soil water dynamics in cropping systems containing gliricidia

Abstract The water dynamics of cropping sys-

tems containing mixtures of Gliricidia sepium

(Jacq.) Walp trees with maize (Zea mays L.) and/

or pigeonpea (Cajanus cajan L.) were examined

during three consecutive cropping seasons. The

trees were pruned before and during each crop-

ping season, but were left unpruned after har-

vesting the maize; prunings were returned to the

cropping area in all agroforestry systems to pro-

vide green leaf manure. The hypothesis was that

regular severe pruning of the trees would mini-

mise competition with crops for soil moisture and

enhance their growth by providing additional

nutrients. Neutron probe measurements were

used to determine spatial and temporal changes

in soil moisture content during the 1997/98, 1998/

99 and 1999/00 cropping seasons for various

cropping systems. These included gliricidia inter-

cropped with maize, with and without pigeonpea,

a maize + pigeonpea intercrop, sole maize, sole

pigeonpea and sole gliricidia. Soil water content

was measured to a depth of 150 cm in all treat-

ments at 4–6 week intervals during the main

cropping season and less frequently at other

times. Competition for water was apparently not

a critical factor in determining crop performance

as rainfall exceeded potential evaporation during

the cropping season in all years. The distribution

of water in the soil profile was generally compa-

rable in all cropping systems, implying there was

no spatial complementarity in water abstraction

by tree and crop roots. However, available soil

water content at the beginning of the cropping

season was generally lower in the tree-based

systems, suggesting that the trees continued to

deplete available soil water during the dry season.

The results show that, under rainfall conditions

typical of southern Malawi, the soil profile con-

tains sufficient stored water during the dry season

(ca. 75–125 mm) to support the growth of gliric-

idia and pigeonpea, and that gliricidia trees

pruned before and during the cropping season did

not deleteriously compete for water with associ-

ated crops. Water use efficiency also appeared to

be higher in the tree-based systems than in the

P. W. ChirwaDepartment of Forest and Wood Science, Faculty ofAgricultural and Forestry Science, StellenboschUniversity, Private Bag X1, Matieland 7602,South Africa

C. K. OngWorld Agroforestry Centre (ICRAF), PO Box 30677,Gigiri, Nairobi, Kenya

J. MaghembeTunisia Road Plot 11, Scan Tanzania, PO Box 60023,Dar Es Salaam, Tanzania

C. R. Black (&)Plant Sciences Division, School of Biosciences,University of Nottingham, Sutton Bonington Campus,Loughborough LE12 5RD, UKe-mail: [email protected]

Agroforest Syst (2007) 69:29–43

DOI 10.1007/s10457-006-9016-7

123

Soil water dynamics in cropping systems containingGliricidia sepium, pigeonpea and maize in southernMalawi

Paxie W. Chirwa Æ Chin K. Ong ÆJumanne Maghembe Æ Colin R. Black

Received: 10 August 2005 / Accepted: 24 July 2006 / Published online: 13 September 2006� Springer Science+Business Media B.V. 2006

sole maize and maize + pigeonpea treatments,

subject to the proviso that the calculations were

based on changes in soil water content rather than

absolute measurements of water uptake by the

trees and crops.

Keywords Cajanus cajan Æ Gliricidia sepium ÆMixed cropping Æ Water availability ÆWater use Æ Zea mays

Introduction

In rain-fed agricultural systems in the semi-arid

tropics, water present in the soil profile origi-

nates primarily from infiltration following rain-

fall. Several studies have shown that a significant

proportion of the water received may be lost by

evaporation from the soil surface or percolation

to deep horizons beneath the crop rooting zone

(Wallace 1991, 1996). In systems where the crop

does not provide complete ground cover during

the growing season, evaporation from the soil

surface may account for 30–60% of the annual

rainfall (Cooper et al. 1983, 1987; Wallace 1991,

1996). Ong et al. (1992) showed that, although

the most effective cropping systems in semi-arid

India used 40% of the annual rainfall, up to 26%

and 33%, respectively, of the annual rainfall was

lost as run-off and deep drainage. Black and

Ong (2000) suggested that the benefits of inter-

cropping in such environments may result pri-

marily from improvements in water use

efficiency (WUE) rather than total seasonal

water use.

