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GROUP 6 Soil Organic Matter Loss

Working group members and contributors

Soil Organic Matter loss

Importance of Soil organic matter

7.1 Soil organic matter is a universal constituent of soils, and plays a vital role in contributing to a range of soil functions. Between a third and one half of the mass of organic matter contained in soils is made from carbon and so management of soil has important wider consequences in the context of greenhouse gas emissions and climate change. Scotland contains more than half of the organic matter in the UK soils, and in this respect Scotland is distinctive within the UK and throughout the rest of Europe (Figure 7.1). Scotland's organic soils alone contain 2735 Mt Carbon (mineral soils excluded) and this compares with a total of 114 Mt Carbon in UK surface vegetation (Bradley et al., 2005). The development of a Scottish Soil Strategy will highlight the importance of this national resource and the need to preserve and enhance this large pool of terrestrial carbon.

7.2 Organic soils can be considered as either peats or organo-mineral soils, with peats being defined as soils that consist of more than 60% of organic matter and exceed 50 centimetres in thickness, and organo-mineral soils as soils having organic surface horizons, less than 50 cm thick (Smith, 2007b). Many of Scotland's soils would not be defined as organic using these criteria and yet still contain large quantities of organic Carbon when compared with other countries (Bradley et al., 2005). Scotland has a responsibility to look after this resource not only from a climate change mitigation perspective but also in recognition of the valuable cultural heritage and habitats these soils support; and this is supported by Scottish Ministers.

7.3 Organic matter in all soils plays a critically important role in enabling soils to deliver a range of ecosystem services. DEFRA ( DEFRA, 2004) have recently defined these as: food and fibre production, environmental interaction (between soils, air and water), support of ecological habitats and biodiversity, protection of cultural heritage, providing a platform for construction and providing raw materials. Scottish soils play a particularly important role in storing Carbon and removing CO ₂ from the atmosphere and in influencing other greenhouse gas emissions. Recent estimates suggest that within the UK, the pool of soil Carbon (currently around 4.5 Tg Carbon, Bradley et al., 2005) is slowly accumulating Carbon at a rate of 0.22 Mt/y in 2000 (as reported to UNFCCC). However, there is a high degree of uncertainty in these estimates, and other studies have suggested that UK soils may be acting as a net source of CO ₂ to the atmosphere, principally as a consequence of agricultural activities and peatland emissions (Janssens et al., 2003). There is also significant spatial heterogeneity, with forest and grassland soils acting as a net sink, whilst arable soils and some peats may be a net source (Dawson & Smith, 2006). Carbon fluxes from soils as a result of land use change make an important contribution to Scotland's net greenhouse gas flux from the land use change and forestry sector (Baggott, 2007). Their contribution is an overall net flux to the atmosphere of 1096 Gg Carbon, but within this land use change to forests and grasslands results in a net sink of 1169 Gg Carbon, whereas land use change to cropland and to settlements results in a net source of 2234 Gg Carbon. Many of these fluxes come from land use change that occurred before 1990.

Figure 7.1 Organic carbon content (%) in the surface horizon of soils in Europe (van Kamp 2004).

7.4 Soils also play an important role in contributing to the production of the greenhouse gas nitrous oxide with agriculture accounting for 83% of Scottish emissions most of which is attributable to soils (Scottish Executive, 2006a). The use of organic and inorganic fertiliser and emissions from grazing animals are particularly important contributors. Soils (especially peats) also release small amounts of methane, although these make up less than 10% of national emissions (the major sources being livestock, industry and waste recycling). The soil resource is therefore a complex and highly dynamic component of our environment, requiring careful management in order to ensure delivery of a wide range of environmental and societal benefits.

What are the issues relating to SOM loss in Scotland?

