SP1316 - Appendix 1

Literature review on the impacts of working waterlogged land

Helen Balshaw1, Paul Newell Price2 and John Williams3

1ADAS Rosemaund, Preston Wynne, Hereford, UK, HR1 3PG 2ADAS Gleadthorpe, Meden Vale, Mansfield, Nottinghamshire, UK, NG20 9PD 2ADAS Boxworth, Battlegate Road, Boxworth, Cambridgeshire, UK, CB23 4NN

January 2015

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Contents

1. Introduction ................................................................................................................ 1

2. Physico-chemical and bio-physical effects of working waterlogged soils .................... 1

2.1 Cultivation and soil moisture ................................................................................ 1

2.2 Cultivation methods ............................................................................................. 3

2.3 Impacts of working waterlogged soils .................................................................. 5

3. Working waterlogged land within different farming system/soil type combinations ...... 8

3.1 Soil type .............................................................................................................. 9

3.2 Winter combinable cropping .............................................................................. 10

3.3 Roots combinable cropping ............................................................................... 11

3.4 Grassland systems ............................................................................................ 12

4. Effects of working waterlogged land on key ecosystem services .............................. 13

4.1 Crop production ................................................................................................. 13

4.2 Water quality ..................................................................................................... 15

4.3 Flood mitigation ................................................................................................. 16

4.4 Carbon storage.................................................................................................. 17

4.5 Nutrient cycling .................................................................................................. 19

5. Conclusions .............................................................................................................. 20

6. References ............................................................................................................... 21

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What are the impacts of working waterlogged soils in terms of productivity, soil degradation and ecosystem services?

1. Introduction

The main objective of this short study was:

To collate and review information on the impacts of working waterlogged soils in terms of soil degradation, productivity, water quality, flood mitigation, carbon storage and nutrient cycling

To achieve the study objectives, information from review publications was summarised and key words selected for use in literature searches of the Scopus and Web of Knowledge databases (Table 1). The literature search process followed the general principles of a systematic review with search criteria chosen to minimise bias. Citation trails were also followed up and a limited number of web searches (Google, Google Scholar and Bing) carried out for grey literature on the effects of working or trafficking waterlogged soil. The review also drew upon information in ADAS Field Drainage Experimental Unit (FDEU) bulletins that describe best practice soil management under contrasting soil water regimes. Section 2 focuses on the physico-chemical and bio-physical effects of working waterlogged land; section 3 considers the effects of working waterlogged land within different farming systems; and section 3 reviews the effects of working waterlogged land on key ecosystem services (crop production, water quality, flood mitigation, carbon storage and nutrient cycling). The review findings are related to a range of farming systems, crops and soil types. Table 1.1. Examples of key words and phrases used in the literature review searches.

Adaptation Productivity (yield etc) Soil quality

Arable Prolonged surface waterlogging Soil structure

Carbon storage Prolonged wetness Soil wetness

Crop physiology Reduced infiltration rate Soil wetness class

Cultivation Reduced macroporosity Subsoil compaction

Erosion Root architecture Topsoil

Field capacity Root distribution Tyre pressure

Field traffic Saturated Vertical stress

Flood mitigation Soil Water holding capacity

Grassland Soil compaction Water quality

Mean ground pressure Soil degradation Waterlogged

Mitigation methods Soil erosion Wet

Nutrient cycling Soil management Wheat physiology

Poaching Soil processes Wheel load

2. Physico-chemical and bio-physical effects of working waterlogged soils

2.1 Cultivation and soil moisture Spoor (1975) describes cultivation or the working of soil as any form of soil movement which occurs during the crop production process. Activities that move the soil with the intention of improving conditions for crop growth are described as ‘positive’ cultivations, whereas unwanted, but often unavoidable movements arising from non-tillage operations such as harvesting and spraying, or even tractor compaction while carrying out tillage, are described

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as ‘negative’ cultivations. Any type of cultivation carried out when soils are ‘wet’ can result in soil structural damage. The degree of damage caused relates to the degree and direction of force and pressure applied and the frictional and cohesive strength of the soil at any given soil moisture content. However, the effect of soil moisture on structural damage is greater than any effect likely to be due to differences in tractor weight, tyre size or ground pressure (Davies et al., 1973), and moisture content changes of the order of 2 to 3 per cent can have profound effects on soil damage due to changes in soil consistency relative to certain thresholds. The way that a soil responds to an applied load or cultivation changes at critical moisture contents or consistency states known as the:

Shrinkage limit – below which soil bulk strength is not particularly high (due to lack of water and therefore low surface tension forces and low film cohesion) but clod and aggregate strength is very high (due to attractive forces associated with the clay fraction), and working the soil will simply rearrange clods/aggregates with no structural damage

Lower plastic limit – above which clod strength is low and bulk strength is high and soil moisture content is adequate for the water to behave like a lubricant and the risk of structural damage increases with water content

Upper plastic limit – above which the soil is in the ‘liquid’ state, has almost no strength, is readily puddled (compacted) and can be virtually impossible to work

Figure 2.1. Variation in cohesion with moisture content (Spoor, 1975). Soils are most workable at moisture contents within the friable range between the shrinkage limit and the lower plastic limit, with bulk shear strength increasing and clod shear strength decreasing rapidly with moisture content (Spoor, 1975; Figure 1). At the shrinkage limit end of

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the friable range, the risk of structural damage is very low and soil can be cultivated with minimal energy requirement. When soil is in a plastic state (between the lower and upper plastic limits) soil clod and bearing strength declines, resulting in increased wheel slip, rolling resistance and sinkage/ compression. The risk of structural damage increases with soil moisture content to a ‘sticky point’ where bulk shear strength falls and soil sticks to cultivation machinery (Spoor, 1975), resulting in significant:

puddling – the mechanical process whereby wet soil aggregates are disrupted and some clay is dispersed; and

smearing – localised spreading and smoothing of soil by sliding pressure Fuel use increases (efficiency is reduced) and severe soil structural damage can result (Davies et al., 1972). Great care is needed if loading the soil in a plastic condition to prevent severe damage. Once the soil is in a liquid state it should not be worked or trafficked if at all possible as it will be readily puddled and smeared, resulting in severe soil structural damage (Spoor, 1975). Not all soils have these consistency states. Lighter soils with less than 18% clay content will only reach the lower plastic limit at a relatively high moisture content and the lightest soils will not reach a plastic state or have a ‘sticky point’ due to lack of clay content and related adhesion forces. The effects of working land therefore relate to the load applied and the soil moisture content (relative to the plastic state) at the time of working. When soil is in a friable state it is in the optimum state for cultivation. Nevertheless, soil compaction through compression can still occur especially as soil moisture approaches the lower plastic limit, as coarser pores (i.e. greater than 0.05 mm) are still mainly air-filled and able to reduce in volume (Spoor, 1975). Thus, soils at field capacity (start of drainage) are prone to compaction. Clay and medium soils tend to be at greatest risk of structural damage due to the longer periods of time during which the soil is in the plastic state following prolonged or heavy rainfall. Soils also differ in terms of the proximity of field capacity to the lower plastic limit. For example, some clay and medium soils that are above their lower plastic limit at field capacity need several days without rainfall before they can be cultivated (Davies et al., 1972). By contrast, many sandy soils are not plastic and do not have a lower plastic limit. Recently cultivated soils that are mechanically weak are also more liable to damage than a well-structured soil in stubbles or under grassland. 2.2 Cultivation methods In the UK, tillage crops are most commonly established by mouldboard ploughing, typically to a depth of 20-30 cm, which is then followed by secondary cultivations such as harrowing, powered tillage and disc/tines to provide a seedbed suitable for drilling. Ploughing (or inversion tillage) typically buries surface residue and in doing so can help control arable weeds, pests and diseases (Morris et al., 2010). Ploughing is often focused on soils with known drainage issues and those with poor soil structure caused by trafficking when harvesting the previous crop (Morris et al., 2010). Soil moisture content should guide whether or not cultivations can be carried out effectively (Plate 1).

