Báo cáo khoa học nông nghiệp " Irrigating rice crops with waste water to reduce environmental pollution from catfish production in the Mekong Delta " doc

In six field experiments using fishpond waste water for irrigation, decreasing fertiliser N by 33 % and P and K by 50 % had no effect on rice yields.. If waste water application on this

Technical Report

Irrigating rice crops with waste water to reduce environmental pollution from

catfish production in the Mekong Delta

Cao van Phung1, Nguyen be Phuc2, Tran kim Hoang2 and Bell R.W. 3

1 Cuu Long Rice Research Institute, O’Mon, Cantho Province, Vietnam

Email: caovanphung@hcm.vnn.vn

2 An Giang University, Long Xuyen, An Giang Province, Vietnam

3 School of Environmental Science, Murdoch University, Murdoch 6150,

Australia

Abstract

Waste from intensive catfish (Pangasianodon hypophthalmus) aquaculture production

has become a pollutant of surface waters in the Mekong Delta, Vietnam In the

present study, the aim was to treat the wastewater from catfish ponds in the Mekong Delta by land application to padi fields so that the nutrients could be recovered by rice crops as a fertilizer substitute A survey in the dry season 2007 of paired fields in An Giang Province showed that rice yield in 16 paddies receiving waste from fishpond was 1 t/ha higher than in another 16 paddies that did not use wastes In six field experiments using fishpond waste water for irrigation, decreasing fertiliser N by 33 % and P and K by 50 % had no effect on rice yields In other cases, decreasing N by 40

% or P by 50 % did not decrease yield The variation in nutrient composition in waste water among sites, and in yield potential and irrigation requirements especially

1

Corresponding author Cuu Long Rice Research Institute, O’Mon district, Cantho city-Vietnam Phone No (84) 710861452 Fax: (84) 710861457 Email: caovanphung@hcm.vnn.vn

between wet (lower yield potential and lower irrigation requirement) and dry seasons may account for the different extent of fertiliser replacement feasible by waste water without decreasing yield

Ten -20 ha of padi land would be required to use the wastewater produced in the dry season, from 1 ha of fishponds assuming that only wastewater is used for irrigation In the early wet season, rainfall would prevent wastewater irrigation on some days, so that 20-40 ha of padi land may be required for every 1 ha of fishpond If waste water application on this cycle causes excess nutrient loading on padi fields, a further increase in padi land is required to apply this approach as a sustainable strategy for treating fishpond waste water

Keywords: catfish, fishpond waste, nutrients, pollution, rice

Introduction

Catfish culture in the Mekong Delta has been practiced for a long time but this

industry became important for export only after the year 2000 with a subsequent annual growth rate up to 2008 of about 15-20 % per annum Total catfish production

in the Mekong delta was 0.68 million tonnes of fillets in 2007 (Phan et al 2009) Cat fish production expanded to cover about 6,000 ha of ponds in the Mekong delta, Vietnam (Bosma et al 2009) The production of each tonne of catfish consumes 4,023

m3 of water and releases 47.3 kg of N (Phan et al 2009) Wastewater discharged directly from intensive catfish aquaculture production is polluting surface waters in the Mekong Delta, Vietnam From the catfish ponds, large quantities of liquid waste

are discharged to waterways without treatment (Phan et al 2009) It is estimated that about 2754GL of water was discharged annually back to the surface water system of rivers and canals of the Mekong Delta from catfish ponds Consequently, the

pollution of canals or rivers by loading of fishpond waste, rich in nutrients (especially nitrogen, phosphorus and carbon) has emerged as a major concern for sustainability of the industry (Phan et al 2009)

The introduction of the National Environment Law in 2005 which prohibits direct wastewater discharge of aquaculture water into rivers and canals, is an underlying driver for this study Although enforcement and compliance with this regulation currently appears to be low, the future sustainability of fish pond aquaculture relies greatly on the ability of farmers to comply with environmental and export regulations For this reason cost-effective wastewater treatment strategies need to be developed and applied by farmers Presently only 15-24 % of catfish farmers in Cantho and An Giang provinces currently practice recycling of wastewater from their ponds by irrigating rice fields (Cao et al 2009)

