition of organic matter in plant material deposited on the soil surface. In such areas, the accumulation of organic matter is often more rapid than decomposition and an organic surface horizon forms. On level or gently sloping sites the total accumulation can be as much as 7-8 metres.

Although data on the C content of Scottish soils is relatively good, it must be remembered that these cannot be translated directly into carbon stocks; bulk density of each soil horizon must also be considered. Bradley et al. (2005) have reported on the development of a database of soil carbon and land use and have estimated that Scotland contains 48% of the carbon stocks in soils in the UK down to a depth of 100 cm. The carbon stock in the top 30 cm in organic soils under semi-natural vegetation is similar to that contained in the same depth of mineral grassland soil. In both it is estimated to be 16 kg m ^-3. This is despite the fact that soils under semi-natural vegetation having approximately ten times the amount of carbon (as a % weight) than the soils under pasture. The much lower bulk density of organic horizons negates its higher % C content. This paper also indicates that the total stock of carbon in Scottish soils to one metre depth (2,187 Tg C) is split approximately equally between 0-30 cm depth and 30-100 cm depth. Estimates of C stocks below 100 cm depth are currently being determined as part of the ECOSSE project funded by the Scottish Executive, but it is inevitable that this addition will bring Scotland's total contribution well above 50%. Previous estimates have put this figure at 71% (Milne and Brown, 1997) and it will be interesting to compare the new estimate to this. It is impossible to get a truly definitive estimate of SOM, but the importance of soil as a carbon stock is emphasised by the fact that in the whole of Great Britain, it holds between 80 and 90 times more carbon than the vegetation.

2.4.2 Data availability

Organic matter is reasonably well characterised for the period 1978-1987 in the NSIS. The other data held within the MI soils database comprise data with an inherent bias as the soils were selected subjectively in order to characterise soil series, but some very detailed grid and transect data (between 10 and 50 m spacing) also exist and give detailed estimates of spatial variability.

There are also monitoring data from the Countryside Survey and a limited number of Scottish Water and FC data. Data will also be available within the UK Soil and Herbage study (pending) although this is at a much wider sampling density than the NSI. A number of very detailed surveys of peat bogs exist throughout Scotland where depths were measured along transects and a number of attributes measured including degree of decomposition (the Van Post Humification index), moisture and ash content. The peat surveys were largely carried out between 1949 and 1961 and as such provide a unique record of the depth and state of Scottish bogs at that time. There will also be data coming on stream from the EU BioSoil survey and monitoring project. More information on these datasets can be found in Appendix A.

2.5 Gaps in data / evidence

There has been no systematic investigation into reductions in soil C levels, although data are being collected from heathland sites where birch has been planted as part of the SEERAD funded ECOSSE project. Indirect evidence exists in the form of increased concentrations of dissolved organic carbon ( DOC) in stream waters draining upland catchments where soils are dominantly peaty. Data published by McCartney et al. (2003) show a clear rising trend in DOC concentrations in a stream draining a mature forest catchment near Loch Ard in west-central Scotland from around 5 mg l ^-1 in the early 1980s to around 16 mg l ^-1 in 2003 (Fig. 2.3). There also appears to be a trend towards higher values in extreme events from 1995 onwards. A similar although less dramatic trend is seen at the Glensaugh Environmental Change Network ( ECN) site (Fig. 2.4).

Worrall et al (2003) summarised data from 198 streams within the UK (155 in Scotland). These showed a statistically significant increase in DOC concentrations at 77% of the 198 sites. The remaining 23% showed no trend and no sites showed a decrease. The trends were independent of regional effects of rainfall, acid and nitrogen deposition and land use change. The mean annual increase was 0.17 mg l ^-1, which is similar to the increase seen at the Glensaugh ECN site.

Further work has investigated whether these changes can be linked to climate change (Worrall et al., 2004; Freeman et al., 2004) but both have cast doubt on whether climate warming on its own offers satisfactory explanations. Recovery from acidification ( Chapter 7) may be the trigger for these increases in DOC levels.

Figure 2.3 DOC losses from a site near Loch Ard, Central Scotland, over a 16 year period.

Figure 2.4 DOC concentrations from the Glensaugh ECN site.

There is direct evidence of decreases in soil carbon concentrations from England and Wales with the largest changes found in soils with the largest initial carbon concentrations (Bellamy et al., 2005). This study was based on a comprehensive phased and partial resampling of the NSI in England and Wales which achieved a 40% revisiting of the original sample sites. The details are:

• 1994-95 (arable and rotational grassland sites n = 853, 33% of the original)
• 1995-96 (permanent grassland sites n = 771, 49% of the original)
• 2003 (non-agricultural sites n = 555, 37% of the original)

Overall a 40% revisiting of the original sample population was achieved.

Soils with low (<3%) original soil organic carbon values showed very small increases in soil carbon but for the remainder of the soils, all showed decreases in soil organic carbon. These decreases increased in proportion to the original organic carbon content and were greatest for upland soils with organic surface horizons. For soils with carbon contents of greater than 10% (100 g kg ^-1), the rate of decrease was 2% per year. Reference to Figure 4.1 indicates that this is very relevant in the context of Scottish soils although no comparable data are available for Scotland. Without additional data (bulk density and soil depths) they do not prove that there has been any decline in the absolute amounts of C in the soil. What it does say is that there has been a decline in organic carbon within the upper 15 cm of the soil compared with the initial sampling period. Whether these declines are similar in scale to the increases in DOC observed requires more detailed examination.

