BD5001: Characterisation of soil structural degradation under grassland and development of measures to ameliorate its impact on biodiversity and other soil functions Literature Review: Use of Plant Species for Remediation of Soil Compaction February 2011 C N R Critchley1 & F W Kirkham2 1ADAS UK Ltd, c/o Newcastle University, NEFG Offices, Nafferton Farm, Stocksfield, Northumberland NE43 7XD. 2Ecological Research & Consultancy, Far View, Nymet Rowland, Crediton, Devon EX17 6AL. Executive Summary • Plant species vary in their tolerance of soil compaction and their ability to improve soil structure. The aim of this review was to identify plant species that may have potential to remediate soil compaction in grasslands and would also have some practical application in the field. The literature was reviewed, the specific objectives being to identify: 1. Plant species and traits shown to have remedial effects in compacted soils, 2. UK grassland species that could have potential to remediate soil compaction and 3. Ease of establishment, persistence and agronomic value of the candidate species. • There are relatively few relevant studies from grasslands in the UK, most studies having been carried out elsewhere under various climatic conditions and using commercial crop species within arable rotations. Lucerne (Medicago sativa) was the most widely studied species and featured most prominently in the literature. However, these studies do point to a number of consistent traits associated with alleviating soil compaction. Members of the family Fabaceae are frequently reported as being capable of tolerating compaction and improving soil structure. Possible mechanisms for this are linked to Rhizobium bacteria in root nodules and arbuscular mycorrhizal associations. A deep rooting system capable of deep penetration and radial expansion, especially a large tap root, is also a recurrent feature. In general, dicotyledonous species tend to have more strongly penetrating root systems than grasses. Long-lived perennials are also more likely to develop larger root systems than annuals. • Successful establishment of plant species into an existing grassland sward has been linked to ruderality, percentage germination of seeds and autumn germination. Members of the Fabaceae often have poor germination unless seeds are scarified or subjected to prolonged soaking. Generalist species, especially those associated with fertile soils, are more persistent in restoration experiments than habitat specialists and species associated with infertile soils. Persistence in a sward is also related to a species’ tolerance or avoidance of repeated defoliation caused by grazing or cutting. • Nutritive value of forage plants are determined by digestibility, protein content and concentration of essential minerals. Digestibility declines with maturity, the rate varying among species. Stage of growth and ability to absorb nutrients will affect a species’ protein and mineral concentrations. Forbs (including ‘weed’ species) are recognised for their mineral content and palatability but have declined in intensive grassland systems. Fabaceae are valued for their nitrogen-fixing properties and some also have high feeding value. Livestock usually learn to select the more digestible species from a sward and mostly avoid plants with spines. Plants with a low growth habit will often escape being grazed. Plants containing undesirable chemicals and highly invasive or competitive weeds are considered to be actively detrimental to agricultural production. • A total of 14 perennial, deep-rooted forb plant species, either native or naturalised to Britain, were identified by reference to their traits as candidate species with the potential to alleviate soil compaction. Species were only included in this initial list if the published trait data relevant to this study were found. Because of the relatively limited range of information in the literature and incomplete data on rooting characteristics, a further 5 candidate species were added following consultation with i the project Steering Group. Each of these species was further reviewed under the main headings of ease of establishment, persistence, and agronomic value. Based upon this information, 9 candidate species were identified as having potential for alleviating soil compaction in grasslands, plus 2 further species that might be suitable on specific soil types (see table below). The remaining 8 species were rejected because they either have low establishment rates, low forage quality or would be unacceptable in an agricultural sward. • Field experiments are required to test how effective these plant species are at alleviating soil compaction under grassland. The 9 candidate species identified in this review have been selected and included as a mitigation method to be tested in the field experiment phase of Defra project BD5001. The 9 candidate species will be introduced into grassland swards to assess their remedial effects on soil compaction. The other mitigation method tested in the field experimental phase of Defra project BD5001 is mechanical loosening using sward lifters at two specific depths. These experiments will therefore investigate the potential of the selected plant species to alleviate grassland soil compaction in isolation and in combination with mechanical loosening. One hypothesis is that the selected plant species could potentially exploit the mechanically loosened soil and thus increase the duration of the mechanical loosening effect and perhaps increase the soil’s resistance to re-compaction by establishing roots and mycorrhizal associations within and around loosened soil aggregates. ii a) Suitable species: Species Remedial characteristics Establishment & persistence Agronomic value Trifolium pretense Tap-rooted legume, mycorrhizal associations. Possibly not long-term persistent in High. (Red Clover) sward and native varieties might be slow to establish. Lotus corniculatus Tap-rooted legume, high mycorrhizal Establishes better than most other High. (Bird’s-foot trefoil) associations. native forbs. Achillea millefolium Mycorrhizal associations but no tap root. Establishes and persists well but Tolerates grazing; (Yarrow) might not disperse well in productive superior mineral source swards. to grasses and average for forbs. Hypochaeris Tap-rooted, mycorrhizal associations. Less easy to establish than other Reasonably good radicata species and needs to disperse source of minerals and (Cat’s-ear) seeds to persist. selectively grazed. Trifolium repens Superior to Lolium perenne although shallower Establishes and persists well. High. (White Clover) rooting depth than other forbs identified. Cichorium intybus Deep tap root but probably does not occur Establishes well and competes well High mineral content; (Chicory) naturally in grasslands. with legumes. can increase productivity and forage quality. Centaurea nigra Tap-rooted, mycorrhizal associations but no Moderate establishment and Probably low. (Black Knapweed) data on rooting depth found. persistence. Plantago lanceolata Tap-rooted but shallower rooting depth than Establishes and persists well. Medium feed value. (Ribwort Plantain) other forbs identified. Sanguisorba minor Probably deep tap-rooted with mycorrhizal Establishes well, tolerates grazing. Good quality forage. ssp. muricata associations. (Fodder Burnet) iii b) Suitable species but limited by soil type: Species Remedial characteristics Establishment & persistence Agronomic value Knautia arvensis Tap-rooted, mycorrhizal associations. Difficult to establish and limited to Calcium source but low (Field Scabious) infertile, dry and/or calcareous soils in other minerals and with low competition. crude protein. Medicago sativa Tap-rooted legume, mycorrhizal associations. Mainly on light or calcareous soils Comparable to other (Lucerne) and limited persistence in grazed legumes and high grassland. mineral content. iv Contents Executive Summary........................................................................................................ i a) Suitable species:....................................................................................................iii b) Suitable species but limited by soil type: .............................................................. iv Introduction....................................................................................................................1 Methods.........................................................................................................................1 Results...........................................................................................................................2 Review of remedial effects.........................................................................................2 Provisional list of species...........................................................................................5 Medicago sativa (Lucerne/Alfalfa)..............................................................................7 Trifolium pratense (Red clover)..................................................................................9 Lotus corniculatus (Birds-foot trefoil) .......................................................................11 Silene vulgaris (Bladder campion) ...........................................................................13 Knautia arvensis (Field scabious)............................................................................15 Hypochaeris radicata (Cat’s-ear) .............................................................................17