Pollution due to fishpond waste is generally attributed to high organic carbon and nutrients (Pillay, 1992) although high total suspended solids, NH4-N and COD may also downgrade its acceptability for a range of uses Moreover, this form of discharge

is also contributing to the spread of diseases of catfish since downstream operators are pumping the infected water from canals into their ponds (Phan et al 2009) Levels of fish-disease organisms in the river peak at the beginning of the rainy season in the Mekong Delta The quantity of waste produced depends upon the quantity and quality

of feed (Cowey and Cho, 1991) This is associated with lower feed conversion ratios

from manufactured feed pellets than to farm made feed (1.69 vs 2.25; Phan et al

2009) Hence the latter feed results in greater waste production The frequency of

water replacement and stocking densities in the fishponds will also affect the quantity and quality of waste water However, integration of aquaculture into existing

agricultural systems has been reported to improve productivity and ecological

sustainability of both operations through better management and improved soil

fertility arising from waste recycling (Bartone and Arlosoroff 1987) Moreover,

properly managed inputs of waste materials can reduce the need for fertilisers Ardakani et al 1987)

(Falahi-Rice uses large volumes of water, especially in the dry season , when the crop is fully irrigated Instead of using river or canal water to irrigate rice, wastewater, if available from adjacent catfish ponds, could supply most of the water requirements for rice while also providing a significant supply of nutrients The present study aims at recycling waste water from fishponds by using it to irrigate rice The rationale for the treatment was to use the assimilation capacity of rice to absorb nutrients and the filtering processes of the rice padi and associated water distribution canals and sediment traps to improve the quality of water discharged from fishponds before it re-enters the main canals and rivers The objective of the present study was to determine the fertiliser substitution value of the fishpond waste water in order to determine how

to adjust recommended fertiliser rates for rice when using this water to irrigate crops instead of river water

Materials and methods

Field experiments on recycling of waste water were carried out on rice crops commencing with the wet season 2007 and ending with the dry season 2010 Site locations and soil characterisation are given in Table 1

A preliminary survey on the beneficial use of fishpond waste water for rice cultivation on farmers’ fields was carried out in the dry season 2007 at Chau Phu and Phu Tan districts of An Giang province In each district, 16 fields were selected comprising 8 which used waste water from fishponds and the other 8 paired sites which were protected by levees to prevent inflow of waste water Rice samples were harvested from 5 m2 with 3 replications for yield evaluation

Experiments on recycling of waste water for rice production were carried out

at two communes of Chau Phu district namely My Phu during wet season 2007 and dry season 2008 and at Vinh Thanh Trung for dry season 2009 and wet season 2009 Another 2 experiments were conducted during the dry season 2008 at Phu Tan district (two sites) of An Giang province Further wastewater experiments were conducted at CLRRI in the dry season 2009 and at Phong Dieng in the dry season 2009 At the latter site, the wastewater was sourced from a Clarias fish pond, while for the

remaining sites the water was from catfish ponds Nutrient composition of wastewater

at each site is shown in Table 2

There were 6 treatments for experiments at Chau Phu and Phu Tan using chemical fertilisers (N-P-K rates in kg/ha given in parentheses) as follows: T1 (90-26-50); T2 (60-13-25); T3 (30-0-25); T4 (30-26-25); T5 (30-13-50) and T6 (0-13-50) Experiments in My Phu did not include T5 Inorganic fertilisers treatments for

experiments at Vinh Thanh Trung and CLRRI were adjusted as follows: T1 17.5-25); T2 (80-14-25); T3 (60-10.5-25); T4 (40-14-25); T5 (40-10.5-25) Finally,