Several factors influence WUE. Morris and

Garrity (1993) suggested that a key factor con-

tributing to improvements in WUE in intercrop-

ping systems relative to sole crops is that their

more rapid canopy expansion and greater ground-

cover reduces soil evaporation, with the result

that transpiration forms a larger proportion of

evapotranspiration. Secondly, the inclusion of

fast-growing C4 species, with their inherently

higher water use efficiency, may increase yields in

intercropping systems (Black and Ong 2000).

Thirdly, the modified microclimatic conditions

provided by the presence of two or more system

components which differ in their above-ground

canopy structure and growth dynamics may create

an atmospheric environment which enhances

WUE; for example, relative humidity may be

increased and windspeed reduced within the

canopy, reducing evaporative demand.

The addition of trees to conventional annual

cropping systems may increase water use by using

water which cannot be accessed by annual crops

(Ong et al. 2000). The presence of trees may also

modify microclimatic conditions in ways which

improve the WUE of understorey crops. This is

especially true for agroforestry systems, as these

offer substantial scope for spatial and temporal

complementarity of water use resulting from im-

proved exploitation of soil water reserves and off-

season rainfall. Significant complementarity of

water use is obtained when the component spe-

cies have different rooting patterns or exhibit

contrasting temporal characteristics (Ong et al.

2000). Examples of traditional cropping systems

exhibiting these characteristics include the scat-

tered mature trees of the Sahelian parklands, such

as Faidherbia albida, which provide a discontin-

uous overstorey canopy.

However, as most tree species promoted for

use in agroforestry have root systems whose ver-

tical distribution is similar to agronomic crops

(Jonnson et al. 1988; Akinnifesi et al. 1999; Rowe

et al. 1999), they may compete with associated

crops. To minimise competition for water or

nutrients, especially when supplies are limited,

the temporal patterns of below-ground activity by

trees may be modified through management of

their above-ground components (Schroth 1999).

Thus, pruning the tree canopy before the start of

the growing season may allow annual crops to

exploit available water in the surface horizons by

reducing demand by the trees; the trees may also

be able to tap water reserves in the deeper soil

horizons as their canopy regrows during the latter

stages of the season (Droppelmann et al. 2000).

However, some studies suggest that trees sub-

jected to repeated shoot pruning may develop

more extensive lateral rooting systems, thereby

limiting spatial complementarity (van Noordwijk

and Purnomosidhi 1995; Ong and Leakey 1999).

The present study examined the water

dynamics of systems containing mixtures of

gliricidia, maize or pigeonpea and sole stands of

30 Agroforest Syst (2007) 69:29–43

123

each species. The hypothesis was that regular

severe pruning of the trees would eliminate

competition with crops for soil moisture and en-

hance their growth by providing additional

nutrients. The trees in the agroforestry systems

were pruned before and during the cropping

season, but were left unpruned after harvesting

the main crop component (maize). A primary

objective was to establish whether the trees and

crops competed for soil moisture during the

cropping season. Neutron probe measurements

were used to determine spatial and temporal

changes in soil moisture content during the 1997/

98, 1998/99 and 1999/00 cropping seasons. Tree

and crop productivity and soil nitrogen dynamics

within the various systems examined are reported

elsewhere (Chirwa et al. 2003, 2006).

