Increasing Wheat Production While Decreasing Xa0055248 Nitrogen Losses from Ammonium Bicarbonate

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. Sampling and analysis

During the wheat-growing season, plants and soil were sampled at sowing and at Zadoks growth stages Z-20, -31,-41,-51 and -90. The maize was sampled only at maturity. At final harvest, three blocks (each 1x2 m) of winter wheat or maize were removed from each yield sub-plot and weighed for stover dry matter and grain yield after drying. Soil samples, three cores of 2 cm diam, were randomly taken from sampling sub-plots and separated into 0 to 15, 16 to 30, 31 to 50, 51 to 75, and 76 to 100 cm sections, except during 1994-95 when the soil-sampling sections were 0 to 20, 21 to 40, 41 to 60, and 61 to 80 cm. Each section was mixed thoroughly, quickly analyzed for water content, and stored at 4°C pending determinations of total-N and inorganic-N content. Soil-N was determined by the Kjeldahl distillation method using 0.01 M H3BO3 as the absorbing solution after digestion in concentrated sulphuric acid. Soil mineral N was determined using a distillation apparatus (Tecator, Sweden) with 2M KC1. After being dried at 70°C, weighed and milled, plant samples (root, straw, grain and husk separately) were determined for N content by Kjeldahl distillation. The 15N/14]SJ ratios of plant and soil samples were determined by mass spectrometry (ZHT-03, Beijing Analysis Co.).

3. RESULTS AND DISCUSSION

3.1. Yield

Yield differences from season to season were probably due to climatic conditions rather than to soil fertility which was largely uniform. There were significant responses in grain and straw yields to N

180 application, except for the 1994-95 trial (Table VIII) in which straw weights with 75 and 150 kg N ha" 1 were not significantly different, although significantly lower than with 225 kg N ha~l. For the 1996-98 and 1997-98 seasons, straw yields with 150 kg N ha'l were significantly higher than those with 75 kg N ha~l and similar to those with 225 kg N ha~l. The 150 kg N ha~l treatment produced the highest grain yields, although in 1994-95 and 1995-96, they were not significantly higher than those with 75 or 100 kgNha-1.

TABLE VIII. DRY MATTER, STRAW YIELD AND GRAIN YIELD OF WINTER WHEAT AS INFLUENCED BY NITROGEN APPLICATION RATE Growing Fertilization Dry matter Straw yield Grain yield season Code tha"1 LSD tha-1 LSD tha-1 LSD 1994/95 NO.O 12.14 C*B** 7.00 bA 5.14 bB N0.5 14.78 aA 8.39 aA 6.39 aA N1.0 14.70 aA 8.02 aA 6.68 aA N1.5 13.12 bB 7.94 bA 5.08 bB 1995/96 NO.O 11.61 cC 6.91 cC 4.70 bB N0.5 13.63 bB 8.61 bB 5.02 bB N1.0 15.85 aA 9.62 aA 6.23 aA N1.5 16.45 aA 9.99 aA 6.46 aA 1996/97 NO.O 15.77 dC 11.12 bB 4.65 cB N0.5 17.10 cB 10.92 bB 6.18 bA N1.0 19.26 aA 12.49 aA 6.77 aA N1.5 18.21 bA 12.06 aA 6.15 bA 1997/98 NO.O 15.66 cB 10.76 bB 4.90 cB N0.5 16.62 bB 10.67 bB 6.05 bA N1.0 18.21 aA 11.54 aAB • 6.67 aA N1.5 18.23 aA 12.06 aA 6.17 bA

LSDn *LSD,0.01

TABLE IX. HARVEST INDEXES OF WINTER WHEAT AS INFLUENCED BY NITROGEN APPLICATION RATE Growing season Code values LSD 1994/95 NO.O 0.423 ab* A** N0.5 0.432 ab A N1.0 0.454 a A N1.5 0.390 b A 1995/96 NO.O 0.404 a A N0.5 0.368 a A N1.0 0.393 a A N1.5 0.392 a A 1996/97 NO.O 0.294 c B N0.5 0.361 a A N1.0 0.356 a A N1.5 0.337 b A 1997/98 NO.O 0.312 c B N0.5 0.361 a A N1.0 0.366 a A N1.5 0.338 b B LSD0.01