(100-experiments at Phong Dien tested two treatments viz, irrigation with wastewater and reduced inorganic fertiliser (45-13-30, NPK in kg/ha) vs irrigation with river water

and the recommended fertiliser rate (83-21-17, NPK in kg/ha)

Irrigation with wastewater occurrred 5 times for the wet season and 10 times for dry season rice crops The volume of wastewater applied at each irrigation event was

1000 m3/ha (i.e 10 cm depth of water) Nutrients composition of the wastewater at different sites is presented in Table 2

All experiment was laid out in randomized complete block design with 4 replications except Phong Dien which comprised 8 replications, each of 2 treatments Plot size was 8 x 7 m and each plot was separated from others by bunds Soil sampling on each plot was done before planting and after harvesting every crop Yield components were estimated by sampling two 0.5 x 0.5 m quadrats on each plot Actual yield was

measured by harvesting 5 m2 per plot Rice crops were protected against leaf folder, thrips, brown plant hopper and rice blast by using effective pesticides as required

Organic carbon is determined by wet digestion; analysis of nutrients (N, P, K, Ca,

Mg, Fe, Cu, Zn, Mn) followed standard methods for soil (Page et al 1982), plant and water analysis (Chapman and Pratt, 1961)

Statistical analysis was completed with IRRISTAT software version 5.1 by applying a balanced one-way ANOVA

Nutrient (N, P) balances were calculated following the approach of Dobermann and Fairhurst (2000) to estimate total input and output Values reported by Dobermann and Fairhurst (2000) were replaced where possible with locally relevant values Nutrient budgets for N and P under the double rice system were calculated for the scenario where 2/3 of the straw is removed, which is current practice, and for 100 % retention of straw as an indication of the consequences of different straw management strategies

Results

Preliminary investigation of farmers’ use of waste water

The preliminary study showed that rice yields in farmers’ fields using wastewater from fishponds for irrigation were higher than in paddies using an equivalent volume and time of application of river water for irrigation Yield difference between the two methods was about 1 t/ha (Table 3) This indicates that wastewater, applied at appropriate rates, can help to further increase rice yield

Analysis of soil samples at harvest time showed that total nitrogen, phosphorus and potassium in paddies with wastewater application were significantly higher than plots without wastewater application but organic carbon was lower (Table 4) Wastewater was rich in nitrogen, phosphorus, and potassium (Table 2) which is likely why soils receiving it had higher nutrient contents By contrast, the high bacterial loading in waste water may accelerate decomposition of organic matter leaving lower organic C levels but higher mineralized nitrogen levels

The survey also recognized that farmers usually added zeolite, lime and dolomite while cleaning fishponds after harvesting (Bosma et al 2009) This may be the reason for higher contents of calcium and magnesium in paddies receiving wastewater Besides that, iron and manganese were also significantly higher in wastewater-treated fields (Table 4)

Recycling of wastewater for rice cultivation at Chau Phu

At Chau Phu, rice yields of all treatments were in the range 3.9- 4.2 t/ha in the wet season 2007 and not statistically different However, in the dry season 2008 rice

yields of T1 and T2 were significantly higher than the other treatments (T3, T4 and T5) with no added P or N or only 33 % of the recommended N (Table 5) This suggests that irrigation by wastewater from fishponds can save 1/3 of recommended

N, P and K The lower yields in T3 were attributed to the acidity of soils in which phosphorus supply is a key factor for crop growth (Cong et al 1995) Besides that, nitrogen in T3, T4 and T5 was only 33 % of the recommended rate and not sufficient

to achieve potential yields for the dry season Rice yield in the wet season is usually lower than in dry season in the Cuu Long Delta due to lower solar radiation (Hung et al., 1995)

Analysis of soil, straw and grain samples at harvesting time showed no significant difference among treatments in concentrations of N, P and K (data not shown)

