Agronomy Journal Grow Your Career With ASA
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published online 27 June 2006
Published in Agron J 98:1097-1108 (2006)
DOI: 10.2134/agronj2005.0328
© 2006 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Ghosh, P. K.
Right arrow Articles by Subba Rao, A.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Ghosh, P. K.
Right arrow Articles by Subba Rao, A.
Agricola
Right arrow Articles by Ghosh, P. K.
Right arrow Articles by Subba Rao, A.
Related Collections
Right arrow Intercropping Systems

Intercropping

Interspecific Interaction and Nutrient Use in Soybean/Sorghum Intercropping System

P. K. Ghosha,*, M. C. Mannab, K. K. Bandyopadhyayb, Ajayb, A. K. Tripathib, R. H. Wanjarib, K. M. Hatib, A. K. Misrab, C. L. Acharyab and A. Subba Raob

a ICAR Research Complex for NEH Region, Umiam, Meghalaya 793103
b Indian Inst. of Soil Sci., Nabibagh, Berasia Rd., Bhopal, Madhya Pradesh, India 462038

* Corresponding author (ghosh_pk2006{at}yahoo.com)

Received for publication December 7, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Intercropping soybean (Glycine max L.) with sorghum (Sorghum bicolor L.) is common in the semiarid tropics of India. In most intercropping studies, little attention has been paid to belowground interaction and nutrient management other than N while assessing yield advantage. In a 5-yr field experiment (2000–2004), we evaluated the impact of fertilizer and organic manures on below- and aboveground interaction, competitive ability, and economic viability of this intercropping at the Indian Institute of Soil Science, Bhopal for deep Vertisol (isohyperthermic Typic Haplustert) of central India. Above- and belowground growth components as well as biological activities were greatly improved in intercropped sorghum while the value of these except nitrate reductase (NR) activity, soil microbial biomass C (SMBC), and dehydrogenase activity (DHA) were reduced in intercropped soybean indicating interspecies competition between component crops. The increased NR activity, SMBC, and DHA in intercropped soybean revealed interspecies facilitation between the component crops. This showed that interspecies competition concurred with interspecies facilitation in this system. We observed that yield and land equivalent ratio (LER) of both the intercrops increased over sole crops though based on aggressivity and relative crowding coefficient (RCC), sorghum (+) is more competitive than soybean. Interaction of yield with different components indicated that three belowground components, i.e., NR activity in root (r = 0.62, r = 0.63, P < 0.05), root length density (r = 0.36, r = 0.33, P < 0.05), and SMBC (r = 0.71, r = 0.66, P < 0.05) of both intercrop soybean and intercrop sorghum, respectively, had the greater effect on yield advantage in the intercropping system. Soybean did not benefit from intercropping to the same degree as sorghum under N–P–K. Nutrient application influenced LER, RCC, and monetary advantage index and was found in the order of N–P–K plus farmyard manure (FYM) > N–P–K plus poultry manure (PM) > N–P–K plus phosphocompost (PC) > N–P–K > control. However, based on competition ratio, yield advantage was greater under N–P–K plus PM. The results suggest that sorghum is the major contributor to the mixture yield and that the integrated use of N–P–K plus FYM or N–P–K plus PM is an important nutrient management option for sustaining this intercropping system, particularly to benefit the legume component.

Abbreviations: CGR, crop growth rate • CR, competition ratio • DAS, days after sowing • DHA, dehydrogenase activity • FYM, farmyard manure • HI, harvest index • LER, land equivalent ratio • MAI, monetary advantage index • NR, nitrate reductase • PC, phosphocompost • PM, poultry manure • RCC, relative crowding coefficient • RLD, root length density • SMBC, soil microbial biomass carbon • TPF, triphenyl-formazan • WUE, water use efficiency


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
INTERCROPPING IS practiced in many parts of the world (Francis, 1986) because of some of the established and anticipated advantages such as greater yield stability (Jensen, 1996), greater land-use efficiency (Ofori and Stern, 1987), increased competitive ability toward weeds (Hauggaard-Nielsen et al., 2001a), improvement of soil fertility due to the addition of N by fixation (Hauggaard-Nielsen et al., 2001b; Jensen, 1996), and some favorable exudates from the component legume (Willey, 1979; Ofori and Stern, 1987). Almost all published intercropping combinations with a significant yield advantage were nonlegume/legume combinations (Morris and Garrity, 1993). Compared with corresponding sole crops, yield advantages have been recorded in many nonlegume/legume intercropping systems, including maize (Zea mays L.)/soybean (West and Griffith, 1992; Ghaffarzadeh et al., 1994), sorghum/soybean (Elmore and Jackobs, 1986), wheat (Triticum aestivum L.)/mungbean [Vigna radiata (L.) R. Wilczek] (Chowdhury and Rosario, 1994), barley (Hordeum vulgare L.)/medic (Medicago spp.) (Moynihan et al., 1996), canola (Brassica spp.)/soybean (Ayisi et al., 1997), groundnut (Arachis hypogaea L.)/pearl millet [Pennisetum glaucum (L.) R. Br.] (Ghosh and Devi Dayal, 1998), maize/faba bean (Vicia faba L.) (Li et al., 1999), pearl millet/cluster bean [Cyamopsis tetragonoloba (L.) Taub.] (Yadav and Yadav, 2001), groundnut/cereal fodders (Ghosh, 2004), barley/pea (Pisum sativum L.) (Chen et al., 2004), and faba bean/barley (Trydemanknudsen et al., 2004).

