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FERTILIZER MANAGEMENT

Long-Term Fertilization Effects on Crop Yield and Nitrate Nitrogen Accumulation in Soil in Northwestern China

^a The Key Lab. of Arid and Grassl. Agroecol., Lanzhou Univ., Ministry of Educ., Lanzhou, 730000, P.R. China
^b Inst. of Soil and Fert., Gansu Acad. of Agric. Sci., Lanzhou 730070, P.R. China
^c Agric. and Agri-Food Canada, Research Farm, P.O. Box 1240, Melfort, SK, Canada S0E 1A0
^d Gansu Agric. Univ., Lanzhou 730070, P.R. China
^e Inst. of Agric. Sci. of Zhangye Prefecture, Zhangye 734000, P.R. China

^* Corresponding author (fmli{at}lzu.edu.cn).

Received for publication July 22, 2003.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A long-term (1982 to 2000) field experiment was conducted at Zhangye, Gansu, China, on a sandy clay loam (Typic Anthrosol) under wheat (Triticum aestivum L.)–wheat–corn (Zea mays L.) rotation to determine the effects of N, P, and K chemical fertilizers and farmyard manure (M) on grain and straw yield, harvest index (HI), protein concentration, and N uptake in grain and straw and accumulation of nitrate N (NO₃–N) in the soil profile (0–180 cm). The eight treatments from various combinations of fertilizers and M were check, N, NP, NPK, M, MN, MNP, and MNPK. Mean grain yield decreased in the order of MNPK MNP > NPK > MN > NP > M > N > check (i.e., 8.01, 8.00, 7.51, 7.28, 7.00, 5.50, 4.89, and 3.43 Mg ha^–1, respectively). Yield response to applied N and P increased with time since yields in the check plots declined with time. Potassium fertilizer application provided no, slight, and dramatic increase in grain yield during the initial 6 yr, next 5 yr, and last 8 yr, respectively. Response of straw yield to fertilizers and M was similar to the grain yield. Mean HI increased with fertilizers in no-M treatments for both crops. Crude protein concentration and N uptake in grain and straw increased markedly with fertilizers, and M increased it further. Fertilizers (N, NP, and NPK) led to NO₃–N accumulation in most subsoil layers. Combined applications of fertilizers and M reduced soil NO₃–N accumulation in soil compared with fertilizers alone. In conclusion, the findings suggest that it is important to use balanced application of chemical fertilizers and M at proper rates in order to protect soil and underground water from potential NO₃–N pollution while also sustaining high crop production.

Abbreviations: HI, harvest index • M, manure

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
SOIL MANAGEMENT PRACTICES, such as tillage, irrigation, soil-mulching cultivation, and fertilization, usually result in crop yield increases (Burgess et al., 1996; Rasmussen et al., 1997; Li et al., 1999, 2001). Among these agronomic measures, fertilization may be the most important way to maintain high crop productivity and soil quality (Suo and Wang, 2000; Shen, 2002). The contribution of fertilization has reached 50 to 60% of the total increase in grain yields in China (Lu and Shi, 1998).

Continued increases in agricultural production would require increased supply of irrigation water as well as fertilizers. However, some studies have shown that continued use of inorganic fertilizers may result in destruction of soil structure and diminishing of quality and productive capacity of soil (Lai et al., 1992; Doran et al., 1996). Other studies have indicated negative (Jin and Ma, 1996; Zhang et al., 2002), positive (Belay et al., 2002), and no noticeable (Lu et al., 2001) effects on soil productivity. In most long-term experiments, combination of inorganic fertilizers and M has generally given the best yields in many parts of the world (Lin et al., 1996; Wang et al., 2002).