Recycling of wastewater for rice cultivation at Phu Tan

At Phu Tan site 1 in the dry season, rice yields in T1 and T2 were close to 7 t/ha and not significantly different (Table 5) This suggests that irrigation by wastewater from fishponds can save 1/3 of recommended N and ½ of the recommended P and K Further decrease in N fertiliser resulted in reducing yield Omission of P produced the lowest yield at Phu Tan 1, because low P on these soils limits N use efficiency (Cong

et al., 1995) At Phu Tan 2, the maximum yield was only 5.7 t/ha, but T2 again produced the same yield as the recommended fertiliser rate Further decreases in N and P decreased yield significantly

Macro and secondary nutrient uptake at the Phu Tan sites (Table 6) showed that plots with high yield were also high in nutrient uptake (kg/ha) in straw and grain apart from P in straw of Phu Tan 1 In the experiment at Phu Tan 2, nutrient uptake in grain

followed the same trend as in the experiment Phu Tan 1 but K and Ca uptake in straw were not statistically different among treatments (Tables 6)

Recycling of wastewater for rice cultivation at CLRRI

At CLRRI, rice yields of all treatment were not different (Table 7) Even though T1, with the highest applied inorganic N, P and K fertiliser rates (100-18-25 kg/ha), had the highest yield, due to high variability among the plots, the effect was not significant Nitrogen contents in grain at harvesting time of treatments T1, T2 and T4 were among the highest and they were statistically different to others (Table 8) This suggests that differences in N on plots irrigated with waste water from fishpond were not a decisive factor for rice growing on acid soil where P is deficient Low N content

in T3 plot might have resulted from low P application in this treatment as compared to T4 As regard to P content in grain of this experiment, treatment T1 had lower P concentration than others which may result from a dilution effect because this treatment had the highest yield

Recycling of wastewater for rice cultivation at Chau Phu in 2009

At the two Chau Phu sites in the wet and dry seasons of 2009, respectively, there were

no significant yield différences among treatments Even when 40 % of the

recommended N and 67 % of the recommended P were applied as fertiliser, no

decrease in yield was obtained in the rice irrigated with waste water (Table 7) At Chau Phu, the crops were a healthy green colour during growth with 40 kg N/ha This suggests moderate but not excessive levels of N in waste water at Chau Phu, perhaps because there is greater use of settling ponds here

Recycling of wastewater for rice cultivation at Phong Dien

Total fertiliser application in treatment T1 (irrigated with waste water from fishpond)

at planting was about 1/3 of treatment T2 (which used only river water) Later on fertilisers used in treatment T1 was based on plant diagnosis for N (leaf colour), P (tillering capacity) and K (leaf turgour) Overall, T1 had about 45 % less N and 40 % less P application as fertiliser, but a 76 % increase in K in order to minimise insect damage

There was no difference in yields between two treatments over two succeeding crops

in 2009 (Table 15) This demonstrated that 40-45 % decrease in N and P fertilisers for rice irrigated with waste water from Clarias fishpond did not induce any nutrient disorders (Tables 16,17)

Nutrient budgets

Nutrients budgets of 4 sites were presented in Tables 19 and 20 If rice straw was removed, all treatments at Phu Tan and Phong Dien had negative N balances regardless of differences in N dosages of treatments But experiments in Chau Phu and CLRRI showed that only treatment T5 & T6, with 30 kg N/ha applied as fertiliser had negative N balances When straw was recycled in situ, N balance was positive for most cases except treatment T5 in Chau Phu, T6 in Phu Tan and T1 in Phong Dien which all had a small N deficit

Phosphorus was in surplus even when straw was completely removed at all 4 sites except treatment T1 at Phong Dien However, P was in surplus to the extent of 18-70

kg P/ha in the waste water irrigated rice crops if rice straw was retained except if no P fertiliser at all was applied (Table 20)

