Nutrient Cycling in Agroecosystems 63: 139–149, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.
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Nitrogen budgets and environmental capacity in farm systems in a large-scale karst region, southern China Ryusuke Hatano1,2,∗ , Takuro Shinano2 , Zheng Taigen2 , Masahiko Okubo1,2 & Li Zuowei3 1 Field
Science Center for Northern Biosphere, Hokkaido University, Sapporo 060–8589, Japan; 2 Graduate School of Agriculture, Hokkaido University, Sapporo 060–8589, Japan; 3 Science and Technology Department of Gunangxi, Nanning, China; (∗ Corresponding author; e-mail:
[email protected])
Key words: chemical fertilizer, field surplus nitrogen, nitrate leaching, nitrogen budget, nitrogen cycling
Abstract Field surplus nitrogen (N) and farm disposal N are major sources of water pollution in farming systems. These sources are estimated from N budgets in field and whole farms, which are associated with the production and consumption of food. This study was conducted to evaluate these two pollution sources in the steep mountainous karst region of Quibainong, Guangxi Province, southern China. The region is, characterized as an area of upland farms, due to the shallow soils and rapid water drainage through cracks in the limestone. Although field surplus N in 1960 was only 4.1 kg N ha−1 , current field surplus N ranged from 10.1 to 463 kg N ha−1 , with values above 50 kg N ha−1 in farms along roads and less than 40 kg N ha−1 in the farms away from roads. The results obtained in near-road farms were similar to those in a previous study of N budgets in China. There was a significant positive correlation between the field surplus N and N application rate, including when the previous data were incorporated. The proportion of manure to total N application decreased with increase of N application. Chemical fertilizer was applied in greater quantity in economically rich farms. Therefore, the increase of field surplus N in Quibainong may be caused by economic improvement. Although livestock and human excreta were stocked in manure barns, unused excreta N increased with the increase of N excreted. The unused excreta N also increased with the decrease of feed self-sufficiency, but was not related to N application rate. These facts indicate that livestock husbandry in Quibainong is related to economic status of farms, but independently of crop production. The N application rate of more than 160 kg N ha−1 increased field surplus N to an extent greater than crop uptake N, and a N application rate of more than 185 kg N ha−1 increased the potential nitrate-N concentration to more than 10 mg L−1 . Therefore, 160–185 kg N ha−1 is suggested to be the environmental capacity to sustain optimal N cycling in Quibainong. The average value of excreta N produced on near-road farms in Quibainong was 171 kg N ha−1 . If excreta N was used evenly for crop cultivation without chemical fertilizer in whole fields, the optimal N cycling would be maintained. The survey conducted here using a questionnaire was effective in evaluating all kind of N flows in the farming systems. Introduction Human activities have been seriously unbalancing the N cycle and have doubled the N available for plant uptake, which is an environmental issue of major concerns both globally and regionally. Chemical N fertilizer and the cultivation of leguminous crops account for 60 and 25% of total N increased by human activity (Vitousek et al., 1997). According to the FAO
statistics (2001), global chemical N fertilizer use increased from 11 × 109 t N in 1961 to 82 × 109 t N in 1998. In China there was an increase from 0.5 × 109t N in 1961 to 22 × 109 t N in 1998, proportionately the largest increase in the world. Wen and Pimentel (1986) showed that traditional agriculture in China sustained the balance between crop N uptake and N input until the 1950s, whereas the current N input in China is about twice as much as crop
140 N uptake. Ellis and Wang (1997) reported on longterm changes in the rate of N application in the Tai Lake region of southeastern China. From 1000 AD to the early 1950s, the application rate was 70–100 kg N ha−1 , this being supplied as organic manure from human and livestock excreta, green manure crops, crop residues, weeds, water plants and pond sediments. By the late 1950s, the N application rate had doubled, mostly due to increased pig production. From the early 1970s, chemical fertilizer had begun to be used; there was a peak of 800 kg N ha−1 in 1979, followed by the current annual N application rate of 500 kg N ha−1 . Fu and Meng (1994) showed that current N application rate in subtropical paddy area in China ranged from 264 to 805 kg N ha−1 , and the average was 556 kg N ha−1 . Wang and Shao (1994) showed that annual N application rate in various fields in a