Many studies of cereal/legume intercropping have shown that the quantity of N fixed by the legume depends on such factors as the morphology, density, and competitive ability of the legume (Ofori and Stern, 1987); the effectiveness of the rhizobia symbiosis; and the system of intercropping (Rerkasem et al., 1988). The basic physiological and morphological differences between nonlegume and legume benefit their mutual association (Akuda, 2001). The differences in the depth of rooting, lateral root spread, and root densities are some of the factors that affect competition between the component crops in an intercropping system for water and nutrients, and hence input use efficiency. The cereal component, which usually is taller, has a faster-growing or more extensive root system, particularly a larger mass of fine roots (Lehmann et al., 1998), and is competitive for soil inorganic N (Carr et al., 1998, 2004; Carruthers et al., 2000). This forces the legume component to fix N from the atmosphere (Jensen, 1996; Hauggaard-Nielsen et al., 2001b).

Interspecific competition is defined as an interaction between two species that reduces the fitness of one (0,-) or both of them (-,-) (Crawley, 1997). Interspecies interactions, including above- and belowground competition and facilitation, play an important role in determining the structure and dynamics of plant communities in agriculture (Callaway, 1998; Aerts, 1999). For interspecies interactions in an intercropping "ecosystem," more has been reported on interspecies aboveground interactions than belowground interactions (Vandermeer, 1989). Various researchers (Donald, 1958; Martin and Snaydon, 1982; Wilson and Newman, 1987; Wilson, 1988) found that root competition had a greater effect than shoot competition, but there was a positive interaction between the two. In wheat/clover intercropping, Dauro and Mohamedsaleem (1995) also found that root but not shoot interaction significantly affected the yields of the two component crops.

Soybean is the most important oilseed crop grown during the rainy season on Vertisols of the semiarid tropics of central India. It is traditionally intercropped with sorghum and maize by marginal and submarginal farmers during the rainy season (Gupta and Rajput, 2001). Imbalanced nutrient application coupled with low native soil N and P is a major constraint limiting productivity of intercropping systems on Vertisols in central India. Nutrient use in intercropping systems has received considerable attention despite difficulties in quantifying beneficial or competitive effects. The competition for nutrients is important and can begin early in the growth of the component crops in a cereal/legume intercropping system. There is a lot of published information on N acquisition, which has focused on nonlegume/legume combinations in various cereal/legume associations, e.g., barley/pea (Jensen, 1996), maize/bean (Siame et al., 1998), mucuna [Mucuna pruriens (L.) DC.]/maize (Sanginga et al., 1996), faba bean/barley (Trydemanknudsen et al., 2004), and legume genotypes/maize (Mandimba, 1995). Greater amount of biomass and N yields in legume/nonlegume intercropping systems compared with their respective sole crops has been reported (Wahue and Miller, 1978; Rerkasem et al., 1988; Fujita et al., 1990). In addition, P acquisition was studied in wheat/lupin (Lupinus albus L.) (Gardner and Boundy, 1983; Horst and Waschikes, 1987) and pigeonpea [Cajanus cajan (L.) Millsp.]/sorghum associations (Ae et al., 1990). However, little work has been done to see the effect of nutrient application on competitive ability though Senaratne et al. (1993) reported improvement in the competitive ability of the legume in legumes/grass mixture with K application.

