BD2304 Scoping Study to Assess Soil Compaction Affecting Upland and Lowland Grassland in England and Wales
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BD2304 Scoping study to assess soil compaction affecting upland and lowland grassland in England and Wales APPENDICES TO SID5 The appendices give more detail about each part of the project and include all relevant references at the end of each section. APPENDIX 1 Mapping the extent of soil compaction (Work Package 1) APPENDIX 2 The causes of soil compaction (Work Package 2a) APPENDIX 3 The impacts of soil compaction (Work Package 2b) APPENDIX 4 Conflicts and synergies within existing and potential ES options, between objectives relating to soil compaction and its remediation and other scheme objectives (Work Package 3) APPENDIX 5 Responses received at the Stakeholder workshop (Work Package 5) APPENDIX 6 Glossary of terms Appendix to SID5 1 Soil compaction in England and Wales January 2008 APPENDIX3: Impacts of soil compaction 3.1 The impact of soil compaction on soil flora and fauna The following appendix provides more detail on specific studies highlighted in the main report in relation to the impacts of compaction on soil flora and fauna. Soil compaction is generally apparent as an increase in bulk density and hence a decrease in the overall porosity of the soil, with a reduction in the number of larger pores. However, a gross parameter such as this may mask subtle but important changes in the nature of the pore network, including the pore-size distribution and the connectivity and tortuosity of such pores, which will impact on organisms in many ways. The pore network defines the physical habitat for soil organisms, directly providing the living space which they inhabit, and indirectly modulating the dynamics of water, solutes, gases and volatiles, and hence the metabolic processes of the biota. The topology of the pore network will affect the distances that mobile or motile organisms must travel in order to locate new substrate or prey. Where such material is located in relatively small pores, then access to it by potential degraders or predators may be prevented by physical exclusion – organisms can only pass through pores larger than their physical size. Such mechanisms will retard consumption of such physically-protected prey organisms, or degradation of so-called physically occluded organic matter. 3.1.1 Microflora Microbes [prokaryotes, fungi] Effects of soil compaction on soil microbial communities have been studied most often in the context of forest and arable systems, with remarkably few published studies specifically in relation to grasslands1. Amelung et al. (2001) studied the biochemical properties of soils derived from a range of ten prairie grasslands in the US which were subject to different degrees of restoration from cropland states. They found that the ratios of certain amino sugars, which bear some relation to microbial community structure, were correlated with increases in compaction. They reported a significant linear correlation between the glucosamine:muramic acid ratio and bulk density, which can be tentatively interpreted as evidence that fungi were predominant in the top 5 cm of soils in compacted systems. However, amino sugars are not robust bioindicators in that they do not uniquely represent the groups to which they can be associated. Kohler et al (2005) carried out a field experiment where the effects of mowing, fertilisation and animal trampling upon aspects of a cultivable subset of the soil bacterial community in a European montaine grassland proximal to a forest. They determined the concentration of bacterial colony-forming units (CFUs) on tryptone-soy agar media, and the ‘community-level physiological profile’ of the soil using the Biolog Ecoplate technique. They found that there was no significant effect of trampling upon CFUs or overall growth rate within the Ecoplates, nor any distinct effect upon the physiological profile. There were some significant mowing x trampling effects upon utilisation of certain substrate guilds, but these are not presented in detail. However, they did not assess bulk density of soils and hence it is unclear whether trampling actually affected the compaction status of the soils. It must also be stressed that the degree to which assays such as Biolog, (which rely on in vitro expression of soil bacteria), meaningfully reflect the properties of the communities in their original context is doubtful (Preston-Mafam et al. 2002; Ritz 2007). The crucial interaction between soil structure and water was demonstrated by Jensen et al. (1999) who showed that compaction of grassland soil in laboratory microcosms did not affect 1 Web of Knowledge search [Jun 2007] using (microb* + soil + compaction) returns 123 outputs, and adding the further term ‘grass*’ returns 23 publications. Most of the latter do not involve studies where compaction is explicitly addressed. Appendix to SID5 27 Soil compaction in England and Wales January 2008 microbial biomass but resulted in no effect or a reduction in respiration dependent on the water status. Althoff & Thien (2005) studied the effect of military trafficking by