Changes in soil properties

Soil properties of 4 sites were presented in Table 21 There was not much change in nutrient availability for the double rice system (CLRRI) because in this system rice straw was mostly retained In Chau Phu and Phu Tan, where a closed dyke was built

to protect rice crops in the wet season, rice straw is usually burnt to facilitate rapid land preparation Soil data of Chau Phu and Phu Tan showed reduction of organic carbon, total N and available N, P and K but not in available P at Phu Tan Soil analysis at Phong Dien after the 2nd rice crop did not show much variation of soil parameters except available N was sharply reduced, which is consistent with the negative N balances

Effect of wastewater irrigation on crop profit

In the farmer survey in An Giang, rice crops irrigated with wastewater had 1 t/ha extra yield The extra income from 1 t of rice is 4 millions VND The other benefit from using wastewater to irrigate crops is the cash saving from reduced fertiliser rates In the scenario where fertiliser was reduced to 33 % of N, 50 % for P and for K, the saving in fertiliser costs was equivalent to 1,161 million VND /ha If full straw removal is practiced, reducing fertiliser requirements to 50 % of recommended N, 33

% of P and 0 % of K, the saving in fertiliser costs decline to 0.658 million VND /ha (Calculations based on: paddy price= 4,000 VND/kg, urea = 6,000 VND/kg, super phosphate (16% P2O5)=3,600 VND/kg and KCl (60% K2O)= 9,000 VND/ha)

Discussion

Rice fertiliser rates and yield

Two rice crops per year are generally grown in the Mekong Delta and fertiliser is applied by farmers for both crops The CLRRI recommendations indicate that N

levels should be reduced in the wet season when yield potential is lower (Hung et al 1996) The total nutrient supply to 2 crops, following the CLRRI recommendation, would be 140 kg of N, 100 kg of P and 100 kg of K per hectare With straw removal these rates maintain close to neutral N balance, a positive P balance of about 16 kg P/ha/crop and a modest K balance of 5 kg/ha/crop (Cao et al 2010) However, it is common for farmers to exceed these recommendations in their N fertiliser application The cost of these fertilisers has risen substantially in recent years following international price trends Hence any technology that reduces fertiliser input costs while maintaining or increasing yield would be more profitable for rice farmers

A conservative conclusion from the experiments conducted in the present study is that use of wastewater for irrigation in the wet season and dry season could save 33 % of the N, and 50 % of both P and K fertiliser recommended for rice by CLRRI While the wet season crop requires only half as much waste water for irrigation as the dry season crop, due to the smaller water deficit from rainfall, the lower nutrient input is offset by lower yield Hence a similar reduction in fertiliser rate was applicable in both wet and dry seasons

With the exception of the preliminary yield assessment of farmers’ rice crops irrigated with waste water in 2007, the effect of the waste water irrigation was to reduce fertiliser inputs without affecting yield Hence the main economic benefit of using the waste water would be to decrease fertiliser costs, rather than to achieve increased yield When the catfish farmer also produces rice, as is common in An Giang Province (Cao et al 2009), the logistics for the transfer of wastewater to the padi field and for the capture of the reduced fertiliser costs within the family business enterprise seems quite obvious and simple By contrast, in many parts of the Mekong Delta the fish farmers run a specialised business with no rice production (Cao et al 2009) Here

the catfish farmers need to negotiate with rice farmers for disposal of their waste water If water discharge regulations were enforced, catfish farmers would have a strong incentive to negotiate such arrangements In the absence of regulatory inducements, the onus seems to be on convincing rice farmers of the value of the waste water resource, so that they seek arrangements with catfish farmers for access

to waste water A close working relationship between catfish farm operators and rice farmers would be necessary so that the timing of wastewater release can be coordinated with irrigation schedules for rice Catfish farmers may seek to recover some of their costs by a payment for the use of the waste water but would obviously need to set the price at less than the cost of the equivalent fertiliser saved by rice farmers

