Rapid socio-economic development in suburban areas of developing countries has induced changes in agricultural waste and nutrient management, resulting in water pollution. The study aimed at estimating agricultural nutrient cycles and their contribution to the water environment. A material flow model of nitrogen (N) and phosphorus (P) was developed focusing on agricultural activities from 1980 to 2010 in Trai hamlet, an agricultural watershed in Nhue-Day River basin, Vietnam. The model focused on the change in household management of human excreta and livestock excreta, and chemical fertilizer consumption. The results showed that the proportion of nutrients from compost/manure applied to paddy fields decreased from 85 to 41% for both N and P between 1980 and 2010. The nutrient inputs derived from chemical fertilizer decreased 6% between 1980 and 2000 for both N and P. Then, these nutrients increased 1.4 times for N and 1.2 times for P from 2000 to 2010. As of 2010, the total inputs to paddy fields have amounted to 435 kg-N/ha/year and 90 kg-P/ha/year. Of these nutrient inputs, 40% of N and 65% of P were derived from chemical fertilizer. Thirty per cent (30%) of total N input was discharged to the water bodies through agricultural runoff and 47% of total P input accumulated in soil.

INTRODUCTION

For many decades, crop–livestock systems, which involved the intensive application of human/livestock excreta as nutrient sources, were traditional and well-practiced in Asia (Amir & Knipscheer 1989; Devendra 2002; Allen et al. 2007). However, recent rapid economic growth has induced changes in waste management practices, and subsequently in the nutrient flows. Chemical fertilizers were produced significantly and have become a more preferable source of nutrients than human/livestock excreta. The amount of chemical fertilizers applied to paddy fields has been dramatically increasing (Heffer & Prud'homme 2010). Excessive nutrient inputs to paddy fields from chemical fertilizers may be a potential source of water pollution due to agricultural leaching or runoff. In addition, a huge amount of human/livestock excreta unused for agriculture now flows into fish cultivation ponds, or is directly discharged to the nearby water environment. Consequently, environmental pollution in agricultural watersheds is increasing. Moreover, the loss of valuable nutrients due to this discharge is a concern.

To deal with these problems, the changes of farming systems and their corresponding waste management practices as well as the dynamics of fertilizer application have to be understood. Nutrient balances in paddy fields, which are likely affected by these changes, and the contribution of paddy fields to water pollution are necessary information for designing sound nutrient management interventions at a watershed level. Recently, several researchers have conducted this kind of research using material flow analysis (MFA) in developing countries such as Thailand (Whitbread et al. 2003), China (Chen et al. 2008), and Vietnam (Montangero et al. 2007; Nga et al. 2011). MFA is a systematic tool used to understand the levels of flow and the stocks of materials within a system defined at a particular point in space and time (Brunner & Rechbenger 2004). Although nutrient cycles of farming activities have likely changed in developing countries due to the rapid socio-economic development, previous studies did not address historical changes in the role of the paddy fields on nutrient management. Previous research also did not discuss how the changes in this role lead to changes of interaction between agricultural activities and the environment.

This study aimed at understanding the historical changes of agricultural activities; including the practices of waste management and fertilizer application for a typical agricultural situation in the context of a river basin in Vietnam. The Nhue-Day River basin, where agriculture has been actively conducted in its downstream, has long been a platform for exchanging irrigation water with paddy fields. The basin is one of three important basins in Vietnam and has been experiencing an alarming level of nutrient pollution (Environmental Report of Vietnam 2006). One potential source of pollution is diffuse agricultural pollution in the basin. To understand the pollution loads contributed in this manner, the study also aimed at estimating nutrient cycles in paddy fields and their contribution to the watershed environment by developing a material flow model of nitrogen (N) and phosphorus (P).