Statistics showed that N fertilizer consumption is growing rapidly in the developing countries (Chalk et al., 2003). This has resulted in serious environmental consequences, such as NO₃ leaching and eutrophication (Zhu, 1995; Ferguson et al., 1996; Matson et al., 1998). Overfertilization can cause water, air, and soil pollution (Singh et al., 1995; Zhang et al., 1996; Carpenter et al., 1998; Mosier and Kroeze, 2000). Nitrate N leaching is a major problem for calcareous soils in which ammonium (NH₄) is quickly nitrified to NO₃ (Tong et al., 1997). It has become a major concern worldwide, mainly due to increased N fertilizers and farmyard M inputs (Yuan et al., 2000; Di and Cameron, 2002; Zhu and Chen, 2002). The problems of NO₃ leaching and the contamination of ground and surface waters exist widely in Europe, USA, China, and elsewhere (Spalding and Exner, 1993; Zhang et al., 1996). In northern China, analysis of ground water and drinking water showed that 50% of the samples had NO₃ content above the critical value (Zhang et al., 1995); and it was 21.5 and 29.7% for the Shaanxi and Guandong Provinces of China, respectively (Lu and Shi, 1998). Guillard et al. (1995) reported an increase of NO₃–N in soil with increasing rate of N fertilizer. Nitrogen fertilizer applied in excess of crop needs can cause accumulation of NO₃–N in the soil profile (Malhi et al., 1991, 2002).

The long-term approach to minimize movement of NO₃ into groundwater is to develop site-specific improved N fertilization and irrigation management practices to increase N uptake efficiency, decrease N loss, and minimize leaching losses below the root zone. The objective of this study was to determine the effects of chemical fertilizers and farmyard M on grain and straw yield, HI, protein concentration, N uptake in grain and straw, and accumulation of NO₃–N in the soil profile.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A field experiment was conducted from 1982 to 2000 on a calcareous desert soil (sandy clay loam, mixed, Typic Anthrosol) near Zhangye (38°36' N, 100°18' E; 1560 m altitude) in Hexi Corridor of Gansu Province in northwestern China. This region is one of the main grain production areas in China. Mean annual precipitation is 136 mm, and mean temperature is 7.0°C. Annual potential evaporation is about 1990 mm. Annual frost-free period is about 165 d, and approximately 50% of the annual precipitation is received during July to September. At the experimental site, mean annual precipitation during the study period was 127 mm (range of 72 mm in 1985 to 214 mm in 1983), and on average, 53% (range of 36% in 1987 to 85% in 1995) of the annual precipitation was received in July to September. Mean annual air temperature at this site ranged from 6.3 to 9.5°C. Mean monthly temperatures ranged from 19.8 to 23.2°C for July, 18.9 to 21.8°C for August, and 13.3 to 17.5°C for September. Soil test results from the experimental site in 1982 were 8.4 pH, 20.8 g kg^–1 organic matter, 28.1 mg kg^–1 available N (NO₃–N + NH₄–N), 21.7 mg kg^–1 extractable P (0.5 M NaHCO₃), and 99.1 mg kg^–1 exchangeable K (1 M NH₄CH₃COO).

Twenty-four plots (each plot 6.6 by 5.0 m in size) in a split-plot factorial included eight treatments and three replications. Adjacent plots were separated by ridges (50-cm width), and the blocks were separated by irrigation furrows (60-cm width) and two ridges (50-cm width). Main-plot treatments were with M and without M, and subplot treatments consisted of unfertilized check, N, NP, and NPK. The eight treatments were check, N, NP, NPK, M, MN, MNP, and MNPK. The inorganic fertilizer sources of N, P, and K, respectively, were ammonium nitrate or urea, single superphosphate or diammonium phosphate, and potassium chloride commercial fertilizers. The majority of farmyard M was swine M, which is extensively used by the local farmers. The nutrient analyses of M varied from year to year, but on the average, farmyard M contained 29.1 g kg^–1 organic matter, 1.5 g kg^–1 total N, 0.88 g kg^–1 total P, 34 g kg^–1 total K, 163.7 mg kg^–1 available N (NH₄–N + NO₃–N), 60.3 mg kg^–1 extractable P, and 1293 mg kg^–1 exchangeable K.