7.5 There is concern that the organic matter content of Scottish soils may be threatened by a range of environmental pressures. These include; climate change, land use change, pollution, erosion, soil sealing, and excavation (van Kamp, 2004). The threats facing different ecosystems vary considerably. In areas currently used for arable farming, forestry and rotational grassland, there may be opportunities to increase soil organic matter though appropriate management (Smith et al. 2000a). However, in areas where soil Carbon stocks are already high, such as peatland soils, some forest soils and extensive grasslands, it is likely to be more important to focus on conserving current Carbon stocks.

7.6 The major pathway of loss of organic matter is via the atmosphere through the process by which CO ₂ is generated via respiration. Although much of this is returned to soils as a consequence of the photosynthetic activity of plants, the net exchange of Carbon from land surfaces may still be large, exceeding 5 t Carbon ha ^-1 y ^-1 (Janssens et al., 2003; Soussana et al., 2007). Other processes can contribute to soil organic matter loss such as soil erosion (Bilotta et al., 2007) and loss of dissolved organic carbon in stream-water (Billett et al., 2004). Losses by these processes are generally though to be quantitatively less important than direct atmospheric losses, although there remain high levels of uncertainty.

7.7 Conservation of soil organic matter provides a range of wider benefits in relation to climate change (through offsetting greenhouse gas emissions), water pollution (potential reductions in diffuse pollution by phosphates and nitrates), biodiversity (particularly in relation to soil biodiversity), and issues relating to contaminated land (soil organic matter has a high capacity to lock up potentially toxic elements preventing them from entering the food chain and other environmental media).

What data are available and where are the gaps?

7.8 We have relatively good knowledge of the spatial distribution of organic soils (e.g. National Soils database held by the Macaulay Institute) but our estimates of soil Carbon stocks lack measures of statistical certainty as data was primarily derived from soil surveys or were designed to evaluate land for agricultural capability. This absence of reliable, quantitative data limits our ability to determine change over time. Much of the information relating to changes in soil carbon is derived from small-scale research projects, inference from studies in other countries, and modelling. Work by Bellamy et al. (2005) has highlighted potentially large losses of carbon from carbon rich soils in England & Wales in recent years. We are unable to answer specific policy questions at present regarding the impacts of climate change scenarios and land use changes in Scotland such as conversion of land to energy crops as we don't have sufficient data on which to base these. Specific issues relating to cross cutting themes also need to be addressed. For example there is uncertainty regarding the extent to which climate change will exacerbate fire in moorland and forest vegetation or soil erosion both of which could accelerate organic matter loss.

7.9 The current partial resampling of the National Soils Inventory Scotland ( NSIS2) which is planned by the Scottish Government, will provide valuable new evidence regarding changes to soils in Scotland since the first sampling exercise. The Scottish Government is also planning work to develop a soil monitoring scheme for Scotland and is funding work to gain more reliable data on soil carbon stocks alongside the NSIS2 programme. This programme of work needs to needs to be extended and consideration needs to be given to measuring functional aspects of the organic matter such as carbohydrates or readily available organic matter (which relates to compactibility and erosivity) at the same sites. It is essential site measurements within different landuse categories should include the depth of soil horizons and their bulk density. Laboratory measurements should be made using a standardised technique (elemental analyser), complete with bulk density measurements to the appropriate depth. This will enable the monitoring of change to soil carbon stocks over time. The ECOSSE model (Smith, 2007a) developed to simulate the behaviour of organic soils in response to climate and land use change will be improved and validated using new Scottish soils data hence improving its ability to predict changes in soil carbon.

7.10 There is information on land use change from the sample surveys undertaken for the National Country Monitoring Scheme (Mackey et al., 1988) project (based on 1947, 1973 and 1988 aerial photographs) and the Countryside Survey (field based surveys in 1984, 1990 and 1998) (Barr, 1993; Firbank, 2003). These provide good statistical information on land use change at the national and regional scale but cannot help with requirements for detailed local information on land use histories. Satellite-imagery based land cover mapping from the latest Countryside Survey (2007) should become available in 2009/2010 and be comparable with Land Cover Map 2000, enabling an assessment of land cover change across the whole country. Soil sampling is being undertaken as part of the latest Countryside Survey (2007), with soil cores being analysed for biological and chemical characteristics, including organic matter concentration. Sampling will be done in each 1km sample square, 203 of which are in Scotland.

