Role of organic matter and nutrients

In rice cultivation, as in any other form of agriculture, it is necessary to sustain soil fertility by returning plant nutrients to the soil. Plant residues, green manure from intercrops or the aquatic plant Azolla and its associated N-fixing blue-green alga Anabaena azollae, human faeces and animal manure are the most important forms of organic fertilizers used in rice crops. Since the 'green revolution' in the 1960s introduced new varieties with increased yields and nutrient requirements, these organic amendments have been generally complemented in many regions by mineral forms of fertilizer. A major difference between organic and mineral fertilizers, in the context of methanogenesis, is that organic fertilizers contain, in addition to plant nutrients, energy sources (for example carbohydrates) that stimulate soil microbial activity. The energy content in organic amendments declines rapidly with time during aerobic decomposition. Applied freshly, however, organic materials lead to large CH4 emissions. For example straw, incorporated shortly before flooding, produces three to four times larger emissions compared to when it is incorporated 30 or more days before flooding and is partly decomposed before conditions turn anoxic (Yan et al, 2005). Globally, offseason incorporation of rice straw has the potential to mitigate emissions of about 4Tg CH4 each year (Yan et al, 2009).

Composting organic fertilizers off-site can be even more effective, however, it involves additional labour to transport material from and to the field. Whether decomposed on-site or composted off-site, plant nutrients, including a large proportion of the nitrogen, are retained while the energy content is reduced. In contrast, burning of straw removes all energy but also almost all the nitrogen, while other nutrients are still retained. Straw burning is already practised on 30 per cent of the rice growing area (Yan et al, 2009). However, in some areas the burning of straw is prohibited. Where few arguments other than legal restrictions are preventing straw from being burned, easing those restrictions may be the easiest mitigation option.

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