The effect of biofumigation on soil microbial activity

Abstract

This study sought to investigate the effects of biofumigation on soil microbial activity. A number of experiments that included a soil incubation, a soil paste, a microbial plate culture and solarisation simulation were conducted. In the main biofumigation experiment, a Leptosol was separately amended and incubated with leaves from Nemat brassica plant, Caliente Brand Mustard 61 brassica plant and grass, together with an incubation containing 2-phenylethyl isothiocyanate (PITC) and a no treatment control. In the incubations containing plant residues, these where added to the soil with and without myrosinase enzyme. At intervals during the incubation period, soil samples were taken and analysed for urease and arginase activity, respiration rate, and organic matter content. The soil paste was set up to study the effect of varying concentrations of PITC on Azotobacter chroococcum. The microbial plate culture experiment was set up to study the effect of the plant residues and PITC on culturable soil microorganisms respectively. The soil solarisation simulation experiment was conducted as a comparative analysis of the effect that solarisation has on the soil’s microbial mediated activities measured for biofumigation. In the biofumigation experiment, urease activity seemed to be affected to some extent by the inclusion of these biofumigants in the soil. The activity in the untreated soil was low, reflecting low levels of organic matter. The addition of plant material marked an increase on the respiration rate. However, during the course of the incubation, the rate of respiration decreased in both the brassica amended soil and the grass amended soil and matched that of the no treatment control soil after 7 days of incubation, reflecting a decrease in readily mineralisable organic matter. Contrary to what was shown for urease activity, arginase activity increased over time for all incubations. It is difficult to conclude whether myrosinase addition was effective or not in accelerating biofumigation. The addition of PITC to the soil had no adverse effect either on the respiration or on the enzyme activity. The organic matter quantity in the soils was not significantly different. The soil paste technique showed that A. chroococcum was inhibited by increasing concentration of PITC. In the microbial plate count technique, the highest mean number of colonies was registered in the no treatment control plates (95.5 colonies), indicating that PITC, did affect the growth of soil bacteria on the nutrient agar plates. Together with the A. chroococcum soil paste experiment, this experiment confirms that PITC and biofumigation affect soil microorganisms differently and highlights the extraordinary diversity of the soil microbial population. In the solarisation experiment, the results obtained indicated that the microorganisms in the soil were mainly thermophilic since at 80 °C there was an increase in the rate of respiration, and in the urease activity. Overall, biofumigation does not appear to exert a direct toxic effect on microbial guilds, instead the changes induced reflect a general microbial response to the addition of fresh organic matter in the soil.