Materials and methods

Experimental site

The study was carried out at Makoka Research

Station in southern Malawi (latitude 15�30¢S,

longitude 35�15¢ E) during the 1997/98, 1998/99

and 1999/00 cropping seasons. Rainfall in the area

is characterised by a long dry season (April–

October) and short wet season (November–

March). Mean daily temperature varies between

16�C and 24�C, while daily maximum and mini-

mum values range between 21–34�C and 10–19�C,

respectively. The soils are Ferric Lixisols accord-

ing to the FAO classification. The topsoils are

sandy loams with a pH in water of 5.1–5.6, organic

carbon content of 1.33%, total nitrogen content

of 0.09%, cation exchange capacity of 6.2–

10.0 cmol kg–1, bicarbonate–EDTA extractable

phosphorus and potassium concentrations of

5.1 mg kg–1 and 0.19 cmol kg–1, respectively, and

KCl extractable calcium and magnesium

concentrations of 6.4 cmol kg–1 and 1.7 cmol kg–1

(Ikerra et al. 2001).

Experimental design and treatments

The experiment was established during the 1995/

96 rains as a randomised block design with three

replicates (Chirwa et al. 2003); the plots were

11.5 m · 11.0 m in area. Treatments included

Gliricidia sepium (Jacq.) Walp. trees (provenance

Retalhuleu from Guatemala) intercropped with

maize (Zea mays L. hybrid variety MH41), with

or without pigeonpea (Cajanus cajan L.), and sole

maize. Sole gliricidia and sole pigeonpea treat-

ments were not initially included, but were added

prior to the 1998/99 season by converting spare

plots. The treatments examined were:

• SM—sole maize planted at a spacing of 90 cm

within rows and 75 cm between rows. Three

seeds were sown on the ridges at each planting

station; no fertiliser or green leaf manure

(GLM) was added.

• MP—maize was intercropped with pigeonpea

on the ridges; maize was planted at the same

spacing as sole maize and two pigeonpea seeds

were sown on the ridges in the spaces between

adjacent maize plants. No fertiliser or GLM

was added.

• GM—gliricidia was intercropped with maize

planted at the same spacing as in the sole crop.

Tree seedlings were planted in alternate fur-

rows between the ridges at a spacing of 0.5 m

within rows and 1.5 m between rows. GLM

from the gliricidia was incorporated into the

cropping area.

• GMP—gliricidia was intercropped with maize

and pigeonpea planted on the ridges at the

same spacing as in the MP treatment. Glirici-

dia was grown at the same spacing as in the

GM treatment and GLM from the gliricidia

was incorporated into the cropping area.

• SG—sole gliricidia: trees were planted at the

same spacing as in the GM treatment but were

managed as woodlots, i.e. they were not

pruned during the cropping season.

• SP—sole pigeonpea was planted at the same

spacing as in the MP and GMP treatments; no

fertiliser or GLM was added.

Plant management

Gliricidia seedlings planted on 15 December 1995

were first pruned 1 year later. During each annual

cycle, the trees were pruned to 30 cm above

ground level 2–4 weeks before the anticipated

planting date of the crops; GLM from individual

Agroforest Syst (2007) 69:29–43 31

123

plots was incorporated into the ridges on which

the crops would be grown. The trees were pruned

again when maize reached ca. 60 cm in height; the

prunings were applied as top-dressing. The trees

were also pruned at other times if they began to

shade the maize and prunings were incorporated

into the soil during normal weeding activities.

Maize was sown in late November or December

and pigeonpea was planted ca. 28 days later.

Planting dates for maize were: 12 December 1997,

29 November 1998 and 1 December 1999; maize

was replanted on 11 January 2000 as the initial

sowing failed due to the late arrival of the rains.

Maize was harvested on 29 May 1998, 22 April

1999 and 29 May 2000, while pigeonpea was har-

vested at maturity in late September or early

October. GLM was applied on 4 September 1997,

24 November 1997 and 26 January 1998 (1997/98

cropping season), 2 September 1998, 15 December

1998 and 29 January 1999 (1998/99 cropping sea-

son), and 13 September 1999, 19 November 1999

and 26 January 2000 (1999/00 cropping season).

