Tuesday, 11 July 2006

Effects of Methyl Bromide and Chloropicrin Fumigation and Steam Sterilization on Soil Nitrogen Dynamics and Microbial Community.

Takehiko Yamamoto1, Sota Tanaka2, Katsutoshi Sakurai3, and Kozo Iwasaki3. (1) The United Graduate School of Agricultural Sciences, Ehime University, 3-5-7 Tarumi, Matsuyama, 790-8566, Japan, (2) Graduate School of Kuroshio Science, Kochi University, B200 Monobe Otsu Nankoku 783-8502, Nankoku, Japan, (3) Faculty of Agriculture, Kochi University, B200 Monobe Otsu Nankoku 783-8502, Nankoku, Japan

In Japan, because the production and usage of Methyl Bromide (MeBr) as soil fumigant have been prohibited since 2005 following the Montreal Protocol, the expansion of the application of ChloroPicrin (CP) fumigation and Steam Sterilization (SS) is considered to be one of the countermeasures. In this study, the effects of these soil disinfection methods on nitrogen dynamics and microbial community were evaluated. The soil packed in a PVC pipe (11cm dia., 25cm height) was treated with a usual dosage of MeBr, CP and SS as well as a control without disinfection, followed by melon planting (Cucumis melo L. cv. Earth Miyabi) after 9 days of the treatment. The soil samples at the depth of 5-15 and 15-25 cm were collected before the treatment, at transplanting and 10 days, 1 month and 2 months after transplanting. The samples were analyzed for the amounts of microbial biomass N (MBN) and mineral N, microbial activities by the Biolog method and community structure by the analysis of PhosphoLipid Fatty Acids (PLFAs). The amount of MBN decreased and NH4-N increased in the CP and SS treatments. The amount of decreased MBN was almost equivalent to that of increased NH4-N. In the MeBr treatment, the decrease in the amount of MBN was small although the amount of NH4-N decreased as in the CP and SS treatment. Average well color development value obtained by the Biolog analysis decreased in the order of SS>MeBr>CP, and recovered at 10 days after transplanting to the initial level. Total amounts of PLFAs also decreased with the treatments. In the MeBr treatment, remarkable decreases in poly-unsaturated fatty acids, especially 18:2ω6, were observed, suggesting that soil fungi was strongly affected by the treatment. The CP and SS treatments reduced the amounts of all of the PLFAs, but non-specifically compared with the MeBr treatment. At 10 days after transplanting, although the amounts of PLFAs in the MeBr treatment recovered to the level of the control, those in the CP and SS treatments remained low. These results indicated that the CP and SS treatments affected the soil microbial community considerably and the effects persisted for longer duration compared with the MeBr treatment. In the CP and SS treatments, increased NH4-N could be mostly derived from dead microbial cells. On the other hand, in the MeBr treatment, although the microbial community was affected strongly, the influence was short-lived. The community could be recovered for relatively shorter duration, which might be one of the reasons for small decrease in the amount of MBN compared with the increase in NH4-N.

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