Manure or biodigester effluent as fertilizer for duckweed

Kaensombath Lampheuy, San Thy* and T R Preston**

National University of Laos (NUOL)
Vientiane, Laos
lampheuylam@hotmail.com
** UTA (Cambodia), Phnom Penh 3, PO Box 2423, Cambodia
santhsimon@yahoo.com
** UTA (Colombia), AA #48, Santander, Colombia
regpreston@utafoundation.org

Abstract

An experiment to compare the growth response of duckweed (Lemna minor) to increasing concentrations of N from two sources of substrates was conducted. The substrates were raw cow manure (M) and effluent from biodigesters charged with cow manure (E). The experiment took place in An Giang University, Long Xuyen district, An Giang province, Vietnam. Raw cow manure and biodigester effluent were used as the fertilizers with five levels of N (0, 50, 100, 150 and 200 kg/ha), according to a 2 x 5 factorial arrangement with 2 replications per treatment with the split-plot design. Average air and water temperature during the test were 26 to 31 ^oC and 25 to 27 ^oC with the highest in the midday, when temperature was 38 and 29 ^oC, respectively. Plastics baskets lined with plastic film were used as experimental ponds. The surface of water in each basket was 0.16 m² with 10 cm depth, and the volume was 16 liters. Duckweed (Lemna spp) was inoculated at a rate of 200 g/m² (30 g/basket). The yield of duckweed was calculated by subtracting the inoculum from the total biomass production measured every 48 hours and was expressed as fresh duckweed yield g/m²/day. The fertilizers were applied five times in equal amounts during the experiment (20 days), according to the levels in each treatment.

There was no significant (P=0.85) interaction of  N level*fertilizer source for biomass production. Duckweed yield increased linearly with level of N in the fertilizer and was higher for effluent than for manure. Root length was shorter when the ponds were fertilized with the effluent, root length being negatively correlated with protein content of the duckweed (R² = 0.82 for E and R² = 0.71 for M) and with biomass yield (R² = 0.54 for E and M combined). Crude protein content in duckweed increased with N level applied and was higher for effluent than for manure.

It is suggested that biodigester effluent is a better N fertilizer than the raw manure for duckweed production in small ponds.  Root length can be used as a simple and cheap indicator of biomass yield and crude protein content in duckweed. Further studies concerning the sources of N for increasing the N content of duckweed production are suggested to improve the efficiency of integrated farming systems in developing countries.

Key words: Biodigester effluent, duckweed, Lemna minor, raw cow manure,

Introduction

Duckweed (Lemna spp.) is a water plant that is rich in nutrients. It has  been used as a main protein supplement for pigs (Bui Hong Van et al 1997) and ducks (Bui Xuan Men et al 1995; Nguyen Duc Anh et al 1997b) and also as a source of minerals for ruminants (Leng et al 1995). Duckweed has received research attention because of its high nutritive value, especially the high protein content and also because of its capacity to grow rapidly on nutrient-rich waste water and produce biomass rich in protein (Leng et al 1995).

Recently, several studies on duckweed have been conducted on growth and as a supplement feed to animals. In this connection, it has been reported that duckweed has a stimulating effect on live weight gain explained by an additional protein intake (Du Thanh Hang 1998). Also, a diet with 100 % replacement of soya bean by duckweed was the most profitable for farmers raising ducks (Bui Xuan Men et al 1995). On the other hand, several experiments have also been conducted on growth of duckweeds under different conditions. The results from these studies showed that the growth of duckweed is similar to that of any other plant. Moderate conditions of temperature and light and liquid medium with the necessary nutrients are essential for good growth. Also, duckweeds adapt well to a wide range of conditions and are easy to grow (Cross 2001).

It is considered that the use of effluent from biodigesters for growing duckweeds could be a way of increasing feed availability for animals and at the same time reducing problems of pollution to the environment.

Objective

The aim of this experiment was to study the growth and N content of duckweed as influenced by increasing concentrations of raw cow manure and  effluent from biodigesters charged with the same raw cow manure.
 

Material and methods

Location and temperature

The experiment was conducted in the experimental area of An Giang University, An Giang province, Vietnam, and lasted for 20 days. Average air and water temperature during the test were 26 to 31 ^oC and 25 to 27 ^oC with the highest in the midday, when temperature was 38 and 29 ^oC, respectively.

Treatments

Two fertilizers were compared:

• Raw cow manure (M)
• Biodigester effluent (E)

Each fertilizer was used in five treatments equivalent to levels of N of 0, 50, 100, 150 and 200 kg/ha. The raw cow manure was from cattle fed maize and rice straw in a dairy farm near Long Xuyen City, An Giang province, Vietnam. The effluent was from a plastic, plug-flow biodigester installed at the An Giang University (Bui Phan Thu Hang 2003), which was charged with the same cow manure.

Experimental design

The experiment was designed as a split-plot factorial arrangement (2 x 5) with two replications (blocks). Each replication consisted of 10 plastic baskets (5 levels; two types of fertilizer), the treatments being allocated at random within each block (Table 1).

Management and data collection

Plastic baskets lined with plastic film were used as experimental ponds. The surface of water in each basket was 0.16 m² with 10 cm depth giving a volume of 16 liters. Duckweed was inoculated at a rate of 200 g/m² in each treatment. The first day, the different fertilizers with different levels were added to each basket.

Manure was analyzed for N content every five days before being applied. The biodigester effluent was stored in a container (50 liters) and a sample analyzed at the beginning of the experiment. Fertilizer was applied every four days according to the level of N in each treatment (Appendix 1 shows the total amount of fertilizers that were used in each level).

