Water management in irrigated rice - Rice Knowledge Bank ...

consumption (Hamilton 2003). Many traditionalfestivals and religious practices are associated withrice cultivation and rice fields are valued for theirscenic beauty.4.2 Environmental impactsThe production of lowland rice affects the environmentin negative ways, such as the emissionof greenhouse gases and water pollution. In thissection, environmental impacts that have a relationshipwith water and the hydrology of rice fields aresummarized.4.2.1 Ammonia volatilizationAmmonia (NH 3) volatilization from urea fertilizeris the major pathway of N loss in tropical floodedrice fields, often causing losses of 50% or more ofthe applied urea-N (Buresh and De Datta 1990).Ammonia-N emissions from lowland rice fields areestimated to be roughly 3.6 Tg per year (comparedwith a total of 9 Tg y −1 emitted from all agriculturalfields), which is some 5−8% of the estimated 45−75Tg of globally emitted ammonia-N per year (Kirk2004). The magnitude of ammonia volatilizationlargely depends on climatic conditions, field waterstatus, and the method of N fertilizer application.Volatilized ammonium can be deposited on theearth by rain, which can lead to soil acidification(Kirk 2004) and unintended N inputs into naturalecosystems.4.2.2 Greenhouse gasesIrrigated rice systems are a significant sink for atmosphericCO 2(Chapter 4.1), a significant sourceof methane (CH 4), and a small source of nitrousoxide (N 2O). In the early 1980s, it was estimatedthat lowland rice fields emitted some 50−100 Tgof methane per year, or about 10−20% of the thenestimated global methane emissions (Kirk 2004).Recent measurements, however, show that manyrice fields emit substantially less than those investigatedin the early 1980s, especially in northern Indiaand China. Also, methane emissions have actuallydecreased since the early 1980s because of changesin crop management such as a decreased use oforganic inputs (Van der Gon et al 2000). Currentestimates of annual methane emissions from ricefields are in the range of 20 to 60 Tg, being 5–10%of total global emissions of about 600 Tg (Kirk2004). The magnitude of methane emissions fromrice fields is mainly determined by water regime andorganic inputs, and to a lesser extent by soil type,weather, tillage, residue management, fertilizer use,and rice cultivar (Bronson et al 1997a,b, Wassmannet al 2000). Flooding of the soil is a prerequisitefor sustained emissions of methane. Mid-seasondrainage, a common irrigation practice adopted inmajor rice-growing regions in China and Japan,greatly reduces methane emissions. Similarly,rice environments with an uneven supply of water(for example, those suffering from water scarcity,Chapter 1.7) have a lower emission potential thanfully irrigated rice.Few accurate assessments have been made ofemissions of nitrous oxide from rice fields (Abao etal 2000, Bronson et al 1997a,b, Dittert et al 2002),and the contribution to global emissions has not yetbeen assessed. In irrigated rice systems with goodwater control, nitrous oxide emissions are quitesmall except when excessively high fertilizer-Nrates are applied. In irrigated rice fields, nitrousoxide emissions mainly occur during fallow periodsand immediately after flooding of the soil at the endof the fallow period.4.2.3 Water pollutionChanges in water quality associated with rice productionmay be positive or negative, dependingmainly on management practices such as fertilizationand biocide (all chemicals used for crop protection,such as herbicides, pesticides, fungicides, etc.)use. The quality of the water leaving rice fields maybe improved as a result of the capacity of the ricefields to remove nitrogen and phosphorus (Feng etal 2004, Ikeda and Watanabe 2002). On the otherhand, nitrogen transfer from flooded rice fields bydirect flow of dissolved nitrogen through runoffwarrants more attention. High nitrogen pollutionof surface fresh waters can be found in rice-growingregions where fertilizer rates are excessivelyhigh, such as in Jiangsu Province in China (Cuiet al 2000).Contamination of groundwater may arisefrom the leaching of nitrate or biocides and theirresidues (Bouman et al 2002). Nitrate leachingfrom flooded rice fields is quite negligible becauseof rapid denitrification under anaerobic conditions(Buresh and De Datta 1990). For example, in thePhilippines, nitrate pollution of groundwater underrice-based cropping systems surpassed the 10 mgL −1 limit for safe drinking water only when highly36