We hypothesized that nutrient application of both organic and inorganic fertilizer could influence competitive ability of component crops and yield of the intercropping system. Willey and Reddy (1981) tried to separate above and below-ground interaction in groundnut/pearl millet intercropping. However, there is a need to assess relationship of each component with yield in intercropping system. The present study, therefore, attempts (i) to assess and compare below- and aboveground competition and their relationship with yield under a varying set of nutrient applications, (ii) to study interaction of root with other growth components, and (iii) to identify nutrient management options that lead to higher yield and income from the intercropping system under rainfed conditions of the semiarid tropics.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Site
Field experiments were conducted during five rainy seasons (2000–2004) at Bhopal, Madhya Pradesh, India (23°18' N, 77°24' E; 485 m above mean sea level) at a fixed site. The soil was deep Vertisol (isohyperthermic Typic Haplustert) with clayey texture (52% clay) having 62% water-holding capacity, bulk density of 1.34 Mg m–3 at 27% soil moisture content, and moisture retention at 0.033 and 1.5 MPa of 40.6 and 25.6%, respectively, in the plow layer (0–15 cm). The pH of the surface soil was 8.1, organic C 5.2 g kg–1, electrical conductivity 0.52 dS m–1, and cation exchange capacity 46 cmol (p+) kg–1. The available N (alkaline permanganate N at 145 kg ha–1) (Subbiah and Asija, 1956) and available P (Olsen P at 10.7 kg ha–1) were low, and available K (ammonium acetate K at 325 kg ha–1) was high. The experimental site is located in a hot subhumid climate. The 22-yr average annual precipitation is 1016 mm, and average potential evapotranspiration is 1400 mm. Average maximum monthly temperature (40°C) is reached in May while the minimum (10°C) is in January. The total rainfall in 2000, 2001, 2002, 2003, and 2004 during the crop growing season (June–October) was 643, 767, 709, 1008, and 794, respectively (Fig. 1 ). All the 5 yr except in 2003 experienced low rainfall leading to drought conditions during a part of the crop growing season. In comparison to the 22-yr average, the rainfall during the period of experimentation was not only low but also erratic as well. As per the normal trend, most of the rains were received during June–August. Negligible rainfall was received in September and October except in 2003. Of the 5 yr, rainfall during 2003 was comparatively well distributed during the crop growth period (Fig. 1). Soybean, wheat, cotton (Gossypium hirsutum L.), pulses, sorghum, and maize are mainly grown on this soil.


Figure 1
View larger version (19K):
[in this window]
[in a new window]
  		Fig. 1. Total monthly rainfall from January 2000 to December 2004 and the 22-yr average near the experimental site on the research farm of the Indian Institute of Soil Science, Bhopal, India.

 
Preparation of Phosphocompost
Phosphocompost was prepared using fresh cow dung, soil, and wheat straw in the ratio of 100:1:100 (dry weight basis). Mussoorie rock phosphate (100 mesh) at 2.2% P, pyrite (22.2% S) at 10%, and urea N at 0.5% were also added to the mixture on dry weight basis. The materials were allowed to decompose for a period of 90 d. The chemical constituents of organic manures including PC are given in Table 1.


View this table:
[in this window]
[in a new window]
  		Table 1. Constituents of different organic manures.

 
Field Methods and Intercropping System
Every year sorghum cultivar CSH-9 (sole as well as intercrop) was sown on the same day with soybean (cv. JS 335) in the last week of June. Initially three to five seeds of sorghum were planted per hill at a row spacing of 60 cm. One week after emergence, the seedlings were thinned to one plant per hill to achieve a plant density of 50 x 103 ha–1. For soybean, row spacing of 30 cm was maintained to get a plant density of 333 x 103 ha–1 (Fig. 2 ). The overall proportional density of each crop species was equal in both sole cropping and intercropping treatments.


Figure 2
View larger version (9K):
[in this window]
[in a new window]
  		Fig. 2. Sketch of soybean/sorghum intercrop row (each spacing of 30 cm) pattern.

 
The experimental design was arranged in a split plot with three replicates, with intercropping as main plots (sole soybean, sole sorghum, and soybean/sorghum intercropping) and nutrient treatment (control, 75% N–P–K, 100% N–P–K, 75% N–P–K plus FYM at 5 Mg ha–1, 75% N–P–K plus PC at 5 Mg ha–1, and 75% N–P–K plus PM at 1.5 Mg ha–1) as subplots. The N–P–K applications (100% NPK) were equivalent to 30:26.5:25 kg ha–1 for soybean and 90:26.5:25 kg ha–1 for sorghum, respectively. The rate of manures was decided based on N equivalence and was applied to the crops on a dry weight basis 2 wk before sowing. The plot size was 9 by 6 m while the net plot size was 7 by 4 m. The fertilizer requirement of the intercrop was calculated on the basis of proportion of plant population of the component crops. The entire dose of N, P, and K was applied as basal to soybean, whereas in sorghum, 50% N and the entire dose of P and K were applied as basal and the remaining 50% N was topdressed at 30 d after sowing (DAS). The plant protection and other management practices were followed during cultivation. Crops were rainfed. Hand weeding was done one month after sowing to keep the field free from weeds. Soybean and sorghum were harvested manually by sickle from ground level at 100 and 105 DAS, respectively.

Laboratory Analysis
Plant Dry Matter
Plant samples from each plot were collected for growth analysis at 15-d intervals between 30 DAS and harvest. The plant samples were oven-dried at 65°C for 72 h until a constant weight and dry weight were recorded. Crop growth rate (CGR), the increase in dry weight per unit ground area of crop in a unit time, was calculated as

Formula 1[1]
where W1 and W2 are dry weights at times t1 and t2, respectively, and expressed as g m–2 d–1.

Nodule Mass and Nitrogen Content
For recording nodulation, three soybean plants were uprooted with a ball of soil. Keeping the root portion intact, the ball of soil was washed gently with clean running water followed by washing with camel hairbrush to dislodge soil particles adhering to root. Nodules from roots were removed, counted, and dry mass of nodule was measured (Vincent, 1970). Nitrogen concentration of shoot from a bulk sample was estimated at 60 DAS using micro-Kjeldahl method as described by Kuo et al. (1997).