Soil water measurements

Soil water content was measured at 4–6 week

intervals during the cropping season and less fre-

quently at other times between 1997 and 2000 using

a Wallingford neutron probe (Bell 1987); alumin-

ium access tubes were installed to a depth of

165 cm in all plots. Treatments were categorised

into main and secondary treatments. The former

comprised the tree-based systems (glirici-

dia + maize and gliricidia + maize + pigeonpea),

sole maize and the maize + pigeonpea intercrop-

ping system; four access tubes were installed in all

replicate plots. The secondary treatments were

introduced in 1999/00, when sole pigeonpea (SP)

and sole gliricidia (SG) treatments were added for

comparison; two access tubes were installed in all

replicates.

Neutron probe calibration

To enable volumetric water content (VWC) to be

calculated from neutron probe counts, five cali-

bration tubes were installed at each of two loca-

tions adjacent to the experimental site. Neutron

probe readings were taken in a drum containing

water to provide a water count before making

measurements in the calibration tubes at 15 cm

intervals to a depth of 165 cm. Soil samples were

collected from the same horizons at two locations

30 cm from each calibration tube to determine

gravimetric soil moisture content after drying at

100�C for 24 h. Gravimetric values were con-

verted to VWC using bulk density values deter-

mined for undisturbed soil cores of known volume

sampled from the vertical faces of soil pits located

close to the calibration tubes. This procedure was

repeated three times during the 1998/99 cropping

season to span the range between extreme soil

wetness and dryness and establish the mean rela-

tionship between VWC and probe count for all

sampling depths (r2 = 0.63; Chirwa 2002).

Soil moisture release curves

A moisture release curve was constructed to

establish soil moisture content at field capacity

(FC), permanent wilting point (PWP) and inter-

mediate values; FC was used to establish the

moisture content at which drainage began. Three

undisturbed soil samples were collected using pF

rings for the 0–30 cm, 30–60 cm, 60–90 cm, 90–

120 cm and 120–150 cm horizons from pits dug

close to the experimental site. These were placed

in a pressure membrane chamber and completely

wetted before being subjected to tensions of

0.03 MPa, 0.5 MPa, 1.0 MPa and 1.5 MPa to span

the range between FC and PWP; samples were

equilibrated for 2–3 days before completing the

measurement. VWC was determined for each soil

sample for all tensions applied; mean VWC val-

ues for each soil depth were plotted against the

corresponding neutron probe count to determine

the moisture release curve (Chirwa 2002).

Seasonal water use

Water use (WU) in the various land use systems

was estimated for the period between maize

planting and maturation of pigeonpea approxi-

mately 10 months later as

WU ¼ Ri þ SWChi ð1Þ

where R denotes rainfall and SWCh represents

the change in soil water content within the


123

0–150 cm soil profile between successive neu-

tron probe measurements (i). Runoff and

drainage beneath the maximum measurement

depth were assumed to be negligible as the

experimental site was located on flat land and

water content throughout the soil profile was

below field capacity except at depths greater

than 90 cm during the period of peak soil re-

charge, when these horizons reached or ex-

ceeded FC.

Statistical analysis

A randomised complete block design (RCBD)

analysis of variance (ANOVA) was used to test

for treatment effects. A split-plot model was used

to test for effects on soil water content, in which

the cropping system represented the main plot

and soil depth the sub-plot. Time series analyses

were used to test for significant changes with time

during each annual cycle.

Results

1997/98 cropping season

Water availability

Rainfall distribution was good during the 1997/

98 season (Fig. 1a), with >840 mm being

received during the maize cropping period

between December 1997 and May 1998. Plant-

available water, calculated as the difference

between the measured water content for each

horizon and the corresponding value at perma-

nent wilting point (PWP) summed over the

0–150 cm soil profile, was closely correlated with

rainfall (Figs. 1a and 2a). Available soil water

increased sharply between December and

January in response to the good rainfall in both

months and ranged between ca. 150–200 mm in

all treatments in January and March 1998.

Thereafter, available water declined between

March and August (P < 0.001, Table 1); the

reduction was most pronounced in the tree-

based gliricidia + maize and gliricidia + maize +

pigeonpea systems.

Water distribution

Figure 3 shows the seasonal changes in the pro-

files of VWC; significant variation with depth was

apparent for all sampling dates (P < 0.001). The

soil was already relatively moist in December

1997, particularly at depths below 30 cm, where

VWC exceeded 0.25 cm3 cm–3 in most treatments

(Fig. 3a); VWC values were lowest in the

maize + pigeopea treatment, particularly in the

30–60 cm horizon (P < 0.001). Substantial wet-

ting of all horizons occurred between December

1997 and January 1998 (Fig. 3b; P < 0.01), during

the vegetative growth of maize. The marked in-

crease in VWC during this period at depths below

30 cm demonstrates the occurrence of substantial

downwards percolation of water; VWC profiles

Fig. 1 Monthly total rainfall and mean potential evapo-ration during the 1997/98 (a), 1998/99 (b) and 1999/00 (c)cropping seasons at Makoka Research Station, southernMalawi


123

were neutral at depths below 30 cm. VWC was

generally lowest in the maize + pigeopea system

and highest in the sole maize and glirici-

dia + maize treatments. By April, VWC had de-

creased greatly in the 0–30 cm horizon, and to a

lesser extent in the 30–60 cm and 60–90 cm

horizons (Fig. 3c); little change was apparent in

the deeper horizons and no significant treatment

differences were detected. Broadly similar pro-

files were apparent in May (Fig. 3d), except that

VMC was lower in all horizons of the glirici-

dia + maize + pigeonpea system than in all other

treatments for all horizons below 60 cm

(P < 0.05). Maize was harvested on 20 May 1998.

By September, three months after the last sig-

nificant rainfall (Fig. 1a), VWC was still relatively

high at depths below 30 cm despite continued

drying of the 0–30 cm horizon (Fig. 3e). At this

time, when pigeonpea was producing pods and

gliricidia was left unpruned, VWC was generally

slightly lower in the tree-based gliricidia + maize

treatment than in the sole maize and

maize + pigeonpea systems. Seasonal changes in

VWC were greatest in the 0–30 cm horizon, for

which values in the gliricidia + maize treatment

decreased from a maximum of 0.25 cm3 cm–3 in

January to a minimum of 0.12 cm3 cm–3 in Sep-

tember.

Water use

The difference in estimated seasonal water use

between the gliricidia + maize treatment (901 mm)

and the sole maize, maize + pigeonpea and

gliricidia + maize + pigeonpea systems (855–

867 mm) approached significance (P = 0.06;

Table 1). WUE, calculated using the total above-

ground biomass produced by each system and the

corresponding water use values, was much lower

in sole maize than in all other treatments

(P < 0.001), primarily because biomass produc-

tion was substantially lower (P < 0.001; Table 1).


Water availability

Rainfall was unusually high (1123 mm) but

poorly distributed, as almost 500 mm was re-

ceived in December 1998 but only 230 mm was

received during the three months between the

end of January 1999 and maize harvest in April

(Fig. 1b). Available soil water ranged between

90 cm and 120 cm in November 1998, but in-

creased following heavy rainfall in November and

December to a maximum of 170–190 mm in all

treatments in February and March 1999 (Fig. 2b).

The values tended to be lowest under sole maize

between January and May, although no signifi-

cant treatment effects were detected. Available

soil water did not differ significantly between

treatments following maize harvest on 22 April

1999, although values were generally lowest in

the gliricidia + maize system. Seasonal variation

in available soil water was significant for all

treatments (P < 0.01).

Table 1 Total biomass, water use and water use efficiencyin the sole maize (SG), sole gliricidia (SG), sole pigeonpea(SP), maize + pigeonpea (MP), gliricidia + maize (GM)

and gliricidia + maize + pigeonpea (GMP) treatmentsduring the 1997/98, 1998/99 and 1999/00 cropping seasonsat Makoka Research Station, southern Malawi

Croppingsystem

Above-ground biomass (kg ha–1) Water use (mm) Water use efficiency (g kg–1)

1997/98 1998/99 1999/00 1997/98 1998/99 1999/00 1997/98 1998/99 1999/00

SM 4470 1300 3320 863.5 881.8 612.5 0.52 0.15 0.54MP 10610 5160 5290 866.9 886.0 606.6 1.23 0.62 0.87GM 7520 19600 11220 901.1 917.8 596.0 1.43 1.97 1.88GMP 9100 17240 14890 855.4 914.8 617.8 1.82 1.76 2.41SP N/A N/A 2100 N/A N/A 601.1 N/A N/A 0.35SG N/A N/A 30040 N/A N/A 619.7 N/A N/A 4.86P value < 0.001 < 0.001 < 0.001 0.06 ns ns < 0.001 < 0.001 < 0.001SED 980 640 1484 14.8 19.4 22.7 0.12 0.09 0.24CV% 11.0 7.7 16.0 2.1 2.6 5.0 12.1 10.0 16.0

N/A: no data are available for these treatments in 1997/98 and 1998/99; ns: no significant difference


123

Water distribution

As in 1997/98, extensive variation in VWC with

depth was apparent for all sampling dates

(P < 0.001). No significant treatment effects on

the distribution of soil moisture were detected in

January, February or March 1999 (Fig. 4a–c), al-

though the values were consistently lower under

sole maize than in the gliricidia + maize + pi-

geonpea treatment, particularly at depths below

30 cm. In February, March, April and May, VWC

in the 90–120 and 120–150 horizons reached or

exceeded field capacity (0.31 cm3 cm–3) in the

tree-based systems, indicating significant down-

wards percolation of water associated with the

extremely high rainfall between December and

March (820 mm; Fig. 4b–e). VWC was high

throughout the profile in April, during the

reproductive phase of maize (Fig. 4d); although

no significant treatment effects were apparent at

this time; values below 90 cm tended to be

greatest in the gliricidia + maize + pigeonpea

treatment. By May, considerable drying of the 0–

30 cm horizon had occurred and VWC values in

the gliricidia + maize + pigeonpea treatment

were much greater than in all other treatments at

depths below 60 cm (P < 0.05; Fig. 4e). In

October, during the dry season, there was again

no significant variation in VWC between treat-

ments, although the values were generally slightly

lower in the tree-based gliricidia + maize treat-

ment, especially at depths below 60 cm. As in

1997/98, seasonal changes in VWC were much

greater in the 0–30 cm horizon than in deeper

horizons (Fig. 4f; P < 0.001).

Water use

Seasonal water use did not differ significantly

between cropping systems; the values were simi-

lar for the tree-based gliricidia + maize and gli-

ricidia + maize + pigeonpea systems (ca.

915 mm; Table 1) and the sole maize and

maize + pigeonpea treatments (882–886 mm).

WUE was substantially greater in both tree-based

treatments (P < 0.001), again primarily because

of their much greater biomass production

(P < 0.001).


Water availability

Rainfall was low and poorly distributed; over

125 mm was received in November 1999, before

the cropping season began, but only 35 mm was

received in December (Fig. 1c). There was no

further rainfall for over two weeks, resulting in

complete failure of the maize planted on 1

December 1999 and necessitating replanting on

Fig. 2 Available water in the 0–150 cm soil profile for thesole maize (SM), sole gliricidia (SG), sole pigeonpea (SP),maize + pigeonpea (MP), gliricidia + maize (GM) andgliricidia + maize + pigeonpea (GMP) treatments duringthe 1997/98 (a), 1998/99 (b) and 1999/00 (c) croppingseasons at Makoka Research Station, southern Malawi.Vertical bars show standard errors of the differencebetween treatment means