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Plate 1: Ploughing is most effective when soil is in a friable state. Secondary cultivations involving the use of flexible tines, power-rotated tines or discs are particularly high risk operations for soil structural damage in 'wet’ conditions. Tines lift the soil and in wetter conditions are likely to be least damaging of these three implements. Discs (Plate 2), however, tend to smear the soil in ‘wet’ conditions due to their pressing and scooping action (Shepherd et al., 2002). Similarly, power harrows tend to cause smearing and because they break down soil more finely than passive tines or discs, subsequent rainfall can cause the soil to ‘slump’ into a compacted, structureless condition up to 10-15 cm thick, with reduced permeability increasing the risk of prolonged waterlogging over winter (Davies et al., 1972).

Plate 2. Discing can smear soils in ‘wet’ conditions By contrast, non-plough based cultivation practices are known as reduced tillage. Generally crops are established in a reduced tillage system after shallow cultivation, typically to a depth of 10-15 cm using discs or tines (Davies et al., 1972). There is a third option of zero tillage where seed is drilled directly into an uncultivated soil surface, however, issues in some areas

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with weed pressure, disease and compaction have so far limited uptake to around 4% of the arable area in England and Wales (Defra, 2010). When soils are ‘wet’, ploughing is often the preferred option, since the mechanical disturbance drains and aerates a greater depth of soil than reduced cultivation. Ploughing wet soil increases the risks soil structural degradation. The majority of ploughs are pulled by tractors with one wheel in the furrow bottom, which can cause severe compaction at plough depth due to compression and wheel slip. The proportion of the field affected will depend on the number of furrows that the plough pulls in each pass (Shepherd et al., 2002). Soil structural damage is greater in ‘wet’ conditions or with high draft forces (Davies, 1972; Spoor, 1975). The cumulative effect over a number of years can create a ‘plough pan’ in the upper subsoil or ‘transition layer’, which itself can give rise to further compaction in the future due to reduced hydraulic conductivity and increased wetness in the layer above the ‘pan’ (Batey, 2009). Such a ‘plough pan’ can also hinder crop root development severely affecting crop growth. Damage by tractor wheels in furrow bottoms can be avoided (to some extent, depending on soil moisture content) if the towing vehicle travels on the soil surface (e.g. crawler tractors) or where a powerful wheeled tractor pulls a wide plough, since some of the compaction by wheelings at the surface can be removed by the plough. However, soil structural damage risks are greater in ‘wet’ field conditions when soils are in the plastic or liquid state, no matter what the method of cultivation (Davies et al., 1972). 2.3 Impacts of working waterlogged soils When soils are cultivated in a plastic state, compaction results from reduced pore size and continuity due to compression and soil smearing, often causing an impenetrable layer to plant/crop roots as a result (Forbes and Watson, 1996). Macroporosity can be reduced by 10-60% (Singleton et al., 2000; Drewry et al., 2008) resulting in reduced water infiltration and greater likelihood of subsequent waterlogged conditions. When soil water is perched over a compacted layer anaerobic conditions can rapidly develop as microbes consume the available oxygen and start to use other electron donors to provide energy for respiration (Brady and Weil, 2008). Root growth can therefore be impacted by a combination of physical restriction, through reductions in pore size and increases in penetration resistance (Whitmore et al., 2010), and chemical changes related to anoxic conditions particularly where organic material has been incorporated into the topsoil (Brady and Weil, 2008). Under such circumstances, prolonged waterlogging can give rise to degradation of leaf chlorophyll, purpling and senescing of leaves and emissions of ethylene, which can inhibit root proliferation to depth and stem elongation as well as accelerating leaf senescence. However, it is also thought that ethylene can facilitate the development of adventitious roots, which can act as a functional replacement for existing roots that fail to supply the plants with water and nutrients under waterlogged conditions (Kozlowski, 1984), and can protect the growing root from injury (Shiono et al., 2008). The overall effect of waterlogging on plant growth and ultimate yield is related to the:

Duration of waterlogging

Soil temperature

Particular crop growth stage at which waterlogging occurs Poor root development can have knock-on effects on soil drainage since the crop’s ability to dry out the soil is reduced and the likelihood of trafficking or working waterlogged soil is further increased (Shepherd et al., 2002). When the soil surface is ‘wet’ and already compacted from previous cropping, ploughing can invert as a continuous mass that can form an anaerobic layer at the plough share depth (Potato Council, 2012). The risk of compaction is greater on heavy soils, imperfectly drained soils (usually with a slowly permeable subsoil) and soils with low soil organic matter content