Plant Enzyme, Photosynthesis, and Chlorophyll Content
The second fully expanded leaf from the top of soybean and sorghum plants from each plot was used for studying enzymes, pigments, and photosynthesis. Photosynthesis was recorded by Infrared gas analyzer (CID-510). Five-gram fresh leaf was taken and assayed for NR activity following the method of Hageman and Hucklesby (1971). Total chlorophyll contents in fresh leaves were analyzed adopting the nonmacerating procedure described by Hiscox and Israelstam (1979).

Root
Root samples were collected at 60 DAS using root-sampling cores (6 cm height, 8.6 cm diam.) up to a depth of 30 cm. After thorough washing and staining with methyl violet, root length was determined by Delta-T (Cambridge, UK) scanner. The root length was divided by core volume to estimate root length density (RLD). Roots were then dried at 65°C to a constant weight and weighed.

Soil Microbial Biomass Carbon
For analysis of SMBC, a 40-g moist soil sample (2 mm) was kept for 3 d of preincubation at 25°C before attaining basal respiration condition. The SMBC was determined by the ethanol-free chloroform fumigation extraction method (Vance et al., 1987) using Kc value of 0.45 (Jenkinson and Ladd, 1981).

Dehydrogenase Activity
Dehydrogenase activity was measured by the method of Casida et al. (1964). Moist soil samples (4 g) were taken in test tubes, adding 1 mL of 3% aqueous solution of triphenyl tetrazolium chloride, 40 mg of CaCO3, and 2.5 mL of distilled water. The suspension was mixed with a glass rod and incubated for 24 h at 37°C. Triphenyl-formazan (TPF) was extracted by transferring the soil with the aid of methanol from each tube, and the color intensity was determined in a spectrophotometer. The DHA was expressed as µg TPF g–1 24 h–1 on dry weight basis.

Estimation of Climatological and Agrobiological Parameters
Yield advantage of intercropping was calculated according to Ofori and Stern (1987). The LER, an accurate assessment of the biological efficiency of the intercropping situation, was calculated as

Formula 2[2]
where Yaa and Ybb are yields as sole crops of a and b and Yab and Yba are yields as intercrops of a and b. Values of LER greater than 1 are considered advantageous. The LER has also been used to calculate monetary advantage.

Relative crowding coefficient (RCC) is a measure of relative dominance of one component crop over another in an intercropping system. For crop a in association with crop b,

Formula 3[3]
where Xab is the sown row proportion of a in mixture of b and Xba is the sown row proportion of b in mixture of a. For crop b in association with a, the Kba was calculated in the similar way. The product of two coefficients (KabKba) = K; if K obtained in the system is greater than 1, there is a yield advantage; if K obtained in the system equals to 1, there is no yield advantage; if K in the system is less than 1, there is a yield disadvantage.

Aggressivity is another index that represents a simple measure of how much the relative yield increase in a crop is greater than that of b crop in an intercropping system. It was calculated as

Formula 4[4]
If Aab = 0, both crops are equally competitive; if Aab is positive, a is dominant; if Aab is negative, a is the dominated crop.

Willey and Rao (1980) suggested competition ratio (CR), instead of taking the difference of two terms in aggressivity. The CR represents simply the ratio of individual LERs of the two component crops, but it takes into account the proportion of the crops in which they were initially sown.

Formula 5[5]
where CRa is the competition ratio of dominated crop. If CRa < 1, there is a positive benefit and the crop can be grown in association; if CRa > 1, there is a negative benefit. The reverse is true for CRb.

Farmers are concerned mostly with total profit and the marginal benefit: cost ratio from investment on different inputs, particularly labor and seed. The yield and economic performance of the intercropping were evaluated to decide whether soybean yield and additional sorghum yield are sufficient to justify farmers to adopt an intercropping system. Moreover, none of the competition indices explain the economic advantage of the intercropping system. Thus, monetary advantage index (MAI) was computed as

Formula 6[6]
The higher the index value, the more profitable is the cropping system.

Harvest index (HI) was calculated as

Formula 7[7]

Water-use efficiency (WUE) was calculated as

Formula 8[8]

Evapotranspiration (ET) was estimated from water balance equation as

Formula 9[9]
where P is precipitation, I is irrigation, Cp is contribution through capillary rise from ground water, Dp is deep percolation, Rf is runoff, and S is change in soil moisture storage in the profile.

Yield Trend and Statistics
To determine yield trends (slope) over the years and to test the hypothesis that yield trends throughout the experimentation period are not significantly different from zero, a linear regression analysis as Y = a + bt for all 5 yr of data for an individual treatment was performed, where Y is the yield (kg ha–1), a constant, t the year, and b the slope or magnitude of the yield trend. The P value indicates the level of significance of the observed yield changes.