MATERIALS AND METHODS

Study site

The study area is the Trai hamlet, a suburban district of Hanoi, the capital of Vietnam. The hamlet is located on the bank of the Nhue River, 40 km from the river's upstream (Figure 1). General information regarding this hamlet is summarized in Table 1. Farming is the major occupation of the residents and rice is the dominant crop. The area of paddy fields, which has not changed between 1980 and 2010, covers more than 90% of the area. Human and livestock excreta are applied to paddy fields together with chemical fertilizers here, as in several other agricultural communities in the basin (Nga et al. 2011), as well as in Indonesia (Harashina et al. 2003), and China (Chen et al. 2009). Water from rivers in the watershed is utilized for irrigating paddy fields. Agricultural wastewater from paddy fields is pumped back to the river after harvesting seasons.

Table 1
   Data
 
Information Symbol Unit 1980 2010 
Population  People 505 800 
Household number  Household – 240 
Livestock number 
 1. Pig  Head 49 103 
 2. Poultry  Head 1094 3026 
 3. Cattle  Head 16 
Paddy field area  ha 52.6 52.6 
Total area  ha 56.1 56.1 
   Data
 
Information Symbol Unit 1980 2010 
Population  People 505 800 
Household number  Household – 240 
Livestock number 
 1. Pig  Head 49 103 
 2. Poultry  Head 1094 3026 
 3. Cattle  Head 16 
Paddy field area  ha 52.6 52.6 
Total area  ha 56.1 56.1 
Figure 1

Map of Nhue-Day River basin and Trai hamlet.

Figure 1

Map of Nhue-Day River basin and Trai hamlet.

Material flow model development

A mass flow model, shown in Figure 2, was developed based on Giang et al. (2012), which focused on nutrient balances in paddy fields. This model was applied for the hamlet. Table 2 summarized reaction processes present in the paddy field component through matrix expression. All of the components in the system are listed across the top of the table with symbols. The model is composed of eight components labeled (X1−8). (X1) represents households, (X2) represents livestock, (X3) represents fishponds, (X4) represents paddy fields, all of which are within the boundary. (X5) represents a market, (X6) represents water bodies, (X7) represents both soil and groundwater, and (X8) represents the atmosphere. (X5−8) are outside of the boundary.

Table 2

Matrix expression on the description of reaction processes (adapted from Harada et al. (2010) and Giang et al. (2012))

Component Process          Reaction rate of Pi (kg/year) 
 Human excreta discharge −1         
 Grey-water discharge −1         
 Kitchen waste discharge −1         
 Rain water supply        −1  
 Pig excreta discharge  −1        
 Cattle excreta discharge  −1        
 Poultry excreta discharge  −1        
 Chem. fertilizer application    +1 −1     
 Irr. water supply    +1  −1    
 N fixation    +1    −1  
 Agri. product harvesting     +1     
 Agricultural runoff    −1  +1    
 N emission    −1    +1  
 Soil accumulation and discharge    −1   +1   
Net reaction rate for (kg/ha/year)  (*) Obtained from Berk & Zeki (1997)  
Component Process          Reaction rate of Pi (kg/year) 
 Human excreta discharge −1         
 Grey-water discharge −1         
 Kitchen waste discharge −1         
 Rain water supply        −1  
 Pig excreta discharge  −1        
 Cattle excreta discharge  −1        
 Poultry excreta discharge  −1        
 Chem. fertilizer application    +1 −1     
 Irr. water supply    +1  −1    
 N fixation    +1    −1  
 Agri. product harvesting     +1     
 Agricultural runoff    −1  +1    
 N emission    −1    +1  
 Soil accumulation and discharge    −1   +1   
Net reaction rate for (kg/ha/year)  (*) Obtained from Berk & Zeki (1997)  
Figure 2

Material flow model.

Figure 2

Material flow model.

