ANAEROBIC SOIL DISINFESTATION IN MICROCOSMS OF TWO SANDY SOILS.

In recent years, anaerobic soil disinfestation (ASD) has been proposed as an alternative control method of soil-borne plant pathogens. It involves adding a labile carbon source, irrigating the soil to stimulate decomposition of organic material and then covering the soil with air-tight plastic to limit gas exchange. During the ASD process, soil microorganisms switch from aerobic to anaerobic metabolism. As a result, by-products of anaerobic metabolism are released into the soil environment such as various organic acids and gases. These by-products are reported to have a negative effect on survival of soil-borne plant pathogens. However, the efficacy of ASD to reduce soil-borne pathogens in practice may vary significantly. Therefore, we studied the efficacy of the ASD process in two different soils. In addition, it was investigated whether a pre-treatment with an anaerobic bacterial inoculum prior to ASD affected the efficacy of the process. Two sandy soils (dune sand and glacial sand) were inoculated in 2 L soil microcosms. We tested the efficacy of ASD treatment against the potato cyst nematode Globodera pallida. For each soil, three treatments were used: control treatment (no Herbie addition, aerobic incubation), ASD 1 (organic substrate addition, anaerobic incubation) and ASD 2 (organic substrate and anaerobic bacterial inoculum addition, anaerobic incubation). Soil microcosms were incubated in the dark at 20°C for two weeks. We observed that anaerobic soil disinfestation treatments were highly effective against Potato Cyst Nematode (PCN), with pathogen being eradicated totally in all but one ASD treatment (glacial sand ASD2) within two weeks. The relative abundance of Firmicutes (spore-forming bacteria, often fermentative) in total bacteria increased significantly in ASD treated soils. Numbers of these bacteria correlated positively with increased concentrations of acetic and butyric acids in soil water phase in ASD treatments.

Quasi-steady state approximation to a fungal growth model.

In a previous paper, we proposed a fungal growth model (Lamour et al., 2001 IMA J. Math. Appl. Med. Biol., 17, 329-346), describing the colonization and decomposition of substrate, subsequent uptake of nutrients, and incorporation into fungal biomass, and performed an overall-steady-state analysis. In this paper we assume that where nutrient dynamics are much faster than the dynamics of fungal biomass and substrate, the system will reach a quasi-steady-state relatively quickly. We show how the quasi-steady-state approximation is a simplification of the full fungal growth model. We then derive an explicit fungal invasion criterion, which was not possible for the full model, and characterize parameter domains for invasion and extinction. Importantly, the fungal invasion criterion takes two forms: one for systems where carbon is limiting, another for systems where nitrogen is limiting. We focus attention on what happens in the short term immediately following the introduction of a fungus to a fungal-free system by analysing the stability of the trivial steady state, and then check numerically whether the fungus is able to persist. The derived invasion criterion was found to be valid also for the full model. Knowledge of the factors that determine invasion is essential to an understanding of fungal dynamics. The simplified model allows the invasion criterion to be tested with experimental data.

Modelling the growth of soil-borne fungi in response to carbon and nitrogen.

Growth of soil-borne fungi is poorly described and understood, largely because non-destructive observations on hyphae in soil are difficult to make. Mathematical modelling can help in the understanding of fungal growth. Except for a model by Paustian & Schnürer (1987a), fungal growth models do not consider carbon and nitrogen contents of the supplied substrate, although these nutrients have considerable effects on hyphal extension in soil. We introduce a fungal growth model in relation to soil organic matter decomposition dealing with the detailed dynamics of carbon and nitrogen. Substrate with a certain carbon : nitrogen ratio is supplied at a constant rate, broken down and then taken up by fungal mycelium. The nutrients are first stored internally in metabolic pools and then incorporated into structural fungal biomass. Standard mathematical procedures were used to obtain overall-steady states of the variables (implicitly from a cubic equation) and the conditions for existence. Numerical computations for a wide range of parameter combinations show that at most one solution for the steady state is biologically meaningful, specified by the conditions for existence. These conditions specify a constraint, namely that the 'energy' (in terms of carbon) invested in breakdown of substrate should be less than the 'energy' resulting from breakdown of substrate, leading to a positive carbon balance. The biological interpretation of the conditions for existence is that for growth the 'energy' necessary for production of structural fungal biomass and for maintenance should be less than the mentioned positive carbon balance in the situation where all substrate is colonized. In summary, the analysis of this complicated fungal growth model gave results with a clear biological interpretation.

Modelling the growth of soil-borne fungi in response to carbon and nitrogen.

Growth of soil-borne fungi is poorly described and understood, largely because non-destructive observations on hyphae in soil are difficult to make. Mathematical modelling can help in the understanding of fungal growth. Except for a model by Paustian & Sch urer (1987a), fungal growth models do not consider carbon and nitrogen contents of the supplied substrate, although these nutrients have considerable effects on hyphal extension in soil. We introduce a fungal growth model in relation to soil organic matter decomposition dealing with the detailed dynamics of carbon and nitrogen. Substrate with a certain carbon : nitrogen ratio is supplied at a constant rate, broken down and then taken up by fungal mycelium. The nutrients are first stored internally in metabolic pools and then incorporated into structural fungal biomass. Standard mathematical procedures were used to obtain overall-steady states of the variables (implicitly from a cubic equation) and the conditions for existence. Numerical computations for a wide range of parameter combinations show that at most one solution for the steady state is biologically meaningful, specified by the conditions for existence. These conditions specify a constraint, namely that the 'energy' (in terms of carbon) invested in breakdown of substrate should be less than the 'energy' resulting from breakdown of substrate, leading to a positive carbon balance. The biological interpretation of the conditions for existence is that for growth the 'energy' necessary for production of structural fungal biomass and for maintenance should be less than the mentioned positive carbon balance in the situation where all substrate is colonized. In summary, the analysis of this complicated fungal growth model gave results with a clear biological interpretation.

Control of soilborne plant pathogens by incorporating fresh organic amendments followed by tarping.

ABSTRACT A new method for the control of soilborne plant pathogens was tested for its efficacy in two field experiments during two years. Plots were amended with fresh broccoli or grass (3.4 to 4.0 kg fresh weight m(-2)) or left nonamended, and covered with an airtight plastic cover (0.135 mm thick) or left noncovered. In plots amended with broccoli or grass and covered with plastic sheeting, anaerobic and strongly reducing soil conditions developed quickly, as indicated by rapid depletion of oxygen and a decrease in redox potential values to as low as -200 mV. After 15 weeks, survival of Fusarium oxysporum f. sp. asparagi, Rhizoctonia solani, and Verticillium dahliae in inoculum samples buried 15 cm deep was strongly reduced in amended, covered plots in both experiments. The pathogens were not or hardly inactivated in amended, noncovered soil or nonamended, covered soil. The latter indicates that thermal inactivation due to increased soil temperatures under the plastic cover was not involved in pathogen inactivation. The results show the potential for this approach to control various soilborne pathogens and that it may serve as an alternative to chemical soil disinfestation for high-value crops under conditions where other alternatives, such as solarization or soil flooding, are not effective or not feasible.