Simulation of Nitrogen Balance in Rice–Wheat Systems of the Indo-Gangetic Plains

Crop growth simulation models provide a means to quantify the effects of climate, soil, and management on crop growth and biogeochemical processes in soil. The Denitrification and Decomposition (DNDC) model was evaluated for its ability to simulate N dynamics and balance in the rice (Oryza sativa L.)–wheat (Triticum aestivum L.) cropping systems in the Indo-Gangetic Plains with various N and water management practices. The observed crop yield, N uptake, and losses of N were in good agreement with the values predicted by the model. In the rice–wheat systems, current annual inputs of N through fertilizer, manure, biological fixation, atmospheric deposition, and irrigation were 98, 37, 17, 8, and 7 kg N ha, respectively, while outputs through uptake, volatilization, leaching, and denitrification were 175, 14, 12, and 4 kg N ha, respectively. The northwestern transects of the Indo-Gangetic Plains (Punjab and Haryana) showed greater yields and N uptake because of a higher amount of N use and more favorable climatic conditions than those in the eastern transects (Uttar Pradesh, Bihar, and West Bengal). Volatilization was the dominant N loss mechanism in Punjab and West Bengal while NO3 leaching was dominant in Bihar, Uttar Pradesh, and West Bengal. The simulated balance of N was negative in all the states. The largest depletion of soil N was estimated in Bihar, followed by Uttar Pradesh, Haryana, Punjab, and West Bengal. The study suggests that better N management is required to arrest the depletion of soil N. RICE AND WHEAT are the two most important cereals for world food security. The rice–wheat production system occupies 21 million ha of cultivated land in the Asian subtropics (Dawe et al., 2003). In South Asia, the system occupies about 13 million ha, extending across the Indo-Gangetic floodplain (IGP) into the Himalayan foothills. The rice–wheat production system provides staple grain for more than 400 million people. During the Green Revolution era in the 1960s, production increases resulted from increases in both rice–wheat area and system productivity. But little additional land is available now and traditional farmlands are increasingly lost to urbanization. In addition, since most of the land is already doubleand even triple-cropped, increasing cropping intensity is not a possible option for increasing production. Therefore, future demand for food will have to be met mainly through increases in production per unit of harvested area (Ladha et al., 2003). In the rice–wheat systems, rice is grown in the summer months (June–October) under monsoon climatic conditions and wheat is grown during the cooler and drier winter months (November–March) after rice harvest. Generally, two to three rice seedlings (4–5 wk old) are transplanted in the puddled lowland field at 20by 15-cm spacing. Normally the fields are flooded (2–4 cm) until 2 wk before the rice harvest. Fertilizer dose varies in different regions of the IGP. Nitrogen is applied as urea in three splits, 50% at transplanting of rice and the remaining two splits top-dressed at tillering and panicle initiation stages. Phosphorus and K are applied as basal fertilizer on the day of planting. Manure is incorporated into soil 15 d before transplanting. Wheat (100–120 kg seed ha) is sown in rows and three to four irrigations are given at the sowing, crown root initiation, maximum tillering, and flowering stages. All P and K and a half dose of N are applied at the sowing of wheat. The remaining N is topdressed in one or two splits at crown root initiation and maximum tillering stages with irrigation. Crops are harvested by combine in the upper IGP (Punjab, Haryana, and western Uttar Pradesh) and the straw is burnt before sowing wheat. In the lower IGP (eastern Uttar Pradesh, Bihar, and West Bengal), crops are harvested manually close to the ground using sickles and straw is removed from the field. Nitrogen is the most widely used fertilizer nutrient and its consumption has increased substantially in recent decades. Before World War II, global N fertilizer application to agricultural soils was only 3 Mt, which rose to 84 Mt in 2002. Nitrogen fertilizer consumption in India has also grown dramatically (Fig. 1). In 1950–1951, consumption of N fertilizer in the country was only 0.06 Mt, which increased to 11.31 Mt in 2001–2002, an increase of about 190-fold in the last 50 yr (Fertilizer Association of India, 2003). It rose steeply after the Green Revolution in the 1960s, mainly because of the availability of N fertilizer at low cost and the rapid adoption of modern, high-yielding N-responsive rice and wheat cultivars. The amount of compost consumed in Indian agriculture increased from 231.79 Tg in 1980–1981 to 286.30 in 1993– 1994 but decreased thereafter (Fertilizer Association of India, 2003). In 1999–2000, the consumption of compost was reduced to 137.58 Tg. Similarly, the area under green manure crop was about 5 to 6 million ha during 1980–1981 to 1993–1994 but has been reduced to only 2.25 million ha since 1996–1997. Therefore, fertilizer consumption in India needs to increase further in the coming years, as the country has to produce more food to feed its increasing population and the contribution of N through manure is not likely to increase. Nitrogen can escape from soil–plant systems to water bodies and the atmosphere, creating pollution problems. H. Pathak, Unit of Simulation and Informatics, Indian Agricultural Research Inst., New Delhi, India; C. Li, Inst. for the Study of Earth, Oceans, and Space, Univ. of New Hampshire, Durham, NH; R. Wassmann, Inst. for Meteorology and Climate Research, GarmischPartenkirchen, Germany; and JK. Ladha, International Rice Research Inst., India Office, New Delhi 110 012, India. Received 26 May 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:1612–1622 (2006). Nutrient Management & Soil & Plant Analysis doi:10.2136/sssaj2005.0165 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: DNDC, denitrification and decomposition model; FYM, farmyard manure; GIS, geographic information systems; IGP, Indo-Gangetic Plains; SOC, soil organic carbon. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 1612 Published online August 3, 2006