village, southern China was 150 kg N ha−1 for tea plants, 257 kg N ha−1 for citrus trees, 73 kg N ha−1 for plum trees and 637 kg N ha−1 for rice. The increase of the N application rate resulted in an increased N concentration in aquatic environments. Yan et al. (1998) showed that the total N concentration of Lake Chaohu, southern China, increased from less than 0.1 mg N L−1 in 1976 to more than 4 mg N L−1 in 1991. Dokulil et al. (2000) also reported that more than 90% of 31 large lakes in China were eutrophic, with values of TN >0.6 mg N L−1 and Chl-a >7 µg L−1 in 1992. Chen et al. (2000) showed that the N concentration of the Yangtze River was 5–10 times higher than that in 1960. Zhang et al. (1996) reported that groundwater nitrate-N concentration in an area of 140 000 km2 in northern China exceeds the 11 mg N L−1 allowance limit for drinking. A comprehensive approach to preparing a N budget based on calculation of N flow associated with production and consumption of food, together with biological processes for N transformation, has been developed and used to determine the impact of N cycling in farm, community and regional systems (Matsumoto et al., 1992a–c; Guo and Bradshaw, 1993; Watson and Atkinson, 1999; Zebarth et al., 1999; Matsumoto, 2000; Nagumo and Hatano, 2000). Watson and Atkinson (1999) suggested that ignorance of biological N transformations such as denitrification and ammonia volatilization led to more than a 50% overestimate of field surplus N. Matsumoto et al. (1992a–c) and Matsumoto (2000) showed that the disposal excreta N in a Japanese urban area was considerably larger than field surplus N due to the human diet. Nagumo and Hatano (2000) reported that the annual
disposal N of 2713 kg N ha−1 in another Japanese urban area became a point source of N pollution of stream water through the sewage treatment facility, while field surplus N ranged from 69 to 99 kg N ha−1 y−1 . On the other hand, Guo and Bradshaw (1993), who estimated the annual N budget in a small village in the Lake Tai region, southern China, and showed that all human and livestock excreta was applied to fields together with other manure composed of water plants or mud from fish ponds and chemical fertilizer, and consequently the surplus N was estimated to be 155 kg N ha−1 . However, the surplus N accounted for 69% of N derived from water plants and mud from fishponds, indicating that water plants and phytoplankton play an important role in recycling N discharged from fields. Some recent studies have indicated that the estimated field surplus N is almost equal to the N discharged from the fields (Barry et al., 1993; Goss and Goorahoo, 1995; Hayashi and Hatano, 1999). Hayashi and Hatano (1999) showed that the annual surplus N in an onion field in central Hokkaido, Japan, corresponded to the annual nitrate-N discharge measured in tile drainage. Barry et al. (1993) and Goss and Goorahoo (1995) indicated that the field surplus N provided a good prediction of mean nitrate concentration in soil drainage water. However, the amounts of discharged N measured for the large-scale watersheds were significantly less than the estimated field surplus N, probably due to denitrification and N uptake by trees in the riparian zone (David et al., 1997; Jordan et al. 1997). Therefore, field surplus N may help only in an estimation of potential N leaching. Qibainong in southern China is steep mountainous karst regions, which is characterized by shallow soil depth, rapid water drainage through cracks in limestone, and possesses only upland farming associated with pig production; there are no rivers and no paddy fields. Therefore, N will be directly discharged from fields and pollute groundwater if the N management in Quibainong exceeds the environmental capacity related to N cycling in farm systems. The purpose of this study was to evaluate the influence of agricultural activity on N cycling in the farming systems in Quibainong by the N budget approach using a questionnaire survey.
141 and drinking. Some water storage tanks 3 m in diameter and 3 m in depth, have been established in each community. There is a national road through the center of Quibainong. The 13 farms in a community along the road were surveyed in December 1999. Most farms in the community used a considerable amount of chemical fertilizer. In order to check the influence of the access to the road on chemical fertilizer use, two other farms in a community far from the road were also surveyed in April 2000. The far-road community was accessed by walking along a narrow trial through steep mountains for at least 2 h. Two farms in a community near the center of Da Hua Xian prefecture outside the Quibainong region were also surveyed in April 2000 to provide comparative information. Questionnaire survey Figure 1. N flow model in farm systems.
Materials and methods Study site Qibainong is located in Da Hua Xian prefecture of Guangxi Province, southern China, which is close to Vietnam (about 24◦ N and 107◦ E). It is a subtropical monsoon are. According to the meteorological station of Da Hua Xian, annual mean temperature and precipitation are 19.5 ◦ C and 1500 mm, respectively. The annual cultivation period is about 300 days. Climatic conditions are suitable for crop production, but the topography is harsh due to the steep mountainous (limestone) karst. Qibainong means ‘700 holes’ in Chinese. There are 1124 ‘holes’ called Doline, which are circular flat places some 100–300 m in diameter, which are surrounded by 3570 steep mountains 600– 1000 m in height. Flat lands and terraced fields made on slopes under 30◦ are cultivated with a double cropping system composed of maize and sweet potato or maize and soybean. The sloping land is mainly grows maize. Pig and chicken productions form the main livestock husbandry, though some farms also have goats and cows. A rapid appraisal showed that the heavy rain in May and June in Qibainong leads to surface runoff which can be severe enough to resemble a water-fall, because of the shallow soil depth on the slopes of the steep mountains. Water infiltrating the soil drains quickly through the limestone along the cracks. This leads to a lack of water for both crop production
In order to calculate N flows, the following information was collected through the questionnaire: (1) crop species cultivated; (2) cultivated area; (3) amounts of chemical fertilizer and manure applied for each crop; (4) crop yields; (5) livestock species and population; (6) amounts of feeds consumed; (7) livestock grazing period; (8) livestock yields; (9) human population; (10) amounts of food consumed; (11) amounts of food and feed imported; (12) amounts of agricultural products exported. The income and expenditures of each farm were also investigated to check the accuracy of the data. All survey information providing average values for 1960 was also collected from the community head in a near-road community in December 1999. Estimation of N budget Figure 1 shows the farm scale N flow model used in this study, which was constructed based on the survey. The model is similar to the regional scale model proposed by Matsumoto et al. (1992). The model consists of internal N cycling in the farm system, which is connected with the trading system, N application and N load. Internal N cycling consists of the N flowing through the field, crop, livestock, human and manure. The trade system exchanges N through the export of agricultural products and the import of food and feed. Grazing transfers N from forests and pastures to the farm. Atmospheric deposition, N fixation, manure and chemical fertilizer application are the inputs to the fields. Denitrification, ammonia volatilization and N leaching (surplus N) are the outputs from the
142 fields. Ammonia volatilization also occurs during the handling and storage of human and livestock excreta. Atmospheric deposition of N was assumed to be 10 kg N ha−1 y−1 , which is an average value for South Asia (Bouwman and van Vuuren, 1999). N content in crop uptake, chemical fertilizer and manure applied, import and export of food and feed, consumption of food and feed were calculated by multiplying the dry or fresh weight values investigated and N content values measured. Excreta N was assumed to be 80% of intake N based on McKown et al. (1991), and therefore 20% of intake N was retained as an increment N in livestock and human. The amount of ammonia volatilized during excreta storage and after manure application was assumed to be 28 and 10% of total N in excreta and manure applied, respectively (Barry et al., 1993). Denitrification was estimated as 20% of ammonium applied (Ryden, 1983). Biological N2 fixation was assumed to be 5 kg N ha−1 y−1 (Stewart, 1975). Field surplus N was estimated from a N budget in field by the following equation: Field surplus N = chemical fertilizer N + mannure N + atmospheric deposition N + N fixation – ammonia volatilization from manure
Table 1. Measured N content used for calculation of N flows gN/gFW Crop cultivated Maize (grain) Maize (leave+stem) Soybean(grain) Soybean(leave+stem) Sweet potato (root) Sweet potato (leave) Vegetable
0.0128 0.0014 0.0640 0.0090 0.0016 0.0025 0.0025
Food and feed imported Corn Rice Pork Chicken Goat meat
0.0125 0.0106 0.0240 0.0320 0.0250
Compost Manure Liquid manure
0.0050 0.0004
Chemical fertilizer Ammonium carbonate Urea
0.1700 0.4600
application-denitrification – crop N uptake (1) A proportion of excreta, which was not used in manure application, was stored in manure barns. The unused excreta was also estimated from the N budget in a whole farm as follows: Unused excreta N = imported food and feed N + grazing N + chemical fertilizer N + atmospheric deposition N + N2 fixation–exported product N–denitrification–field surplus N–ammonia volatilization from manure application and excreta storage –increment N in livestock and human
(2)
Estimation of potential nitrate-N concentration Assuming that all the field surplus N in upland fields is leached away after mineralization and nitrification each year, the annual mean nitrate-N concentration
in drainage water from fields was predicted by dividing the amount of field surplus N by drainage water volume. Drainage water volume was approximated as the difference between mean annual precipitation (1500 mm) and evapotranspiration. In order to estimate mean annual evapotranspiration, heat balance and precipitation were measured at the near-road community in 1999. The results showed an annual precipitation of 1961 mm and an annual evapotranspiration of 780 mm. Using these results, the proportion of drainage water to precipitation was estimated to be 60%, and the mean annual drainage water volume was approximately 900 mm. Although N leaching is generally related to soil processes, such as denitrification, immobilization, mineralization, ion exchange, hydrodynamic dispersion or diffusion, such reactions were not taken into consideration. Therefore, the prediction should be termed the potential nitrate-N concentration in drainage water.
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Figure 2. Field N budgets in farm systems surveyed in 1999–2000 including for average of 1960.
Results Characteristics of farms surveyed The human population in the farms of Quibainong ranged from three to seven persons per family, which was similar to that outside Quibainong. Pigs and chickens were the predominant livestock husbandry in Quibainong, although livestock husbandry was not popular in 1960 (Table 1). All the farms fed chickens, with three to 52 head per family. Pigs, with one to 27 head per family, were raised in all the farms except one in the near-road community. Goats were raised in 65% of farms in the near-road community, but there were no goats in the other communities. Cattle husbandry was relatively uncommon in Qibainong, probably due to the absence of paddy fields. Arable land ranged from 0.022 to 0.2 ha capita−1 (Table 2). The mean arable land area in China was 0.10 ha capita−1 in 1999 (FAO, 2001). Corn, soybean and sweet potato were the predominant crops in Quibainong. The annual yield ranged from 2.3 to 7.1 t ha−1 for corn, and from 0.3 to 1.5 t ha−1 for soybean. Leaves and roots of sweet potato were used as livestock feed, with fresh weights ranging from 2.5 to 87 t ha−1 for leaves and 1.0 to 150 t ha−1 for roots. Although these values varied over a wide range, they were possible
values when compared to previous values. Some other vegetables were also cultivated in Quibainong. The annual income ranged from 123 to 1127 US$ family−1 (Table 2). According to the IEA statistical database (2001), the GDP in China was 574 US$ capita−1 in 1996. The sale of agricultural products contributed 89% of the total income on average. However, all the income of the three farms in the near-road community, which had an income of only 140 US$ family−1 , was obtained from outside work. Only livestock production was exported in all farms. The values showed wide ranges: 0 to 650 kg family−1 for pigs; from 0 to 243 kg family−1 for chickens; 0 to 150 kg family−1 for goat. However, the maximum value for pig export was shown outside Quibainong. The annual expenditure ranged from 116 to 500 US$ family−1 , of which chemical fertilizer accounted for 3–33% (Table 2). The proportion of chemical fertilizer expense to total expenditure was high in economically poor farms. Although there were differences between income and expenditure due to deposits, annual expenditure in almost all farms was close to the annual income. Corn, rice and pork were imported for food and feed. Imports in Quibainong ranged from 0 to 804 kg family−1 for corn, 0 to 350 kg family−1 for rice and 0 to 120 kg family−1 for pork, while outside Quibainong, corn was not imported; however, a large
144 Table 2. Some characteristics of farms surveyed in 1999 and 2000 including for average in 1960
Average in 1960 NR01 NR02 NR03 NR04 NR05 NR06 NR07 NR08 NR09 NR10 NR11 NR12 NR13 FR01 FR02 OC01 OC02
Human capita family−1
Pig head family−1
Chicken head family−1
Goat head family−1
Cattle head family−1
Arable land ha capita−1
Annual income US$ family−1
Annual expenditure US$ family−1
Proportion of agricultural income to total income %
Proportion of chemical fertilizer expense to expenditure %
7 5 6 4 3 3 7 3 7 4 5 5 4 5 2 4 7 7
1 6 27 0 2 1 7 4 3 3 20 13 3 16 20 8 21 3
5 3 42 10 13 9 7 27 9 11 52 20 3 14 33 4 14 14
0 3 0 0 1 6 5 0 5 3 4 0 4 0 0 0 0 0
0 0 0 0 0 0 1 0 0 3 2 0 0 0 1 0 0 3
0.067 0.043 0.054 0.067 0.067 0.053 0.038 0.022 0.043 0.080 0.056 0.040 0.053 0.033 0.200 0.050 0.024 0.057
0 123 518 140 924 140 205 475 496 130 328 290 140 1127 230 132 1556 1175
0 116 500 153 385 127 248 279 448 144 302 178 124 377 208 150 1488 560
– 31 43 0 9 0 79 47 100 57 83 100 0 6 73 53 77 100
– 33 11 20 8 18 18 3 12 26 11 21 29 15 4 3 7 13
NR, near-road farm; FR, far-road farm; OC, farm outside Qibainong.
amount of rice and pork was imported, with maximum values of 2500 and 250 kg family−1 , respectively. Field N budget Figure 2 shows the field N budgets in the farm systems investigated. Total N inputs showed a wide range, with values from 74 to 736 kg N ha−1 . However, total N inputs in near-road farms and outside Qibaing exceeded 100 kg N ha−1 , while those in far-road farms and the average value for 1960 were less than 100 kg N ha−1 . The total N input of 74 kg N ha−1 in 1960 was similar to that in traditional agriculture reported by Ellis and Wang (1997) and Zhu (1997). Chemical fertilizer application accounted for more than 50% of total input in all farms, though chemical fertilizer was not used in 1960. Crop N uptake ranged from 52 to 148 kg N ha−1 , which accounted for 14.2–83.4% of total N input. Therefore, 16.6–85.8% of total N input was lost from the fields. Denitrification and ammonia volatilization
accounted for 7.1–16.7% and 0.2–4.4% of total N input, respectively. Therefore, field surplus N accounted for 5.6–69.1% of total N input, with values ranging from 4.1 to 463 kg N ha−1 . However, field surplus N in near-road farms and outside Quibainong exceeded 50 kg N ha−1 , while that in far-road farms and the average value for 1960 was less than 40 kg N ha−1 . Farm N budget Figure 3 shows the N budgets in the whole farm system. Total N input showed a wide range from 52 to 736 kg N ha−1 . In the near-road farms, total N input exceeded 150 kg N ha−1 , with chemical fertilizer accounting for more than 50%. This was similar to the farms outside Quibainong. On the other hand, in the far-road farms, total N input was less than 150 kg N ha−1 , with chemical fertilizer accounting for less than 50%. In 1960, total N input was 52 kg N ha−1 , none of which was chemical fertilizer. Grazing or collecting feed from forest and pasture was an important N input
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Figure 3. N budgets in whole farm system surveyed in 1999–2000 including for average for 1960.
Figure 4. Relationship between N application rate and field surplus N and crop uptake N.
Figure 5. Relationship between N application rate and potential nitrate-N concentration.
in some farms, which accounted for up to 62% of the total N input. Atmospheric deposition and N2 fixation accounted for 2.1–15.9% of the total N inputs. N output associated with the export of livestock products was the only N output in terms of economic activities in the farms, which ranged from 0 to 56.1 kg N ha−1 . Therefore, 72.8–668 kg N ha−1 was recycled and remained or discharged in the farms, in which 0– 118 kg N ha−1 was lost by denitrification in fields, and
6.9–104 kg N ha−1 was lost by ammonia volatilization during composting of excreta and manure application. Increment N in livestock and human bodies ranged from 5.8 to 80.4 kg N ha−1 . As field surplus N ranged from 4.4 to 463 kg N ha−1 , unused excreta was estimated to be 0–215 kg N ha−1 , which accounted for 0–62% of the total N input.
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Figure 6. Relationship between chemical fertilizer N application rate and proportion of manure N to total N application.
Figure 7. Relationship between extreta N and unused excreta N.
Characteristic relationships among the values in N budgets Field surplus N and crop N uptake were significantly correlated with N application rate (Figure 4). The equations are y = 0.70x − 39.0 (R 2 = 0.966, P < 0.001) for field surplus N and y = 0.12x + 56.5 (R 2 = 0.464, P < 0.01) for crop N uptake. These equations indicate that field surplus N did not appear in the range of N application rate under 55 kg N ha−1 and field surplus N became larger than the crop N uptake, when N application rate exceeded 160 kg N ha−1 . The estimated potential nitrate-N concentration in drainage water was also significantly correlated with N application rate as follows: y = 0.078x − 4.33 (R 2 = 0.966, P < 0.001) (Figure 5). This shows that when the N application rate increased to more than 185 kg N ha−1 , the potential nitrate-N concentration exceeded 10 mg N L−1 , the maximum contamination level permitted for drinking water. The proportion of manure N application in the total N application (expressed as rates) was negatively cor-
Figure 8. Relationship between feed self-sufficiency and unused excreta N.
related with chemical fertilizer application rate: y = −0.146 Ln(x) + 0.977 (R 2 = 0.750, P < 0.01) (Figure 6). This indicates that the increase in total N input in fields in Quibainong was due mainly to increases in the rate of chemical fertilizer application. Unused excreta N was positively correlated with excreta N: y = 0.611x − 33.6 (R 2 = 0.886, P < 0.001) (Figure 7). This shows that about half of the excreta was unused. Furthermore, unused excreta N was also negatively correlated with feed self-sufficiency; which was estimated as a proportion of self supplied feed N to total feed N as follows: y = −107 Ln(x) − 29.2 (R 2 = 0.654, P < 0.01) (Figure 8). These suggest that economically richer farms conducted more intensive livestock production and increased the amount of unused excreta. However, there was no significant relationship between unused excreta N and the N application rate. Unused excreta was not disposed of, but stored in manure barns, although the amount of manure stock was not investigated.
Discussion Although previous studies showed that governmental census data and statistical data were useful for estimating N budgets on regional and community scales (Guo and Bradshow, 1993; Matsumoto, 2000; Nagumo and Hatano, 2000), official data on import and export of food and feed and consumption of chemical fertilizers are not often available on the farm scale. For this reason, the data obtained from the questionnaire were used in this study to estimate N budgets on a farm scale. As a start to making the N budget estimation, the accuracy of the survey data was checked by comparingincome and expenditure (Table 2). This
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Figure 9. Relationship between income and chemical fertilizer N application rate.
comparison showed a relatively good correspondence between income and expenditure, although there were some discrepancies due to deposits. However, the N flow values in N budgets showed that there were meaningful significant relationships among them (Figures 4–8). Therefore, the questionnaire survey in this study was reliable enough to evaluate N budgets. The current field surplus N in Quibainong showed a wide range from 10.1 to 463 kg N ha−1 , while the value 1960 was 4.1 kg N ha−1 (Figure 2); the field surplus N was significantly related to N application rate (Figure 4). According to the N balance study for the whole of China reported by Zhu (1997), the mean surplus N in 1952, 1979, 1983 and 1987 was −19.0, 22.2, 38.8 and 50.0 kg N ha−1 , respectively, while the N application rate was 21.3, 124, 143 and 151 kg N ha−1 , respectively. Abe et al. (1999), Guo and Bradshaw (1993) and Yan et al. (1999) reported values for field N balance in 1990s on the village scale in central and southern China. These authors reported values for field surplus N of 567, 155 and 397 kg N ha−1 , respectively, and for the N application of 746, 433 and 682 kg N ha−1 . Regression analysis including these values together with the values in Figure 4 showed that field surplus N was significantly correlated with the N application rate (R 2 = 0.947, P < 0.001). Therefore, the increase of N application rate simply results in an increase in field surplus N. Although the mean N application rate for the whole of China reported by Zhu (1997) was relatively low, local values of N application rate in the Lake Tai region of southern China reported by Ellis and Wang (1997) were considerably higher, with 70 to 100 kg N ha−1 applied only as manure in the 1950s, while the current N application rate was 500 kg N ha−1 with chemical fertilizer and manure following the peak of
800 kg N ha−1 in 1979. The data obtained in the present study gave a N application rate in 1960 of 59 kg N ha−1 in Quibainong, which was higher than the mean value for the whole China reported by Zhu (1997), but slightly less than the value reported by Ellis and Wang (1997). On the other hand, the current N application rate in Quibainong showed a wide range from 34.5 to 654 kg N ha−1 (Figure 3). Figure 9 shows the relationship between application rate of chemical fertilizer N and household income, and indicates that the application rate of chemical fertilizer N increased with increasing income in Quibainong, while in the farms outside Quibainong the chemical fertilizer N application rate was lower despite the incomes than those of the farms in Quibainong. Figure 9 also shows that the chemical fertilizer N application rate in far-road farms was considerably less than those in the near-road farms. The proportion of expense of chemical fertilizer to total expenditure in far-road farms (Table 2) was also considerably less than in near-road farms. These results suggest that there is an unrestricted application of chemical fertilizer in near-road farms and therefore economic and topographic conditions may be a factor influencing the variability in the rate of N application. In Chinese traditional agriculture, organic manure from human and livestock excreta, green manure crops, crop residues, weeds, water plants and pond sediments have been applied to fields to sustain highly productive soils (Wen and Pimentel, 1986). Chemical fertilizer is an alternative for organic manure, and therefore the manure application rate has decreased with the increasing rate of chemical fertilizer application and resulted in increase of unused excreta (Ellis and Wang, 1997). The present study showed that the proportion of manure N to N application decreased with an increase in application of chemical fertilizer N, but did not relate to the amount of unused excreta N. Unused excreta N simply increased with an increase of excreta N (Figure 7). Taking into consideration that unused excreta N was negatively correlated with feed self-sufficiency (Figure 8), but not correlated with N application rate, the amount of livestock is influenced by the economic activity of individual farms and is independent of crop production. Therefore, chemical fertilizer was applied regardless of manure application. Zebarth et al. (1999) showed that the annual minimum field surplus N required to obtain optimal crop yield in British Columbia, Canada, was 50 kg N ha−1 . In the present study, a N application rate above 160 kg N ha−1 increased field surplus N more than crop
148 uptake N (Figure 4), and a N application rate above 185 kg N ha−1 increased the potential nitrate-N concentration to values above the 10 mg L−1 drinking water limit (Figure 5). These N application rates show the minimum N application rate to obtain optimal crop yield and minimum N contamination. Therefore, a N application of 160–185 kg N ha−1 is the environmental capacity to sustain optimal N cycling in Qibainong. Using the relationship between field surplus N and N application rate shown in Figure 4, the minimum field surplus N was estimated to be 73– 91 kg N ha−1 . The difference between the minimum field surplus N values estimated in the present study and that by Zebarth et al. (1999) may be due to the difference in dilution effect depending on amount of precipitation. The mean value of excreta N in near-road farms was 171 kg N ha−1 . If all excreta N was evenly used for crop cultivation without chemical fertilizer in whole fields, the optimal N cycling would be maintained. Zebarth et al. (1999) made a similar suggestion. As mentioned above, an increase in imported feed increases excreta N. Aarts et al. (2000) indicated that nitrate-N concentration in ground water in a dairy farm area of The Netherlands decreased from 200 to 50 mg N L−1 within a few years through N management, in which inputs of N in imported feed and chemical fertilizer decreased by 56 and 78%, respectively. Therefore, the total N input in farms in Quibainong, which keeps field N input within 160– 185 kg N ha−1 and does not produce unused excreta, should be required to obtain optimal crop yield and minimum N contamination.
Conclusion Previous research has suggested that traditional farming systems maintained optimal N cycling and minimum N load in farms by using all the excreta from livestock and humans as manure. The import of chemical fertilizers and feed has increased agricultural productivity, but it has also induced a much larger environmental N load than the N retained by agricultural products. It seems that there is no positive linkage between fertilizer application and feed consumption to sustain environmental quality in the current agricultural system. This suggests that it is crucial to evaluate manure resources and the productivity of crops and livestock in farming systems quantitatively in connection with the environmental capacity related to N
cycling. The questionnaire survey used in this study proved effective in providing quantitative estimates of N flow.
Acknowledgments This study was partly supported by Research of the Future Program, Japan Society of the Promotion of Science (JSPS).
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