Yield of duckweed and root length of the duckweed were determined at each harvest every two days.  The root length was measured with a graduated ruler (Rodriguez and Preston 1996). At  each harvest, all the biomass was removed from each basket and weighed. Thereafter, 30 g of duckweed were returned to the basket as inoculum. In every harvest, samples of duckweed were taken and bulked for analysis. DM content in duckweed was determined by drying the sample in a microwave radiation (Undersander et al 1993). The first and the last harvest of duckweed were analyzed for N content, and also for every three successive harvests of duckweed samples were taken and mixed for determination of N content following standard procedures (AOAC 1995). Water extractable DM and N in duckweed were analyzed in the last harvest according to Ly and Preston (1997). In addition, at the beginning and every five days,  water from each basket was sampled for analyzing pH with a glass electrode in a digital pH meter, and NH₃-N by steam distillation of samples treated with 40% NaOH..

Data analysis

Data were subjected to analysis of variance following the GLM option of the ANOVA software of Minitab version 13. Sources of variation in the model were sources of fertilizer, levels of fertilizer, interaction source*level and error.

Results and discussion

Biomass yield increased with level of fertilizer and was higher for the effluent than for the manure (Table 2; Figure 1).  This agrees with the findings of  Le Ha Chau (1998) who also compared biodigester effluent with manure but at a fixed N level of 150 kg/ha. There was no interaction level*fertilizer for biomass production.

Root length decreased with increasing N application and was shorter in the treatments with effluent compared with manure.  The interaction level*fertilizer was significant for root length the difference between the effluent and the manure increasing as the level of N was increased (P= 0.008). Root length was negatively correlated with protein content of the duckweed (Figure 2; R² = 0.82 for effluent;  Figure 3; R² = 0.70 for manure), and with biomass yield (Figure 4; R² = 0.54). These findings are similar to those reported by Le Ha Chau (1998).

The percentage of N in duckweed was increased (P=0.005) by fertilizer N level and was higher for effluent than for manure. There was no interaction (P=0.45) for N level*fertilizer.

Figure 1. Biomass yield of duckweed cultivated with increasing levels of  effluent and raw cow manure

There was a close relationship (R² = 0.80 for effluent and R² = 0.70 for manure) between root length and crude protein content of duckweed (Figures 2 and 3). It has been claimed that duckweed root length can be used as a  indicator of protein content in this water plant (Rodriguez and Preston 1996; Nguyen Duc Anh and Preston 1997a,b, 98; Le Ha Chau 1998; Phengsavanh Phonepaseuth 2001). The results of the present study confirm the usefulness of this relationship.

Figure 2: Relationship between root length and crude protein content of duckweed grown in water fertilized by biodigester effluent	Figure 3: Relationship between root length and crude protein content of duckweed grown in water fertilized by raw cow manure

There was a good relationship ( R² = 0.54) between root length and biomass yield, as when the roots were long, the duckweed biomass yield was low. Therefore the relationship between root length and biomass yield can be used as indicator for the yield and crude protein content of duckweed, in agreement with findings of Le Ha Chau (1998).

Figure 4. Relationship between root length and biomass yield of duckweed

The interaction N level*fertilizer was significant (P= 0.001)(Figure 5) for water extractable DM which increased linearly (R² = 0.95)  with increasing N from effluent but showed a curvilinear relationship (R² =0.80) with N level in the manure. The results with the effluent are similar to those reported by Phengsavanh Phonepaseuth (2001); however, this parameter does not appear to have been studied with manure as fertilizer.

Figure 5. Effect of fertilizer N level in effluent and manure on water extractable DM of  duckweed

There was no effect of fertilizer N level, or source, on the DM content of the duckweed nor on the water-extractable N (Table 3).  Crude protein increased with fertilizer N level but the rate of increase was more marked with the effluent (Figure 6). The increase in crude protein content with N level in the effluent is in agreement with the findings of Phengsavanh Phonepaseuth (2001).

The results obtained from the present study with high levels of biodigester effluent were similar to the data of duckweed grown under ideal conditions and harvested regularly (DM of around 5%; and crude protein of 34%) as reported by Leng et al (1995).

Figure 6. Crude protein content of duckweed grown in water fertilized with
different sources and levels of N

The  ammonia nitrogen content in the pond water increased with fertilizer level but was  not different  between effluent and manure (Table 4).  The water in the  pond contained more NH₃-N when the level of N was increased, rising to to 3.47 to 4.18 mg/litre in the water, at the fertilizer level of 200 kg N/ha. The pH value of the water in the pond was in the range of 6.9 to 7.4 and 6.8 to 7.2 for effluent and raw cow manure, respectively. There was a good relationship (R² = 0.61) between root length and ammonia N in water; the roots were shorter when the value of ammonia increased (Figure 7). The crude protein content of the duckweed was directly related with the the ammonia N level in the water fertilized with effluent  (R² = 0.91) (Figure 8).

The ammonia concentration in the pond water increased with level of added N but there was no difference between the effluent and manure treatment (Table 4). The pH was higher in the ponds fertilized with effluent compared with manure.

Conclusions

• Fertilizing the pond water with effluent from biodigesters charged with cow manure supported higher yields of duckweed biomass, which was of higher crude protein content, as compared with the use of the raw cow manure.
• Increasing levels of N fertilizer in the pond  increased biomass yield and crude protein content of the duckweed.
• The root length of the duckweed was inversely related to protein content, and this index is an excellent indicator of the protein status of this aquatic plant.

Acknowledgements

We are grateful to the MEKARN Program, supported by SIDA-SAREC, for providing the financial support for this experiment. Thanks are also given to the staff of  UTA (Cambodia), and of An Giang University,  who provided practical support and advice and assistance in the laboratory.
 

References

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Received 21 February 2004; Accepted 25 February 2004

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