INCREASING WHEAT PRODUCTION WHILE DECREASING XA0055248 NITROGEN LOSSES FROM AMMONIUM BICARBONATE XIANFANG WEN, JIARONG PAN, QING CHEN Institute for Application of Atomic Energy, Chinese Academy of Agricultural Science, Beijing ZHANFENG GAO, LIANG CHEN, XIAORONG XU, HONGMEI WANG Institute of Agricultural Physics and Physiology-Biochemistry, Hebei Academy of Agricultural Sciences, Shijiazhuang China Abstract The objectives of a 4-year field experiment were i) to investigate the effects of rate and timing of application of ammonium bicarbonate on N-uptake efficiency by irrigated winter wheat, ii) to determine the fate of fertilizer N in wheat followed by maize, and iii) to study nitrate dynamics in the soil after N-fertilizer application to evaluate groundwater pollution by leaching. Nitrogen-application rates significantly affected wheat grain yields and straw dry matter. Grain yields were higher with 150 than with 225 kg N ha" 1, whereas the highest fractional recoveries of N from ammonium bicarbonate occurred with 75 kg N ha~l (38.5% in 1994-95 and 33.5% in 1996-97). On the basis of grain yield, N recovery and soil-N balance, ammonium bicarbonate at 150 kg N ha"', was the optimum rate, when applied basally and as a top dressing to wheat. Subsequent yields of maize stover and grain were affected by N applied to the wheat, suggesting that fertilizer recommendations, in terms of rate and timing, should be made on the basis of effects on the cropping rotation as a whole. Water-holding capacity of the soil was poor, therefore large applications of N are likely to cause nitrate pollution of ground water. 1. INTRODUCTION Nitrogen is the most important limiting nutritional factor for crop production. Fertilizer inputs are, therefore, required to obtain acceptable yields, but the amounts of fertilizer N that are taken up by crops are usually relatively low, 20 to 50%, and dependent on several factors including management practice. In China, approximately 45% of fertilizer N is applied as ammonium bicarbonate, estimated at 45 Mt annually. The fraction of N applied as ammonium bicarbonate that is taken up by crops can be as low as 25%, due to instability and volatilization [1]. Improper use further adds to losses of N [2]. Since the 1970s, there have been several studies on the physical and chemical characteristics of ammonium bicarbonate [3, 4] with a view to improving its efficiency of uptake by crops, with some success [5, 6, 7, 8]. The objective of this work was (a) to investigate the effects of rate and timing of ammonium bicarbonate on N-uptake efficiency by winter wheat under irrigation in Hebei province; (b) to investigate the fate of fertilizer N on current and subsequent crops; and (c) to study dynamics of nitrate in the soil after fertilizer application and evaluate groundwater pollution caused by leaching. 2. MATERIALS AND METHODS 2.1. Experiment site The experiment was conducted from October 1994 through August 1998, at the Agricultural Experiment Station of Hebei Academy of Agricultural Sciences in Shijiazhuang city, 38°02'N 114°25'E. This is in Hebei province, part of the North China Plain, which is important in winter-wheat production in rotation with maize. The fertilizers chiefly used are ammonium bicarbonate and urea. Because of ammonium bicarbonate's powder formulation and losses of N by volatilization, farmers generally prefer urea. 177 In this region, fertilizer applications to winter wheat are generally 150 kg N ha" 1 and 75 kg P2O5 ha~l for soils of medium fertility. The climate is semi-arid with a cold, dry winter, and hot summer. Relative humidity is high in summer and low in winter. Climatic characteristics are shown in Table I. The experiment field was 80 to 100 m above sea level and the water table was at 18 m; fertilizer experiments had not previously been done at this site. The field layout is shown in Fig. 1. TABLE I. THE CLIMATE CHARACTERISTICS IN SHIJIAZHUANG, HEBEI PROVINCE Climate indexes Recorded data Annual mean temperature(°C) 12.9 Mean temperature of February (°C) -2.7 Mean temperature of July (°C) 26.7 Lowest temperature (°C) -26.5 Highest temperature (°C) 42.7 Annual mean rainfall (mm) 556 Annual mean Frostless period (days) 194 Data recorded from December 31,1984-December 31,1993 provided by Shijiazhuang Meteorology BureauV •« — 110m ». 1996-97 1997-98 40m 1995-96 1994-95 FIG. 1. Design of experimental site, 1994-98. 2.2. Soil The soil has been classified as cinnamon or drab; physical and chemical characteristics are described in Tables II and III. TABLE II. SELECTED CHEMICAL CHARACTERISTICS OF THE SOIL COLLECTED FROM THE EXPERIMENT FIELD Indexes Measured value Classification Total Nitrogen (%) 0.092 Available Nitrogen(mg kg-1 soil) 90.7 Medium response to N Available P2O5 ( mg kg~l soil) 43.4 Medium response to P Available K-2O( mg kg"* soil) 240 Low response to K Organic matter ( %) 1.88 pH (in water) 7.88 TABLE III. SELECTED PHYSICAL CHARACTERISTICS OF THE SOIL COLLECTED FROM THE EXPERIMENT FIELD Code Depth Sand Silt Clay O.M. B.D. s.w.c. 3 (cm) (W, to/. (0/. (g cnr ) Al 0-15 53.4 32.3 14.3 1.91 43.5 1.17 A2 16-26 54.4 31.3 14.3 1.73 28.1 1.55 A3 27-47 51.9 30.3 17.8 0.82 33.9 1.40 A4 48-75 50.9 31.1 17.8 0.67 36.2 1.35 A5 76-107 49.9 32.3 17.8 0.59 32.0 1.44 A6 108-146 47.9 28.3 23.8 0.71 • 40.8 1.32 aB.D. = bulk density, O.M.= organic matter, CS.W.C.= saturated water content 178 2.3. Crops The rotation used in this experiment was wheat/maize/wheat. Winter wheat (Triticum aestivum L. cv. 71-3) was sown around 10 October and harvested around 10 June of each year. Maize (Zea mays L. cv. Yedan 13) was planted immediately after the wheat and harvested around 25 September each year. 2.4. Fertilizers From 1994 to 1997, the N was applied as ammonium bicarbonate (17% N, moisture content 3%) and 15N_enrjched (5.54% abundance) ammonium bicarbonate. For the 1997-98 growing season, urea (Ncontent 46%, H2O content 2.7%) and 15N-enriched urea (5.32% 15N) were used. The unenriched fertilizers were provided by Zhengdong Chemical fertilizer factory, Shijiazhuang city, and the ^N- enriched counterparts were supplied by the Shanghai Research Academy of Chemical Industry. 2.5. Treatments 2.5.1. Rates and timing ofN The N-rate design is shown in Table IV. The application-timing design was as shown in Tables V and VI; applications were variously made at sowing and/or at jointing. TABLE IV. NITROGEN APPLICATION RATES( kgN ha"l) Code 1994/95 1995/96 1996/97 1997/98 NO.O Control 0 0 0 0 N0.5 Low 50% of optimum rate 75 50 75 75 N1.0 Optimum rate 150 100 150 150 N1.5 Above 50% of optimum rate 225 150 225 225 *The nitrogen fertilizer used in 1994/95, 1995/96 and 1996/97 was ammonium bicarbonate, the nitrogen fertilizer used in 1997/98 was urea. TABLE V. DESIGN OF NITROGEN FERTILIZATION IN MICROPLOTS DURING IRRIGATED WINTER WHEAT SEASONS OF 1994/95 AND 1996/97 Code At sowing(ZS** 00) Atjointing(ZS31) NO.O 1/3* 2/3* N0.5 1/3* 2/3* N1.0 1/3* 2/3* N1.5 1/3* 2/3* * ]5N labeled nitrogen fertilizer applied * * FEEKES scale TABLE VI. DESIGN OF NITROGEN FERTILIZATION IN MICROPLOTS DURING IRRIGATED WINTER WHEAT SEASONS OF 1995/96 AND 1997/98 Code At sowing(ZS 00**) Atjointing(ZS31) 1/3* 2/3 N1.0 1/3 2/3* 1/3* 2/3* 1/3* 2/3 N1.5 1/3 2/3* 1/3* 2/3* * 15N labeled nitrogen fertilizer applied ** FEEKES scale 179 2.5.2. Plots During the 1994-95 and 1995-96 growing seasons, plot size was 9.0x5.5 m; for 1996-97 and 1997-98, it was 7.5x7.0 m. Each was divided into two equal sub-plots, one for sampling and the other for yield. The ^N micro-plots, 1.0x0.6 m, were located within the yield sub-plots. During 1994-95 and 1996-97, each No.5, Nj Q and NJ 5 yield sub-plot contained one micro-plot and for 1995-96 and 1997- 98, each Nj 0 and Nj 5 yield sub-plot contained three micro-plots (Tables IV, V and VI). All treatments were randomly located and replicated three times. 2.5.3. Fertilization All N-fertilizer applications were incorporated to a depth of 10 cm (Table IV, V and VI). In addition, calcium monophosphate was applied just before sowing at a rate of 75 kg P2O5 ha~l. Nitrogen and P were not applied to the subsequent maize crops. 2.5.4. Irrigation Irrigation timing and amounts were as recommended by local farmers. Each winteF-wheat crop was irrigated six times (Table VII) by flooding. TABLE VII. IRRIGATION TIMING AND AMOUNT DURING THE GROWING SEASON OF WINTER WHEAT Growing Irrigation 1st 2nd 3rd 4th 5th 6th season 1994/95 Date 10/20/94 11/20/94 03/13/95 04/15/95 05/05/95 05/26/95 Amount 4 m3 per plot 1995/96 Date 10/19/95 12/01/95 04/01/96 04/24/94 05/16/96 05/28/96 Amount 4 m3 per plot 1996/97 Date 10/20/96 12/03/96 03/15/97 04/10/97 05/02/97 05/24/97 Amount 4 m3 per plot 1997/98 Date 10/12/97 11/29/97 03/24/98 04/15/98 05/11/98 05/28/97 Amount 4 m3 per plot 2.6.