Nitrogen fertilizer was applied at 120 and 150 kg N ha^–1 to wheat during 1982–1990 and 1991–2000, respectively, and at 240, 300, and 450 kg N ha^–1 to corn during 1982–1990, 1991–1999, and 2000, respectively. Half of N fertilizer was applied to wheat as base fertilizer 1 d before sowing, and the other half was broadcast at three-leaf stage. For corn, 30, 30, and 40% of the N fertilizer was applied at sowing, jointing/elongation, and 10- to 12-leaf (pretasseling) stages, respectively. Rates of P and K fertilizers, respectively, were 26 to 33 kg P ha^–1 and 50 to 63 kg K ha^–1 for wheat and 53 to 99 kg P ha^–1 and 101 to 189 kg K ha^–1 for corn. Farmyard M (with average moisture content of 15%) was applied at 60 and 75 t ha^–1 to both wheat and corn during 1982–1990 (8.4 kg ha^–1 available N, 77.0 kg ha^–1 total N, 3.6 kg ha^–1 available P, 52.5 kg ha^–1 total P, 77.1 kg ha^–1 available K, and 173.4 kg ha^–1 total K) and 1991–2000 (10.4 kg ha^–1 available N, 96.2 kg ha^–1 total N, 4.5 kg ha^–1 available P, 65.6 kg ha^–1 total P, 96.4 kg ha^–1 available K, and 216.7 kg ha^–1 total K), respectively. All of the M, P, and K fertilizer were applied before sowing as basal dressing (i.e., mixed evenly to a depth of 15 cm with plowing).

The experiment rotation was wheat–wheat–corn. The wheat cultivars were Zhangchun no. 9, Zhangchun no. 11, and Zhangchun 3131-1 in the first 10 yr (1982–1991) and Zhangchun no. 20 and Zhangchun 920 in the later 9 yr (1992–2000). The cultivar of corn was Hudan no. 1, Yedan 476, and Hudan no. 1 in 1984, 1987, and 1990, and Zhongdan no. 2, Shendan no. 10, and Zhongdan no.16 in 1993, 1997, and 2000. Spring wheat was seeded in mid-March at a rate of 300 kg ha^–1, and corn was planted at a rate of 75000 seeds ha^–1 in mid- to late April. The row spacing was 15 cm for wheat and 50 cm for corn. All plots were irrigated three to four times for wheat and six to seven times for corn during each growing season. The irrigation was implemented when soil moisture was depleted to 45% of field capacity. The amount of each irrigation was 53 to 67 mm of water.

Grain and straw yields of wheat and corn were determined by hand harvesting of plants from central rows (14 rows harvested for wheat and six rows harvested for corn). Nitrogen use efficiency was expressed as kg grain kg^–1 applied N ha^–1. Harvest index was calculated by dividing grain yield by grain plus straw yield. Grain and straw samples were dried at 55°C, ground to pass a 0.3-mm sieve, and analyzed for total N–Kjeldahl N (Bremner, 1965a) to determine the concentration of crude protein. Protein (crude) concentration was calculated by multiplying total N concentration by 5.70 for wheat grain and by 6.25 for wheat straw, corn grain, and corn straw. Uptake of N in grain (or straw) was calculated by multiplying yield of grain (or straw) by percentage N in grain (or straw). After corn harvest in 2000, soil samples from each plot (five composite samples or cores per plot) were collected from the 0- to 20-, 20- to 60-, 60- to 100-, 100- to 140-, and 140- to 180-cm soil depths. The soil samples were air-dried, ground to pass a 2-mm sieve, and analyzed for NO₃–N using Cd reduction method (Bremner, 1965b). The data were subjected to analysis of variance (SAS Inst., 1989). Differences among treatments were determined with least significant difference (LSD) at the 0.05 probability level. Cumulative yields were regressed using linear regressions.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Grain Yield, Straw Yield, and Harvest Index
The cumulative grain yields over the 19-yr period showed generally consistent trends of increased responses to chemical fertilizers and M with time (Fig. 1 and 2) . The linear correlations between cumulative grain yields and years were highly significant. This indicated that the yield responses to applied fertilizers and M had similar patterns over years. The differences between linear graphs from various chemical fertilizers and M treatments increased as experiment progressed. The cumulative grain yield was 65.17, 92.84, 132.90, 142.67, 104.46, 138.30, 151.93, and 152.24 Mg ha^–1, respectively, for the check, N, NP, NPK, M, MN, MNP, and MNPK. The mean cumulative grain yield was 28.36 Mg ha^–1 greater for the M plots than the no-M plots (Fig. 2). Manure-alone treatment (M) produced 73% grain yield of that with NPK.