Data were subjected to the analysis of variance (ANOVA) appropriate to the design as given by Gomez and Gomez (1984). Test of significance of the treatment difference was done on the basis of a t test. The significant differences between the treatments were compared with the critical difference at a 5% level of probability.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Above- and Belowground Component
The productivity of a crop depends on photosynthesis, partitioning, and transfer of assimilates to the economically important parts. The CGR of sorghum was marginally less than the CGR of soybean at the initial stage (30–45 DAS), but during 60–75 d, both crops attained their peak (Fig. 3 ). The CGR of sole soybean decreased slightly after 30–45 DAS while that of sole sorghum and soybean/sorghum intercropping increased up to 60–75 DAS. Soybean/sorghum intercropping system recorded significantly greater above- and belowground biomass than monoculture of soybean (Table 2). The higher biomass production in intercropping was attributed to the enhanced growth of the nonlegume component because it, being taller than the legume, could intercept relatively high solar radiation. The higher CGR of intercropping (Fig. 3) compared with sole soybean was, thus, obviously because of the nonlegume component in the intercropping system. Further, the greater N accumulation by a nonlegume crop intercropped with a legume is frequently reported in the literature (Francis, 1986; Vandermeer, 1989; Stern, 1993). Thus, biomass production of the nonlegume is more closely related to improved N nutrition.


Figure 3
View larger version (47K):
[in this window]
[in a new window]
  		Fig. 3. (a) Crop growth rate and (b) chlorophyll content of soybean and sorghum under sole and intercropping under various nutrient management. Data following common letters are not significantly different at 5% probability as per Duncan's multiple range tests. H, harvest; FYM, farmyard manure; PC, phosphocompost; PM, poultry manure.

 

View this table:
[in this window]
[in a new window]
  		Table 2. Effect of cropping system and nutrient management on soybean equivalent yield, biomass, soil biological activities, and water use efficiency of soybean and sorghum.{dagger}

 
The leaf chlorophyll content is one of the key factors in determining the rate of photosynthesis and dry matter production (Bellore and Mall, 1975). Sole soybean recorded comparatively higher chlorophyll content than intercropped soybean. The converse was observed in case of sorghum (Fig. 3 and Table 3). Nitrate is the predominant N form in most soils. However, sufficient NR activity is a prerequisite for an optimal utilization of soil nitrogen (Beevers and Hageman, 1969). In the present study, the NR activities and chlorophyll content in intercropped sorghum were relatively greater than sole sorghum (Table 3). Our results further showed higher NR activities in leaves followed by roots (Table 3 and Fig. 4 ), indicating higher nitrate concentration in leaves than in roots. The level as well as the distribution of enzyme activity between the different plant organs, however, varies among species apparently due to their natural habitat (Lee and Stewart, 1978) and also depend on the developmental stage of the plant (Pate, 1980). Lexa and Cheeseman (1997) suggested that changing the location of nitrate reduction did not change growth significantly and caused only subtle changes in N concentration. The favorable effect of the intercropping system in increasing chlorophyll content and NR activity may have resulted in higher dry matter production.


View this table:
[in this window]
[in a new window]
  		Table 3. Various growth attributes of soybean and sorghum at 60 d after sowing.{dagger}

 

Figure 4
View larger version (61K):
[in this window]
[in a new window]
  		Fig. 4. Trends in nitrate reductase activity (NRA) in sole and intercropping under different nutrient management. Data following common letters are not significantly different at 5% probability as per Duncan's multiple range tests. FW, fresh weight; FYM, farmyard manure; PC, phosphocompost; PM, poultry manure.

 
Because of the differences in canopy height of soybean and sorghum, the two species not only competed for nutrient and water but also for sunlight (Hauggaard-Nielsen et al., 2001b; Ghosh, 2004). The shading effect of tall intercropped sorghum may have adversely affected the biomass, nodule mass, N uptake, chlorophyll, and photosynthesis of intercropped soybean (Table 3). There was a 30, 13.5, 8.2, 46, 37, and 24% reduction in aboveground biomass, photosynthesis, N uptake, nodule mass, root biomass, and RLD, respectively, in intercropped soybean over sole soybean. In contrast, there was a 52, 1.2, 10.1, 23.5, and 55% improvement in aboveground biomass, photosynthesis, N uptake, root biomass, and RLD in intercropped sorghum over sole sorghum (Table 3). Such effect was also evident between soybean rows. These above- and belowground components were less in soybean middle rows than those adjacent to sorghum (Fig. 5 and 6) . Substantial reduction of these components in rows adjacent to sorghum compared with middle rows was because of less photosynthesis due to more effective shading of tall sorghum (Fig. 5). In general, both the intercrops recorded higher SMBC (3.8 to 10.3%) and DHA (4.9 to 8.0%) than sole crops (Table 3). Variation in the SMBC value of middle and adjacent rows was inconsequential (data not presented).


