It has been known for many years that animal manures, oil-cakes, residues from leguminous crops and other materials with a low C/N ratio can be added to soil to control plant-parasitic nematodes (see reviews by Muller and Gooch 1982; Rodriguez-Kabana 1986; Stirling 1991). Although there is some evidence that such amendments increase populations of microorganisms antagonistic to nematodes, the main mechanism is thought to be the release of nematicidal compounds such as ammonia during the decomposition process. Since relatively high concentrations of ammonia are needed to achieve control, there is a direct relationship between the amount of N in an amendment and its effectiveness (Rodriguez-Kabana 1986). Thus amendments with N contents greater than 2% are usually used and application rates are typically greater than 10 t/ha.
Although the nematicidal effects of ammonia are well established (Eno et al. 1955; Rodriguez-Kabana et al. 1982; Oka and Pivonia 2002; Tenuta and Ferris 2004) and lethal concentrations are achievable with nitrogenous amendments, the commercial use of such amendments is limited by cost and by concerns about the environmental impact of large quantities of nitrogen. Most recent studies have therefore sought to achieve efficacy at lower application rates. One successful approach involved adding a nitrification inhibitor (nitrpyrin) with the amendment to slow the oxidation of ammonia to nitrite and nitrate, therefore allowing ammonia concentrations to build up for an extended period. When the inhibitor was applied with chitin or cottonseed amendments, ammonia levels were higher for longer periods than in amended soils without the inhibitor, and this was associated with reduced egg production and galling from Meloidogyne javanica (Oka and Pivonia 2002). Alkaline additives have also improved the effectiveness of nitrogenous amendments by increasing soil pH and therefore shifting the equilibrium between the NH4+ and NH3 to the latter form, which is nematicidal (Oka et al. 2006a).
Other work in the USA and Israel has shown that specially formulated organic amendments can cause nematode mortality through mechanisms other than ammonia production. De-watered municipal biosolids applied at 1.1% w/w did not affect Heterodera glycines, but the nematode was killed when the biosolids were stabilised with alkaline materials such as cement kiln dust, fly ash or quicklime (Zasada 2005). Nematode mortality was associated with a rapid increase in the pH of the soil solution (to a pH > 10), and this occurred when CaO in the amendment reacted with water to form Ca(OH)2 (Zasada and Tenuta 2004; Zasada 2005). The contribution of ammonia production to the nematicidal effect was unclear in the American studies, but work with similar products in Israel suggested that it was important there (Oka et al. 2006b). However, the mechanism is clearly chemical rather than biological, as experiments with autoclaved materials indicated that microbes associated with the amendment were not involved (Zasada 2005).
Whether it will eventually be possible to use nitrogenous amendments in nematode management programs remains a moot point. Enormous quantities of organic and inorganic wastes and industrial by-products are available in most countries and there is a need to find uses for them as commercial fertilisers and soil conditioners. Alkaline-stabilised organic amendments are effective against plant-parasitic nema-todes, but fine tuning will be needed before they can be used routinely in nematode management. Thus there is a need to determine the application rates required to achieve consistent nematode control; develop methodologies to prevent overproduction of ammonia and ensure that pH does not increase excessively; understand the long-term effects of these amendments on soil physical properties, soil chemistry and soil microbial ecology; and find ways of integrating the practice into the soil and crop management programs used for specific nematode-susceptible crops.
Although most recent research on organic amendments for nematode control has focused on nitrogenous materials, the possibility of using materials with a much higher C/N ratio has also received attention. McSorley and Gallaher (1995) used a composted mixture of sticks, leaves, branches, grass clippings and wood chips from the urban environment (C/N ratio = 36) as an amendment or mulch and found that it had little effect on plant-parasitic nematodes in vegetable crops planted immediately after the amendment was applied. However, in another study that continued for 3 years, population densities of plant-parasitic nematodes on maize were reduced in the third season, once the woody compost material had broken down and levels of soil organic matter had increased in amended plots (McSorley and Gallaher 1996).
Three studies in Australia have also shown that amendments with minimal amounts of N have suppressive effects on nematodes. In the first of these studies, apple trees mulched with sawdust for 5 years had much lower populations of Pratylenchus jordanensis in years 2-5 than non-mulched trees or trees growing in fumigated or nematicide-treated plots. In years 4 and 5, yields from mulched trees were as good as those obtained with methyl bromide fumigation (Stirling et al. 1995). A second study in which tomato was planted into field plots that had been amended over the previous 2 years with sawdust and urea showed that the amended soil was highly suppressive to M. javanica and that the level of nematode control was significantly better than that obtained with the nematicide fenamiphos. Plants in amended plots were almost free of galls, whereas the untreated controls were heavily galled (Vawdrey and Stirling 1997). The third study involved an amendment of sugarcane residue (the tops and leaves remaining in the field after sugarcane is mechanically harvested). Sugarcane was planted 23 weeks after the residue was incorporated into soil, and 24 weeks after planting there were 95% fewer lesion nematodes (Pratylenchus zeae) in roots growing in amended soil than in roots from the non-amended control (Stirling et al. 2005).
Results from these studies indicate that amendments with a high C/N ratio act much more slowly than nitrogenous amendments. When soil is amended with materials such as yard waste, sawdust or sugarcane residue, suppressiveness may take months or years to develop whereas it develops almost immediately when the amendment has a relatively high N content. Interestingly however, suppressiveness is soon lost with nitrogenous amendments. Thus when soil was assessed 4 and 7 months after it was amended with N-rich materials (e.g. lucerne hay, feedlot manure, poultry manure, chitin and a waste product from sugar mills known as mill mud), it was not suppressive to M. javanica or P. zeae (Stirling et al. 2003). In contrast, materials with a much higher C/N ratio (e.g. sawdust, sugarcane residue and grass hay) were suppressive to both nematodes.
Although the suppressiveness generated by high C/N amendments has not been studied in detail, the evidence currently available suggests that physical or biological rather than chemical mechanisms are responsible. Relatively large predators (e.g. nematodes and arthropods) may be able to operate more effectively when soil structure is improved with organic matter, while in the Australian studies discussed previously, one common observation was that fungi appeared to be associated in some way with suppressiveness. For example, a suppressive, sawdust-amended soil had high numbers of fungal-feeding nematodes (Vawdrey and Stirling 1997), while low concentrations of nitrate nitrogen, a fungal-dominant soil biology and high numbers of omnivorous nematodes were associated with suppression in one of the other experiments (Stirling et al. 2003). In an experiment where P. zeae was suppressed after soil was amended with sugarcane residue, an unidentified predatory fungus was found in the amended but not the non-amended soil (Stirling et al. 2005). It is therefore possible that fungal predation on nematodes was responsible for these suppressive effects. The predatory hyphomycetes and several genera of wood-decaying basidiomycetes are commonly found in habitats that are rich in cellulose and lignin and are thought to have evolved the capacity to scavenge for additional N in low N environments by preying on nematodes (Barron 1992; Tzean and Liou 1993). Thus when high C/N amendments are added to soil, these fungi may utilise free-living nematodes as a food source and coincidently capture plant-parasitic species.
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