Most of flows were calculated by unit value method: 
formula
1
 
formula
2
where Ii: an input flow of a component (kg/ha/year); Oj: an output flow of a component (kg/ha/year); Uk, Ul: unit composition data of good k, l (g/unit amount/year); Ck, Cl: amount of good k, l (amount); Rk, Rl: ratio of good k, l transferred from one component to another component.
The flow which could not be calculated by unit value method was calculated based on mass conservation law: 
formula
3
where n, m: total input and output flows of a component.
The rows in Table 2 show 14 reaction processes that occur within each paddy field component (P1P14). The reaction rate (ρi) for each process (Pi) is in the rightmost column of the matrix. The main text of the matrix conveys the proportion  of a process that occurs in each component. Therefore, the net reaction rate of a single component is affected by a number of different processes, which can be seen by moving down the column representing a component. Based on the law of mass conservation, the net reaction rate r4 for component X4 was considered in the mass balance: 
formula
4
where i is the process (i = 1–14) and  (ha) is the total area of the study site.

Data collection

The necessary data for flow calculation appear in Table 3. Structured questionnaire surveys were used to acquire data on waste management including human excreta, livestock excreta, kitchen waste, grey-water, and agricultural waste (Giang et al. 2012). The data of waste composition data, e.g. N and P amount in human excreta or in chemical fertilizers, were collected from the literature. It was assumed that waste composition data had not changed from 1980 to 2010, except for the N and P concentration in the Nhue River. This is because the water quality of the river is substantially affected by the rapid socio-economic development occurring in the area. Although data in 2010 were available, the oldest available data for total N (TN) and total P (TP) concentrations in Nhue River were those from 2007 by the Vietnam Environment Administration (VEA) Centre for Environmental Monitoring (2012). Despite accelerated socio-economic growth in the area from the middle of the 2000s, the authors assumed that the concentrations had not changed significantly from 1980 to 2007. The concentration data in 2007 were used as those in 1980, 1990, and 2000, as the impact of TN and TP concentrations in 2007 was limited on the flows to paddy fields.

