J Sci Food Agric 1995,69,67-71

Selection of Annual Legumes for Incorporation into Rice Farming Systems U R Sangakkara Faculty of Agriculture, University of Peradeniya, Peradeniya, Sri Lanka (Received 5 September 1994; revised version received 1 March 1995; accepted 31 March 1995)

Abstract: Legumes are often recommended for inclusion into rice farming systems to provide organic matter and nitrogen. Thus, a study evaluated the growth, yields and nitrogen dynamics of four popular legumes, namely Sesbania, Mungbean, Cowpea and French bean using "N, when grown in a rice soil. Sesbania produced the highest quantity of nitrogen-rich dry matter. In contrast, French beans produced the lowest quantity, which was low in nitrogen. Dry matter contents of other legumes available for incorporation were greater than in French beans. Nitrogen dynamics illustrated the very high nitrogen fixing capac- ity of sesbania, while the eficiency of symbiotic fixation was low in French beans. The net nitrogen balance of sesbania was high, followed by cowpea and mung- bean. The study illustrated the benefits of using sesbania for the purpose of adding green manure and nitrogen. If a food legume is to be used, species such as cowpea or mungbean was more suitable to achieve the stated objectives than French bean, which produced the highest economic yield.

Key words: legumes, nitrogen, rice farming systems.

INTRODUCTION

Legumes are widely cultivated for food, fodder, shade, fuel and timber, as cover or green manure crops. Thus they are an integral part of cropping, grazing, plan- tation and agroforestry systems, all of which are common in tropical developing countries (Peoples and Craswell 1992). However, the role of legumes in rice systems of the tropics is limited, although the crop pro- duces the staple diet of the region from the most common cropping pattern (George et al 1992). This is due to the cultivation of rice under puddled conditions and the use of mineral fertilisers (Bacon 1990).

Agricultural enterprises at the present time are becoming increasingly concerned with a desire of main- taining sustainability of tropical cropping systems. This is due to the reduction in yields of most conventional chemical based cropping systems (Odum 1989; Bohlool et al 1992). However, rising costs of chemical fertilisers and the removal of subsidies has led to reduced use of these inputs, resulting in lower yields. Furthermore, a decline in rice yields under continued use of chemical fertilisers has also been recorded in Asia.

Nitrogen is an important nutrient in rice culture, which is often limiting in most tropical farming systems

67

(Peoples and Herridge 1990). In addition, flooding of rice soils leads to the loss of nitrates (Buresh et a1 1989). Thus, research is aimed at seeking possible methods of maintaining the balance of nitrogen. The use of legume to provide organic matter and nitrogen to these systems is considered a suitable option to achieve this objective.

In most Asian rice farming systems, there is short dry period of 2-3 months between the cropping seasons (George et al 1992). These short dry periods are ideal times for the incorporation of legumes to provide organic matter and nitrogen. However, the contribution of nitrogen by a legume to a succeeding crop is depen- dant upon its ability to fix a greater quantity of atmo- spheric nitrogen than that removed by the harvested product (Peoples el al 1993).

Analysis of several studies in different locations in the tropics (Peoples and Craswell 1992) illustrate that some legumes remove a greater quantity of nitrogen than that fixed, causing a loss of nitrogen from the rhizosphere. Thus, this phenomenon becomes an important criterion in the selection of legumes for cropping systems, to obtain a positive contribution of nitrogen to the rhi- zosphere and the succeeding crop. However, compara- tive studies on the merits of different tropical legumes to provide nitrogen and organic matter to rice systems

J Sci Food Agric 0022-5142/95/$09.00 CI 1995 SCI. Printed in Great Britain

68 U R Sangakkara

have not been widely reported within the Asian mon- soonal climates, where two crops are grown every year. Thus, a study was carried out using "N-enriched nitro- gen to identify the nitrogen-fixing capacity of four popular tropical legumes, which could easily be incorp- orated into rice farming systems during the fallow periods. The objective of the study was to determine the net nitrogen balance of the legumes using "N, thereby evaluating the suitability of the crops for incorporation into rice farming systems to supply organic matter and nitrogen to the rhizosphere.

MATERIALS AND METHODS

The experiment was conducted at the plant house of the Faculty of Agriculture (University of Peradeniya, Sri Lanka). The planting medium used was made up of soil obtained from a rice field and washed river sand, mixed in the proportions of 75 : 25, respectively. Thus, the soil mixture had a sandy clay loam texture, with a pH ( 1 :2.5 H 2 0 ) 7.15 f 0.16; total N content of 0.32 0.04% ; organic carbon content of 0.42 5 0.1 1 % and a CEC of 1084 meq per 100 g soil f4.9.

Plastic pots (capacity 4.0 litre) were filled with 4 kg of air dried soil. A uniform application of phosphorus and potassium (22 mg P and 32 mg K per plant) was added to each pot. Uniform seeds of the selected legumes, namely sesbania (Sesbania rostrata), mungbean ( Vignaradiata), French beans (Phaseolus vulgaris), cowpea (Vigna unguiculata) and finger millet (Elucine coracana) which was the test crop, were planted and thinned to two plants per pot soon after germination. The soil moisture of all pots was maintained between field capacity throughout the study. A uniform rate of nitrogen (10 mg per plant) was added in the form of 10% excess 15N-enriched NH,SO, to each pot at plan- ting. The experiment thus had five species as treatments and was laid out in a randomized block design with five replicates per crop.

Sampling was carried out at V2, V4, V6 and V8 growth stages in each of the selected legumes and dry

weights determined after drying at 80°C for 48 h. This data was used for the calculation of relative growth rates of the individual legumes as described by Fageria (1992). The relative growth rates were based on increments of dry matter per unit dry matter over time.

The seed legumes (mungbean, sesbania and cowpea) were grown until seed maturity, (R8 growth stage), and French bean until pod development (R6 growth stage). However, at flowering (R2) and harvest (R8) growth stages of each legume, the dry weights of shoots and roots of each legume and of millet were determined. Seed yields of mungbean, cowpea, sesbania and fresh pod yields of French beans were recorded at the R8 growth stage.

The total nitrogen contents (Kjeldhal) of shoots and roots of the legumes and millet at the R2 and R8 growth stages were determined. The 'N enrichments of these were measured by emission spectrometry (Axmann 1990). The nitrogen dynamics were calculated by the equations described by Kumarasinghe et a1 (1 992).

RESULTS AND DISCUSSION

Sesbania had the highest relative growth rate over the vegetative phase (Table 1). This was consistent with the dry weights of this species at the R2 and R8 growth stages. Thus, at both samplings, sesbania could add a greater quantity of organic matter to the rice farming system, thereby enhancing soil properties and sustaina- bility, as shown by Ladha et a1 (1988). In contrast, French beans had the lowest relative growth rates and dry weights. Thus, this species provided the lowest rela- tive growth rates and dry weights and this species pro- vided the lowest quantity of organic matter.

Cowpea had a lower relative growth rate than mung- bean, although dry weights were greater (Table 1) because it grows better under dry conditions (Pandey and Anake 1985) and did not thrive under conditions of this experiment. However, cowpea provided a greater

TABLE 1 Dry matter accumulation and yields of selected legumes

Species Dry wt' ( g per plant) RGR* Yield ( g g- ' per week) ( g per p h i )

R2 R8

Sesbania 7.45 21.24 0.282 2.84 (seed) Mungbean 6.18 17.30 0.174 5.58 (seed)

Cowpea 7.25 19.42 0.165 7.24 (seed) LSD ( P = 0.05) 0.146 16-42 O W 4 0.963 (seed)

French bean 4.85 12.92 0.1 14 204.16 (pod)

' R2 and R8 represent the growth stages of flowering and harvest, respectively. RGR, relative growth rate.

Legumes for rice farming sysrems 69

quantity of organic matter than mungbean due to its plant structure.

All species produced an economic yield although quantities differed significantly. The highest yield was produced by French beans in the form of fresh pods. Cowpea and mungbean seed yields were similar, and were edible products. In contrast, the low seed yield of sesbania could only be used for planting. As farmers would prefer an edible or saleable product, cowpea, mungbean or French beans would be preferred to sup- plement incomes from rice farming systems.

Sesbania had the highest nitrogen content (Table 2). Cowpea residues had a greater quantity of nitrogen than mungbean, while French beans had the lowest values. This indicated the value of using either mung- bean or cowpea for rice farming systems, if a food legume is included as a green manure.

Evaluation of sources of nitrogen (Table 2) illustrated the high biological nitrogen fixing capacity of sesbania. Thus, the utilisation of soil and fertiliser nitrogen by this species was comparatively low. In contrast, 27% of the total nitrogen content of French beans was obtained from fertiliser, while 13% was from the soil. The nitro- gen fixing capacity of cowpea was greater than in mungbean. Thus cowpea was less dependent on soil and fertiliser nitrogen than mungbean, which makes it a more suitable species for rice farming systems, where nitrogen contents are low (George et al 1992).

Nitrogen balances of all species at the two samplings are presented in Table 3. At the first sampling, there is no removal of nitrogen from the rhizosphere, due to the absence of a harvest. Thus, the optimal time of incorp- oration of legumes into rice farming systems for green

TABLE 3 Nitrogen removed by the harvest and the net balance of

atmospheric N of selected legumes

Species N removed at harvest Net balance of (mg per plant) atmospheric N

(ms per plant)

Sesbania + 274 R2" -

R8" 84 + 573

+ 159 R2 -

R8 248 + 135

R2 - R8 1843 - 1639

+ 206 R2 - R8 283 + 177

Crop 24.9 11.9 Harvest 32.1 66.8

Mungbean

French bean + 86

Cowpea

LSD ( P = 0.05)

" R2 and R8 represent the growth stages of flowering and harvest, respectively.

manuring would be at flowering, as reported by Buresh and Datta (1991) and Roger and Ladha (1992); although farmers would often be reluctant to undertake this operation with food legumes. At the final sampling, which corresponded to harvesting, where nitrogen is removed from the rhizosphere in the form of a bio- logical yield in the food legumes (seeds in mungbean and cowpea and fresh pods in French bean), the net

TABLE 2 Nitrogen contents and uptake of the selected legumes

Species Total N content NdFF N d F A NdFS' Ratio of N d : NdFF + N d F S (mg per plant) (mg per plant) (mg per plant)

Sesbania R2' 348 35 274 39 3.75 R8' 817 98 657 62 4.13

R2 215 29 160 26 2.94 R8 549 106 385 59 3.03

R2 181 44 86 51 0.90 R8 351 98 205 48 1.39

R2 287 34 206 47 2.57 R8 592 101 46 1 30 3.53

Crop 42.41 45.09 108.94 31.06 Harvest 20.40 17.04 144.05 4.94

Mungbean

French bean

Cowpea

LSD ( P = 0.05)

' NdFF, NdFA and NdFS correspond to nitrogen derived from fertiliser, atmosphere and soil, respectively. R2 and R8 represent the growth stages of flowering and harvest, respectively.

70 U R Sangakkara

nitrogen balance due to sesbania increased. This is due to the greater quantity of fixed nitrogen of the stem and root nodules of this species rather than seeds (Pingali et a1 1990). In contrast, removal of a greater quantity of seeds or pods from food legumes reduced the net nitro- gen balance. French beans removed a greater quantity of nitrogen than was added by biological fixation. This indicated its unsuitability as a green manure, especially if a crop is harvested. This species would remove nitro- gen from the soil, thus depriving the subsequent rice crop of this important element, although farmers would prefer this species due to the high quantity of pods pro- duced. A greater quantity of nitrogen was added to the rhizosphere by cowpea residues than mungbean. This is due to the ability of this species to fix atmospheric nitrogen in most tropical ecosystems (Mulongoy 1985). In contrast, mungbean added a lower quantum of nitro- gen to the soil, which suggested its lower efficiency in fixing atmospheric nitrogen under conditions of this study.

The efficiency of biological nitrogen fixation of the selected species in relation to the use of soil and fertil- iser nitrogen illustrated the benefits of using sesbania (Table 3). The efficiencies of cowpea and mungbean were lower. The efficiency of French bean was the lowest, due to the poor nitrogen fixing capacity of this species (Sangakkara 1991). This confirmed the poor utility value of French beans as a green manure, espe- cially if pods are harvested.

The ratios between nitrogen fixed to that obtained from the soil and fertiliser increased with time. However, incorporation of mungbean and cowpea would add a lower quantity of nitrogen to rhizosphere for the succeeding rice crop than sesbania, even at maturity. This again suggested the benefits of sesbania for green manuring rather than mungbean or cowpea, from which an edible produce is harvested.

CONCLUSIONS

Smallholder farmers in the tropics do not generally plant legumes to enhance organic matter and nitrogen status of soils, although research (eg Ladha et al 1988) has highlighted its benefits. However, they try to obtain maximum production from their lands. Thus, most farmers would expect an economic return from a legume included into their rice farming systems inbet- ween the two major rice seasons.

Most food legumes do not provide a substantial quantity of organic matter or nitrogen, due to the removal of a nitrogen rich product at harvest, which generates an income. This calls for the careful selection of legumes to achieve the primary objective of farmers to add organic matter and nitrogen to the rhizosphere.

The greenhouse study clearly illustrated the benefits of using a green manure crop such as sesbania and the

disadvantages of using a food legume such as French beans. While the former species fixed large quantities of atmospheric nitrogen which was available for incorpor- ation, the latter removed a significant quantity of both soil and fertiliser nitrogen from the soil. Thus a compro- mise would be a species such as cowpea and mungbean. These produce a relatively high quantity of organic matter, provide an economic product and also add nitrogen to the rhizosphere. However, further studies on the benefits of these and other legumes in terms of effi- ciency of symbiotic nitrogen fixation and contribution to the non legume crop need to be carried under field conditions. These studies would quantify the beneficial role of food and other legumes in increasing the pro- ductivity of rice farming systems by utilising the land at a time it is generally left fallow.

ACKNOWLEDGEMENTS

Gratitude is expressed to Ms AMU Attanayake and Mr ER Piyadasa for research assistance, the University of Peradeniya for partial funding and Professor Dr J Nos- berger for facilities to prepare this manuscript.

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