7.11 A valuable assessment of soil quality has recently been undertaken by the UK Soil Indicators Consortium, which is a group of public stakeholders developing a UK set of soil indicators and a UK soil monitoring scheme. Its aims are to develop a set of policy relevant and scientifically robust indicators of soil quality for all soil functions and develop a framework for UK soil monitoring. Fifteen priority indicators were selected as suitable for UK monitoring including topsoil SOC%, soil pH and C/N ratio as the top three. The UKSIC commissioned the Scottish and Northern Ireland Forum for Environmental Research ( SNIFFER) to undertake a national soil monitoring network review and assessment study to assess existing monitoring schemes and their suitability to report on soil quality. Following this a new project was instigated in order to design a UK Soil Monitoring Scheme ( SP0558). This project is due to report early in 2008.

7.12 In Scotland, the Scottish Executive funded a parallel project for the Development of a Soil Monitoring Scheme for Scotland ( CR/2006/14). One strong recommendation of the report was that "Scotland needs to obtain data on soil carbon stocks and given the nature of its highly differentiated soil horizons this should be measured in terms of full soil horizon depth on a pedological (horizon) basis." DEFRA have also been funding research "To halt the decline of soil organic matter caused by agricultural practices in vulnerable soils by 2025, whilst maintaining, as a minimum, the soil organic matter of other agricultural soils, taking into account the impacts of climate change." This primarily applies to England, but some projects have received a UK label (e.g. a Soil Carbon workshop).

Management actions to conserve and enhance SOM

7.13 There is an extensive literature reporting on land management options that can be adopted in order to increase carbon sequestration in soils. Much of this has been developed as a consequence of the need to reduce greenhouse gas emissions, and has been reported in recent IPCC publications (Table 7.1; IPCC, 2007). Within a Scottish context it is likely that the management activities that will be particularly important will include improved crop land management, improved water management, improved grazing management, the management of peat soils and forest management. These represent estimates of potentialCarbonsequestration rates for temperate environments, with uncertainties represented by the maximum and minimum values. Uncertainties can be constrained by reference to a number of recent research programmes that have attempted to quantify carbon sequestration potentials of different types of management. Highest carbon sequestration is generally reported from forest and grassland systems, although this depends on the nature of management and its interaction with site conditions. Measured rates of carbon sequestration by European forests at rates of 0.1-1.1 t Carbon ha ^-1 y ^-1 were reported by Cannell (1999) and Liski et al. (2002), whilst that in grasslands ranges between 0.3-3 Carbon ha ^-1 y ^-1 (Soussana et al. 2004) Management of arable land using a range of management activities is thought to have a potential to achieve a carbon sequestration potential in the UK of 0.1-0.7 t Carbon ha ^-1 y ^-1, (Smith et al., 2000a; Smith et al., 2000b; Smith et al., 2005) and undisturbed peatlands have been shown to sequester around 0.7 t Carbon ha ^-1 y ^-1 (Cannell et al., 1999). A recent review (Reynolds, 2007) concluded that changing land use on organo-mineral soils from semi-natural/grazed grassland and moorland to forest has a relatively small effect on SOC storage but there are still uncertainties that need further research. It is likely that novel management practices could contribute to carbon sequestration such as deep ploughing which could include incorporation of compost, or the blocking of drains to increase soil wetness. Again further research would be required to quantify the potential benefit of these activities, pollution swapping (particularly if this involved greenhouse gases) and a broader range of environmental impacts such as flooding and alterations in vegetation type.

