RESEARCH PAPER Ecophysiological study on weed seed banks and weeds in Cambodian paddy fields with contrasting water availability AKIHIKO KAMOSHITA, 1 * HIROYUKI IKEDA, 2,3 JUNKO YAMAGISHI 2 and MAKARA OUK 4 1 Asian Natural Environmental Science Center, 2 Institute for Sustainable Agro-Ecosystem Services, Graduate School of Agricultural and Life Sciences,The University of Tokyo,Tokyo, 3 AG. Chem. R&D Department, Biological Research Laboratories, Nissan Chemical Industries, Saitama, Japan and 4 Cambodian Agricultural Research and Development Institute, Phnom Penh, Cambodia Weed infestations are a major cause of yield reduction in rice (Oryza sativa) cultivation, particu- larly with direct-seeding methods, but the relationship between weed dynamics and water availability in Cambodian paddy fields has not been documented previously.We surveyed the weed abundance and weed seed banks in the soil of paddy fields with inferred differences in their water regime in 22 farm fields in three provinces of Cambodia in the 2005 and 2006 rainy seasons.We studied rain-fed lowland fields in upslope and downslope topographic positions and fields at different distances from the irrigation water source inside an irrigation rehabilitation area.The weed seed banks were estimated by seedling emergence in small containers and weed abundance and vigor were estimated by a simple scoring system.The estimated weed seed bank in the top 5 cm of soil ranged from 52.1 to 167 ¥ 10 3 seeds m -2 (overall mean of 8.5 ¥ 10 3 seeds m -2 ) and contained a high proportion (86%) of sedge species, such as Fimbristylis miliacea L. and Cyperus difformis. Several fields had particularly large seed banks, including one near the reser- voir. No clear difference was found in the weed seed banks between the irrigated fields that were located close to (upstream) and distant from (downstream) the water source or between the irrigated and rain-fed lowland fields, but the weed scores were larger in the rain-fed fields and the downstream fields within the irrigated area.A water shortage during the late growing season in 2005 led to a proliferation of weeds in some fields and an associated increase in weed seedbank size in 2006.However,the weed scores in 2006 were more strongly associated with that year’s water conditions than with the weed seedbank size. Keywords: Cambodia, direct seeding, irrigation rehabilitation, rain-fed lowland rice. Weed infestations reduce the grain yield of many Asian rice fields, particularly with direct seeding for crop estab- lishment, a practise that has been increasing in Southeast Asia since the late 1980s (Pandey &Velasco 2002; Ikeda et al. 2008; Hayashi et al. 2009). Studies have been con- ducted on weed ecology in paddy fields under different environmental conditions (e.g. Tomita et al. 2003b), but no study has been conducted on weed seed banks.The term “weed seed bank” refers to the store of viable seeds in the soil and soil surface (Roberts 1981); these seeds play a significant role in weed population dynamics, along with field water availability and crop management methods.The control of weed populations through the manipulation of the weed seed bank is an important weed management option (McIntyre 1985; Wilson et al. 1985; Sago 2000); however, for Cambodian or other tropical Asian rice fields, information is lacking on the important attributes of weed seed banks, such as changes Communicated by T.Yoshioka. *Correspondence to: Akihiko Kamoshita, Asian Natural Environ- mental Science Center,The University of Tokyo, 1-1-1 Midoricho, Nishitokyo 188-0002, Japan. Email: akamoshita@anesc.u-tokyo.ac.jp The authors have no commercial interest in the findings presented. Received 7 July 2008; accepted 25 September 2010 Weed Biology and Management 10, 261–272 (2010) © 2010 The Authors Journal compilation © 2010 Weed Science Society of Japan doi:10.1111/j.1445-6664.2010.00393.x