Interspecific facilitation of nutrient acquisition by intercropped … Long et al... · Nutrient Cycling in Agroecosystems 65: 61–71, 2003. 61 2003 Kluwer Academic Publishers. Printed

Interspecific facilitation of nutrient acquisition by intercropped maizeand faba bean

Li, L., Zhang, F. S., Li, X. L., Christie, P., Sun, J. H., Yang, S. C., & Tang, C. (2003). Interspecific facilitation ofnutrient acquisition by intercropped maize and faba bean. Nutrient Cycling in Agroecosystems, 65, 61-71.

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Nutrient Cycling in Agroecosystems 65: 61–71, 2003. 61 2003 Kluwer Academic Publishers. Printed in the Netherlands.

Interspecific facilitation of nutrient uptake by intercropped maize andfaba bean

1,2 1, 1 1,3 2 2*Long Li , Fusuo Zhang , Xiaolin Li , Peter Christie , Jianhao Sun , Sicun Yang and4Caixian Tang

1Department of Plant Nutrition, China Agricultural University, 100094 Beijing, People’s Republic of China;2Gansu Academy of Agricultural Sciences, Institute of Soils and Fertilizers, 730070 Lanzhou, People’s

3Republic of China; Department of Agricultural and Environmental Science, The Queen’s University of4Belfast, Newforge Lane, Belfast BT9 5PX, UK; Soil Science and Plant Nutrition, The University of

*Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia; Author for correspondence (e-mail:[email protected]; phone: 186-10-62892499; fax: 186-10-62891016)

Received 24 November 2000; accepted in revised form 3 August 2001

Key words: Multiple cropping, Nitrogen, Phosphorus, Potassium, Root barrier, Root interactions

Abstract

Interspecific complementary interactions in N, P and K uptake between intercropped maize (Zea mays L. cv.Zhongdan No. 2) and faba bean (Vicia faba L. cv. Linxia Dacaidou) were investigated in a field experiment. A rootbarrier study was also set up in which belowground partitions were used to determine the contribution ofinterspecific root interactions to crop nutrient uptake. Nitrogen uptake by intercropped faba bean was higher than

21(no P fertilizer) or similar to (33 kg P ha of P fertilizer) that by sole faba bean during the early growth stages21(first to third sampling) of faba bean, and was similar to (no P fertilizer) or higher than (33 kg P ha of P

fertilizer) that by sole faba bean at maturity. Nitrogen uptake by intercropped maize did not differ from that by solemaize at maturity, except when P fertilizer was applied. Intermingling of maize and faba bean roots increased Nuptake by both crop species by about 20% compared with complete or partial separation of the root systems.Intercropping also led to some improvement in P nutrition of both crop species. Maize shoot P concentrations weresimilar to those of sole maize during early growth stages and became progressively higher until they weresignificantly higher than those of sole maize at maturity. Intercropping increased shoot P concentration in fababean at the flowering stage and in maize at maturity, and increased P uptake by both plant species at maturity.Phosphorus uptake by faba bean with root intermingling (no root partition) was 28 and 11% higher than withcomplete (plastic sheet) and partial (400 mesh nylon net) root barriers, respectively. Maize showed similar trends,with corresponding P uptake values of 29 and 17%. Unlike N and P, K nutrition was not affected by the presenceof root barriers.

Introduction vantage in terms of interspecific interactions has been¨focused mainly on interspecific competition (Bohr-

Total biomass and grain yields of intercropped maize inger and Leihner 1997; Braconnier 1998; Dauro andand faba bean were significantly higher than those of Mohamedsaleem 1995; Dupraz et al. 1998; Heleniusmaize and faba bean in the corresponding sole crops and Jokinen 1994; Piepho 1995; Jolliffe and Wanjauon a calcareous soil in a field study by Li et al. (1999). 1999). Although interspecific facilitation (or positiveAccording to Vandermeer (1989), intercropping ad- interaction) in which one plant species enhances thevantage depends on the net effect in the trade-off survival, growth, or fitness of another has been dem-between interspecific competition and facilitation. onstrated in many natural plant communities (Calla-However, most of the research on intercropping ad- way and Pugnaire 1999), there have been few studies

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on facilitation (especially of nutrient uptake) in inter- potential evaporation. The soil in the experimental21cropping systems. Effects on P availability that may field contained 7.20 g kg organic matter, 0.51 g

21 21qualify as facilitation have been reported by Gardner kg N, 6.4 mg kg Olsen-P and 170 mg exchange-21and Boundy (1983), Horst and Waschkies (1987) and able K kg soil. The soil is classified as an Aridisol

Ae et al. (1990). Ae et al. (1990) showed that pigeon with a pH of 8.2.pea could increase P uptake by associated sorghum.Gardner and Boundy (1983) and Horst and Waschkies The major field experiment(1987) indicated that white lupin could increase Puptake by intercropping with wheat. In addition, some The experimental design was a split-plot with threework has been conducted on the N budget in inter- replicates in which the main plot treatments were

21cropping systems between cereals and legumes in fertilizer P (33 kg P ha as triple superphosphate)which the cereals obtain some part of the N from the and no fertilizer P. Sub-plot treatments consisted ofassociated legumes (Midmore 1993; Stern 1993). sole maize (Zea mays L. cv. Zhongdan No. 2), soleCereals, on the other hand, can compete for N in the faba bean (Vicia faba L. cv. Linxia Dacaidou) andrhizosphere of cereal / legume mixtures, leading to N maize / faba bean relay intercropping. Relay intercrop-depletion in the rhizosphere of the legumes, and this ping plots consisted of three strips (six rows of maizestimulates increased N fixation by the legumes and six rows of faba bean) with two rows of maize2

(Boucher and Espinosa 1982). plants grown in alternating 1.2-m wide strips with twoThere is only one cropping season in most areas of rows of faba bean. There were 18 plants per row for

northwest China due to temperature limitations. Many maize or 21 plants for faba bean in the intercropping.relay intercropping systems with early spring sowing The maize inter-row spacing was 0.40 m, the fabaof crops with later sown crops, such as faba bean/ bean inter-row spacing was 0.20 m and the distancemaize, wheat / soybean and wheat /maize have there- between maize and adjacent faba bean rows was 0.30fore been widely adopted by farmers, and these play m in the intercropping treatment. The sole croppingan important role in the food production of this plots consisted of 6 rows with 24 plants per row forregion. In a previous paper (Li et al. 1999), we sole maize or 16 rows with 31 plants per row for solereported some beneficial effects of maize and faba faba bean. The maize inter-row spacings were 0.80 mbean intercropping on crop yields and suggested that and 0.40 m for wide and narrow rows, respectively,these resulted mainly from interactions between the and the corresponding faba bean inter-row spacingsroot systems of the two crop species. This paper were 0.3 m and 0.15 m, with alternating wide andpresents data on crop nutrient uptake from the same narrow row spacing in the plots. All plots were 3.6 3

2field site. The objectives of the study were to (i) 6 m, giving an area of 21.6 m (Figure 1). A 0.4-mexamine the possible occurrence of interspecific wide ridge between plots was built to separate thefacilitation of nutrient uptake between faba bean and plots from each other, and there was a 0.5-m widemaize, and (ii) evaluate the contribution of inter- irrigation furrow plus two 0.4-m ridges betweenspecific root interactions to facilitation of nutrient blocks. The densities of sole maize and faba bean

22uptake in the maize / faba bean association. were 10.12 and 22.95 plants m , respectively. Two-thirds of each intercropped area was occupied bymaize and one-third by bean. The densities of inter-

Materials and methods cropped maize and faba bean were therefore 6.75 and227.64 plants m , respectively, so that the overall

Study area proportional density of each crop species was equal inboth the sole and intercropping treatments.

The field experiment was conducted in 1997 at Jing- Seeds were sown on 27 March (faba bean) and 16yuan village, Jingyuan county, Gansu province, China April (maize). Faba bean was harvested on 25 July(378059 N, 1048409 E) at an altitude of 1645 m above and maize on 20 September. All plots were given a

21sea level. Annual mean temperature is 6.6 8C and the basal application of 225 kg N ha as urea. Both thefrost-free period is 160–170 days. Total solar radia- N and P fertilizers were evenly broadcast and incorpo-

22 21tion averages 6162 MJ m year . The region is rated into the top 20 cm of the soil prior to sowing. Allclassified as arid with a continental climate and has plots were irrigated during the growing season to200–250 mm of annual precipitation and 2369 mm of prevent water stress. Six irrigation events (with a

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Figure 1. Diagram showing the arrangement of the rows of maize (1) and faba bean (s) in the field experiment and the positions of thepartitions in the microplots: (a) sole maize, (b) sole faba bean, and (c) maize and faba bean intercropping.

water depth of 70 mm on each occasion) were carried limit adverse effects on the main experiment (Figureout on April 20, May 18, June 13, July 10, August 1 1). Roots did not penetrate the nylon mesh at finaland September 3, respectively for intercropping plots harvest.and sole maize. Only the first four irrigation eventswere used for sole faba bean. The faba bean /pea intercropping study

The root barrier study In this experiment, there were three treatments: (i)sole cropping pea (Pisum sativum), (ii) sole cropping

A root barrier study was conducted in the treatments faba bean and (iii) faba bean/pea intercropping.receiving P fertilizer: (i) an impermeable plastic sheet Fertilizer N and P were applied at total rates of 225

21partition inserted into the ground between adjacent and 33 kg ha . The peas were sown on 25 March andstrips of maize and faba bean to a depth of 0.70 m to harvested on 5 July, while the faba beans were sownprevent interspecific root interactions, (ii) a 400 mesh on 27 March and harvested on 25 July. The area-

22nylon net partition (nominal aperture of 37 m) inserted based density of pea was 56 plants m under bothinto the ground between the two crop species to intercropping and monocropping treatments.prevent direct root contact but allow interactions bymass flow and diffusion, and (iii) a control treatment Data collectionwith no partition between the two crop species toallow complete intermingling of their root systems. Above-ground parts of 10 faba bean, 10 pea and 4The root barriers were inserted just after wheat emer- maize plants were harvested from each plot at 20-daygence. Final harvesting dates for faba bean and maize intervals from faba bean emergence (5 May). Shootwere the same as in the main field experiment. The dry matter yield was measured by oven drying at 65length of each root barrier was restricted to 1 m to 8C. At the final harvest, grain yields and above-ground

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dry matter of maize, faba bean and pea at maturity Resultswere determined by harvesting two rows of maize,two rows of faba bean and two rows of pea in the Main field experimentintercropping treatment, from two rows of maize inthe sole maize, and from four rows of faba bean and Shoot nutrient concentrationspea in the sole faba bean and pea treatment. All of the There was no significant difference in N concentrationplants from 1 m were harvested in the root barrier between intercropped and sole maize, except at thestudy at maturity and the aboveground yield and 5th sampling (12 August) when the N concentrationbiomass were determined as described above. Further was lower in the intercropped maize. This indicatesinformation on the experiments and the yield data has that the N concentration in maize was not increasedbeen given by Li et al. (1999). by the associated faba bean. Intercropping also did not

Nitrogen, P and K concentrations were determined affect shoot N concentration in the faba bean duringon ground subsamples of oven-dried plant material its entire growth period (Table 1).after digestion in a mixture of concentrated H SO At the early stages of maize growth (20 days after2 4

and H O . Nitrogen was measured by the micro- maize emergence), shoot P concentration was sig-2 2

Kjeldahl procedure, P by the vanadomolybdate meth- nificantly lower in the intercropped than in the soleod and K by flame photometry. maize. Shoot P concentrations in the intercropped

maize subsequently increased and exceeded those ofStatistical analysis sole maize after faba bean was harvested; the con-

centration in the intercropped plants increased by 11%Analysis of variance was performed using a split-plot at the final harvest. Intercropping increased the Pmodel to test for significance of treatments by analysis concentration in faba bean at the flowering stageof variance (ANOVA) and means were compared by compared to the sole faba bean. However, at maturityleast significance difference (LSD) (SAS Institute the P concentration in the intercropped faba bean did1985). not differ from that in the sole faba bean (Table 1).

Table 1. Concentrations (% of dry matter) of N, P and K in shoots of maize and faba bean grown in the sole and intercropping systems in thefield at different harvest dates.

Crop Cropping system Sampling date

25 May 15 June 5 July 25 July 12 August 3 September 20 September

Nitrogen1Maize Sole 3.69 4.77 2.03 ND 2.46 0.803 0.98

Intercropped 3.58 4.62 2.22 ND 1.50 0.886 1.032Significance NS NS NS ND ** NS NS

3Faba bean Sole 3.03 1.32 2.37 2.53 H H HIntercropped 3.31 1.45 2.33 2.64 H H HSignificance NS NS NS NS H H H

Phosphorus1Maize Sole 0.377 0.280 0.194 ND 0.112 0.108 0.128

Intercropped 0.356 0.263 0.192 ND 0.122 0.129 0.142Significance * NS NS ND NS NS *

Faba bean Sole 0.265 0.088 0.158 0.210 H H HIntercropped 0.266 0.118 0.191 0.206 H H HSignificance NS ** NS NS H H H

PotassiumMaize Sole 3.79 4.01 3.83 ND 1.74 1.35 1.38

Intercropped 3.81 3.82 3.22 ND 1.14 0.98 1.07Significance NS NS NS ND NS * *

Faba bean Sole 3.12 1.33 1.36 1.38 H H HIntercropped 2.91 1.32 1.34 1.39 H H HSignificance NS NS NS NS H H H

1 2ND, not determined. Significance of difference between intercropped and sole by analysis of variance; **, P,0.01; *, P,0.05; NS, not3significant. H, after final harvest of faba bean.

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Intercropping did not reduce maize shoot K con- sampling date (15 June: elongation stage) P uptake bycentration before 12 August (Table 1), indicating that intercropped maize was significantly lower than thatthere was no significant competition from faba bean by sole maize where P fertilizer was applied. On thefor K when the two crops co-existed. However, after third sampling date (5 July) the difference in P uptakethe final harvest of the faba bean, shoot K concen- between intercropped and sole maize had declined,trations in the intercropped maize were lower than in and this trend continued until the fifth sampling datethe sole maize, especially at sampling on 3 September (3 September, grain filling stage). During the last 20and 20 September when the differences were signifi- days of the grain filling stage, P uptake of inter-cant (P # 0.05). There was no significant intercrop- cropped maize was significantly higher than that ofping effect on faba bean shoot K concentration sole maize (Table 3). Application of P generallythroughout its growth period. increased P uptake irrespective of cropping system or

crop species.Nutrient uptake Potassium uptake of intercropped faba bean wasWithout P fertilizer, significant differences in N ac- slightly higher than of sole faba bean only at maturity.quisition between intercropped and sole faba bean Potassium uptake of intercropped maize was similarwere observed at all harvests. By contrast, with P to that of sole maize on the first two sampling dates,fertilizer application, there was no difference at the but was then lower than that of sole maize andfirst three harvests. At maturity, intercropping in- eventually similar to sole maize (Table 4).creased N uptake of faba bean by 29% without P

21fertilizer and by 58% with 33 kg P ha . Nitrogen Root barrier studyuptake by intercropped maize was not increased byassociated faba bean, with the exception of P-fertil- Nitrogenized plots at final harvest (Table 2). Application of P There were no significant differences in maize or fabafertilizer tended to increase N uptake, especially of bean plant N concentration (weighted mean for shootintercropped maize. and grain) among the three partition treatments in the

Phosphorus uptake of intercropped faba bean was root barrier study (Table 5). This supports the conclu-significantly higher than of sole faba bean except at sion that the supply of N was adequate for both cropsthe first sampling with P fertilizer application. The and consequently there were no interspecific N inter-difference became significant (P , 0.05) only at the actions in the field experiment. Without any rootsecond sampling (15 June). At maturity, it was 56% partition, N uptake by faba bean was 22% and 18%and 18% higher than of sole faba bean in the plots higher than with the plastic sheet partition and thewith P fertilizer and without P fertilizer, respectively nylon mesh partition, respectively. Similarly, N up-(Table 3). There was no difference in P uptake take by maize without any root partition was 22% andbetween intercropped and sole maize during early 20% higher compared with plastic sheet and nylonstages of growth (seedling stage), but on the second mesh. The nylon mesh partition, which prevented

22 21Table 2. Nitrogen uptake (g N m ) by faba bean and maize grown without fertilizer P or with 33 kg fertilizer P ha in the sole andintercropping systems (Experiment 1).

P fertilizer application Cropping system and crop species Sampling date

25 May 15 June 5 July 25 July 12 August 3 September 20 September1No P fertilizer Sole faba bean 5.54 4.69 14.3 28.3 H H H

Intercrop faba bean 7.44 9.98 27.8 36.4 H H H2133 kg P ha Sole faba bean 7.35 7.01 21.0 27.2 H H H

Intercrop faba bean 7.64 7.33 23.1 43.0 H H H2LSD 1.21 2.44 9.8 8.8 H H H0.05

3No P fertilizer Sole maize 0.25 4.9 17.6 ND 31.7 16.5 26.0Intercrop maize 0.25 4.7 12.4 ND 26.2 20.5 26.4

2133 kg P ha Sole maize 0.28 5.5 19.3 ND 34.2 22.5 26.4Intercrop maize 0.31 5.1 20.4 ND 24.3 21.3 35.8

LSD 0.04 0.8 4.6 ND 11.9 4.3 7.00.05

1 2 3H, after final harvest of faba bean. LSD , least significant difference by analysis of variance at the 0.05 level. ND, not determined.0.05

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22 21Table 3. Phosphorus uptake (g P m ) by faba bean and maize grown without fertilizer P or with 33 kg fertilizer P ha in the sole andintercropping systems (Experiment 1).







LSD 0.01 0.06 0.65 ND 0.52 0.92 0.550.05


direct root-to-root contact but would have allowed Potassiummass flow and diffusion of substances between the There were no significant root partition effects onrhizospheres of the two crops, did not increase N maize or faba bean shoot K concentrations and uptakeuptake by either crop species (Table 5). in the root barrier study (Table 5). This suggests that

there were no interspecific root interactions betweenmaize and faba bean because of an adequate supply of

Phosphorus soil K for both species.Although P concentration in faba bean shoots was notaffected by the root partitions, P uptake by inter-

The faba bean /pea intercropping studycropped faba bean was lowered by the plastic sheetpartition inserted between faba bean and maize roots(Table 5). Phosphorus uptake by faba bean using the Nutrient concentrationnylon mesh partition was intermediate between that Nitrogen concentration in intercropped faba bean wasfor the plastic sheet partition and no partition. Both P significantly lower than in sole faba bean at the thirdconcentration in the maize shoots and P uptake by the sampling (5 July), indicating that faba bean inter-maize were higher without any root partition than cropped with pea was completely different from fabawith the plastic sheet or nylon mesh partition (Table bean intercropped with maize as described above. In5). contrast, P concentration in intercropped faba bean

22 21Table 4. Potassium acquisition (g K m ) by faba bean and maize grown without fertilizer P or with 33 kg fertilizer P ha in the sole andintercropping systems (Experiment 1).







LSD 0.07 0.68 10.0 ND 5.7 8.6 7.10.05


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21Table 5. Nutrient concentration (% of dry matter) and uptake (mg nutrient m per 2 rows) in the above-ground parts of maize and faba bean21 21with three types of below-ground partition. All plants received 33 kg P ha and 225 kg N ha and were harvested at maturity (25 July for

faba bean and 20 September for maize).1Nutrient and partition type Nutrient concentration Nutrient acquisition

Maize Faba bean Maize Faba bean

Nitrogen2Plastic sheet 0.992a 2.734a 14.2b 24.0b

Nylon net 0.910a 2.701a 14.6b 23.3bNo partition 1.091a 2.672a 17.3a 28.8aPhosphorus

2Plastic sheet 0.115b 0.227a 2.78c 1.19bNylon net 0.126ab 0.240a 3.25b 1.32abNo partition 0.134a 0.227a 3.58a 1.52aPotassium

2Plastic sheet 1.069a 1.419a 25.8a 7.59aNylon net 1.072a 1.212a 27.4a 6.58aNo partition 1.120a 1.404a 29.6a 9.30a1 2Weighted averages based on the proportions of straw and grain. Within each column, means followed by the same letter are not significantlydifferent by LSD at the 0.05 level.

was significantly higher than in sole faba bean at the Nutrient uptakesame sampling. No significant difference in K con- Although intercropping improved P uptake by fabacentration between intercropped and sole faba bean bean intercropped with pea at final harvest (25 July),was observed at any sampling occasion (Table 6). it did not increase N uptake by faba bean intercroppedThere was no significant difference in N, P and K with pea, and this is different from N uptake by fabaconcentration between intercropped and sole pea. bean intercropped with maize (Table 7). This may be

21 21Table 6. Nutrient concentrations (%) in faba bean and pea grown with 33 kg fertilizer P ha and 225 kg fertilizer N ha in the sole andintercropping systems.

Nutrient and crop species Cropping system Sampling date

25 May 15 June 5 July 25 July

NitrogenFaba bean Sole 3.11 1.54 2.77 2.52

Intercropped 2.68 1.41 1.92 1.86LSD 1.61 0.71 0.84 1.270.05

Pea Sole 3.44 2.57 1.74 HIntercropped 3.91 2.58 1.82 HLSD 0.47 1.12 0.90 H0.05

PhosphorusFaba bean Sole 0.276 0.101 0.119 0.218



PotassiumFaba bean Sole 3.33 1.44 1.26 1.24



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22 21 21Table 7. Nutrient acquisition (g m ) by faba bean and pea grown with 33 kg fertilizer P ha and 225 kg fertilizer N ha in the sole andintercropping systems.

Nutrient and crop species Cropping system Sampling date

25 May 15 June 5 July 25 July

NitrogenFaba bean Sole 7.4 7.0 21.0 27.2



PhosphorusFaba bean Sole 0.65 0.47 0.88 2.33



PotassiumFaba bean Sole 7.8 6.7 9.4 13.4

Intercropped 8.2 6.8 9.0 21.8LSD0.05

Pea Sole 12.2 19.4 12.2 HIntercropped 10.9 20.2 12.8 HLSD H0.05

why faba bean/pea intercropping is less successful 1990). In these two studies, legumes only improvedthan faba bean/maize intercropping (Li et al. 1999). the P nutrition in cereals. However, the present results

indicate that intercropping improved not only P nutri-tion in maize but also in faba bean. If the improve-

Discussion ment in P nutrition in wheat / lupin and sorghum/pigeon pea intercropping is termed ‘asymmetrical’

Improved P nutrition in intercropped maize and facilitation [(1, 0) or (1, 2)] (Crawley 1997), thenfaba bean the P nutrition improvement in maize / faba bean may

be considered as ‘symmetrical’ (1, 1). In otherThe present study demonstrates that intercropping words, there was mutual interspecific facilitation in Pfacilitated P nutrition in both faba bean and maize. nutrition in maize / faba bean intercropping.Firstly, P concentration was higher in the intercropped Various mechanisms can be proposed for the facili-plants than in the sole cropping plants during the tation in P nutrition by intercropping. The improved Pflowering (15 June) and pod-setting (5 July) stages for nutrition in maize could have resulted from an in-faba bean and grain filling (3 September) to mature creased uptake of P released during the decomposition(20 September) stages for maize. Secondly, intercrop- of root residues of faba bean. It was evident that Pping increased total uptake of P by faba bean at all concentration and uptake in maize were increasedsampling stages except at first sampling and by maize mainly by intercropping at later growing stages afterat later stages, irrespective of P fertilization. Thirdly, the faba bean had been harvested. Alternatively,when faba bean and maize grew together, both P although the crops received fertilizer N (urea), fabaconcentration and P uptake in maize, and P uptake in bean, as a legume, was better nodulated when inter-faba bean were significantly higher when their roots cropped than in monoculture (Table 8) and may havewere not separated than when their roots were par- fixed more atmospheric N . When fixing N , legume2 2

titioned by a plastic sheet. plants take up more cations than anions and release1 1Similar facilitation in P nutrition has been found in H from the roots (Tang et al. 1997). The H ions

wheat / lupin associations (Horst and Waschkies 1987) released are particularly important in dissolving P inand sorghum/pigeon pea intercropping (Ae et al. calcareous soils. Furthermore, root nodulation and N2

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fixation of legumes have been shown to increase when The symmetrical facilitation in P uptake betweenthe legume is intercropped with a species unable to fix intercropped maize and faba bean has not been foundN (Boucher and Espinosa 1982; Stern 1993; Sangak- in most of the legume/cereal intercropping systems2

kara 1994). In the present study, N uptake in faba investigated. Although overall P uptake exceeds solebean by intercropping presumably increased through cropping in most of the intercropping studies re-improved N fixation (discussed below). Total acid ported, an increase in nutrient uptake by one species is2

production was found to be highly correlated with usually at the expense of a decrease in the othertotal N fixation in legumes by Tang et al. (1997). species. For example, in cassava /peanut intercrop-2

Therefore, the increase in N fixation in intercropped ping, cassava acquired more P while peanut acquired2

faba bean may have led to increased proton excretion less compared to the respective sole cropping sys-by faba bean, and this may have contributed to the tems, although overall P acquisition by intercroppingmobilization of sparingly soluble phosphate in the was higher than that in the sole cropping (Manson etrhizosphere and thus improved P nutrition in both al. 1986). Impaired P acquisition by maize intercrop-

¨crop species. ped with cowpea was found by Hardter and HorstThe improvement in maize P nutrition by intercrop- (1991). In maize /mung bean intercropping, P absorp-

ping could also result partly from the increased vol- tion decreased by 5–43% in maize and by 31–58% inume of soil exploited by the maize roots for P absorp- mung beans as a result of intercropping (Chowdhurytion. The root barrier study showed that P uptake in and Rosario 1992). Therefore, the symmetrical inter-maize was greater without separation of its roots from specific facilitation in P acquisition between inter-faba bean roots than when the roots of the two crops cropped species that we found in the maize / faba beanwere partitioned with nylon mesh. This was consistent intercropping has important implications for inter-with the higher intercrop yields obtained when maize cropping ecology and plant community ecology.and legume roots intermingled in the soil compared toseparate planting of the two species (May and Mis-angu 1982), or when the roots of the species were Increased N uptake by intercropped faba beanpartitioned by a plastic sheet (Li et al. 1999). Inaddition, there is evidence of interspecific nutrient Intercropping increased N uptake by faba bean in thetransfer through vesicular-arbuscular mycorrhizal hy- present study. High concentrations of nitrate canphae (Chiariello et al. 1982). In a tallgrass prairie impair nodulation and depress N fixation in legumes2

plant community, P is also transferred among neigh- (Unkovich and Pate 1998; Tang et al. 1999). Growingbouring species by mycorrhizal hyphal interconnec- cereals, e.g. maize in the present study, would effi-tions and there is differential transfer among co-oc- ciently utilize soil nitrate and thus there would be lesscurring species (Walter et al. 1996). Interspecific P adverse effect on N fixation by legumes. In our other2

transfer via vesicular-arbuscular mycorrhizal hyphae experiment on faba bean/maize intercropping, inter-to an inefficient P acquisition species from an efficient cropping increased the weight of single nodules ofone may therefore be one of the mechanisms of faba bean under combined fertilizer treatments of 0 kginterspecific P uptake facilitation in faba bean/maize N/53 kg P, 300 kg N/0 kg P and 300 kg N/53 kg P

21intercropping. ha (Table 8). A greater stimulus for nodulation inbeans was also noted when the beans were inter-cropped with maize (Boucher and Espinosa 1982),and the nodulation and nodule activity of plants in

21 close proximity to maize or cassava was higher thanTable 8. Weight of single root nodules (mg nodule ) in sole fabain that of central rows in mung bean/maize and mungbean and faba bean intercropped with maize grown with different

21fertilizer N and P treatments (kg ha ) at the Jingtan site in 1998. bean/cassava intercropping (Sangakkara 1994). Inpea /barley intercropping, the intercropping advantageTreatment Intercropped Sole faba

faba bean bean was mainly due to the complementary use of soilinorganic and atmospheric N sources by the inter-0 N/0 P 19.4 16.9cropped pea and barley, resulting in reduced competi-0 N/53 P 23.7 14.2

300 N/0 P 14.0 8.4 tion for inorganic N, rather than a facilitative effect, in300 N/53 P 16.7 12.4 which symbiotically fixed N was made available to2LSD 2.90.05 the barley (Jensen 1996). Furthermore, there was no

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significant difference in N uptake between intercrop- Referencesped and sole maize in the present study, indicating

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have important implications where land resources and maize sole cropping and maize /cowpea mixed cropping systemsfertilizer supply are limited, and where intercropping on an alfisol in the northern Guinea savanna of Ghana. Biol.systems can improve efficiency of resource use and Fertil. Soils 10: 267–275.

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¨(Lupinus albus L.). Z. Pflanzenernahr Bodenkd. 150: 1–8.Jensen S. 1996. Grain yield, symbiotic N fixation and interspecific2We are grateful to the Major State Basic Research

competition for inorganic N in pea-barley intercrops. Plant SoilDevelopment Programme of the People’s Republic of 182: 25–38.China (Project number G1999011707) and the Na- Jolliffe P.A. and Wanjau F.M. 1999. Competition and productivitytional Natural Science Foundation of China (Project in mixtures: some properties of productive intercrops. J. Agric.

Sci. (Cambridge) 132: 425–435.number 30070450) for generous financial support.

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