ABSTRACT

INTRODUCTION About 75% of the 143 million hectares of rice land are wetlands where rice grows under flooded conditions during part or all of the crop cycle. Flooding leads to the differentiation of macro-and microenvironments differing in redox state, physical properties, light status and nutrient status, which allows all kinds of N2-fixing micro-organisms (aerobes, facultative and strict anaerobes, heterotrophs, phototrophs, free-living and symbiotic) to grow in wetland rice fields (Roger and Watanabe, 1986) (Fig. 12.1). This results in a unique agroecosystem in which moderate but constant yields have been obtained for centuries after continuous cropping without N fertiliser addition. A compilation of 211 balance studies in rice soils indicated that in the absence of N fertiliser application, biological N2 fixation (BNF) averaged 31 kg N crop"1, among which 2 /3 could be roughly attributed to photodependent organisms, mostly free-living cyanobacteria (Table 12.1) (Roger and Ladha, 1992).