Wastewater from fishponds supplies comparatively large amounts of N, P and K when used to irrigate rice crops At the dry season irrigation rate, equivalent to 1000

mm depth of waste water (10 lots of 100 mm depth of water), the amounts of N more than doubled those in recommended N fertiliser while the P additions in wastewater more quadrupled the P fertiliser addition Clearly, the use of wastewater for irrigation affords the opportunity for farmers to greatly reduce their fertiliser costs Indeed, unless farmers reduce N fertiliser rate, they risk decreasing rice yield when applying wastewater due to increased risk of crop lodging Such experiences have been

reported to discourage farmers in the past from irrigating with wastewater On the other hand, some An Giang farmers had, through trial and error, reduced their N rate

to 20 kg N/ha, about 20 % of the recommended N rate When observed, all Phu Tan plots were very dark green suggesting excessive N supply, even when only 40 kg N/ha was applied (40 % of the recommended rate) Only 3 irrigations with waste water had occurred The farmer has over the last 5 years of using the wastewater to

irrigate his rice chosen 20 kg N/ha as the optimal N rate to apply to avoid excessive N

or risk crop lodging Hence the present recommendations are conservative and

especially with greater straw retention could be further reduced, as some farmer

practice suggests

Nutrient balance

The N additions in 1500 mm of wastewater/yr supplied an additional 50 % to that recommended for rice, suggesting that use of wastewater without adjusting fertiliser rates will cause large N surpluses in rice fields (Table 15) The main consequence of excess N is lodging of rice crops which may decrease yield Decreasing N fertiliser additions to 33 % of the recommended rate and relying on wastewater irrigation for the remaining N requirements would still generally supply more total N than is required by rice Straw removal would diminish the surplus of N carried forward to the following crop, but in the case of 33 % of the recommended N fertiliser rate would result in a deficit of N

In order to integrate wastewater irrigation with fertiliser practices, simple tools are required by farmers The composition of wastewater may vary from place to place and time to time as shown in the present study (Table 2) Hence, it is important that farmers are tooled with means to make decisions on how to adjust fertiliser application to supply the crop demand without applying excess or too little For N, the leaf colour chart is already in use by rice farmers to manage N fertiliser requirements The leaf colour chart could be used to determine whether supplementary N fertiliser was required while irrigating with wastewater On the other hand if the leaf colour chart indicates that a crop already contains adequate N, wastewater irrigation should be deferred in favour of river water irrigation

The P additions in wastewater were greatly in excess of rice crop requirements and more than double the fertiliser addition rate The fate of the surplus P depends on the

P sorption capacity of the soils On acid sulfate soils with very high P sorption, most

of the P that is not removed in harvested rice grain would be adsorbed by soils Indeed

in a related study testing the use of fishpond solid waste as a fertiliser substitute for rice, the increse in extractable soil P from positive P balance was equivalent to only 2-

3 % of the P surplus (Cao et al 2010) Hence, on acid sulfate soils in the Mekong Delta, the capacity for soils to sorb the surplus P seems adequate at present to prevent the release of P On alluvial soils, the capacity for positive P balances to result in marked increases in soil P are much greater However, continued application of such large P surpluses would eventually exceed the capacity of the soil to adsorb P and at this point soils would begin to release P into surface water creating a risk of eutrophication Straw removal was not sufficient to significantly decrease the surplus

of P Clarias fishpond wastewater which contained much lower P did not produce significant P surpluses

Relative to crop requirements and fertiliser additions, wastewater contained relatively modest amounts of K Nevertheless, addition of wastewater in addition to recommended fertiliser would result in a K surplus after every crop except if straw was completely removed

At CLRRI, when using wastewater to fully irrigate rice fields, it is estimated from the nutrient budget calculations that fertiliser rates should be adjusted from the recommended rate to 20 % of N, 0 % of P and 0 % of K if straw is retained If straw is removed, then fertiliser should be applied at 50 % of recommended N, 0 % of P and