Table 3

Data collection

Symbol Explanation Unit 1980 1990 2000 2010 References 
 Ratio of human excreta go to fish ponds – 0.05 1) 
 Ratio of human excreta go to paddy fields – 0.97 0.97 0.93 0.52 1) 
 Ratio of human excreta go to water bodies – 0.07 1) 
 Ratio of human excreta go to septic tank – 0.36 1) 
 N transfer coefficient in septic tank (leachate) – 0.90 0.90 0.90 0.90 2) 
 P transfer coefficient in septic tank (leachate) – 0.81 0.81 0.81 0.81 2) 
 N transfer coefficient in septic tank (sludge) – 0.10 0.10 0.10 0.10 2) 
 P transfer coefficient in septic tank (sludge) – 0.19 0.19 0.19 0.19 2) 
 Ratio of grey-water go to fish ponds – 0.16 0.16 0.16 0.16 1) 
 Ratio of grey-water go to paddy fields – 0.25 0.25 0.25 0.25 1) 
 Ratio of grey-water go to water bodies – 0.59 0.59 0.59 0.59 1) 
 Ratio of kitchen waste go to live stock – 0.27 0.27 0.27 0.27 1) 
 Ratio of kitchen waste go to fish ponds – 0.04 0.04 0.04 0.04 1) 
 Ratio of kitchen waste go to water bodies – 0.07 0.07 0.07 0.07 1) 
 Ratio of kitchen waste go to soil/groundwater – 0.62 0.62 0.62 0.62 1) 
 Ratio of pig excreta go to households (biogas) – 0.01 0.01 0.01 0.15 1) 
 Ratio of pig excreta go to fish ponds – 0.04 0.04 0.15 0.53 1) 
 Ratio of pig excreta go to paddy fields – 0.96 0.95 0.84 0.32 1) 
 Ratio of cattle excreta go to paddy fields – 0.67 0.67 0.67 0.67 1) 
 Ratio of cattle excreta go to soil/groundwater – 0.33 0.33 0.33 0.33 1) 
 Ratio of poultry excreta go to households (biogas) – 0.01 1) 
 Ratio of poultry excreta go to fish ponds – 0.03 0.04 0.07 0.09 1) 
 Ratio of poultry excreta go to paddy fields – 0.76 0.75 0.73 0.70 1) 
 Ratio of poultry excreta go to soil/groundwater – 0.21 0.21 0.20 0.20 1) 
 Ratio of agri. residue go to livestock – 0.03 0.03 0.03 0.03 1) 
 Ratio of agri. residue go to paddy fields – 0.73 1) 
 Ratio of agri. residue go to soil/groundwater – 0.27 1) 
 Ratio of agri. residue go to agri. production (rice) – – – – 0.53 1) 
 Ratio of agri. residue go to agri. production (bean) – – – – 1) 
 N amount in human excreta g/cap/day – – – 8.1 3) 
 P amount in human excreta g/cap/day – – – 1.2 3) 
 N amount in grey-water g/cap/day – – – 0.4 4) 
 P amount in grey-water g/cap/day – – – 0.4 4) 
 N amount in kitchen waste g/cap/day – – – 0.65 5) 
 P amount in kitchen waste g/cap/day – – – 0.83 5) 
 N amount in rain water mg/L – – – 0.25 6) 
 P amount in rain water mg/L – – – 0.06 6) 
 Average rainfall mm/year – – – 1,612 7) 
 N amount in pig excreta g/head/day – – – 20.33 1) 
 P amount in pig excreta g/head/day – – – 4.59 1) 
 N amount in cattle excreta g/head/day – – – 31.66 1) 
 P amount in cattle excreta g/head/day – – – 5.13 1) 
 N amount in poultry excreta g/head/day – – – 0.36 1) 
 P amount in poultry excreta g/head/day – – – 0.08 1) 
 N amount in chemical fertilizer 5–46% (depending on fertilizer types) 1) 
 P amount in chemical fertilizer 10–16% (depending on fertilizer types) 1) 
 N amount in irrigation water (Nhue river) mg/L 2.6 2.6 2.6 7.7 8) 
 P amount in irrigation water (Nhue river) mg/L 0.17 0.17 0.17 0.66 8) 
 Irrigation water consumption m3/ha – – – 16,200 9) 
 N amount in rice kg/kg – – – 0.0114 10) 
 P amount in rice kg/kg – – – 0.0026 10) 
 N amount in bean kg/kg – – – 0.0064 11) 
 P amount in bean kg/kg – – – 0.0019 11) 
 Rice production kg/year 292,143 342,446 570,744 630,720 12) 
 Bean production kg/year 50,400 12) 
 N emission factor of excreta – – – – 0.2 13) 
 N emission factor of chemical fertilizer – – – – 0.1 13) 
 Ratio of runoff in case of nitrogen – – – – 0.3 14) 
 Ratio of runoff in case of phosphorus 
 Rice – – – – 0.01 14) 
