Effect of cashew nut shell liquid on rice growth and methane emission from paddy soil

Rice cultivation and ruminants are the top two sources of methane (CH4), a potent greenhouse gas, in agriculture. The supplementation of ruminants with cashew nut shell liquid (CNSL), a by-product of the cashew nut industry, can reduce CH4 production through alteration of the rumen microbiota. To assess the effects of CNSL addition and its rate on rice growth and CH4 emissions from a paddy soil, we carried out four experiments. CNSL was added at 0 (control) to 1000 ppm by volume in water or agar. In 3-week in vitro anaerobic incubation of two soils, Fluvisol and Andosol, CNSL significantly (P < 0.05) reduced CH4 production dose-dependently, by 54-61% at 1000 ppm relative to the control. In 7-day seed germination testing, CNSL concentration did not affect the final germination percentage (96-99%). In 19-day early seedling growth, CNSL significantly increased the maximum root length by the same amount at all concentrations. In addition, the number of root hairs notably increased with increasing CNSL concentration. In pot culture of rice, the seasonal total CH4 emission was not affected by CNSL concentration. However, mixing CNSL at 1000 ppm with soil before transplanting tended to reduce the initial 30-day CH4 emission by 41%, whereas addition to surface water after plant establishment had no effect. Rice plant growth and dry matter production in pot cultivation were not affected by CNSL concentration. Altogether, the results indicate that CNSL addition can reduce CH4 production in a flooded paddy soil without major harm to rice growth. It is necessary to develop an effective method for the application of CNSL to paddy fields for enhancing the inhibitory effect of CNSL on soil CH4 emission.