Figure 5
View larger version (53K):
[in this window]
[in a new window]
  		Fig. 5. Trend in (a) shoot biomass, (b) N uptake, and (c) photosynthesis rate in middle rows of soybean and rows adjacent to sorghum. Data following common letters are not significantly different at 5% probability as per Duncan's multiple range tests. FYM, farmyard manure; PC, phosphocompost; PM, poultry manure.

 

Figure 6
View larger version (46K):
[in this window]
[in a new window]
  		Fig. 6. Trends in (a) root length density, (b) root biomass, and (c) nodule mass of middle rows of soybean and rows adjacent to sorghum. Data following common letters are not significantly different at 5% probability as per Duncan's multiple range tests. FYM, farmyard manure; PC, phosphocompost; PM, poultry manure.

 
The tall-growing intercropped sorghum significantly affected N fixation ability of soybean (Table 3). Limited ability to obtain sunlight by the soybean shoots might translate into major competitive limitation (Midmore, 1993) that strongly influences the interspecific competitive ability. The reduced light energy affects N2 fixation by restricting photosynthesis and the energy supply to roots, thereby reducing nodulation and nodule size (Nambiar et al., 1986). Low N concentration of soybean shoot in the soybean/sorghum intercropping (Table 3) could be attributed mainly to lower N fixation and reduced N uptake. This is presumed to be associated with stronger competitive ability of sorghum compared with soybean for soil inorganic N (Jensen, 1996; Anil et al., 1998). In accordance with the findings of Nambiar et al. (1986) and Sprent and Minchin (1985), our results demonstrated that at a low fertility level (control), irrespective of soybean rows, nodulation in soybean/sorghum intercropping was better or similar to 100% N–P–K (Fig. 6). Such effect on nodulation could be due to the fact that in the intercropping system, half of the N (45 kg N ha–1) topdressed to sorghum rows at 30 DAS might have reached soybean rows spaced only 30 cm away from sorghum rows. Integrated use of organics and N–P–K recorded higher above- and belowground biomass, nodule mass, NR activity, chlorophyll content, SMBC, and DHA. Among the nutrient treatments, 75% N–P–K + PM produced significantly higher above- and belowground biomass than other treatments except 75% N–P–K + FYM where they did not show significant differences (Table 2). The N–P–K + FYM being similar to N–P–K + PC recorded significantly higher nodule mass than other nutrient treatments. The NR activity was highest in 100% N–P–K followed by 75% N–P–K + FYM (Fig. 4). However, chlorophyll content in both the crops was relatively higher in organically treated plots and also in 100% N–P–K plots (Fig. 3). The organic manures provided better soil environment in terms of physical and chemical properties for nitrogenase activity (Shepherd and Withers, 1999; Hati et al., 2001), improved biological activities (Table 2), and a relatively enhanced supply of essential nutrients (Table 1) to crops from continuous slow mineralization of organic manures.

The intercropping systems have an advantage over sole cropping also because of spatial difference in root mass that causes the system to utilize a greater soil volume. In general, roots of soybean and sorghum were confined to a 0- to 15-cm soil depth (Fig. 7 ). At 15- to 30-cm soil depth, the presence of sorghum root was not clearly seen. There was an improvement in the RLD of intercropped sorghum by 114.8% compared with sole sorghum in the 0- to 15-cm soil layer (Fig. 7). The increase in root growth of sorghum is presumed mainly to be associated with the higher competitive ability of sorghum for soil inorganic N sources, the lower number of sorghum plants in the intercrop, and competition from soybean for growth factors other than soil N (Lehmann et al., 1998). Further, the RLD of soybean in the rows adjacent to sorghum was 20.7% lower than that that in the middle rows (Fig. 6), indicating that soybean rows adjacent to sorghum experienced more competition with sorghum than the middle rows. Our previous work (Ghosh, 2004) also supported the fact. This was attributed to less receipt of light energy to adjacent rows of soybean. Reddy and Willey (1981) reported similar observations in intercropped groundnut.


Figure 7
View larger version (42K):
[in this window]
[in a new window]
  		Fig. 7. Root length density of sole and intercrop of soybean and sorghum as influenced by nutrient management. Data following common letters are not significantly different at 5% probability as per Duncan's multiple range tests. FYM, farmyard manure; PC, phosphocompost; PM, poultry manure.

 
The improved root growth in the intercropping system had a significant influence on nutrient and water use, as well as biological activities in soil. The WUE, N uptake, SMBC, and DHA in soil were relatively higher in intercropping than in sole soybean because of higher root biomass (Table 2) and RLD (Fig. 7). This was further ascertained by the positive and significant relationship observed with root mass and SMBC (r = 0.87), root mass and DHA (r = 0.71), root mass and N uptake (r = 0.93), and root mass and water use (r = 0.49).