 Bean – – – – 0.02 14) 
Symbol Explanation Unit 1980 1990 2000 2010 References 
 Ratio of human excreta go to fish ponds – 0.05 1) 
 Ratio of human excreta go to paddy fields – 0.97 0.97 0.93 0.52 1) 
 Ratio of human excreta go to water bodies – 0.07 1) 
 Ratio of human excreta go to septic tank – 0.36 1) 
 N transfer coefficient in septic tank (leachate) – 0.90 0.90 0.90 0.90 2) 
 P transfer coefficient in septic tank (leachate) – 0.81 0.81 0.81 0.81 2) 
 N transfer coefficient in septic tank (sludge) – 0.10 0.10 0.10 0.10 2) 
 P transfer coefficient in septic tank (sludge) – 0.19 0.19 0.19 0.19 2) 
 Ratio of grey-water go to fish ponds – 0.16 0.16 0.16 0.16 1) 
 Ratio of grey-water go to paddy fields – 0.25 0.25 0.25 0.25 1) 
 Ratio of grey-water go to water bodies – 0.59 0.59 0.59 0.59 1) 
 Ratio of kitchen waste go to live stock – 0.27 0.27 0.27 0.27 1) 
 Ratio of kitchen waste go to fish ponds – 0.04 0.04 0.04 0.04 1) 
 Ratio of kitchen waste go to water bodies – 0.07 0.07 0.07 0.07 1) 
 Ratio of kitchen waste go to soil/groundwater – 0.62 0.62 0.62 0.62 1) 
 Ratio of pig excreta go to households (biogas) – 0.01 0.01 0.01 0.15 1) 
 Ratio of pig excreta go to fish ponds – 0.04 0.04 0.15 0.53 1) 
 Ratio of pig excreta go to paddy fields – 0.96 0.95 0.84 0.32 1) 
 Ratio of cattle excreta go to paddy fields – 0.67 0.67 0.67 0.67 1) 
 Ratio of cattle excreta go to soil/groundwater – 0.33 0.33 0.33 0.33 1) 
 Ratio of poultry excreta go to households (biogas) – 0.01 1) 
 Ratio of poultry excreta go to fish ponds – 0.03 0.04 0.07 0.09 1) 
 Ratio of poultry excreta go to paddy fields – 0.76 0.75 0.73 0.70 1) 
 Ratio of poultry excreta go to soil/groundwater – 0.21 0.21 0.20 0.20 1) 
 Ratio of agri. residue go to livestock – 0.03 0.03 0.03 0.03 1) 
 Ratio of agri. residue go to paddy fields – 0.73 1) 
 Ratio of agri. residue go to soil/groundwater – 0.27 1) 
 Ratio of agri. residue go to agri. production (rice) – – – – 0.53 1) 
 Ratio of agri. residue go to agri. production (bean) – – – – 1) 
 N amount in human excreta g/cap/day – – – 8.1 3) 
 P amount in human excreta g/cap/day – – – 1.2 3) 
 N amount in grey-water g/cap/day – – – 0.4 4) 
 P amount in grey-water g/cap/day – – – 0.4 4) 
 N amount in kitchen waste g/cap/day – – – 0.65 5) 
 P amount in kitchen waste g/cap/day – – – 0.83 5) 
 N amount in rain water mg/L – – – 0.25 6) 
 P amount in rain water mg/L – – – 0.06 6) 
 Average rainfall mm/year – – – 1,612 7) 
 N amount in pig excreta g/head/day – – – 20.33 1) 
 P amount in pig excreta g/head/day – – – 4.59 1) 
 N amount in cattle excreta g/head/day – – – 31.66 1) 
 P amount in cattle excreta g/head/day – – – 5.13 1) 
 N amount in poultry excreta g/head/day – – – 0.36 1) 
 P amount in poultry excreta g/head/day – – – 0.08 1) 
 N amount in chemical fertilizer 5–46% (depending on fertilizer types) 1) 
 P amount in chemical fertilizer 10–16% (depending on fertilizer types) 1) 
 N amount in irrigation water (Nhue river) mg/L 2.6 2.6 2.6 7.7 8) 
 P amount in irrigation water (Nhue river) mg/L 0.17 0.17 0.17 0.66 8) 
 Irrigation water consumption m3/ha – – – 16,200 9) 
 N amount in rice kg/kg – – – 0.0114 10) 
 P amount in rice kg/kg – – – 0.0026 10) 
 N amount in bean kg/kg – – – 0.0064 11) 
 P amount in bean kg/kg – – – 0.0019 11) 
 Rice production kg/year 292,143 342,446 570,744 630,720 12) 
 Bean production kg/year 50,400 12) 
 N emission factor of excreta – – – – 0.2 13) 
 N emission factor of chemical fertilizer – – – – 0.1 13) 
 Ratio of runoff in case of nitrogen – – – – 0.3 14) 
 Ratio of runoff in case of phosphorus 
 Rice – – – – 0.01 14) 
 Bean – – – – 0.02 14) 