View larger version (31K): [in this window] [in a new window]   Fig. 1. Regressions of cumulative grain yield (kg ha–1) of crops with years in wheat–wheat–corn rotation from 1982–2000, treated annually with various combinations of N, P, and K fertilizers and unfertilized check, without and with manure (M) on a calcareous sandy loam soil under irrigation near Zhangye, Gansu, China (LSD0.05 for differences between all treatments is 4.451).

View larger version (17K): [in this window] [in a new window]   Fig. 2. Regressions of mean cumulative grain yield (kg ha–1) of crops with years in wheat–wheat–corn rotation from 1982–2000, treated annually with various combinations of N, P, and K fertilizers and unfertilized check, without manure (UM) and with manure (M) on a calcareous sandy loam soil under irrigation near Zhangye, Gansu, China (LSD0.05 for differences between main-plot treatments is 3.420).

The order of cumulative or average grain yield was MNPK MNP > NPK > MN > NP > M > N > check. This showed that the highest crop yield in the present study was obtained when both chemical fertilizers and M were applied together. Our results agree with the findings of other researchers who also attained the best crop yield with combined applications of chemical fertilizers and M (Lal and Mathur, 1989; Kabeerathumma et al., 1993; Lin et al., 1996; Vats et al., 2001; Wang et al., 2002). This is most likely because M improves soil physical properties (Lal and Mathur, 1989; Kurual and Tripathi, 1990) and provides stable supply of both macro- and micronutrients (Kabeerathumma et al., 1993) and thus supports the maximum yield.

The lowest grain yield was obtained in no-fertilizer check plots. As the experiment progressed, grain yields in the check treatment generally decreased. Grain yield of corn in the check in 2000 (2.615 Mg ha^–1) was only 28.2% of that measured in 1984 (9.260 Mg ha^–1), and wheat grain yield in 1999 (1.610 Mg ha^–1) was 33.7% of that attained in 1982 (4.780 Mg ha^–1). This indicates that soil productivity decreased with time, most likely due to depletion in soil fertility (Kumar et al., 2000).

For both the M and no-M treatments, the yield response to applied N increased with years since the yield of the check plots decreased significantly during the experimental period, as earlier observed by Jin and Ma (1996). However, grain yield decreased in the N treatment for both crops as the experiment progressed. Also, the positive effect of P, K, and M applications became more evident in later years of the experiment, clearly showing that N alone was unable to maintain crop productivity and soil quality to sustain high crop yield.

Addition of P fertilizer increased grain yield, and the yield response to applied P for both with- and without-M treatments increased substantially with time. This suggests that the soil became deficient in available P with time, as evidenced by decline in extractable P in the soil from 22 mg P kg^–1 in 1982 to 4 mg P kg^–1 in 2000 in the N treatment. In another long-term experiment on a calcareous soil in northeast China, crops did not respond to P fertilizer during the first 3 yr, but significant yield response was obtained in later years (Zhang et al., 2000). The response of grain yield to P fertilization was relatively less in M treatments. This was most probably due to that M contained P which became available to crops (Vats et al., 2001). In addition to check, grain yield in the N and NP treatments also decreased dramatically in the last 7 yr. This suggests that soil became deficient in nutrients other than N and P with time and N and P fertilizers were not enough to sustain high grain yields.

There was no grain yield response by either crop to K fertilizer application in the first 6 yr from 1982–1987. From 1988–1993, grain yield tended to be greater with NPK compared with NP in no-M treatments only. From 1993–2000, grain yield of both M and no-M treatments generally increased significantly with K fertilization. Until 1993, the maximum grain yield was attained with the MNP treatment. From 1994–2000, the maximum grain yield was attained when K fertilizer was also applied. Extractable K in the soil decreased from 99 mg K kg^–1 in 1982 to 56 mg K kg^–1 in 2000 in the NP fertilizer treatment. Thus, the grain yield difference between NPK and NP treatment became evident after 6 yr without M and after four rotations with M. This indicated that without K fertilizer application, inherently K-rich calcareous desert soil in Gansu would become deficient in K after 6 and 12 yr, without and with M application, respectively. Wang et al. (2003) found decrease in the K-supplying capacity of soil with zero-K application for 11 yr. Our findings suggest that producers, who are using high rates of N and P fertilizers, must pay attention to the K fertility of soils for sustainable crop production in these K-rich soils. For example, M can provide a stable supply of K to crops under intensive cropping systems (Kumar et al., 2000).