Based on the above- and belowground components, it was observed that during the co-growth of the two crops in intercropping, apparent interspecies interactions resulted in increasing sorghum growth (+) and decreasing soybean growth (–). This demonstrated that there was interspecies competition between intercropped soybean and intercropped sorghum during the co-growth stage where sorghum acted as the suppressing species and soybean as the suppressed species. This is similar to contramensalism (one species increased and the other decreased) in microorganism communities (Hodge and Arthur, 1996). However, when two plants grow near one another, basic physiological principles suggest that they will almost always compete, whether or not facilitation is operative (Vandermeer, 1989). The results also demonstrate that biomass, N accumulation, chlorophyll, root growth, and nodulation (Table 3) in intercropped soybean decreased significantly during the co-growth stage in the intercropping system. At the same time, an increase in NR activity in leaf, shoot, and root and SMBC and DHA in soil (Table 3) in intercropped soybean over sole cropping revealed interspecies facilitation between the intercropped soybean and sorghum during the co-growth stage. This showed that interspecies competition and interspecies facilitation coexist in the soybean/sorghum intercropping system. The coexistence of positive and negative interactions in the same ecosystem has also been found in forests between Abies lasiocarpa and Pinus albicaulis (Callaway, 1998), in the shrub Reama sphaerocarpa and herb Marrubium vulgare community (Pugnaire et al., 1996), in other ecosystems (Callaway, 1998), and in field crops wheat and maize or wheat and sorghum (Li et al., 2001).

Seed Yield of Soybean and Sorghum
The seed yield of intercropped sorghum as compared to sole sorghum increased by about 100% (Table 4) while that of intercropped soybean increased by only 2.2%. It appears that soybean did not benefit from the intercropping to the same degree as sorghum. Thus, the results ascertain that sorghum was the major contributor to the mixture yield. Ghaffarzadeh et al. (1994) also observed yield increment of maize and yield decrement of soybean in a maize/soybean strip intercropping system as compared with the corresponding sole crops. The greater ability of sorghum to absorb limited soil factors increased the interspecific competition in the intercrop (Trenbath, 1976). Greater increase in intercropped sorghum yield was evident mainly from the increase in above- and belowground biomass, chlorophyll, and NR activity (Table 3). The yield increment of sorghum in soybean/sorghum intercropping may also be due to transfer of N fixed by soybean to sorghum (Agboola and Fayemi, 1972; Burten et al., 1983).


View this table:
[in this window]
[in a new window]
  		Table 4. Seed yield of sole and intercropped soybean and sorghum as influenced by integrated nutrient management.{dagger}

 
As Wilson (1988) suggested that there is a positive interaction between root and shoot competition, a positive interaction between soybean and sorghum (interspecies facilitation) for NR activity in all parts, SMBC, and DHA perhaps effected a yield advantage in the soybean/sorghum intercropping system in the present study. It was also observed that among aboveground components, only photosynthesis (r = 0.63) of intercrop soybean and NR activity in shoot (r = 0.46) and N uptake (r = 0.45) of intercrop sorghum were positively and significantly correlated with yield. However, among belowground components, correlation of yield was significant and positive with NR activity in root (r = 0.61, r = 0.63), RLD (r = 0.36, r = 0.33), and SMBC (r = 0.71, r = 0.66) of both intercrop soybean and intercrop sorghum, respectively. Thus, it is clear that interaction among these three belowground components of both the intercrops obviously had the greater effect on yield advantage in the intercropping system.

The reproductive sink size and its relative strength appear to have an innate bearing on photosynthesis and consequently the seed yield. Duncan et al. (1978) observed partitioning of photosynthate to pods as the most influential physiological factor in yield determination. In the present study, the HI was more in soybean/sorghum intercropping than sole soybean. Comparatively more reproductive sink mass as reflected in HI (Table 2) might be the reason for higher yield in soybean/sorghum intercropping.

Declining yield trends due to fertilizer treatment in all crops under both sole and intercropping systems were statistically nonsignificant (Table 5). It was also observed that among sole and intercrop soybean and sorghum, yield trend declined more in sole sorghum, especially with inorganic fertilizer treatments.


View this table:
[in this window]
[in a new window]
  		Table 5. Yield trend analysis of soybean and sorghum under different nutrient management.{dagger}

 
Both sole and intercrop soybean responded up to 75% N–P–K while sorghum responded up to 100% N–P–K. We observed that yield of intercrop soybean as compared with sole soybean decreased under N–P–K. Increase in seed yield of intercrop soybean was observed only under integrated use of organics and inorganic, the highest being under 75% N–P–K plus FYM treatment. It appears that under integrated use of organic and N–P–K, the shading effect of tall sorghum had no significant adverse impact on seed yield of soybean (Table 4) and nodule mass (Fig. 6). These nutrient treatments also registered higher WUE than inorganic and control, but the HI was not different. Thus, to benefit the legume component, application of organic along with N–P–K is necessary. Seed yield of soybean between PC and PM plots and sorghum yield between FYM and PC plots did not show discernible difference.