Table 7.1 Potential Carbon sequestration by different land management activities Values in t CO ₂-C ha ^-1 y ^-1. IPCC, 2007

*(Kowalski et al. 2004)

7.14 Scotland's warming climate coupled with altered patterns and amounts of rainfall are likely to influence the organic carbon content of its soils, but peatland soils with their higher carbon contents may be particularly vulnerable. This has led to speculation that peatlands may become sources of Carbon with changing climate, and in some circumstances could contribute to catastrophic carbon losses to the atmosphere (Evans et al., 2006). Indeed, peatlands are already on the margin as Carbon sinks (Waddington & Roulet, 2000). Weather perturbations, especially drought and high temperatures, can reverse the sign of annual CO ₂ flux from sink to source in consecutive years (Waddington & Roulet, 2000). But consensus does not yet exist on the longer-term fate of the peatlands Carbon sink. It is possible that climate change could cause an increase in Carbon sequestration due to an increase in woody biomass replacing moss-dominated landscapes (Laine et al., 1996). Warm, dry years may create a competitive advantage of vascular plants compared to mosses and bring about successional changes in peatland vegetation, altering the relative strength and direction of annual Carbon fluxes in ways that are not well understood. Conversely drier summers with episodic but more intense rainfall events could lead to rapid peatland erosion and Carbon loss. Enhanced losses of dissolved organic carbon from UK peatlands have recently been cited as evidence of the effects of climate change (McCartney et al., 2003). It is possible that peat soils could be managed in order to reduce or reverse carbon loss e.g. by blocking drains and gulleys and re-vegetating erosion channels (Petrone et al., 2001; Waddington & McNeill, 2002), however, the extent to which carbon loss could be controlled by such measures remains uncertain.

7.15 The production of bio-fuels and bio-energy are likely to become an important activity in the coming years in order to achieve government targets for renewable energy production, leading to potentially significant land use change. This will include the production of grain for bio-ethanol production, oilseeds, grasses and short rotation coppices. Recent studies by Smith et al have suggested that bio-fuel production in the UK has a mitigation potential of more than 3 Tg Carbon y ^-1, which alone would offset approximately 3.2% of the UK's 1990 CO ₂ production. However, these estimates of carbon sequestration are relatively uncertain, and it remains difficult to predict the consequences for nitrous oxide and methane exchange (Crutzen et al., 2007) which could counteract the benefits provided by carbon uptake, underlying the need for additional research in this area.

Policies to be considered

7.16 At present there is no direct policy incentive that is used to encourage land managers and farmers to protect or manage the amount of carbon present in their soils. Potential mechanisms by which this can be achieved have been widely discussed, involving both regulatory and voluntary initiatives, but at present there is little consensus on which approach would be most effective (Renwick et al., 2002). A wide range of policies does however contribute indirectly to maintaining and enhancing the organic matter, although the extent to which they achieve this remains uncertain. These include the developing Soil Thematic Strategy, which is likely to highlight losses of soil organic matter as a specific concern. The UNFCCC and the Kyoto protocol require reporting of greenhouse gas emissions at a national level (Baggott et al., 2007) but estimates are also made for the individual countries of the UK. Land use change (except change to/from forestry) is not currently included in the Kyoto protocol inventories, BUT this could change in line with future agreements that are put in place following Kyoto in 2012.

7.17 In developing the Soil Thematic Strategy it will be important to develop policies that contribute to multiple objectives. Thus minimising organic matter loss from soils can be achieved through application to land of organic wastes, and through minimisation of erosion although again in the case of organic waste applications there is a need to assess pollution swapping issues ( PTEs, nutrients etc). Soil organic matter conservation will often maximises biodiversity objectives, and can reduce sources of diffuse pollution. These broader linkages need careful consideration when framing the Scottish Soils Strategy, and they need to avoid conflicts with other areas of policy.

7.18 The Common Agricultural Policy (particularly Good Agricultural and Environmental Condition rules and changes to set-aside rules), the Water Framework Directive, and Prevention of Environmental Pollution from Agricultural activity provide a range of advice and requirements that are placed upon farmers in order to encourage good environmental practices. These include careful use of manures and fertilisers and advice on a wider range of management activities that are designed to deliver specific environmental benefits. Specific guidance on management of soils to enhance soil organic matter is included in the "Good Agricultural and Environmental Condition" guidelines. Many of these measures however will often contribute indirectly to the preservation or enhancement of soil organic matter stocks.