181 TABLE X. NITROGEN YIELD OF WINTER WHEAT AS INFLUENCED BY NITROGEN APPLICATION RATE Growing Whole Straw Grain 1 1 1 season Code kg ha LSD0C>5 kg ha" LSDo.os kg ha LSD005 1994/95 NO.O 161.9 d 46.1 b 115.8 b N0.5 218.1 b 73.3 a 144.9 a Nl.O 232.7 a 74.4 a 158.3 a N1.5 199.9 c 82.2 a 117.7 b 1995/96 NO.O 213.0 b 61.6 b 151.4 c N0.5 254.3 a 90.0 a 164.3 b Nl.O 262.5 a 83.9 a 178.6 a N1.5 266.2 a 85.5 a 180.7 a 1996/97 NO.O 189.8 c 63.5 c 126.3 c N0.5 217.7 b 74.6 b 143.1 b Nl.O 271.8 a 88.9 a 182.9 a N1.5 226.4 b 83.4 a 143.0 b 1997/98 NO.O 196.0 c 59.6 b 136.4 c N0.5 223.5 b 70.3 b 153.2 b Nl.O 265.7 a 86.7 a 179.0 a N1.5 226.4 b 88.7 a 137.7 c

*LSDn

TABLE XI. NITROGEN RECOVERY OF WINTER WHEAT AS INFLUENCED BY NITROGEN APPLICATION RATE AND TIMING Growing Straw Grain

season Code Timing % LSD005 % LbD005 1994/95 N0.5 l/3*+3/2* 12.62 a 25.89 a Nl.O l/3*+3/2* 10.12 b 22.18 b N1.5 l/3*+3/2* 8.75 c 13.67 c 1995/96 l/3*+3/2 18.31 37.23 a N1.0 1/3+3/2* 10.56 21.00 cd l/3*+3/2* 10.28 20.89 d l/3*+3/2 14.48 b 30.60 b N1.5 1/3+3/2* 7.94 d 16.78 e l/3*+3/2* 10.45 c 22.09 c 1996/97 N0.5 l/3*+3/2* 10.46 a 23.04 a Nl.O l/3*+3/2* 9.34 a 21.65 a N1.5 l/3*+3/2* 7.09 b 17.78 b 1997/98 l/3*+3/2 13.57 28.45 N1.0 1/3+3/2* 17.42 18.48 l/3*+3/2* 9.67 23.04 l/3*+3/2 12.45 b 23.04 b N1.5 1/3+3/2* 13.58 b 20.45 c l/3*+3/2* 10.02 c 23.43. b * l5N-labeled fertilizers applied

When the N-application rate was increased to 225 kg N harl, grain yield decreased and, in some cases, e.g. 1994-95, was not significantly higher than that of the zero-N control, probably because excessive N affected nutrient balance, thus increasing plant susceptibility to deficiencies in moisture and P and K. Another possible reason was that excessive fertilizer decreased the normally positive effects of N on photosynthesis, which affected grain filling and final grain yield [9], as indicated by the significantly lower harvest index with 225 kg N har 1 than with 150kgNha~l (Table IX).

182 TABLE XII. THE BALANCE OF FERTILIZER NITROGEN AS INFLUENCED BY NITROGEN APPLICATION RATE AND TIMING (% OF APPLIED) season Code Timing recovery in in soil Unaccounted plant (0-105 cm) kg N ha"' % of applied 1994/95 N0.5 l/3*+3/2* 38.51 38.40 17.31 23.09 N1.0 l/3*+3/2* 32.35 46.77 31.39 20.92 N1.5 l/3*+3/2* 22.45 61.58 88.15 39.18 1995/96 l/3*+3/2 55.51 35.35 3.14 9.44 N1.0 1/3+3/2* 31.17 41.19 18.40 27.64 l/3*+3/2* 32.07 38.97 30.06 30.06 l/3*+3/2 45.07 40.24 7.34 14.68 N1.5 1/3+3/2* 24.71 45.77 29.61 29.51 l/3*+3/2* 32.54 47.07 30.22 20.15 1996/97 N0.5 l/3*+3/2* 33.50 34.40 22.65 32.20 N1.0 l/3*+3/2* 30.99 48.29 30.11 20.07 N1.5 l/3*+3/2* 24.87 47.49 62.14 27.62 1997/98 l/3*+3/2 42.02 37.20 10.38 20.77 N1.0 1/3+3/2* 35.90 43.57 20.35 20.53 l/3*+3/2* 31.71 44.01 36.42 24.28 l/3*+3/2 35.59 35.91 21.45 28.60 N1.5 1/3+3/2* 34.03 40.43 38.31 25.54 l/3*+3/2* 33.45 44.21 50.26 22.34 * 15N-labeled fertilizers applied