Interestingly, FYM was found effective for soybean and PM for sorghum in terms of seed yield and root growth under both sole and intercropping systems (Table 4 and Fig. 7). This could be attributed to the fact that approximately 74% of total P and 40% of total N in PM were in available form (Shepherd and Withers, 1999) and sorghum being a heavy feeder of nutrients, particularly N, utilized N faster than soybean from the available pool of applied PM.

Yield Advantage and Monetary Benefits
In general, the nonlegume crop is considered a suppressing crop in legume/nonlegume associations like sorghum/pigeonpea (Tobita et al., 1994, 1996), groundnut/cereal fodders (Ghosh, 2004), and berseem (Trifolium alexandrinum L.)/barley (Ross et al., 2004). This was shown to be true in soybean/sorghum intercropping in the present study as indicated by aggressivity.

The LER gives an accurate assessment of the biological efficiency of the intercropping situation. The trade-off between increasing the yield of suppressing species and decreasing that of the suppressed species has three possible outcomes for intercropping systems, i.e., yield advantage (LER > 1), yield disadvantage (LER < 1), and the intermediate result (LER = 1) (Vandermeer, 1989). The results of the present experiment showed crop complementarities in soybean/sorghum intercropping and yield advantage, as LER and RCC values are greater than unity (Table 6). This corroborated the findings of Willey (1979) and Reddy and Willey (1981).


View this table:
[in this window]
[in a new window]
  		Table 6. Assessment of yield advantage through different competition indices (unit price, Rs kg–1 of soybean and sorghum represent 10.9 and 5.7, respectively).{dagger}

 
According to Willey and Rao (1980), CR gives a better measure of competitive ability of the crops and can prove a better index as compared with RCC and aggressivity. The CR of soybean was <1, and the CR value of sorghum was >1. This indicates an advantage in yield compared with sole crops. This further suggests that soybean in the intercropping system is less competitive than the associated sorghum.

A number of studies indicated that the LER of intercropping tends to be higher under low-N conditions (Martin and Snaydon, 1982; Ofori and Stern, 1987). In contrast, in our study, we did not find any significant differences in LER among control, 75% N–P–K and 100% N–P–K. The reason is not clear. However, greater response of intercrops to N–P–K owing to very low soil N and P content may be one of the possible reasons. We also observed that LER increased further with integrated use of organics with N–P–K (Table 6), the highest being with 75% N–P–K + FYM (1.33). This further confirmed the practical implication for use of organics with inorganic N–P–K. Yield advantage in terms of RCC values for 75% N–P–K + FYM (4.71) and 75% N–P–K + PM (4.61) were higher than inorganic alone and the control (Table 6). The data in Table 6 also revealed that the CR values of sorghum were greater under 75% NPK+ PM, and the corresponding CR value of soybean was less. This indicates that based on CR, yield advantage in intercropping was greater under 75% NPK + PM. When monetary advantage was considered, 75% NPK + FYM (5486) gave maximum MAI, which might be due to higher LER and RCC and lower CR value. The MAI among organics did not vary. The lowest monetary benefit was recorded in the control (1337).

The observations on competition indicators in the present study corroborated the Crimes theory of competitive success in which the species with greater capacity for resource capture will be the superior competitor (Grace, 1990). Accordingly, sorghum was the superior competitor during the co-growth stage. For assessing belowground competitive ability, root growth was correlated with biological activities involved in nutrient uptake. Moreover, the extent of belowground competition experienced by an individual is a function of both the size and number of neighbors (Casper and Jackson, 1997). Initial crop size might also influence the competitive ability of species (Gerry and Wilson, 1995), which needs further attention.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Intercropping soybean and sorghum increased LER to values exceeding 1.0, indicating yield advantage in this study. The aboveground component (biomass, photosynthesis, chlorophyll, NR activity in leaf and shoot as well as N uptake), belowground component (root biomass, RLD, NR activity in root, SMBC and DHA), and yield of intercropped sorghum greatly improved while the aboveground components, except NR activity, and belowground components, except SMBC and DHA, were reduced in intercropped soybean. Thus, interspecific competition and interspecific facilitation coexisted in this study. Sorghum is a more competitive component crop than soybean. The root mass was positively related with SMBC, DHA, water use, and N uptake. Among aboveground components, only photosynthesis (r = 0.63) of intercrop soybean and NR activity in shoot (r = 0.46) and N uptake (r = 0.45) of intercrop sorghum were positively and significantly related with yield while among belowground components, relationship of yield was significant and positive with NR activity in root (r = 0.61, r = 0.63), RLD (r = 0.36, r = 0.33), and SMBC (r = 0.71, r = 0.66) of both intercrop soybean and intercrop sorghum, respectively. Thus, interaction among these three belowground components of both the intercrops obviously had the greater effect on yield advantage in the intercropping system.

Soybean did not benefit from intercropping to the same degree as sorghum in N–P–K. However, integrated use of organics and N–P–K is effective in increasing yield, LER, RCC, and MAI. Therefore, use of N–P–K plus FYM or N–P–K plus PM is an important nutrient management option for sustaining this intercropping system, particularly to benefit the legume component.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar