Agrochemicals influencing nitrogenase, biomass of N2-fixing cyanobacteria and yield of rice in wetland cultivation
Introduction
Rice is widely cultivated in wetlands having high temperature, low soil organic carbon and high humidity. Such environmental conditions are favorable for cyanobacteria mainly due to their ability to fix atmospheric nitrogen and carbon. Cyanobacteria are considered as key biocatalysts in the N2 cycle (Latysheva et al., 2012, Vitousek et al., 2002). Cyanobacteria were reported as first agent to fix atmospheric nitrogen in flooded rice soils (Singh, 1961) and maintenance of natural soil fertility in such ecosystem was attributed mainly due to these organisms (De, 1939, Singh, 1961). Before introduction of chemical fertilizers, rice has been cultivated year after year without losing soil fertility which was considered mainly due to cyanobacteria and thus role of these organisms in N- economy of rice fields has been widely advocated (Singh, 1961, Venkataraman, 1972, Roger and Kulasooriya, 1980, Swarnalakshmi et al., 2006). With the introduction of high yielding rice varieties, agrochemicals are invariably being used to obtain high yields. Thus, simultaneous or sequential application of agrochemicals such as herbicides, insecticides and nitrogenous fertilizers is indispensible in modern rice farming. But these chemicals affect other plants, animals and microorganisms which prevail in such habitat (Chen et al., 2007; Aktar et al., 2009; Casida, 2009; Geisseler and Scow, 2014). When two or more chemicals are applied together as a mixture or one after another during a cropping season, they interact and cause synergistic, additive or antagonistic responses (Tammes, 1964; Akobundu et al., 1975; Magnusson et al., 2010; Padhy and Rath, 2015). Therefore, information on the interactions of these chemicals or their degradation products with rice field ecosystem is very important. It is particularly relevant in case of cyanobacteria, which are the dominating flora of the tropical rice fields and are known to play important role in building soil organic carbon and nitrogen in the flooded rice ecosystem (Venkataraman, 1972; Singh, 1978; Singh and Bisyoi, 1989; Whitton, 2000; Nayak et al., 2004; Swarnalakshmi et al., 2006). Effects of pesticides on cyanobacteria in laboratory cultures were reported earlier (Singh, 1973, Das and Singh, 1978, Tiwari et al., 2001, Chen et al., 2007, Padhy and Rath, 2015). Recently, Das et al. (2015) reported from pot experiment conducted with planted rice that herbicide (butachlor) application adversely affected native and inoculated cyanobacteria whereas insecticide (metacid) application was favorable to them.
The information on the interaction of different agrochemicals with cyanobacteria in paddy fields is inadequate. Therefore, in present investigation, an experiment was performed to analyse the impact of commonly used agrochemicals (i.e. herbicide, insecticide and N-fertilizer) in isolation and in combination for three consecutive seasons on native cyanobacterial biomass, nitrogenase activity in terms of acetylene reduction assay (ARA), cyanobacterial N-yield as well as growth and yield of rice.
Section snippets
Experimental design and treatments
A field experiment was carried out for three consecutive seasons (two dry seasons and one wet season) with native cyanobacteria in plots of 5 m×2 m size using urea (CH4N2O), Benthiocarb (trade name Saturn) (S-(4-Chlorobenzyl) N, N- diethylthiolcarbamate; C12H16CINOS) and carbofuran (trade name furadan) (2,2-dimethyl-3H-1-benzofuran-7-yl) N-methylcarbamate; C12H15NO3) as N fertilizer, herbicide and insecticide, respectively. The experiment had a completely randomized block design with three
Growth of cyanobacteria as influenced by agrochemicals in rice field
The growth of the native cyanobacteria in terms of dry weight (kg ha−1) was found maximum at 60 days after transplanting (DAT) and minimum at 30 DAT. The cyanobacterial growth in wet season (WS) was comparatively lower than that in the dry season (DS) (Table 1; Fig. 1). Urea applied at rate of 60 kg N ha−1 was observed to be highly inhibitory for growth of cyanobacteria. It reduced their growth at 60 DAT by 55% and 62% over the control. The herbicide benthiocarb applied at rate of 1.0 kg a.i. ha−1
Discussion
Among different food crops, rice ranked second to wheat in terms of area harvested, but in terms of importance as a food crop rice provides more calories than any other cereal crop (De Datta, 1981). The sustainability of agriculture and ecosystems is considered crucial and therefore increasing demand of improving yields using extensive agrochemicals and other modern technologies should also ensure protection of environmental integrity and biodiversity conservation. Since agrochemicals are
Conclusion
The cyanobacteria are known to play an important role in maintaining soil fertility in flooded rice ecosystem. With the demand of intensive rice cultivation to obtain high yields, high doses of chemical N-fertilizers are being used. Herbicides and insecticides have also become essential schedule in rice cultivation. Hence, it is essential to have reliable informations from field studies on the effects of these chemicals on the beneficial microorganisms. The experiment was conducted using a
Acknowledgements
Authors would like to thank Director, Central Rice Research Institute, Cuttack for providing experimental facilities and also thankful to Head, Department of Botany, B.H.U., Varanasi and Indian National Science Academy, New Delhi, India.
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