TABLE XIII. YIELD OF NEXT CROP (MAIZE) Amouivt of N in soil at the end of Previous previous crops (kg N ha"1) Yield of maize (kg ha"1) Growing season fertilizer code 0-30cm 31-50cm Straw Grain 1995 NO.O 3286 1988 4954 3925 N0.5 3962 2268 6192 4237 N1.0 4244 2380 6284 4153 N1.5 4325 2604 6509 4846 1997 NO.O 3324 1889 4500 4400 N0.5 4096 2347 5639 5850 N1.0 4332 2456 6277 5910 N1.5 4468 2565 6194 7472

3.2. Nitrogen content

Plant N significantly increased with applied N up to 150 kg N ha" 1, but with 225 kg N ha~l was significantly lower than with 150 kg N ha~l (Table X). Straw N with 150 kg N.ha~l was similar to that with 225 kg N ha~*, whereas the grain N was significantly higher with 150 kg N har*. These data indicate that excessive N supply may adversely affect N uptake by winter wheat.

3.3. Nitrogen recovery

Recovery of applied 15>j varied depending on fertilizer type, N rate and timing (Tables XI and XII). When one third of ammonium bicarbonate was applied at sowing and two thirds at jointing (Z-31), the highest fractional recovery was obtained from 75 kg N ha"l, at 38.5% in 1994-95 and 33.5% in 1996-97. With 100 and 150 kg N ha"1 as ammonium bicarbonate, the recoveries were 32%, 32.5% and 31% in 1994-95, 1995-96 and 1996-97, respectively. When the application rate was 225 kg N ha"1, the recovery was as low as 22% in 1994-95 and 25% in 1996-97.

183 TABLE XIV. FERTILIZER NITROGEN UPTAKE BY NEXT CROPS Amount of fertilizer N in soil at the end of Fertilizer N uptake by next crops Growing Previous previous crops (kg N ha"1) (kgNha"1) season fertilizer code 0-30cm 31-50cm Straw Grain 1995 N0.5* 23.46 5.34 2.46 5.13 N1.0* 46.69 23.47 7.89 7.89 N1.5* 104.24 34.33 13.45 18.92 1997 N0.5* 22.46 3.40 1.65 3.01 N1.0* 47.74 25.68 6.03 6.80 N1.5* 78.09 28.77 12.37 19.86 *- 15N labeled fertilizer applied both at sowing(l/3) and atjointing(2/3).

TABLE XV. SOIL WATER CONTENT WITH RELATION TO IRRIGATION (N0.0) Irrigation datf Sampling date Soil profile Water content prior to sampling (cm) (% vol.) 1994-10-05 0-20 16.46 21-40 14.70 41-60 17.25 61-80 14.03 1994-10-20 1994-10-26 0-20 18.06 21^0 16.97 41-60 16.16 61-80 12.34 1994-11-18 1994-11-21 0-20 23.25 21-40 20.35 41-60 19.24 61-80 17.89 1995-03-13 1995-03-26 0-20 12.95 21^0 15.68 41-60 16.85 61-80 18.74 1995-04-15 1995-04-26 0-20 19.10 21^0 18.11 41-60 16.45 61-80 16.32 1995-05-26 1995-06-15 0-20 20.40 21^0 19.23 41-60 17.56 61-80 19.31

As a general rule-of-thumb, when 75 kg N ha'l are applied as ammonium bicarbonate, approximately 150 kgN ha"* N will be removed as grain from the system, therefore, even with the return of straw to the soil, 75 kg N ha'l would be lost. Under such circumstances, soil fertility will decrease rapidly. With 150 kg N ha~l applied, the rate recommended in Hebei province, and 150 kg N ha~l removed in grain, if the straw is returned then the soil N should be maintained. In this work, application of 150 kg N ha~l produced the best yield, with recovery of applied N in plant and soil of around 32%.

The recovery of ^N from ammonium bicarbonate, when applied only at sowing, was significantly higher than when applied at jointing and significantly higher than when one third was applied at sowing and two thirds at jointing. These data indicate that ammorium bicarbonate is particularly useful as a basal fertilizer.

When urea was applied at 150 kg N ha"', N recoveries were relatively high, 42%, 36% and 33%, when applied only at sowing, only at jointing, or one third at sowing and the rest at jointing, respectively. With 225 kg N ha"1, 35.5%, 34% and 33% of N was recovered when applied only at sowing, only at jointing, or one third applied at sowing and two third at jointing, respectively.

184 TABLE XVI. SOIL WATER CONTENT WITH RELATION TO IRRIGATION (N0.0)

Irrigation date Sampling date Soil profile Water content prior to sampling (cm) (% vol.) 1995-10-05 0-15 15.64 16-30 16.70 31-50 16.98 51-70 15.87 71-100 15.65 1995-12-01 1996-12-08 0-15 11.41 16-30 12.12 31-50 14.37 51-70 15.54 71-100 17.47 1995-12-01 1996-03-19 0-15 13.06 16-30 14.87 31-50 11.88 51-70 12.27 71-100 18.53 1996-04-01 1996-04-09 0-15 10.86 16-30 11.71 31-50 11.89 51-70 11.71 71-100 12.35 1996-04-24 1996-05-14 0-15 7.22 16-30 8.44 31-50 9.55 51-70 9.99 71-100 9.96 1996-05-28 1996-06-13 0-15 " 13.92 16-30 12.11 31-50 10.58 51-70 10.69 71-100 11.86

3.4. Fertilizer-N balance

With increases of N-application rate, the fertilizer N remaining in the soil, 0 to 1053m, also increased (Table XII). Also, as N application increased, the amount of unaccounted-for N also increased.

The residual N in soil as a fraction of that applied was lower when applied at sowing than when applied only at jointing or both at sowing and at jointing; between the latter two, there was no significant difference.

3.5. Residual N

The yields of maize stover and grain, for the Nj 5 treatment to the wheat, were higher than those for the No.5 and Nj 0 treatments (Table XIII). These data suggest that it is advisable to consider N utilization for current crops and succeeding crops, i.e. the optimum rate and timing of applied fertilizer would be that which maximizes uptake of N by the rotational cropping system as a whole.

When N was applied both at sowing and jointing, about 60 to 70% was recovered in plant and soil (Table VII), of which the latter might be available to the subsequent crop. Part of fertilizer N applied to the wheat was taken up by the maize (Table XIV).

185 TABLE XVII. SOIL WATER CONTENT WITH RELATION TO IRRIGATION (N0.0) Irrigation date Sampling date Soil profile Water content prior to sampling (cm) (% vol.) 1996-10-05 0-15 15.76 16-30 16.65 31-50 14.78 51-70 15.43 71-100 16.32 1996-10-20 1996-12-01 0-15 13.63 16-30 15.53 31-50 14.84 51-70 16.17 71-100 18.69 1996-12-03 1997-03-05 0-15 17.62 16-30 18.78 31-50 18.97 51-70 18.17 71-100 20.42 1996-03-15 1997-04-09 0-15 15.60 16-30 16.01 31-50 16.03 51-70 17.54 71-100 19.75 1996-04-10 1997-05-01 0-15 11.15 16-30 13.02 31-50 15.50 51-70 16.36 71-100 17.54 1995-05-24 1997-06-13 0-15 13.91 16-30 14.22 31-50 13.99 51-70 13.52 71-100 13.96

3.6. Nitrate pollution

3.6.1. Water movement

The winter wheat depended mainly on irrigation due to little rain from October to June (about 350 mm). At sowing, soil-water content was around 16%, and not significantly different between upper and lower layers. For a few days after each irrigation, the water content in the upper sol layers was higher than that in the lower (Tables XV, XVI and XVII). However, eventually the moisture content of deeper soil layers increased and that in upper soil layers decreased. At 3 or 4 weeks after irrigation, the water content deep in the soil generally was much higher than that in the surface layer. These data show that this soil did not retain water for long periods.

3.6.2. Nitrate movement

With N application rates of 50, 75 and 100 kg N ha"l, the soil-nitrate contents at harvest were lower than at sowing, but with rates of 150 and 225 kg N ha~l, the nitrate content at harvest was higher than at sowing (Tables XVIII, XIX and XX).

186 TABLE XVIII. SOIL NITRATE CONTENT AT SOWING AND AT THE END OF WINTER WHEAT (1994/95) Soil profile Nitrate content Sampling time Fertilization Code (cm) (mg kg"1) At sowing 0-15 10.35 (1994-10-05) 16-30 9.52 31-50 11.12 51-70 4.73 71-100 3.67 At the end of winter NO.O 0-20 5.70 wheat (1995-06-15) 21-40 4.79 41-60 1.45 61-80 3.23 N0.5 0-20 9.68 21-40 7.89 41-60 5.19 61-80 1.87 Nl.O 0-20 5.12 21-40 5.72 41-60 9.37 61-80 15.12 N1.5 0-20 14.28 21-40 26.24 41-60 31.50 61-80 29.25

TABLE XIX. SOIL NITRATE CONTENT AT SOWING AND AT THE END OF WINTER WHEAT (1995/96)

Soil profile Nitrate content Sampling time Fertilization Code (cm) (mg kg"1) At sowing 0-15 11.53 16-30 9.34 (1995-10-05) 31-50 12.09 51-70 6.78 71-100 7.35 At the end of winter NO.O 0-15 3.53 16-30 2.76 wheat (1996-06-13) 31-50 1.05 51-70 0.89 71-100 0.54 N0.5 0-15 3.13 16-30 3.49 31-50 2.20 51-70 1.48 71-100 0.59 Nl.O 0-15 8.78 16-30 8.84 31-50 4.85 51-70 2.41 71-100 1.36 N1.5 0-15 5.88 16-30 10.94 31-50 3.10 51-70 0.79 71-100 0.66

187 TABLE XX. SOIL NITRATE CONTENT AT SOWING AND AT THE END OF WINTER WHEAT (1996/97)

Soil profile Nitrate content Sampling time Fertilization Code (cm) (mgkg-1) At sowing 0-15 10.87 16-30 10.23 (1996-10-05) 31-50 9.09 51-70 10.89 71-100 5.67 At the end of winter NO.O 0-15 4.04 16-30 1.11 wheat (1997-06-13) 31-50 1.11 51-70 1.10 71-100 0.74 N0.5 0-15 1.45 16-30 2.57 31-50 0.72 51-70 0.72 71-100 0.75 0-15 9.41 N1.0 16-30 5.06 31-50 2.86 51-70 2.14 71-100 12.50 0-15 10.72 N1.5 16-30 15.93 31-50 12.58 51-70 11.10 71-100 10.75

TABLE XXI. SOIL NITRATE CONTENT AFTER FERTILIZATION (1994/95) Soil profile Nitrate content (mg kg"l) Fertilization Code (cm) 1995-04-26 1995-05-20 NO.O 0-20 2.73 6.73 21-40 2.10 4.56 41-60 1.10 1.23 61-80 3.26 2.45 N0.5 0-20 9.95 10.76 21-40 5.93 8.67 41-60 3.93 6.57 61-80 3.90 3.78 N1.0 0-20 14.70 6.54 21-40 11.60 8.43 41-60 4.37 9.35 61-80 5.20 15.78 N1.5 0-20 44.90 15.34 21-40 20.30 24.90 41-60 12.31 28.65 61-80 9.03 30.38 - fertilization date was 1995-04-10