M1A1 tanks, weighing in excess of 50 tonnes and travelling at high speeds upon prairie soil communities. Microbial biomass was generally unaffected by such treatment, even when soils were wet. In contrast, Peacock et al. (2001) found declines in microbial biomass and distinct shifts in PLFA profiles associated with increasing intensity of military training in a different training ground. However, these sorts of studies suffer from compaction effects being confounded with variation in other factors. The above studies are essentially the sum total that addresses soil compaction effects on grassland microbes. Indeed, there are few studies that rigorously determine direct effects of compaction as opposed to circumstances where compaction is confounded with other factors such as fertiliser additions, manure inputs or animal grazing. Many studies also rarely report bulk density measures and use arbitrary ‘compaction indices’. Across the range of systems studied, reported effects of increased soil compaction on biomass or activity of soil microbes show a full range of responses ranging from neutral through an increase or decrease in magnitude of a wide range of parameters (Table 5). Similarly, responses to compaction in community structure have been shown to be variable. In a study of the effects upon compaction of forest soils by heavy-duty logging Schnurr-Putz et al. (2006) reported evidence that compaction resulted in eukaryote- dominated communities via phenotypic profiling based upon phospholipid fatty acid (PLFA) analyses which showed significantly greater eukaryotic:prokaryotic ratios in soils below wheel tracks than in adjacent less-compacted soils. However, Shestak & Busse (2005) found virtually no effects of compaction upon a broad range of microbiological properties in both field and laboratory experiments with an arable soil. In a highly-controlled laboratory system, Harris et al. (2002) demonstrated that the spatial extent (and by implication biomass) of the fungus Rhizoctonia solani increased with increasing bulk density of an arable soil between 1.2-1.4 Mg m- 3, and stabilised thereafter up to a density of 1.6 Mg m-3 (Figure 9). 3.1.2 Microfauna Protozoa Griffiths & Young (1994) reported little impact of thoroughly grinding a soil upon protozoan populations. Protozoa rely upon water films for motility and hence are sensitive to changes in the water status of soils, particularly in relation to the continuity of water-filled pores. Griffiths & Young (1994) showed a 30-fold decline in protozoan biomass following compaction of a mineral soil which they argued was not directly attributable to compaction but indirectly to its influence on moisture content and aeration status. Bass & Bischoff (2001) reported a decrease in densities of gymnamoebae with increasing compaction in subsoils. Nematodes Soil bulk density appears to affect different species of nematodes in different ways, which can often be associated with their functional roles based upon 6 feeding guilds: microbivorous, fungivorous, omnivorous, predatory, animal-parasitic, entomopathogenic (Zunke and Perry, 1997). For example, Portillo-Aguilar et al. (1999) showed that survival of Heterorhabditis bacteriophora decreased, but Steinernema glaseri increased, with increasing bulk density, whilst survival of S. carpocapsae was unaffected. H. bacteriophora and S. glaseri infected larvae of the greater wax moth, Galleria mellonella, with the incidence of infection unaffected by bulk density, but infection rates by S. carpocapsae increased with bulk density. This concluded that rates of movement and infection by the nematodes were strongly correlated with the amount of soil pore space having dimensions similar to or greater than the diameters of the nematodes. Bouwman & Arts (2000) investigated the effect of a variety of degrees of soil compaction by machinery upon nematode populations in an arable soil. Total numbers of nematodes were not affected by compaction within arable soils, but changes to the relative proportion of individuals or species within the aforementioned guilds (Bouwman & Arts, 2000) are likely. It is suggested this is due to a decrease in habitable pore space for bacterivores and an increased in feeding sites for herbivores, though no direct measure of food availability was made in this study. Griffiths et al. Appendix to SID5 28 Soil compaction in England and Wales January 2008 (1991) found that increasing the bulk density of soil from 1.0 to 1.3 Mg m-3 in pot-grown barley increased the number of nematodes in the rhizosphere but not the bulk soil. Nematodes are multi-cellular prokaryotes and show a greater range of body sizes compared to protozoa, with lengths from <0.5 up to c. 5 mm in soils. As such it may be expected that they would show more sensitivity to soil compaction where the habitable pore space at pertinent dimensions to such body sizes may be more affected, although this issue is rarely considered (c.f. Ritz & Trudgill 1999). Althoff & Thien (2005) found no effect of soil disturbance, which included an increase in bulk density, induced by tank trafficking upon nematode populations in a prairie grassland military training area.