7.19 The EC Habitats Directive identifies a number of habitats and species requiring protection through the designation of Special Areas of Conservation ( SAC). Several of the habitats are supported by soils with high organic matter contents and some; particularly raised bog, blanket bog, bog woodland and alkaline fen are in part defined by their peat soil. The Scottish Executive Changing our Ways: Scotland's Climate Change Programme document set ambitious annual carbon saving targets for the forestry sector of 0.6 MtC by 2010, 0.8 MtC by 2015 and 1 MtC by 2020 through a range of policy measures. The Scottish Forestry Strategy (Scottish Executive 2006) sets out a programme of action to achieve these targets by highlighting the potential for forestry to make a significant contribution to mitigating and adapting to climate change over the coming decades. These actions include promoting biomass as a renewable energy source and increasing carbon sequestration and retention. Much of the proposed carbon saving will contribute to increased soil organic matter contents and therefore contribute to improving a range of soil quality parameters.

7.20 The Nature Conservation (Scotland) Act 2004 provides the mechanism for the notification of Sites of Special Scientific Interest and safeguarding their interest features, including geological and geomorphologic interests, i.e. including soils. It also confers a Biodiversity Duty on Public Bodies and their officers. The Muirburn Code ⁶ identifies the legislation relevant to muirburn as well as describing good practice. This includes the identification of areas which should not be burnt, several of which relate to peat soils. Finally the Scottish Executive published a review Scotland's Climate Change policies in 2006 (Scottish Executive, 2006a). It proposed that the Scottish target to reduce Carbon emissions should exceed the share of UK emissions by 1 million tonnes by 2010. The new Scottish Government has expressed a further commitment to cutting greenhouse gas emissions and is considering an 80% cut in greenhouse gas emissions by 2050 in forthcoming legislation. Such ambitious targets would inevitably require a contribution from land use change and agriculture to increase carbon sequestration by soils through a range of management activities.

References

Baggott, S. L. Cardenas, L., et al. (2007) UK greenhouse gas inventory, 1990 to 2005: Annual report for submission under the Framework Convention on Climate Change. (2007) Didcot, Oxfordshire, UK, AEA Technology for Defra.

Barr, C. J. Countryside Survey 1990, Main Report. (1993) London, Department of the Environment.

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.

Billett, M. F., Palmer, S. M., et al. (2004) Linking land-atmosphere-stream carbon fluxes in a lowland peatland system. Global Biogeochemical Cycles 18, art- GB1024.

Bilotta, G. S., Brazier, R. E. and Haygarth, P. M. (2007) Processes affecting transfer of sediment and colloids, with associated phosphorus, from intensively farmed grasslands: erosion. Hydrological Processes 21, 135-139.

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, 363-369.

Cannell, M. G. R. (1999) Growing trees to sequester carbon in the UK: answers to some common questions. Forestry 72, 237-247.

Cannell, M. G. R., Milne, R., et al. (1999) National inventories of terrestrial carbon sources and sinks: The UK experience. Climatic Change 42, 505-530.

Crutzen, P.J. Mosier, A.R., Smith, K.A., and Winiwarter, W. (2007). N ₂O release from agro-biofuel production negates global warming reduction by replacing fossil fuels Atmos. Chem. Phys. Discuss., 7, 11191-11205.

Dawson, J. C. and Smith P. Review of carbon loss from soil and its fate in the environment. SP08010. 2006.

DEFRA . The first soil action plan for England: 2004-2006. 2004.

Evans, M., Warburton, J. and Yang, J. (2006) Eroding blanket peat catchments: Global and local implications of upland organic sediment budgets. Geomorphology 79, 45-57.

Firbank, L. (2003) Countryside Survey 2000. Journal of Environmental Management 67, 205-290.

IPCC . Fourth Assessment Report; Working Goup Three Mitigation of Climate Change. 2007. IPCC.

Janssens, I. A., Freibauer, A., et al. (2003) Europe's terrestrial biosphere absorbs 7 to 12% of European anthropogenic CO ₂ emissions. Science 300, 1538-1542.

Kowalski, A. S., et al. (2004) Paired comparisons of carbon exchange between undisturbed and regenerating stands in four managed forests in Europe. Global Change Biology 10, 1707-1723.

Laine, J., Silvola, J., et al. (1996) Effect of water-level drawdown on global climatic warming: Northern peatlands. Ambio 179-184.

Liski, J., Perruchoud, D. and Karjalainen, T. (2002) Increasing carbon stocks in the forest soils of western Europe. Forest Ecology and Management 169, 159-175.

Mackey, E. C., Shewry, M. C. and Tudor, G. J. Land cover change: Scotland from the 1940s to the 1980s. 1988. Edinburgh, The Stationery Office.

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.

Petrone, R. M., Waddington, J. M. and Price, J. S. (2001) Ecosystem scale evapotranspiration and net CO2 exchange from a restored peatland. Hydrological Processes 15, 2839-2845.

Renwick, A., Ball, A. S. and Pretty, J. N. (2002) Economic, biological and policy constraints on the adoption of carbon farming in temperate regions. Philosophical Transactions of the Royal Society of London Series A-Mathematical Physical and Engineering Sciences 360, 1721-1740.

Reynolds, B. Implications of changing from grazed or semi-natural vegetation to forestry for carbon stores and fluxes in upland organo-mineral soils in the UK. Hydrology and Earth System Sciences 11[2], 61-76. 2007.

Scottish Executive . Changing our ways; Scotland's climate change programme. 2006a. Edinburgh, Scottish Executive.

Scottish Executive . Scottish Forestry Strategy. 2006b. Edinburgh, Scottish Executive.

Smith, P. Ecosse - estimating carbon in organic soils; sequestration and emissions. 2007a. Edinburgh, Scottish Executive.

Smith, P. Ecosse - estimating carbon in organic soils sequestration and emissions. 2007b. Scottish Executive.

Smith, P., Andren, O., Karlsson, T., Perala, P., Regina, K., Rounsevell, M. and van Wesemael, B. (2005) Carbon sequestration potential in European croplands has been overestimated. Global Change Biology 11, 2153-2163.

Smith, P., Milne, R., Powlson, D. S., Smith, J. U., Falloon, P. and Coleman, K. (2000a) Revised estimates of the carbon mitigation potential of UK agricultural land. Soil Use and Management 16, 293-295.

Smith, P., Powlson, D. S., Smith, J. U., Falloon, P. and Coleman, K. (2000b) Meeting the UK's climate change commitments: options for carbon mitigation on agricultural land. Soil Use and Management 16, 1-11.

Soussana, J. F., Allard, V., et al.. (2007) Full accounting of the greenhouse gas (CO2, N2O, CH4) budget of nine European grassland sites. Agriculture Ecosystems & Environment 121, 121-134.

Soussana, J. F., Pilegaard, K., et al. (2005). Annual greenhouse gas balance of European grasslands. First results from the GreenGrass project. Weiske, A. 25-30. 2004. Leipzig. Greenhouse gas emissions from agriculture - mitigation options and strategies.

Towers, W., Grieve, I. C., Hudson, G., Campbell, C. D., Lilly, A., Davidson, D. A., Bacon, J. R., Langan, S. J. and Hopkins, D. W. Scotland's Soil Resource - Current State and Threats. 2006.

van Kamp, L. Reports of the technical working groups established under the thematic strategy for soil protection. Volume - III, organic matter. EUR 21319 EN/3. (2004) European Commission.

Waddington, J. M. and McNeill, P. (2002) Peat oxidation in an abandoned cutover peatland. Canadian Journal of Soil Science 82, 279-286.

Waddington, J. M. and Roulet, N. T. (2000) Carbon balance of a boreal patterned peatland. Global Change Biology 6, 87-97.

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