123

11 January 2000. Rainfall peaked in February

before decreasing in March and April during

the grain filling period; maize was harvested on 29

May 2000 after receiving only 2 mm of rain dur-

ing the preceding month (Fig. 1c). Available soil

water did not differ significantly between treat-

ments in December or January (Fig. 2c), but in-

creased to a maximum in all treatments in

February and March (P < 0.01) in response to

the relatively high rainfall during the preceding

months. Available soil water then decreased in

all treatments to a minimum of < 30 mm in sole

gliricidia and sole pigeonpea in June, compared

to 120–130 mm in all other cropping systems

(P < 0.001). Rapid recharge of available soil

water occurred in sole gliricidia and sole

pigeonpea following unexpected rainfall in June

(>75 mm), with the result that no significant

treatment differences were apparent in

September.

Water distribution

In January 2000, steep gradients of VWC were

apparent in all treatments, with the 0–30 cm hori-

zon being substantially drier (0.18–0.20 cm3 cm–3)

than all deeper horizons (Fig. 5a; P < 0.001).

By February, VWC had increased throughout the

soil profile following further rainfall (P < 0.01;

Fig. 5b), particularly in the 0–30 cm horizon,

Fig. 3 Profiles of volumetric water content (VWC) for thesole maize (SM), maize + pigeonpea (MP), glirici-dia + maize (GM) and gliricidia + maize + pigeonpea(GMP) treatments in December 1997 (a), January 1998

(b), April 1998 (c), May 1998 (d) and September 1998 (e)at Makoka Research Station, southern Malawi. Verticalbars show standard errors of the difference betweentreatment means


123

where values reached 0.23–0.25 cm3 cm–3. Values

were generally lowest in the maize + pigeonpea

and greatest in the gliricidia + maize + pigeonpea

treatments, although no significant treatment

effects were observed in either month. In March

and April, VWC values for the 0–30 cm horizon

(Fig. 5c, d) were lower than in February, although

the values were broadly comparable for all

treatments at depths below 30 cm. The observa-

tion that VWC values were greater at depths below

30 cm in March and April than in January is

again indicative of significant downward percola-

tion of water to the deeper horizons. In June,

shortly after maize was harvested on 29 May 2000,

VWC was much lower in the sole gliricidia and

sole pigeonpea systems than in all other cropping

systems (P < 0.001; Fig. 5e). At this time, VWC

ranged between 0.14 cm3 cm–3 and 0.22 cm3 cm–3

in the former treatments compared to

0.19 cm3 cm–3 and 0.31 cm3 cm–3 in the latter. By

September, VWC in the 0–30 cm horizon was

much lower than in the deeper horizons and was

also lowest under sole gliricidia at all depths

(Fig. 5f; P < 0.001). As in previous years, VWC

was lower in the 0–30 cm horizon than in deeper

horizons on all measurement dates (P < 0.001).

Fig. 4 Profiles of volumetric water content (VWC) for thesole maize (SM), maize + pigeonpea (MP), glirici-dia + maize (GM) and gliricidia + maize + pigeonpea(GMP) treatments in January 1999 (a), February 1999

(b), March 1999 (c), April 1999 (d), May 1999 (e) andOctober 1999 (f) at Makoka Research Station, southernMalawi. Vertical bars show standard errors of thedifference between treatment means


123

Water use and water use efficiency

Estimated total water use did not differ signifi-

cantly between treatments, ranging between

596 mm and 620 mm (Table 1). WUE was

greatest in sole gliricidia (ca. 4.86 g kg–1;

P < 0.001), followed by the gliricidia + maize +

pigeonpea and gliricidia + maize systems,

although the values were less than 50% of that for

sole gliricidia. As in both previous years, WUE

was lowest in sole maize and sole pigeonpea

(0.54 g kg–1 and 0.35 g kg–1, respectively;

P < 0.001).