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(Shepherd et al., 2002). Above its plastic limit a soil that is compressed, through mechanical cultivations for example, will retain the compressed form, and is therefore at greatest risk of structural damage (Davies et al., 1972). Working waterlogged soil can reduce macroporosity, water holding capacity and available water capacity, increase bulk density; re-orientate clay particles, reduce aggregate stability, increase size, angularity and platy nature of soil structure, reduce water infiltration rates, reduce continuity of pores and links to any drainage systems; damage swards and increase the percentage of bare soil in grassland (Brady and Weil, 2008). Strudley et al. (2008) noted that soil tillage practices can potentially affect soil hydraulic properties and processes which in turn can impact upon crop growth and translocation of nutrients within the soil. One of the most influential factors on soil physical properties is the method of soil tillage. Strudley et al (2008) identified that soil tillage method can affect soil properties due to the repetitive nature of the field operations, the variation in depth to which these operations are carried out and the effect of tillage method (e.g. discing cf. ploughing) on residue management. Munkholm and Schjonning (2004) note that severe topsoil structural degradation can occur when ‘wet’ soils are cultivated or trafficked. They also highlight that due to improvements in tractor power and tyre technology (e.g. low ground-pressure tyres) it is now possible for tillage to be carried out under wetter soil conditions than was achievable previously. Horn et al. (1995) found that the influence of cultivation forces on soil structural degradation will be dependent on the internal strength of the soil which will be linked to the frequency and intensity of applied loading. Repeated wheelings can cause severe degradation of aggregates which can also lead to the formation of massive structures and, as a result, loss of soil strength. Such soil degradation can lead to reduced soil aeration, water infiltration and root development. Horn et al (1995) therefore recommended that when cultivating or trafficking the soil that loadings should take account of the internal strength of the weakest horizon in the soil in order to avoid soil structural damage. Marshall and Holmes (1988) suggest that a decline in aggregate strength is associated with increased water content as the structural units within the soil become weaker as more water becomes absorbed. This has important implications for the use of machinery or grazing of livestock in ‘wet’ soil conditions. Machinery can affect the soil by compression, shear, slip, bounce and vibration (Shepherd et al., 2002). The increase in machinery power also makes some field operations possible even when draught is heavy and soil conditions are not ideal (Munkholm and Schjonning, 2004). Tillage implements are most effective when soil moisture is in the friable range. When soil water content is at or above ‘field capacity’ and soils are plastic (particularly in soils with >18% clay: medium and heavy soils), tillage implements work less effectively and can damage soil structures to the depth of working by compression, deformation and smear. Aggregate structure and surface conditions as a result of tillage are influenced heavily by soil water content (Watts et al., 1996). Hamblin (1987) noted that structural damage in the form of surface crusts, resulting in poor seed germination and delayed emergence can be a result of inappropriate tillage. Watts et al. (1996) also found that higher tillage energy in certain soil water contents would result in higher clay dispersion or puddling. Similarly, Munkholm and Schjonning (2004) identified that high energy input during secondary tillage operations when the soil is ‘wet’ can be potentially damaging to soil structure. The consequences can be decreased aggregate stability which can lead to increased risk of surface crusting and erosion (Bullock et al, 1988; Schjonning et al, 1997; Munkholm and Schjonning, 2004). Wheels have least effect on soil structure when soils are dry and when their consistency is firm or slightly soft. When soils are ‘moist’ and plastic, wheels compress and deform soil structure and reduce pore size and continuity. The compacting effect of wheels can extend up to 20 cm below the point of pressure application. Pressure has the greatest influence on the

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degree of compaction and load influences the depth of soil compaction (Ansorge and Godwin, 2008), so soil structural damage is usually reduced where low ground pressure (LGP) tyres are used (Smith et al., 2013). The compaction caused by wheels is sometimes sufficient to reduce water infiltration rates and impede the growth and extension of roots (Smith et al., 2013). When soils are ‘wet’ and very plastic, the use of machinery and lifting of root crops can result in deeply rutted ground with an almost complete loss of macropores (Shepherd et al., 2002). Soil macropores aid the free movement of water and air, which helps to replenish the soil gas environment (Davies et al., 1972). When a soil is compressed it leads to an increase in bulk density due to the pressure being exerted on the soil and therefore a reduction in pore size and space, causing air and water to be ‘squeezed’ out (Davies et al., 1972). In sandy soils these pores can include the spaces between grains. In all well-structured soils there are usually cracks and fissures formed between soil structural units, along with root channels which allow the movement of air and water through the soil profile. These pores however can be prone to closure by compression (Potato Council, 2012). Signs of compaction in the field due to cultivation or trafficking in ‘wet’ conditions include:

Roots travelling horizontally or only down cracks, earthworm channels and between soil structural units (rather than through them)

Horizontal cracking in the soil: an indication of ‘platy’ structure

Soil packed solid with little evidence of visible porosity

Layers of wetness in the soil held up by soil structural damage

An unpleasant odour as plant remains rot in anaerobic conditions The distortion and compression of soil structures results in loss of pore space and reduced aeration and hence root restriction and soil wetness following rainfall (Shepherd et al., 2002). Continuing microbial activity in the absence of oxygen can quickly lead to products that are damaging or toxic to plant roots e.g. ammonia, sulphides, ferrous-iron, methane and ethylene (Brady and Weil, 2008; also see section 5 on prolonged waterlogging). If soil compaction (and greater soil strength) means that roots cannot push soil aside or compress the soil, they grow laterally along the upper surface of the compacted layer until they can find a vertical pore or fissure to grow into the soil below (Batey, 2009). Rooting patterns of this kind are commonly found above compacted layers and also branching or forking of tap roots in crops such as oilseed rape and sugar beet (Shepherd et al., 2002). The available water in the lower part of the soil profile can be largely cut off, sometimes resulting in reduced crop yield due to drought effects in dry years that can be far greater than the effect of waterlogging (Batey, 2009). How long soil structural degradation persists depends on soil texture, organic matter content, drainage, the crop type and subsequent weather conditions; particularly the number of wetting and drying cycles (Davies et al., 1972). The effect of soil structural damage on crop performance therefore depends on the depth and extent of compaction, the crop type (spring crops tend to be more susceptible) and subsequent weather conditions (drought or extreme ‘wet’ conditions are unfavourable). Cereals are mainly susceptible to soil conditions that delay or inhibit secondary roots, but are generally tolerant of a range of tilth and soil conditions, as is grassland. Sugar beet, brassica crops, carrots and spinach tend to be least tolerant with peas and beans intermediate (Shepherd et al., 2002).

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3. Working waterlogged land within different farming system/soil type combinations

Rainfall in the UK is relatively evenly distributed over the course of the year. However, it is more likely for waterlogging to occur in winter months when evapotranspiration rates are lower (Smith and Trafford, 1976; Varapertian and Jackson, 1987; Dickin et al., 2009). Dickin et al (2009) draw attention to the likelihood of increased amount and intensity of winter rainfall in the UK in line with climate change models which could potentially increase the potential threat of winter waterlogging and therefore impact upon factors such as crop production. These are issues highlighted in MAFF (2000) and BBSRC (2004). Field capacity is defined as the moisture content at which a soil starts to drain. The return to ‘field capacity’ date varies across England and Wales according to rainfall, evapotranspiration, cropping and soil type (including, for ‘impermeable’ soils, whether or not an effective underdrainage system is in place); and marks the date after which there are limited opportunities to access and work the land with machinery without causing soil structural damage (Morris et al, 2010). Singh et al. (1998) noted that a soil water regime is made up of a balance of four processes, namely: evaporation, transpiration, infiltration and internal drainage. Factors such as tillage and residue management can impact on soil hydraulic properties and therefore impact the soil water regime (Singh et al., 1998). Defra’s Code of Good Agricultural Practice (2009) recognises that ‘wet’ soils are more easily damaged by field operations than dry soils and therefore timeliness of operations is crucial to maintain soils in good structural condition. As cultivations commonly take place in late autumn to prepare for winter cropping there is an increased risk of cultivating soils at moisture contents in the plastic range, especially for late autumn drilled winter cereals, which can result in deformation and smearing of the soil (Defra SP1605B; Plate 4). The risks of causing soil structural damage will vary between farming systems and crop rotations according to the specific sequences of harvesting and establishment dates, and differences in competing activities at key times of the year. Crop rotations that include late harvested crops are at particularly high risk. Harvesting and establishment dates also vary from year to year due to variations in weather conditions.

