Soil Biology & Biochemistry 38 (2006) 3359–3371 Microbial community response to soil solarization in Nepal’s rice–wheat cropping system S.W. Culman, J.M. Duxbury, J.G. Lauren, J.E. Thies à Department of Crop and Soil Sciences, Cornell University, 719 Bradfield Hall, Ithaca, NY 14853, USA Received 3 March 2006; received in revised form 13 April 2006; accepted 19 April 2006 Available online 6 June 2006 Abstract The Indo-Gangetic Plains of South Asia support 13.5 million hectares of rice–wheat cropping systems, which currently feed over one billion people. Intensified agriculture has resulted in a more than two-fold increase in rice and wheat yields since the 1970s; however, this continuous cropping has also exacerbated weed, pest and disease problems. Soil solarization is an accessible, low-risk management practice for small-holder farmers that has ameliorated these problems in some settings and has the potential to dramatically improve yields. Field trials were conducted at two sites in Nepal to test whether soil solarization: (i) had a lasting effect on soil bacterial, fungal and nematode communities; (ii) altered the rhizosphere communities of rice nursery seedlings and (iii) improved crop growth and yield in the rice–wheat cropping system. Rice seedlings were grown in nursery plots that were solarized for 28 days or left untreated and were transplanted to field plots that were also either solarized for 28 days or not in a randomized complete block design with four replications. Rice was grown to maturity and harvested, followed by a complete wheat cropping cycle. Solarization of main field plots increased counts of fungal propagules and decreased root galling and nematode counts and decreased weed biomass. Terminal restriction fragment length polymorphism (T-RFLP) analyses of extracted soil DNA revealed significant shifts in fungal community composition following soil solarization, which was sustained throughout the entire rice cropping cycle at both field sites. The bacterial community composition was similarly affected, but at only one of the two sites. Despite the observed changes in soil microbial community composition over more than one cropping period, solarization had no impact on crop productivity at either site. Nevertheless, such changes in soil microbial communities in response to solarization may be responsible for increased yields observed at other sites with greater pathogen pressure. This practice has shown promising results in many farmers’ fields in South Asia, but further elucidation of the mechanisms by which solarization increases productivity is needed. r 2006 Elsevier Ltd. All rights reserved. Keywords: Additive main effects and multiplicative interaction model; AMMI; Indo-Gangetic plains; Nematodes; PCR fingerprinting; Rice–wheat; Soil bacteria; Soil ecology; Soil fungi; Soil microbial community; Solarization; T-RFLP 1. Introduction The rice–wheat cropping systems of the Indo-Gangetic Plains encompass over 12 million hectares and account for 90% of the food produced in this area (Reeves, 2001). The intensification of agriculture in this region since the 1970s has rewarded farmers with more than a two-fold increase in rice and wheat yields. However, yields in some regions are stagnating and even declining despite increased use of agrochemical inputs (Harrington et al., 1989; Byerlee and Siddiq, 1994; Fujisaka et al., 1994; Gill, 1994; Singh and Paroda, 1994; Hobbs and Morris, 1996; Duxbury, 2001). This decline in productivity is attributed to the loss of soil organic matter and mineral nutrients, increased pressure from weeds, pests and diseases, and the loss of soil aggregate stability and subsequent decline in soil structure from continuous rice–wheat rotations (Gill, 1994; Hobbs and Morris, 1996; Cohen et al., 1998). Infestations of insects, pathogens, nematodes and weeds may lead to an estimated average yield loss of 20% in rice and 30% in wheat (Sehgal et al., 2001). Soil solarization is a manage- ment tool that has shown promise in ameliorating these biological constraints and increasing productivity in these ARTICLE IN PRESS www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2006.04.053 à Corresponding author. Tel.: +1 607 255 5099; fax: +1 607 255 8615. E-mail address: jet25@cornell.edu (J.E. Thies).