Various fertilizers affected grain yields differently in various rotation periods (data not shown). The grain yield–increasing effect of N fertilization apparently enlarged as the experiment progressed. Application of N fertilizer increased grain yield by 13.2 to 21.1% in the first to third rotation periods and by 65.9 and 95.2% in the fourth and fifth rotation periods. In the first 6 yr (1982–1987), grain yield showed moderate response to M application (Fig. 2). From 1987–2000, grain yield of both wheat and corn increased markedly with M application. The application of M increased grain yield by 5.0 to 5.3% in the first two rotation periods and by 19.5 to 58.5% in the following four rotation periods. This showed that M had cumulative effect during the duration of the experiment. This is in agreement with the previous findings in the black soil in northeastern China (Zhang et al., 2000; Liu et al., 2001). Furthermore, cumulative residual effect of M in the present study was greater than that of 9.8, 7.6, and 7.0%, respectively, in the no-fertilizer, N-alone, and NP treatments of a 13-yr experiment reported by Liu et al. (2001). This was probably related to the difference in organic matter content in the two soils, which was much higher (54.0 g kg^–1) in the black soil (Liu et al., 2001) than that in the calcareous desert soil of the present study (29.1 g kg^–1).

The average of data from all years showed that application of N increased crop yields by 42.4 and 32.4% in the no-M and M treatments, respectively. In the presence of N fertilizer, P application increased grain yield by 43.2 and 9.9% in the no-M and M treatments, respectively. The K fertilization increased grain yield by 7.4% when no M was used and by only 2.7% when M was used. The reduced yield response to applied fertilizers with time in the M treatments compared with no-M treatments was most likely due to availability of these nutrients to crop plants in the growing season in M plots. Based on the 19-yr data (13 seasons of wheat and six seasons of corn), grain yield increase from different fertilizers followed an order of N > M > P > K, with the average grain yield increase over check of 36.2, 26.2, 23.3, and 3.6%, respectively.

Straw yield was determined in 10 of the 19 yr from 1988 to 1997. The response of straw yields to fertilizer and M applications was similar to the grain yields, and thus straw yields are not presented here. On average, straw yield for wheat was 3.47, 5.64, 8.60, 9.22, 5.56, 8.98, 10.36 and 10.43 Mg ha^–1, respectively, for the check, N, NP, NPK, M, MN, MNP, and MNPK. The corresponding straw yield for corn was 6.68, 9.88, 14.62, 15.50, 12.50, 15.30, 16.93, and 17.66 Mg ha^–1. The mean straw yield for no-M and M treatments, respectively, was 6.73 and 8.84 Mg ha^–1 for wheat and 11.67 and 15.59 Mg ha^–1 for corn.

Harvest index (ratio of grain yield to grain + straw yield) was determined in 10 of the 19 yr (1988 to 1997), in which both grain and straw yields were recorded. Mean HI increased with application of chemical fertilizers in no-M plots for both crops (Fig. 3) . For wheat, M-only plots had greater HI than that of check, and there was no further increase in HI from chemical fertilizers. For corn, M-only treatment did not increase HI compared with check, but application of chemical fertilizers increased HI. This was probably due to higher N requirements of corn than that of wheat for the maximum yield. The mean HI for wheat was 0.36, 0.38, 0.40, 0.41, 0.41, 0.40, 0.39, and 0.39 for check, N, NP, NPK, M, MN, MNP, and MNPK, respectively. In the same order, the HI values for corn were 0.40, 0.43, 0.43, 0.44, 0.40, 0.43, 0.44, and 0.43. In the first 7 yr (1988–1994), there was usually no increase in HI with application of chemical fertilizers, except in 1990 for corn (data not shown). Application of M increased HI in 3 of 7 yr in the check treatment, but application of M in combination with chemical fertilizers did not cause any further increase in HI. In the last 3 yr (1995 to 1997), HI increased with application of chemical fertilizers in both without- and with-M treatments. Application of M-only increased HI over check plot in all 3 yr, but M had no effect on HI in the plots receiving chemical fertilizers. For the average of HI in different rotation periods, chemical fertilizers and/or M did not show any increase of HI in the 1988–1990 and 1991–1993 rotation periods. But in 1994–1997, HI increased from 0.379 in the check to 0.42 in N, 0.43 in NP, and 0.46 in NPK treatment. Application of M alone increased HI to 0.42 in the M treatment. This indicates that M has long-term cumulative residual effect on grain yield (Zhang et al., 2000) and subsequently increased HI in the present study.

View larger version (59K): [in this window] [in a new window]   Fig. 3. Harvest index (ratio of grain yield to grain + straw yield) of crops from 1988–1997, treated annually with various combinations of N, P, and K fertilizers and unfertilized check, without and with manure on a calcareous sandy loam soil under irrigation near Zhangye, Gansu, China (LSD0.05 for differences between main-plot treatments is 0.005 for wheat and 0.007 for corn; LSD0.05 for differences among subplot treatments is 0.018 for wheat and 0.012 for corn).

View larger version (54K): [in this window] [in a new window]   Fig. 4. Concentration of protein (g protein kg–1) in grain of crops from 1988–1997, treated annually with various combinations of N, P, and K fertilizers and unfertilized check, without and with manure on a calcareous sandy loam soil under irrigation near Zhangye, Gansu, China (LSD0.05 for differences between main-plot treatments is 2.8 for wheat and 1.9 for corn; LSD0.05 for differences among subplot treatments is 9.3 for wheat and 11.0 for corn).

Uptake of Nitrogen and Recovery of Applied Nitrogen in Grain and Straw
The N uptake increased considerably in grain and straw with chemical fertilizers in all years, and M treatments increased it further, but the results on N uptake in grain + straw are only presented in this report (Fig. 5) . The total N uptake in wheat grain + straw was 48.8, 95.6, 149.6, 155.9, 95.5, 194.8, 220.0, and 212.0 kg N ha^–1, respectively, for the check, N, NP, NPK, M, MN, MNP, and MNPK treatments. For corn, the corresponding N uptake in grain + straw was 103.6, 212.9, 253.3, 281.8, 176.5, 275.2, 299.7, and 268.2 kg N ha^–1. Mean N uptake in grain + straw was greater in M plots than in no-M plots (180.6 vs. 112.5 kg N ha^–1 for wheat and 254.9 vs. 212.9 kg N ha^–1 for corn). Other researchers have also reported significant improvement in nutrient uptake by crop when farmyard M was applied in addition to chemical fertilizers (Lal and Mathur, 1989). The relative differences among fertilized treatments were greater for crop N uptake than for yield due to increase in both yield and concentration of N in grain and straw with fertilizer application. In both M and no-M treatments, N uptake in grain and straw was greater with N and NP compared with check. Addition of K to NP increased N uptake slightly in grain or straw in the no-M plots, but in the M plots, there was no increase in N uptake in grain or straw (in fact, there was a slight decrease in N uptake). This was most likely due to decline in total N concentration in grain and straw, probably because of dilution effect from the increased yield with K fertilizer.

View larger version (49K): [in this window] [in a new window]   Fig. 5. Total N uptake (kg N ha–1) in grain + straw of crops from 1988–1997, treated annually with various combinations of N, P, and K fertilizers and unfertilized check, without and with manure on a calcareous sandy loam soil under irrigation near Zhangye, Gansu, China (LSD0.05 for differences between main-plot treatments is 16.9 for wheat and 18.5 for corn; LSD0.05 for differences among subplot treatments is 17.6 for wheat and 12.8 for corn).

Without M, the recovery of applied chemical fertilizer N in grain was increased considerably with application of P fertilizer in combination with N, and it was increased further when K was also applied along with NP (data not shown). With M, the N recovery from application of P and K fertilizers in combination with N increased slightly for wheat but decreased for corn. Mean recovery of applied fertilizer N in wheat grain was 26.8, 53.6, 57.5, 43.8, 46.2, and 49.1%, respectively, for the N, NP, NPK, MN, MNP, and MNPK treatments. For corn, the corresponding values for N recovery in grain were 21.1, 35.3, 41.6, 21.6, 24.3, and 17.9%. The mean recoveries of applied N in grain in no-M and M plots were similar for wheat (46.0 vs. 46.4%), but for corn, the N recovery was greater in the no-M treatment than in the M treatment (32.7 vs. 21.3%). The recovery of applied N in straw showed a trend similar to grain. The average recovery of applied N in wheat straw was 7.6, 20.5, 21.3, 29.2, 45.4, and 36.6%, respectively, for the N, NP, NPK, MN, MNP, and MNPK treatments. The corresponding values for N recovery in corn straw were 16.0, 15.4, 18.8, 11.9, 17.5, and 13.2%. Unlike grain, mean recoveries of applied N in straw were less in no-M treatment than in the M treatment for wheat (16.5 vs. 37.1%), but for corn, the N recoveries in straw were similar in no-M and M treatments (16.7 vs. 14.2%).

Nitrate Nitrogen in Soil Profile
Mass of NO₃–N in soil profile was significantly influenced by annual applications of various fertilizers and M for 19 yr (Fig. 6) . Sole application of N fertilizer increased NO₃–N in the soil profile only slightly compared with the check treatment. Earlier, Campbell et al. (1994) found that fertilized wheat had more NO₃–N accumulation in the 0- to 120-cm soil depth than that of the unfertilized wheat. However, in present study, application of N in combination with P and/or with K fertilizers (NP and NPK) had markedly greater NO₃–N accumulation in the soil profile compared with N alone. This is not consistent with earlier results that higher amounts of NO₃–N accumulated in the N and NP treatments compared with 100% NPK and the NO₃–N accumulation was inversely related to cumulative N uptake by the crops (Benbi and Biswas, 1997). This displayed that in spite of much lower crop yields with N alone than NP or NPK, there was no buildup of NO₃–N in soil in the N treatment and substantial buildup of NO₃–N in soil in the NP or NPK treatments. The findings of another study showed that the loss of NO₃–N under arable soil system ranged from 4 to 107 kg N ha^–1, with an average value of 37.8 kg N ha^–1 when N was applied no more than 200 kg N ha^–1 yr^–1 (Di and Cameron, 2002). Because of low yields resulting in low uptake of N, buildup of NO₃–N was expected in the N alone treatment, but this did not happen in the present study. The possible reason may be that the applied N transformed into N₂, NH₃, or other organic N form lost from the soil–plant system. This suggests that further research is needed to determine the fate of applied N in the future.

View larger version (40K): [in this window] [in a new window]   Fig. 6. Nitrate N mass (kg N ha–1) in different soil layers after 19 annual applications (1982–2000) of various combinations of N, P, and K fertilizers and unfertilized check, without and with manure on a calcareous sandy loam soil under irrigation near Zhangye, Gansu, China (for the 0- to 20-, 20- to 60-, 60- to 100-, 100- to 140-, and 140- to 180-cm depths, respectively, the LSD0.05 for differences between main-plot treatments is 0.6, 5.7, 8.1, 7.2, and 8.9, and the LSD0.05 for differences among subplot treatments is 1.5, 7.7, 9.6, 4.9, and 7.3).

The NO₃–N mass was less with MNP or MNPK compared with NP or NPK (Fig. 7) . Similar findings have been made in other long-term experiments (Tong et al., 1997; Yang et al., 1998; Laegreid et al., 1999). Nitrate N accumulation in soil caused by M happened in only the MN treatment. This suggests that when M was applied together with N and P and/or with K fertilizers (MNP and MNPK), it decreased NO₃–N accumulation in soil significantly.

View larger version (43K): [in this window] [in a new window]   Fig. 7. Nitrate N mass (kg N ha–1) in the 0- to 180-cm soil depth after 19 annual applications (1982–2000) of various combinations of N, P, and K fertilizers and unfertilized check, without and with manure on a calcareous sandy loam soil under irrigation near Zhangye, Gansu, China (LSD0.05 for differences between main-plot treatments is 5.3; LSD0.05 for differences among subplot treatments is 13.3).

Total amounts of NO₃–N recovered in the 0- to 180-cm soil depth were 10, 16, 247, 325, 28, 122, 94, and 73 kg N ha^–1 in the check, N, NP, NPK, M, MN, MNP, and MNPK treatments, respectively. This indicated that annual applications of chemical fertilizers and M over a period of 19 yr had a considerable effect on the amounts of NO₃–N accumulated in the soil profile. There was substantial movement of NO₃–N to the depth of 180 cm sampled in this study, especially in plots receiving chemical fertilizers (Fig. 6). Other researchers have also reported increase in concentrations of residual NO₃–N in soil profile at high N rates in various cropping systems (Malhi et al., 1991, 2002; Guillard et al., 1995). Soil NO₃–N below the effective root zone of crops is susceptible to leaching, and the loss of NO₃–N through deep percolation can result in N contamination of groundwater with irrigation and can have a potential risk to groundwater quality and soil health (Zhang et al., 1996; Vasconcelos et al., 1997; Yuan et al., 2000).

The amounts of NO₃–N in soil with and without M treatments indicated that M applied with chemical fertilizers could markedly reduce NO₃–N accumulation in the soil profile while also increasing crop yields. The findings suggest that to reduce the risk of downward movement of NO₃–N out of rooting zone, M should be applied in a combination with N, P, K, and other fertilizer nutrients lacking in the soil. But, it is also necessary to apply M and chemical fertilizers at proper rates to protect soil, underground water, and the atmosphere from potential NO₃–N pollution while also sustaining high crop yields.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
During 19 yr, grain yield in the no-fertilization treatment (check) declined, and yield response to applied fertilizer and M treatments increased with time. The impact of fertilizers on grain yields was N > M > P > K. The K fertilizer showed no grain yield–increasing effect in the initial 6 yr, and its effect on grain yield increased as the experiment progressed. Based on the 19-yr data, the grain yields decreased in the order of MNPK MNP > NPK > MN > NP > M > N > check. Generally, the response of straw yield to fertilizer treatments was similar to grain yield. Mean HI increased with application of chemical fertilizers in no-M plots for both crops, but application of M alone increased HI over check for wheat. Manure alone plots usually had greater HI than check, especially for wheat. Chemical fertilizers and M significantly increased protein concentration and N uptake in grain and straw. From the point of increasing crop yield and quality, MNPK was the best treatment. Fertilization of wheat and corn for 19 successive years had a marked effect on NO₃–N accumulation in the soil profile. Accumulation of NO₃–N in the 20- to 140-cm depth soil profile with application of inorganic fertilizers (NP and NPK) is considered a potential danger for pollution to soil environment as well as the groundwater. Compared with the chemical fertilizers alone, the application of M along with the commercial fertilizers (MN, MNP, and MNPK treatments) reduced residual soil NO₃–N to some extent. The findings suggest that it is necessary to use balanced application of chemical fertilizers and M at proper rates to protect soil and underground water from potential NO₃–N pollution while sustaining high crop production.

	ACKNOWLEDGMENTS

 
This research was partly supported by NKBRSF Project G2000018603, National Important Basic Research Pre-Arrangements Special Project, NSFC (Grant no. 39970184), International Foundation for Science (C/3571-1), and Gansu Foundation for Sciences (QS031-C31-13).

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Contribution from the Key Laboratory of Arid and Grassland Agroecology, Lanzhou University, Ministry of Education, Lanzhou, 730000, P.R. China; Institute of Soil and Fertilizer, Gansu Academy of Agricultural Sciences, Lanzhou 730070, P.R. China; Gansu Agricultural University, Lanzhou 730070, P.R. China; and Institute of Agricultural Sciences of Zhangye Prefecture, Zhangye 734000, P.R. China.

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