100 % of K However, when no P was applied in several experiments, rice yield was strongly depressed This suggests that the supply of P in the wastewater was

insufficient or possibly the timing was not optimal for rice Adequate P is required by rice at sowing or transplanting for rot growth and tillering It appears that waste water does not supply sufficient for early growth and some P fertiliser is required at planting

The above calculations were based on wastewater quality at CLRRI Variable quality

of wastewater will affect the calculated balances and hence the adjustments needed for fertiliser rates For N, adjustments can be made using the leaf colour chart, but all the decisions about fertiliser P have to be made at sowing While topdressing of K is worthwhile for crops, there are no tools apart from leaf analysis that could be used in the season to adjust K fertiliser rates Leaf analysis is available but not readily practiced by farmers or extension agents, and relatively costly

Land requirements for wastewater irrigation

The calculations above were based on 500 mm of irrigation water to wet season crops and 1000 mm to dry season crops However, the required volumes of water may be much less For example, Hoa et al (2006) measured irrigation water applied to rice crops at CLRRI and determined it was only 400 mm From a case study at Chau Phu,

An Giang Province, it was reported that growing 3 rice crops per year required 12-13 irrigations, each supplying about 75 mm of water Hence in this location, 900-1000

mm of irrigation water was required per year Many locations only grow 2 rice crops per year, but in Chau Phu where 3 crops are grown, the main wet season crop uses no irrigation water Hence, the estimate of 800-1000 mm annual irrigation requirement seems reasonable for either 3 crops or 2 crops, although perhaps a conservative estimate

Large volumes of water need disposal from fishponds Assuming an average depth of

3 m for fishponds and a 9.2 cm depth of wastewater application to a rice field, 1 ha of fishpond requires about 33 ha of padi fields for each time the pond is fully emptied and re-filled This assumes rice uses 4.6 mm per day (evaporation and transpiration)

so that re-application of 9.2 mm wastewater to a field needs to occur every 2 days In practice farmers apply 50-100 mm of irrigation water every 10 days in the dry season and every 18-22 days in the early wet season The replacement of 33 % of pond water every day is a common practice (Cao et al 2009; Phan et al 2009) so full replacement notionally takes a 3-day cycle Hence 10-20 ha of padi land would be required to use the wastewater produced in the dry season, from 1 ha of fishponds assuming that only wastewater is used for irrigation In the early wet season, rainfall would prevent wastewater irrigation on some days, so that 20-40 ha of padi land may be required for every 1 ha of fishpond If waste water application on this cycle causes excess nutrient loading on padi fields, a further increase in padi land is required to apply this approach as a sustainable strategy for treating fishpond waste water Possibly 60-80

ha or more would be required for every 1 ha of fish pond Since about 5000 ha of land

is occupied by fishponds, > 350,000 ha of rice land may be required to fully treat wastewater in the Mekong Delta

Long term effects on soils

The long term effects of wastewater application on soil properties are unclear Clogging of soil pores can be a consequence of using wastewater containing suspended particulates However, in padi rice, slowing of percolation rates by pore clogging is not likely to harm crop growth If wastewater was used to irrigate crops like maize or vegetables as discussed below, pore clogging leading to sluggish drainage and anaerobic soil conditions may be detrimental to crop growth

Extra organic matter inputs may cause lower redox potential in soils, with the potential to increase Fe toxicity in some rice paddy soils of the Mekong Delta (Dobermann and Fairhurst 2000) The continuous loading of dissolved organic matter may also lead to changes in nutrient forms and availability Given the number of uncertainties, monitoring of soil conditions under long term application of wastewater would seem to be advisable

Management of waste water

While application of wastewater to rice through irrigation is effective as a fertiliser substitute, the capacity of rice to absorb nutrients is limited compared to some other crops Maize when managed to achieve yield of 10 t/ha has a high N, P and K demand (Dierolf et al 2000) and hence may be an alternative crop to irrigate with wastewater, especially in An Phu district of An Giang province where fishponds occur and maize

is an established crop Similarly, vegetables have a high nutrient demand and could be used for wastewater irrigation, although the area of land requiring irrigation is small compared to rice and may not make a significant contribution to recycling waste water

Alternative treatments need to be considered for the management of wastewater During the wet season, there needs to be a means of treating wastewater other than irrigating rice fields because water levels in the fields will already be high due to heavy rainfall Settling ponds, with or without aquatic plants and fish species other than catfish, could be used as holding ponds for wastewater before discharge into canals and the river Further research is needed to determine whether settling ponds would be sufficient treatment to reduce water quality to levels acceptable under the

2005 National Environment legislation

Even if no fertiliser P was added, the amounts of P in wastewater applied at 500 mm

to the wet season and 1000 mm to the dry season would results in large P surpluses It

is possible to reduce the rate of wastewater added per field, which would reduce nutrient surpluses of N and K as well as P However, this strategy is only feasible if sufficient rice land is available in proximity to the fishpond and arrangements between the owner of the fishpond and the rice fields are in place

Other strategies for removing P from wastewater may be needed such as passing the wastewater through a P-sorbant filter before release into paddy fields The acid sulfate soils of the Mekong Delta have high P sorption capacity and may be useful candidate materials for developing filtration systems The recovery of struvite (ammonium magnesium phosphate crystals) is a feasible technology for various types of waste including animal and human waste (Cordell et al 2009) As the planet moves towards the point at which annual global P production from rock phosphate reaches a peak (Peak P), there is increased interest in technologies that capture P in concentrated form so that it can be re-used in place of fertiliser from mineral rock phosphate

of waste water from fishponds to paddies However, if rice is irrigated with wastewater without any reduction in fertiliser addition, lodging of rice crops is likely, and depressed grain yield may be an adverse consequence While nutrient budgets indicate that use of fishpond wastewater can save 50 or more % of nitrogen, and up to

100 % of the phosphorus and potassium currently applied to crops as inorganic fertiliser, field trials showed that yields declined if no fertiliser P was applied and variations in waste water composition meant that such savings were not always assured The removal or retention of rice straw will greatly alter the N and K balances

in fields irrigated with wastewater, but regardless P balances were positive

Recycling of waste from fishponds for rice cultivation has the potential to alleviate water pollution by reducing the quantity of nutrients discharged directly to water sources Ten -20 ha of padi land would be required to use the wastewater produced in the dry season, from 1 ha of fishponds assuming that only wastewater is used for irrigation In the early wet season, rainfall would prevent wastewater irrigation on some days, so that 20-40 ha of padi land may be required for every 1 ha of fishpond

If waste water application on this cycle causes excess nutrient loading on padi fields,

a further increase in padi land is required to apply this approach as a sustainable strategy for treating fishpond waste water To cover these contingencies, 60-80 ha or more of land may be required for every 1 ha of fish pond

Continued monitoring of fields under treatment with fishpond waste is necessary to determine longer term effects on nutrient budgets, soil quality, rice yields and water quality

Acknowledgements

This research was financially supported by CARD project VIE/023/06 The assistance

of staff in the Soil Science Department and a student of An Giang University to carry out this study are greatly appreciated Thanks also to Cuu Long Rice Research Institute and the Ministry of Agriculture & Rural Development, Vietnam for the facilities and services granted to complete this investigation

Cao Van Phung and Bell, R.W (2009) Report on Baseline Surveys in Cantho and An Giang Provinces Project Progress Report CARD Project VIE/06/023 Cuu Long Rice Research Institute, O Mon

Cao, van Phung, Nguyen, be Phuc, Tran, kim Hoangand Bell, R.W., 2010 Nutrient recovery by rice crops as a treatment for aquaculture solid waste: crop yield, nutrient status and nutrient budgets Technical Report CARD Project VIE/06/023 Cuu Long Rice Research Institute, O Mon

Chapman, H.D., Pratt, P F., 1961 Methods of analysis for soil, plant and water Division of Agricultural Sciences, University of California, Riverside