RESULTS AND DISCUSSION

Historical changes of compost/manure application to paddy fields

Figure 3 shows historical changes of nutrient amounts derived from human/livestock excreta. As shown in Figure 3, the total nutrients from human/livestock excreta double increased from 1980 to 2010, for both N and P, due to population growth. The human population increased 1.6 times, and the livestock population increased 2.7 times in this period. However, the proportion of N and P from compost/manure coming to paddy fields sharply decreased after 2000. As a custom in Vietnam, the application of compost/manure to paddy fields has a long history. In 1980, 35 kg N/ha/year and 7 kg P/ha/year were applied to paddy fields, which accounted for 85% of both total N and P from human/livestock excreta in this year. The use of compost/manure as nutrient sources for paddy fields was popular. Recently, when chemical fertilizers became more preferable, the traditional practice of human and livestock excreta recycling has gradually decreased. Only 41% of total N (37 kg N/ha/year) and total P (7 kg P/ha/year) from human/livestock excreta coming to paddy fields in 2010. This result corresponds to the findings of a relevant study in an agricultural watershed in China (Chen et al. 2009). Instead of being intensively applied to paddy fields, human and livestock excreta flows were directed into septic tanks/biogas systems and to fish ponds.

Figure 3

Nutrient amounts from compost/manure derived from human and livestock excreta.

Figure 3

Nutrient amounts from compost/manure derived from human and livestock excreta.

These transitions of N and P load may be the result of the modernization process, which affected waste management and terminally the dynamics of nutrient cycling. Dry chamber toilets, from which human excreta can be collected and reused for agriculture, have been gradually replaced by flush toilets. In 2010, 44% of households using flush toilets stopped using human excreta in agriculture. Instead, they discharged the human excreta to septic tanks or directly to the environment. In addition, a biogas program for the Vietnam Livestock sector, especially for pigs, was implemented in 2003. The program was implemented in both suburban and rural Vietnam in an effort to increase farmers' income and reduce environmental pollution (Dung et al. 2009). The project was introduced to the study site in 2006; however, only 16% of households raising pigs actually constructed biogas systems to treat pig excreta. About 53% of households discharged pig excreta into fish ponds. The amount of pig excreta that flows into fish ponds is expected to continuously increase since the farmers currently believe that fish in ponds fertilized with pig excreta grow faster than fish in ponds supplied with other feeds (Vu et al. 2007). Thus, fish ponds have become a platform for receiving livestock excreta, gradually replacing paddy fields as recipients of the excreta in the study area.

Historical changes of chemical fertilizer consumption

Although it is a custom to use human and livestock excreta as agricultural inputs, chemical fertilizers are now widely applied to paddy fields. Figure 4 illustrates the transition of nutrients derived from chemical fertilizers and compost/ manure over time. As can be seen in the figure, the nutrient inputs derived from chemical fertilizer decreased by 6% from 1980 to 2000 for both N and P. From 2000 to 2010, N and P then increased 1.4 and 1.2 times, respectively. Those changes were contrasted with the transitions of nutrient inputs derived from compost/manure, which peaked in 2000. This indicates that large amounts of compost/manure applied to paddy fields could reduce the usage of chemical fertilizers. Although the government has policies favoring the expansion of livestock production (Agrifood Consulting International (ACI) 2002), the nutrient inputs from manure have been gradually decreasing. Owing to economic development, chemical fertilizers are now more preferable than compost/manure as a source of nutrients. As reported by UNEP in 2011, the chemical fertilizer consumption in East and Southeast Asia was 196 kg/ha. This value is higher than many regions in the world. It is expected to continuously increase in the near future.

Figure 4

Nutrients derived from chemical fertilizer and compost/manure.

Figure 4

Nutrients derived from chemical fertilizer and compost/manure.

Historical changes of nutrient balance in paddy fields

N and P balances in paddy fields in 1980 and 2010 are shown in Figure 5 and 6. The dominant flows of N were chemical fertilizer, irrigation, runoff, and production. The dominant flows of P were chemical fertilizer, production, and soil accumulation and discharge. The total inputs to paddy fields were 435 kg N/ha/year and 90 kg P/ha/year in 2010. These are 1.5 and 1.3 times higher, respectively, than those in 1980. Chemical fertilizer contributed to the 1980 and 2010 total inputs of 47% and 40% in the case of N, respectively and 74% and 65% in the case of P, respectively. In 2010, 174 kg N/ha/year and 59 kg P/ha/year from chemical fertilizer were put into paddy fields. These results of N chemical fertilizer consumption were consistent with that consumed by a hamlet in a watershed in a study done in Indonesia (Harashina et al. 2003). The inputs from irrigation water in the area had a strong impact on the total nutrient inputs in 2010, which represented 29% of total N (125 kg N/ha/year) and 12% of total P (11 kg P/ha/year). The results were quite high compared to Mishima (2006), which indicated that Japanese irrigation water contributed 7% of N to agricultural input. Such differences could be explained by the quality of the irrigation source in the study area. The Nhue River is greatly contaminated by N and P in 2010 (Vietnam Environment Administration (VEA) Centre for Environmental Monitoring 2012). In contrast, low concentrations of N and P in Nhue River in 1980 resulted in no significant impact of irrigation water to the total nutrient inputs in this period. The total inputs to paddy fields were estimated to exceed the recommended level, 200 kg N/ha/year and 52 kg P/ha/year (Bo et al. 2003), in both 1980 and 2010. The excessive application of N and P to paddy fields, especially due to large inputs of chemical fertilizers, caused the greater burden of N in the water bodies and of P in the soil. The differences between N and P load can be partly explained by the higher runoff coefficients of N to surface water, and by the larger fraction of P accumulating in soil (Carpenter et al. 1998).

Figure 5

N balance in paddy fields: 1980 (left) and 2010 (right) (kg/ha/year).

Figure 5

N balance in paddy fields: 1980 (left) and 2010 (right) (kg/ha/year).

Figure 6

P balance in paddy fields: 1980 (left) and 2010 (right) (kg/ha/year).

Figure 6

P balance in paddy fields: 1980 (left) and 2010 (right) (kg/ha/year).

For a sound material cycle, chemical fertilizer consumption needs to be reduced, and the usage of human and livestock excreta in agriculture should be promoted. There were 33 kg N/ha/year and 5 kg P/ha/year from human/livestock excreta discharged directly or via septic tank/biogas systems to the environment in 2010. If those amounts were applied to paddy fields, the chemical fertilizer could be reduced by 19% for N and 8% for P. This might not only contribute to a better nutrient management, but also help to improve the water environment on the watershed scale.

CONCLUSIONS

N and P flows of an agricultural watershed area in the Nhue-Day River basin were examined together with the corresponding historical flows. Recently, the traditional waste recycling practices have been decreasing and chemical fertilizers have become a more preferred source of nutrients than human and livestock excreta. The total inputs for agriculture in 2010 were 435 kg N/ha/year and 90 kg P/ha/year. This is 1.5 and 1.3 times higher, respectively, than those in 1980. As of 2010, the largest input flow to paddy fields was from chemical fertilizers. It contributed 40% of N and 65% of P out of the total input. The total input of N and P to paddy fields was estimated to exceed the recommended level by 2.0 and 3.5 times, respectively. Excessive application of N resulted in the huge N burden to the water environment through runoff. Excessive application of P resulted in excess accumulation in soil and/or contamination of the groundwater.

The study provided basic information for understanding the contribution of paddy fields to pollution in the agricultural watershed environment. This study hence provides information relevant to formulate interventions for better waste and nutrient management on a watershed scale. A proper measure to reduce chemical fertilizer consumption and to promote human and livestock excreta use for agriculture should be proposed for a sound nutrient cycle not only in the Nhue-Day River basin but also in other agricultural watersheds in Asia. Instead of discharging human/livestock excreta directly, or via septic tank/biogas systems, these wastes could be applied to paddy fields. In the study area, if the wastes were applied to paddy fields, the chemical fertilizer consumption could be reduced by 19% for N and 8% for P. It may not only contribute to better nutrient management, but also help to improve the water environment in the whole watershed.

ACKNOWLEDGEMENT

The study was funded by KAKENHI (24254004 and 25870377).

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