Plate 4: The effect of cultivating in ‘wet’ conditions. The area to the left of the spade was ploughed in early autumn in ‘dry’ field conditions; the land to the right was ploughed in late autumn in ‘wet’ field conditions.

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Workable days are defined as days when soils have a bearing strength great enough to support farm machinery and field operations can be carried out without causing soil ‘damage’. Cultivations are only really effective under optimum conditions within a narrow range of soil water contents (Defra, SP1605B). Late season cultivation of ‘wet’ soils is the highest risk practice for increased surface runoff and erosion, compared with no tillage or early cultivation in drier field conditions (Martin, 1999). 3.1 Soil type Heavy and medium soils Working waterlogged land should be avoided for all soil types due to the damage and degradation it can cause. However, different soil types and farming systems will face different issues linked to wet soils and waterlogged land. Heavy and slowly permeable soils remain ‘wet’ for longer periods than other soil types (Morris et al. 2010). Heavy soils also have a higher saturated moisture content, a lower percentage of drainage water and a greater available water capacity than light sand soils. The relationship between soil water release characteristics and rainfall in heavy soils results in fewer workable days compared with free-draining light sandy soils. Heavy soils also have lower bearing strength when wet and are therefore more susceptible to compaction and smearing during trafficking, grazing or cultivation than soils with lower clay content (Holman et al., 2003). However, heavy soils are also better-able to recover from compaction through natural shrink-swell processes (Gregory et al., 2009), and to retain soil organic matter (SOM), based on their tendency to aggregate (physical protection) and adsorb (chemical protection) (Tisdall and Oades, 1982). Sandy and light silty soils Soils with high silt contents or those with low organic matter content, are more susceptible to surface crusting and slaking (Ramos et al, 2003). Compaction of sandy soils may be irreversible without intervention due to their inability to shrink-swell. Subsoil compaction is particularly insidious because it often develops undetected and is very difficult to alleviate in lighter textured soils (Van den Akker et al., 1999). Collins and Davison (2009) recognised that soil erosion is most likely to occur on silty clay loam soils, particularly on arable farms where winter cereals are grown. On sandy and light silty soils there is an increased risk of surface capping where rainfall or cultivation of ‘wet’ soils can displace silt particles leading to slaking and the formation of an impermeable cap up to 10 mm thick (NSRI,2002). This in turn can lead to reduced infiltration and increased surface runoff. The natural porosity of soil can also be notably reduced, leading to limited root development and restricted water and air movement (NSRI, 2002; Defra SP1315). Shallow soils over chalk and limestone Shallow calcareous soils tend to have a silty clay loam texture that can be susceptible to slaking and capping. However, high base saturation provides resilience in terms of the ability to restructure and resist compaction and a permeable substrate allows good drainage and a greater number of workable days in spring and autumn than on heavier soils (Defra, 2005). Nevertheless, shallow calcareous soils on sloping land can suffer high erosion losses, particularly in a combinable crop rotation with late drilled winter wheat, when low vegetation covers can leave the surface susceptible to slaking and capping (Boardman et al., 2003; Robinson and Woodrun, 2008).

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Peaty soils Peaty soils are most susceptible to degradation when they are overgrazed or dry out (Foulds and Warburton, 2007) and can suffer irreversible shrinkage. Lowland peaty soils are susceptible to severe poaching when ‘wet’, but can be quite resilient to short-lived episodes of trafficking or grazing in such conditions due to their high organic matter content and ability to recover rapidly from compression. Nevertheless, peaty soils can suffer from soil degradation due to frequent cultivation and trafficking, often at moisture contents at or above the plastic limit (Davies et al., 1972). Farming systems Defra project SP1315 on Post Harvest Management (PHM) reported that good PHM can help reduce the risk of soil structural degradation. Soil management is important particularly in the post-harvest period in order to minimise the risks of soil degradation, erosion and associated pollutant losses (SP1315). Soil compaction at harvest time can come about from use of heavy machinery, sometimes in ‘wet’ field conditions which can cause compaction and in turn, reduce porosity and detrimental changes to pore size distribution and shape. Compaction is often more likely to occur when harvesting crops such as maize, potatoes and sugar beet as they are harvested later and generally under wetter field conditions and require a lot of heavy machinery (Defra, 2005; Shepherd et al., 2002). Soil management practices will be heavily influenced by the type of farming system and therefore the type of crop rotation being implemented on the farm. Table 3.1 presents the common farm system and soil type combinations considered in this report.

Table 3.1: Farming system and soil type matrix to assess the impact of working waterlogged soils on soil degradation and ecosystem services.

Farming system Heavy Medium Silty/ Sandy

Peaty Chalk & Limestone

Winter combinable

Roots combinable (including potatoes, sugar beet and horticultural crops)

Grassland

3.2 Winter combinable cropping Winter combinable cropping rotations tend to dominate on heavy soils and deep clay soils with a slowly permeable layer, on which workable days in autumn and spring are limited compared with lighter textured soils. An example of a winter combinable crop rotation is: WW; WB; WOSR WW = Winter Wheat WOSR = Winter Oilseed Rape WB = Winter Barley The risk of soil structural damage from working heavier soils in winter and spring usually limits cultivation and drilling to the early to mid- autumn period, before soil water contents have reached ‘field capacity’ (MAFF, 1975). However, in years with wet summer and autumn periods there are inevitably risks of trafficking or cultivating soils when they are ‘wet’, particularly following late-harvested winter wheat. Harvest tends to be delayed in such years and may result in soil structural damage, and there may not be an opportunity to alleviate soil compaction before establishment of the following crop. This means that there may be compacted layers within the soil that inhibit rainwater infiltration and keep upper soil layers

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wetter for longer, thereby increasing the risk of working waterlogged soil (Batey, 2009). Risks are inevitably highest in wet years with crops following later-harvested winter wheat (particularly second or later winter wheat crops) at greater risk than earlier harvested oilseed rape or winter barley. Defra project SP1315 identified that reduced cultivation systems are less suitable in a wet autumn and only appropriate where soil structural problems have been alleviated. Furthermore there is potential for compaction at the bottom of the cultivated zone if reduced tillage systems are carried out in unsuitable conditions (Batey, 2009). Reduced cultivation systems are most likely to be implemented in a winter combinable crop rotation due to the suitability of such systems for winter wheat, winter barley and oilseed rape 3.3 Roots combinable cropping Roots and combinable cropping rotations tend to dominate on ‘sandy and light silty’ and medium soils that are well drained and provide flexibility in terms of the timing of field operations. They tend to be grown on the ‘best and most versatile’ land i.e. Agricultural Land Classification (ALC) Grades 1, 2 and 3a. Autumn and spring workable days are more numerous than on heavier soils, providing greater opportunity to access land for harvest, cultivation and other field operations. An example of a roots/combinable crop rotation is: Pots; WW; (S.Beet); SB; WB; Veg (carrots/cauliflowers*); SB Pots = Potatoes WW = Winter Wheat S.Beet = Sugar Beet (not normally grow in rotation with cauliflowers) SB = Spring Barley WB = Winter Barley * Cauliflowers can carry beet cyst nematode and would not normally be grown in mixed rotation with sugar beet The best drained soils can be accessible within twenty four hours (or less) of heavy rainfall. Nevertheless, the risk of working waterlogged soils within such rotations are high due to the late harvest of crops such as potatoes, sugar beet, carrots and cauliflowers; the heavy machinery used to establish and harvest crops; and the pressures on growers to provide a steady supply of produce to contract through the late autumn and winter months. These late-harvested crops represent the ‘high risk points’ within the rotation as far as soil structural damage is concerned. The highest risk within the example rotation is associated with harvest of the root and vegetable crops and in the late autumn/winter establishment of winter wheat following potato harvest. In the example rotation, sugar beet (harvested September to January) is followed by spring barley, thereby reducing risks associated with establishment of the following crop. However, sugar beet is often followed by winter wheat resulting in a double risk of working waterlogged land during sugar beet harvest and during operations to establish the following winter wheat crop. Cauliflowers are produced all year round in England and Wales, but in the rotation detailed above (without sugar beet) it is most likely that a winter-heading variety would be grown, increasing the risk of soil degradation from the heavy harvesting machinery. Other ‘high risk’ harvesting dates for cauliflowers are:

Oct to early Dec - for early autumn heading varieties

Nov to Dec - for late autumn heading varieties

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Carrots would most likely be grown within the above rotation as a ‘main crop’ (i.e. established in the spring and harvested in late autumn/winter), with a nurse crop established over-winter to prevent wind blow of soil prior to spring establishment. Cultivations and drilling would typically be carried out between March and April and the crop harvested between September and December. Carrots are almost exclusively grown on sandy soils (i.e. less than 18% clay content) and light loamy peats, which allow all year round harvesting in even the most difficult of field conditions. Over-winter carrot crops are drilled in May, and harvested from December through to the following May. The heavy machinery used to form ridges for potato crop establishment also pose a risk of soil compaction, especially as the operations are carried out in early spring. (Potato Council, 2012). The compaction caused at drilling can increase the risk of waterlogging during the growing season following rainfall or irrigation. Even short-term waterlogging can effect yields as a result of premature root death and crop senescence (Allen and Scott, 2001). 3.4 Grassland systems Within grassland systems the risks of working, trafficking or trampling waterlogged soils are mainly associated with growing forage crops such as maize, strip grazing forage crops, re-seeding grassland, harvesting cut grass and early or late season grazing of grassland. On permanent grassland, the main risks are associated with re-seeding, cutting and grazing. However, grass is also grown in rotation. An example of a grassland rotation is: WB; WB; maize; G; G; G; G; G; G WB = Winter Barley G = Grass Bell et al (2011) identified that surface soil structural degradation reducing surface hydraulic conductivity and water infiltration could occur as a result of trampling by grazing animals. Singleton et al. (2000) and Drewry et al. (2008) found that high stocking rates of grazing animals under ‘wet’ soil conditions could lead to reductions in topsoil macroporosity between 10-40% and even as much as 60%. Warren et al. (1986) and Proffitt et al. (1995) found the greatest detrimental effects when livestock were grazed in conditions of high surface soil moisture. Grazing sheep on vegetation left after sugar beet harvesting or on fodder beet during the autumn and early winter when soils are bare can also increase the risk of soil surface compaction and the use of strip grazing systems for beef cattle and sheep can give rise to significant topsoil compaction (Vallentine, 2000; Jones et al., 2012). Removing stock from vulnerable fields and reducing stocking densities has the potential to reduce the risks of topsoil compaction in livestock systems (Newell Price et al. 2011). Forage maize is typically harvested in mid to late autumn using heavy machinery, which can cause compaction, reduce soil water infiltration and increase the risk of surface runoff (Withers and Bailey, 2003). It is not usually practical to establish the crop following maize immediately post-harvest in the autumn/winter as soil conditions are usually too ‘wet’ (Defra project SP1315). The most common current practice is to retain maize stubbles over winter and cultivate them in early spring before establishment of an arable crop or grassland (SP1315). Surface runoff volumes from compacted maize stubbles can be significantly reduced through the use of a chisel plough or over-sown rye-grass (Defra projects SP0404 and WQ0140). The latter is particularly appropriate where grass follows maize, but may be less practical to manage where maize follows maize.

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4. Effects of working waterlogged land on key ecosystem services

Working ‘wet’ or waterlogged soils can have knock-on effects for key ecosystem services such as crop production, water quality, flood mitigation and carbon storage. There are important interactions between these services and between the mechanisms that impact on them, since working wet land is likely to result in reduced size and/or connectivity of pores, greater penetration resistance and reduced water infiltration rates with implications for a number of soil functions; e.g. crop establishment, crop rooting and crop yield can be affected. A persistent reduction in net primary productivity can result in less carbon storage, which without addition of other organic material, can potentially result in lower soil carbon content, reduced soil resilience and lowered yield potential. Reduced infiltration rates can result in higher surface runoff, which impacts on water quality and flood mitigation. A reduction in flood mitigation can also impact on crop productivity through prolonged waterlogging. A soil that is well-structured with a good proliferation of roots into the subsoil and numerous vertical, connected pores for the transmission of air and water provides the ideal conditions for the delivery of all these key ecosystem services. In the following sections the effects of working ‘wet’ or waterlogged land within different farming systems and on different soil types are discussed. The risk ratings assigned to each farming system/soil type combination in Tables 2 to 5 are approximate; providing a best estimate of the relative risk of working waterlogged land for the given ecosystem service; and are based on evidence in the literature and expert opinion. The evidence is sparse for specific aspects of the nutrient cycling service, and so no matrix Table is provided. 4.1 Crop production Working ‘wet’ or waterlogged soils can have significant implications for soil structure and crop yield (e.g. Gooderham and Fisher, 1975). Indeed, there are potentially more serious yield implications for sowing a crop under wet field conditions than with delaying cultivation and drilling until soil conditions are suitable (Defra project SP1315). Working waterlogged land can result in the development of compacted layers with reduced porosity and greater penetration resistance. Such a layer can physically impair root penetration (Batey 2009), which restricts rooting to the near-surface and can lead to poor crop establishment, increased risk of pest damage and a patchy and lower yielding crop.

Compacted layers in the topsoil increase the likelihood of prolonged waterlogging during wet winters, reducing crop establishment and increase the risk of a yield penalty due to restricted

rooting and limited water uptake in dry summers (Batey, 2009; van den Akker 1999, (Whalley et al., 1995). When soil structural damage is significant and crops do not establish

well there may be a need to re-drill the crop or change from winter to spring cropping with significant implications for the farm business. Costs will include the increased energy requirements from working ‘wet’ land in the first place, the cost of an additional tillage operation to establish a replacement crop, as well as the price of seed. Even when a replacement crop is established, there is no guarantee that the soil structural damage caused by the initial cultivation/drilling will not impact on crop yield with a late winter or spring crop yield penalty more likely in lighter soils and later drilled crops (assuming that the replacement crop is drilled in suitable soil moisture conditions - Johnston et al., 2009; Soane et al., 1987). Spring crops are generally more susceptible to poor soil structure than autumn-sown crops reflecting smaller root systems which reduce the uptake of nutrients and water (Johnston et al., 2009; SP1605B). Within combinable crop rotations, the likelihood of working free-draining light sandy and silty soils when ‘wet’ is generally far lower than for poorly drained heavy or ‘deep clay’ soils. However, within root/combinable crop rotations, working lighter soils when ‘wet’ is sometimes

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unavoidable due to the late harvesting of crops such as sugar beet and potatoes. The implications for yield are often more significant on light soils due to their inability to restructure through shrink-swell processes and more unstable soil structure compared with heavier soils (Table 2). Sandy and light silty soils are rated as high risk within the roots and combinable crop rotation in Table 2 because of the greater risk of working or trafficking ‘wet’ soils within these rotations with late autumn harvested crops; the reduced ability of these light soils to re-structure (compared with soils of higher clay content); and the greater likelihood that spring crops will be impacted by compaction or poor seedbeds resulting from working waterlogged land. Resilience tends to increase with clay and organic matter content; hence the lower risk rating for heavy and peaty soils. It is rarely possible to cultivate poorly drained heavy or ‘deep clay’ soils when they are waterlogged (i.e. “the whole of the plough layer is saturated or filled with water”) due to the low bearing strength of these soils when saturated and the lack of traction (or available friction) afforded to machinery under such conditions (Davies et al., 1973). The effects of poor soil structure on crop yield will vary according to crop type, soil type, the nature of the soil structural degradation and the weather conditions (particularly spring and summer rainfall) in each year, but for winter wheat can be up to 0.7 t/ha for each 0.1 g/cm3 increase in bulk density (Soane and van Ouwerkerk, 1995: Whalley et al. 1995) or each 1 MPa increase in soil strength (Whalley et al. 2006, 2008). Gregory et al. (2007) also reported winter wheat yield reductions of up to 3 t/ha in loamy soil that was heavily compacted, and Alakukku and Elonon (1995) found a 4% reduction in yield averaged over eight years following a tractor compaction experiment in Finland.

Table 4.1: The risks associated with working ‘wet’ soils on provision of food (crop production) for different farming system and soil type combinations. The symbols *, ** & *** refer to relative lower, moderate and higher risk (associated with working ‘wet’ or waterlogged land) respectively. The risk categories relate to the relative resistance and resilience of different soil types to degradation processes; their ability to recover from compaction; the ability of crops within each farming system to recover from soil compaction; and the likelihood/risk of working waterlogged land within each farming system.

Farming system Heavy Medium Silty/ Sandy

Peaty Chalk & Limestone

Winter combinable * ** **

Roots combinable (including potatoes, sugar beet and horticultural crops)

** *** * **

Grassland * ** *** * *1

1 reduced likelihood of slaking and capping on limestone and chalk downland soils under grass

Costs of mitigation and yield losses to the farm business Cost values were taken mainly from Nix (2013) using the principles set out below and used in the “Mitigation Methods – User Guide” (Newell Price et al., 2011). Other sources include Soane et al. (1987) and Defra (2012). Costs are divided into:

Increased Costs: costs associated with greater fuel use for cultivation and trafficking in ‘wet’ conditions and later compaction alleviation.

Lost output: in drier years, output may be lost when cultivating or trafficking ‘wet’ soil due to adverse effects on soil physical properties (e.g. increased penetration resistance) and reduced root elongation and water/nutrient uptake (Gooderham and Fisher, 1975). Output may also be reduced if failure of a crop drilled in ‘wet’ conditions in autumn results in the need for re-drilling a crop in spring.

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On medium and light textured soils, which do not have the shrink-swell, restructuring properties of heavy soils, and on which crop yield penalties due to autumn cultivation induced compaction are more likely (Soane et al., 1987), the cost of working ‘wet’ soils in autumn is estimated at c. £70 per hectare. Where the autumn crop fails, the total cost in this context associated with a switch to spring cropping (additional drilling cost and loss of yield) could be nearer c. £390 per hectare. On heavy, chalk/limestone and peaty soils, when cultivating/drilling wet soils for autumn establishment results in winter crop failure and a switch to fallow or spring cropping, the costs associated with additional drilling and loss of yield are likely to be similar. However, on well-draining chalk/limestone and peaty soils, the number of machinery working days is normally greater than on heavy or medium soils (MAFF, 1988); and so the likelihood of working waterlogged land is reduced. 4.2 Water quality Soil erosion risk and associated water quality can be significantly affected by the timeliness of cultivation operations. The risk of soil damage and the mobilisation and transport of sediment and associated water pollutants is significantly increased if timing of operations is not appropriate and soil conditions are not optimal. Cultivation in itself mobilises sediment and can result in increased surface runoff and erosion, even where soil structural condition is good (Martin , 1999). Cultivating in ‘wet’ soil conditions entails greater risk for surface runoff and water quality (Martin, 1999; Newell Price et al., 2011). Soil sediment and associated phosphorus can be mobilised and transported via surface runoff and drain flow into watercourses leading to water quality issues such as eutrophication and increased turbidity and associated loss of biodiversity. In order to prevent issues related to water quality it is important to alleviate any soil compaction within soils or mitigate against it occurring in the first place. Cultivating compacted tillage soils in suitable conditions typically results in phosphorus and sediment reductions in the range of 10-50% (Catt et al., 1998; Chambers et al., 2000; Newell Price et al., 2011), indicating that working ‘wet’ soils can increase erosional losses by up to 2-fold or more in the year of untimely cultivation. Martin (1999) found that late season cultivation of ‘wet’ silt loam soils was the highest risk practice for increased surface runoff and erosion, compared with no tillage or early cultivation in drier field conditions. Superficial tillage (to 10 cm depth) with a light rigid tine cultivator in unfavourable conditions in late autumn resulted in a doubling of surface runoff volumes relative to no tillage or early tillage. Late cultivation also resulted in a seven-fold increase in soil erosion compared with no tillage, and a 20% increase relative to early tillage (in favourable climatic conditions). Chambers et al. (2000) also found that cultivation in wet field conditions, including disrupting tramlines when ‘wet’, was likely to exacerbate soil structure, surface runoff and erosion. For soils under grassland management, compaction caused from travelling on land when ‘wet’ or by high stocking densities in unsuitable conditions can cause increased surface runoff and erosion due to reduced infiltration rates. Grassland is a significant source of surface runoff and sediment in many catchments. For example, in a Plynlimon sub-catchment (a headwater basin of the river Severn) grassland comprised only 40% of the catchment area, but accounted for 66% of the sediment load (Collins et al., 1997). Use of a rainfall simulator and runoff plot monitoring in the Slapton Catchment in South Devon showed that heavy grazing of permanent grassland resulted in an 80% reduction in water infiltration rates (Heathwaite et al., 1990). Compacted grassland soils can therefore be a significant source of surface runoff, which can in turn mobilise sediment and associated water pollutants such as phosphorus and faecal indicator organisms (FIOs) (Newell Price et al., 2011).

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The risk ratings in Table 3 are related to the erosion risk associated with different soil types and farming systems (Defra, 2005); and the risk of working ‘wet’ or waterlogged land, which tends to be greater in roots/combinable crop rotations. The risk is generally lower within grassland systems due to the near permanent cover provided by a grassland sward (Evans, 1990a, 1990b). Table 4.2: The risks associated with working ‘wet’ soils on sediment and phosphorus

losses (i.e. impacts on water quality) for different farming system and soil type

combinations. The symbols *, ** & *** refer to relative lower, moderate and higher risk

(associated with working ‘wet’ or waterlogged land) respectively. The risk categories relate to

the relative resistance and resilience of different soil types to degradation processes; their

susceptibility to erosion; their ability to recover from compaction; and the likelihood/risk of

working waterlogged land within each farming system.

Farming system Heavy Medium Silty/ Sandy

Peaty Chalk & Limestone

Winter combinable * ** **

Roots combinable (including potatoes, sugar beet and horticultural crops)

** *** * **

Grassland * * ** *1 *

1 This relates to lowland peaty soils. Upland landscapes can have very high soil erosion rates and are

very susceptible to overgrazing (McHugh et al., 2002).

4.3 Flood mitigation Soil structural damage caused by working waterlogged soils reduces infiltration and increases surface runoff (Dexter, 1997; Defra, 2004; Jankowska-Huflejt, 2006; O’Connell et al., 2004). Soil compression and smearing can decrease soil porosity by 10-25% (Mooney and Nipattasuk 2003; Gregory et al. 2009; Matthews et al. 2010) and hence the water-holding capacity of soils (Gregory et al. 2009). Furthermore, compression by vehicles or livestock preferentially impacts macropores that are largely responsible for soil drainage (Breland and Hansen 1996; Richard et al. 2001); both the size and continuity of pores can be affected often resulting in changes to the alignment of pores from vertical to horizontal (Servadio et al. 2005). These processes impact on both initial and saturated water infiltration rates (Heathwaite et al. 1990; Kibblewhite et al. 2008; Batey 2009), and hence the vertical transmission of water during periods of intense or prolonged precipitation, which increases the risk of overland flow, flooding and erosion (Dexter, 1988). Decreases in the saturated hydraulic conductivity of soil by up to three orders of magnitude have been reported (Matthews et al. 2010). Robinson and Woodrun (2008) established an inverse relationship between soil organic matter (SOM) content and surface runoff due to observed crusting in agricultural soils from the chalk land of southern England. In Canada, Reynolds et al. (2007) found that saturated hydraulic conductivity in arable clay loam soils with SOM content of 4% were two orders of magnitude lower than in nearby clay loam grassland soils with 6% SOM. Watts and Dexter (1998) and Etana et al. (2009) also found that the amount of clay dispersion increased as OM content declined in a range of arable and grassland soils in England (Rothamsted) and Sweden respectively.

Grassland can be a significant source of surface runoff and flooding risk. For example, in the Dart catchment (Devon) surface erosion of grassland soils contributed to 76-85% of channel bed sediment contribution; and in the Somerset Levels, pasture soils accounted for c. 40% of

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sediment losses (Collins et al., 2010). Clearly, in catchments such as the Dart, where pasture represents c. 80% of the catchment area, the condition of grassland soils can largely determine the level of flooding risk and working or trampling of waterlogged land can have a significant impact on hydrological response. Defra project BD5001 found that mechanical loosening of ‘high bulk density’, clay loam grassland soils to 30 cm depth resulted in a seven- to ten-fold increase in saturated water infiltration rates that persisted into a third year post loosening. Such increases in water infiltration rate have the potential to reduce surface runoff volumes and peak flows and increase soil water storage. Holman et al. (2003) investigated the contribution of soil structural degradation to catchment flooding in 2000 and found that cases of severe degradation were mainly confined to late-autumn harvested crops and autumn-sown crops. Late autumn harvested crops, such as those in the roots combinable system, were characterised by high degradation (Table 4); autumn sown crops by high and moderate degradation; and on grassland, sites were roughly evenly distributed between high, medium and low degradation classes. Severe soil degradation was generally most common on medium and silty soils that experience occasional or prolonged seasonal waterlogging (i.e. soil water regime, not simply topsoil clay content, seemed to influence soil degradation class), but was also common following late autumn harvested crops on freely draining light sand, silty and medium soils (Table 4). The risk ratings in Table 4 are based on the susceptibility of each soil type and farming system combination to surface runoff (Holman et al., 2003; Defra, 2005). The reduced rating for peaty soils in roots/combinable crop rotations is based on the fact that most of these soils are on flat land in low lying areas. By contrast, the higher risk rating for peaty soils in a grassland system is related to the greater preponderance of sloping land and the fact that peaty soils within grassland systems in catchment headwaters can play an important role in flood mitigation. The roots/combinable cropping system has a generally higher risk rating due to the late autumn harvest of root crops resulting in soil compaction, lower water infiltration rates and a generally higher risk of surface runoff on sloping land compared with other systems (Holman et al., 2003).

Table 4.3: The risks associated with working ‘wet’ soils on flooding risk (surface runoff) for different farming system and soil type combinations. The symbols *, ** & *** refer to relative lower, moderate and higher risk (associated with working ‘wet’ or waterlogged land) respectively. The risk categories relate to the relative resistance and resilience of different soil types to degradation processes; their ability to recover from compaction; their susceptibility to capping within each farming system; and the likelihood/risk of working waterlogged land within each farming system.

Farming system Heavy Medium Silty/ Sandy

Peaty Chalk & Limestone

Winter combinable ** ** **

Roots combinable (including potatoes, sugar beet and horticultural crops)

*** *** * ***

Grassland ** ** ** ** **

4.4 Carbon storage Soil erosion by water or wind can lead to significant losses of soil organic matter (SOM) levels in soils, including in the dissolved form (DOC). Organic matter and total nitrogen in eroded sediment can be five times as high as in the original topsoil (Brady and Weil, 2008; Larson et al., 1983). Reductions in soil erosion risk and the associated losses of SOM can be achieved