188 TABLE XXII. SOIL NITRATE CONTENT AFTER FERTILIZATION (1995/96) Soil profile Nitrate content (mg kg-1) Fertilization Code (cm) 1996-04-23 1996-05-14 0-15 2.16 1.23 NO.O 16-30 1.53 0.96 31-50 0.66 0.67 51-70 0.43 0.37 71-100 0.70 0.61 0-15 7.76 6.59 N0.5 16-30 4.76 4.41 31-50 1.63 3.26 51-70 0.67 1.99 71-100 0.83 1.92 0-15 16.20 30.06 N1.0 16-30 9.53 21.06 31-50 3.77 10.39 51-70 1.90 10.03 71-100 1.60 9.85 0-15 26.00 30.68 N1.5 16-30 12.53 16.39 31-50 6.53 10.51 51-70 1.90 9.43 71-100 1.96 9.28 - fertilization date was 1996-04-05

TABLE XXIII. SOIL NITRATE CONTENT AFTER FERTILIZATION (1996/97) Soil profile Nitrate content (mg kg" 1) Fertilization Code (cm) 1997-04-09 1997-05-01 0-15 4.46 9.33 NO.O 16-30 2.98 6.57 31-50 1.87 11.18 51-70 1.51 7.89 71-100 1.54 2.66 0-15 2.24 8.17 N0.5 16-30 1.12 8.18 31-50 0.75 12.34 51-70 0 17.29 71-100 0 11.82 0-15 3.70 23.87 N1.0 16-30 2.97 23.04 31-50 2.23 9.02 51-70 2.25 7.89 71-100 1.90 7.27 0-15 3.34 23.22 N1.5 16-30 3.35 24.89 31-50 2.61 31.45 51-70 1.49 21.05 71-100 1.16 31.08 - fertilization date was 1997-04-10

189 A few days after N fertilization, the soil-nitrate concentrations increased in all layers commensurately with application rate, with more in upper layers than in lower (Table XXI, XXII and XXIII). But by three or four weeks after N application, the nitrate concentration lower in the soil was much higher, particularly with the higher N-application rates. Because this soil retained water poorly, the dissolved nitrate percolated to lower levels relatively easily, therefore increases in Napplication rate must exacerbate the potential for nitrate pollution of groundwater.

4. CONCLUSION

— Under different N-application rates, the straw and grain yields of winter wheat varied significantly. Of application rates of 50, 75, 100, 150 and 225 kgN ha~*, the highest grain yield was obtained with 150 kg N ha" 1, — Considering grain yield, fertilizer-N recovery, and soil-N balance, when ammonium bicarbonate was used as both basal and top-dressing fertilizer for irrigated winter wheat in areas such as Hebei province, 150 kg N ha'l emerged as the optimum rate for future recommendation, — When ammonium bicarbonate was applied only at sowing, N recovery was significantly higher than when applied only or partly at jointing, therefore ammonium bicarbonate is best used as a basal fertilizer, — Water may move readily from upper to lower soil layers, therefore increases of N-application rate pose significant risk of nitrate pollution of groundwater.

REFERENCES

[1] XI, Z.B., et al., Agrochemical characteristics of ammonium bicarbonate, Pedologica Sinica22 (1985)223-232. [2] PENG, K.M., Agrochemistry, Agricultural Press, Beijing (1979) 73 pp. [3] ZHAO, Z.D., Characteristics of ammonium bicarbonate and its application, Soil 4 (1974) 174— 176. [4] LE, H.Z., et al., Thermodynamic analysis of decomposition reaction of ammonium bicarbonate, Soil Fert. 3 (1980)40-41. [5] GU, E.S., et al., Transformation of ammonium bicarbonate applied to soil and its application method, Soil Fert. 5 (1986) 46-48. [6] LIU, Y.C., et al., Yield increase efficiency of mixed granule fertilizer with ammonium bicarbonate for rice, Soil 23 (1991) 316-318. [7] YANG, Z.Q., et al., Relationship of available phosphorous and nitrogen when calcium monophosphate mixed with ammonium bicarbonate, Soil Fert.3 (1983) 18-21. [8] CAO, Y.H., et al., Study on long-term effective ammonium bicarbonate, Pedologica Sinica 17 (1980) 133-143 [9] YU, X.L., et al., Crop Agronomy, Agricultural Press, Beijing (1980) 22 pp.

190