The paper does not attempt to identify the mechanism that has caused this decline or where the carbon has gone. It is fair to say that the findings have surprised the soil science community and that more work is required to identify the causes. Defra have carried out an independent analysis of the raw data and found that the C loss that they calculate is similar to that reported by Bellamy et al (2005). The report also outlines the importance of bulk density (which was not measured) in any calculation of carbon stocks and the absence of any information on management changes that might have been partly responsible for C increase or decrease.

Soils are traditionally thought of as sinks for carbon, primarily peats and other soils with highly organic surface horizons and Chapman et al. (2001) summarized carbon accumulation rates on a range of soils and land use systems. These varied from around 20 up to 200 g C m ^-2 a ^-1 on different types of peatland although it was acknowledged that these rates are very difficult to measure. However, these figures estimate the rate at which organic carbon accumulates at the soil surface suggesting that organic matter (and therefore carbon) depth is increasing, whereas the results produced by Bellamy et al (2005). suggest a reduction in the C concentration (rather than depth). Chapman et al. (2001) also indicate that podzols and other organo-mineral soils have reached equilibrium. It is not clear from the work of Bellamy et al (2005). precisely what type of soil is contributing to the biggest losses, but there do appear to be conflicting messages between C accumulation in peats in their 'building' phase and the reported reduction in C concentration in these soils.

Although we have information on the extent and location of peat erosion (Chapter 6), we do not have any data on whether erosion events are becoming more common, more intense or both? This is clearly another potential source of carbon loss from soil. Climate change may trigger more of these events (Chapter 3) and it is important that we develop a better understanding of peat erosion processes, how to monitor it and if possible to develop mitigation strategies.

Since soil organic matter supports a number of soil functions and the agriculture and land use and forestry sectors account for 20% of GHG emissions in Scotland (Scottish Executive 2006) it is important that work is undertaken to confirm (or otherwise) the extent of C loss from soils across Scotland. A partial resampling of the NSI in Scotland has been commissioned by SEERAD and the results from this will indicate whether similar reductions are found here. A key difference in the approach will be that the soils will be sampled based on a pedological horizon by horizon basis rather than by coring or auguring down from the soil surface. The method of soil sampling has been highlighted within the DEFRA report as an area where potential uncertainty might be introduced.

2.6 Conclusions

• Scotland contains well over half the total carbon contained in British soils and it is one of the principal soil attributes that distinguishes Scottish soils from the rest of the United Kingdom.
• Soil organic matter profoundly influences a whole range of soil functions and can influence a number of the other threats such as erosion risk, compaction and loss of biodiversity
• The importance of organic matter in soils has already been recognized within the GAEC requirements and the Scottish Executive's climate change programme.
• There is emerging evidence that soil organic matter content in Scottish soils may be decreasing but more data are required to confirm this. Increased DOC levels in streams draining catchments containing predominantly peaty soils provides the best indirect evidence of losses. Large reductions in soil organic carbon have been found in similar soils in England and Wales, based on direct measurement of soil organic matter content.
• Better information on peat erosion processes, how to monitor it, and if possible how to mitigate it are required.
• Soil organic matter is a key attribute of Scottish soils and cuts across a number of areas of policy interest to the Scottish Executive; sustainable farming and forestry, climate change, biodiversity for example. It could be argued that it also indirectly affects the tourist industry as it sustains the above ground habitats that many visitors come to Scotland to enjoy.

2.7 Key references

Bellamy, P.H, Loveland, P.J., Bradley, R.I., Lark, R.M. and Kirk, G.J.D. (2005) Carbon losses from all soils across England and Wales 1978-2003. Nature, 437, 245-248

Bradley, R.I., Milne,R., Bell J., Lilly, A., Jordan C. and Higgins, A. (2005) A soil carbon and land use database for the United Kingdom. Soil Use and Management, 21, 4, 363-369.

CEH (2002) Critical Appraisal of State and Pressures and controls on the Sustainable Use of Soils in Wales. Contract report to EA/National Assembly for Wales

Chaney R and Swift R S (1984) The influence of organic matter on aggregate stability in some British soils. Journal of Soil Science, 35, 223.

Chapman, S.J., Towers, W., Williams, B.L., Coull, M.C. and Paterson, E. (2001) Review of the contribution to climate change of organic soils under different land uses. Environment Group Research Programme Research Findings No.17. Scottish Executive Central Research Unit.

Freeman, C., Fenner, N., Ostle, N.J., Kang, H., Dowrick, D.J., Reynolds, B., Lock, M.A., Sleep, D., Hughes, S. and Hudson, J. (2004) Export of dissolved organic carbon from peatlands under elevated carbon dioxide levels. Nature, 430, 195-198.

McCartney, A.G., Harriman, R., Watt, A.W., Moore, D.W., Taylor, E.M., Collen, P. and Keay, E.J. (2003) Long-term trends in pH, aluminium and dissolved organic carbon in Scottish fresh waters; implications for brown trout (Salmo trutta) survival. Science of the Total Environment, 310, 133-141.

Milne, R. and Brown, T.A. 1997. Carbon in the vegetation and soils of Great Britain. Journal of Environmental Management 49, 413-433.

Loveland, P. and Webb, J. (2003) Is there a critical level of organic matter in the agricultural soils of temperate regions: A review. Soil and Tillage Research, 70, 1-18.

Scottish Executive (2006) Changing our ways. Scotland's Climate Change Programme. Scottish Executive Edinburgh 2006.

Scottish Water Draft Sludge Strategy, January 2006

Worrall, F., Burt, T.P and Shedden, R. (2003) Long terms records of riverine carbon flux. Biogeochemistry, 64, 165-178.

Worrall, F., Burt, T.P.? and Adamson, J. (2004) Can climate change explain increases in DOC flux from upland peat catchments? Science of the Total Environment, 326, 95-112.