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through maintenance of good soil structure, which promotes higher water infiltration and hydraulic conductivity. Reduced tillage practices, if sustained over a number of years, can potentially increase SOM content due to reduced soil disturbance, although there are indications that reduced tillage results in a change to the SOM profile with depth rather than total SOM content and gains in organic matter can be reversed through rotational ploughing required to control weeds or resolve soil structural issues (Bhogal et al, 2009). Any tillage practice that reduces the level of soil disturbance is likely to have an impact on SOM levels in the long term, due to a potential reduction in SOM decomposition rates and losses. However, the potential for increased carbon storage will depend on how long the reduced cultivation system can be sustained without the need for inversion tillage. Working waterlogged land usually involves ploughing, which not only increases soil erosion risk due to induced compaction, but can also increase loss of SOM through mineralisation (Bhogal et al., 2009). Restricted rooting depth due to soil compaction can reduce crop yield and impact upon subsoil carbon storage levels (Carter and Gregorich, 2010). In wetter years, soil compaction can produce waterlogged and associated anaerobic conditions, which can also limit root growth and lead to reductions in crop yield due to poor anchorage, reduced nutrient uptake and, where ponding occurs for prolonged periods, crop death. Reduced plant growth over many years can lead to a decline in organic matter returned to the soil which in turn could cause a net loss of soil carbon as the existing carbon stores are mineralised (Defra project SP1601). However, it has also been suggested that if compaction leads to greater anaerobism, decomposition of organic matter may be reduced resulting in maintained carbon levels (SP1601). Peaty soils are fairly resilient to soil structural degradation due to the high level of organic matter present within the soil. Gregory et al. (2009) found that soils with high particulate organic matter content were capable of ‘rebound recovery’ once a compression stress had been removed and therefore had greater resilience to compression than lower organic matter mineral soils. Nevertheless, peaty soils can be compacted by livestock or wheelings and associated reductions in water infiltration rates and increases in surface runoff can result in organic matter loss through erosion. The risk ratings in Table 5 are based on the susceptibility of each soil type to erosion (Defra, 2005), the risk of working waterlogged land in specific farming systems and the preponderance of sloping land within soil type and farming system combinations. For example, peaty soils within roots/combinable crop rotations are not only resilient to soil degradation, but also tend to predominate in landscapes with gentle slopes such as the Fens and the Lancashire Moss Peats. Cultivating these soils results in carbon loss through oxidation, but working the soils when wet is not likely to result in carbon loss that is much higher than this baseline. By contrast, working sandy or light silty soils when waterlogged on sloping land is likely to result in accelerated erosion and associated carbon loss. Heavier soils are also given a lower risk rating due to their higher clay content and related higher organic matter content and resilience (Gregory et al., 2007, 2009; Merrington, 2006).

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Table 4.4: The risks associated with working ‘wet’ soils on carbon storage as influenced by soil erosion for different farming system and soil type combinations. The symbols *, ** & *** refer to relative lower, moderate and higher risk (associated with working ‘wet’ or waterlogged land) respectively. The risk categories relate to the relative resistance and resilience of different soil types to degradation processes (particularly erosion and compaction); and the likelihood/risk of working waterlogged land within each farming system.

Farming system Heavy Medium Silty/ Sandy

Peaty Chalk & Limestone

Winter combinable * ** **

Roots combinable (including potatoes, sugar beet and horticultural crops)

** *** * **

Grassland * ** *** * *

4.5 Nutrient cycling Working waterlogged land and the resulting soil structural degradation may affect nutrient cycling within the soil. Reductions of up to 18% in N mineralisation linked to compaction have been reported by Breland and Hansen (1996). They suggested that an increase in physical protection of soil organic matter and microbial biomass within smaller (compacted) pores made them inaccessible to grazing nematodes. By contrast, Gregory et al. (2007) found that compaction had little effect on microbial biomass, as it is believed the microorganisms inhabit small pores which remain unaffected by stress pressure on soils when compaction occurs, or alternatively that microorganisms can be resilient. Kohler et al (2005) and Jensen et al. (1996) also report little impact on microorganisms. Nitrous oxide emissions may increase in compacted soils that are ‘wet’ or waterlogged, particularly if conditions are warm and ‘wet’ and there is a readily accessible carbon source and mineral nitrogen for denitrifying bacteria (Bateman and Baggs, 2005). Nitrous oxide emissions typically occur where water-filled pore space is greater than 60%; both nitrification and denitrification can be important sources of N2O when water-filled pore space is in the range 50-70%. There is also an increased risk of ammonia (NH3) volatilisation from livestock slurry applied to the surface of compacted soils due to reduced infiltration rates with slurry exposed to the air for longer periods than on well-structured soils (SP1601). Poor soil structure will reduce nutrient use efficiency Poor soil structure can limit a crop’s ability to take up soil phosphorus (P). Phosphorus can only diffuse through soil at a rate of c. 0.1 mm per day (Johnston, 2000; Larsen, 1967); so crop P uptake is partly dependent on the ability of root exudates to acidify the soil around root hairs and convert less readily available P (tricalcium phosphate and monohydrogen phosphate) to readily available P (dihydrogen phosphate). If poor soil structure limits root proliferation, a smaller volume of soil and a reduced amount of soil P is available for crop uptake, thereby limiting crop growth and reducing the cycling of soil P (Johnston et al., 2001).

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5. Conclusions

Waterlogged soils are sensitive to degradation from farming operations through compaction and erosion. In medium and heavy soils the main risks occur when the soil is in a plastic state and there is an increased risk of aggregate dispersion (puddling) and spreading of soil by sliding pressure (smear). In England and Wales, approximately 45% of winter sown crops are grown on slowly permeable soils that require field drainage to maximise opportunities for field work in the autumn, winter and spring. Sandy and light silty soils are more susceptible to compaction through compression, partly due to their inability to re-structure through shrink-swell processes. Soils naturally ‘wet up’ in the autumn as day length shortens and rainfall exceeds evapotranspiration. Late autumn harvesting and establishment of crops is therefore associated with greater risk of structural degradation due to the greater likelihood of working soils when ‘wet’ or in a plastic state. Ground conditions at the time of cultivations will heavily influence the effectiveness of field operations; and in some cases if the ground is too ‘wet’ or waterlogged there is a serious risk of causing damage to the soil, particularly to topsoil structure. Working waterlogged soils can lead to compaction, reduced water infiltration and increased surface runoff and erosion which in turn can lead to impacts on a number of ecosystem services. Crop yield can be reduced due to restricted rooting and impacts on the ability of crops to sustain growth and take up water and nutrients, particularly in a wet late winter/spring or dry spring/summer. Reduced macroporosity and the development of platy structure rather than vertical fissures reduces water infiltration rates, and can both increase soil erosion rates and increase flooding risk, thereby impacting on water quality. Accelerated soil erosion can also impact on soil carbon storage. Finally, the soil structural degradation resulting from working ‘wet’ or waterlogged land can also affect nutrient cycling in terms of the rate of organic matter mineralisation and uptake of soil nutrients.

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Defra projects BD5001 – Characterisation of soil structural degradation under grassland and development

of measures to ameliorate its impact on biodiversity and other soil functions.

SP0404 - Erosion Control in Maize Fields.

SP1605B - The relationship between best practice for managing soils to protect the environment with that for increased productivity

SP1315 - Post Harvest Management for soil degradation reduction in agricultural soils: methods, occurrence, costs and benefits.

SP1316B - Understanding the impacts of prolonged waterlogging on soil quality and productivity.

WQ0140 - Competitive Maize Cultivation with Reduced Environmental Impact.