Discussion

The three seasons examined provided contrasting

rainfall patterns (Fig. 1). Rainfall was good and

well distributed during the 1997/98 maize crop-

ping season, but was unusually high in 1998/99,

with the great majority being received during the

Fig. 5 Profiles of volumetric water content (VWC) for thesole maize (SM), sole gliricidia (SG), sole pigeonpea (SP),maize + pigeonpea (MP), gliricidia + maize (GM) andgliricidia + maize + pigeonpea (GMP) treatments in

January 2000 (a), February 2000 (b), March 2000 (c), April2000 (d), June 2000 (e) and September 2000 (f) at MakokaResearch Station, southern Malawi. Vertical bars showstandard errors of the difference between treatment means


123

early vegetative growth of maize, followed by an

extended period of low rainfall during the

reproductive phase. In 1999/00, rainfall was ini-

tially so limited that maize sown at the normal

time on 1 December 1999 failed and had to be

replanted in January 2000. These contrasting

seasons allowed treatment effects on available

soil water, nitrogen and water use and biomass

production in the various cropping systems to be

examined under a range of water supply condi-

tions. Tree and crop performance and system

productivity differed greatly between treatments

(Table 1). The tree-based cropping systems were

most productive and substantial quantities of

green leaf manure (GLM, 2.4–9.0 t ha–1 year–1), a

primary objective of the gliricidia-based systems

examined here, were produced from the second

or third year after tree establishment (Chirwa

et al., 2003). Significant improvements in maize

yield were obtained within 3 years in the tree-

based systems following regular applications of

GLM, with ca. 3.0 t ha–1 of grain being produced.

Grain yield for maize was much lower in 1998/99

than in both other years, reflecting the extremely

poor rainfall during the reproductive phase

(Fig. 1). No beneficial influence of pigeonpea on

maize grain yield was apparent either in the

presence or absence of gliricidia (Chirwa et al.

2003).

Soil water availability

Competition for water during the cropping season

was not a critical limiting factor for tree and crop

growth as rainfall exceeded potential evaporation

during the maize cropping season in all years

(Fig. 1). The generally lower available soil water

content at the beginning of the cropping season in

the tree-based systems suggests that the trees

continued to deplete available moisture during

the dry season. During the 1998/99 and 1999/00

seasons, available water content tended to be

greater in the gliricidia + maize + pigeonpea

system at the peak of the recharge phase than in

the other treatments, perhaps due to the com-

bined effects of increased infiltration promoted by

improvements in soil structure and reduced soil

evaporation resulting from the greater ground

cover in this treatment; fractional light intercep-

tion was 0.6–0.7 in the tree-based systems com-

pared to 0.1–0.4 in the sole maize and

maize + pigeonpea treatments (Chirwa et al.

2003).

Jackson et al. (2000) reported that the initial

recharge of the soil profile following heavy rain

was greater in agroforestry treatments than in

sole maize or sole tree treatments at Machakos in

Kenya. It has been suggested that percolation of

water through the soil profile may be improved in

agroforestry systems by channels created by tree

roots which have died and decomposed (van

Noordwijk et al., 1991; Schroth 1999). Jackson

et al. (2000) also reported that soil moisture

content in the 1.2–1.6 m horizon of the agrofor-

estry treatments was rapidly depleted following

rainfall, perhaps because competition with asso-

ciated crops in the surface horizons forced the

trees to abstract water from deeper horizons and

the maximum rooting depth was limited to

150 cm in their study. No equivalent rapid

depletion of water in the deeper horizons was

apparent in the present study as soil moisture

content in the 120–150 cm horizon of the glirici-

dia + maize and gliricidia + maize + pigeonpea

treatments reached field capacity (0.31–

0.32 cm3 cm–3) in March and April of all seasons

(Figs. 3–5). Soil depth at our study site exceeded

2 m, although the deepest horizons consisted of

highly fragmented weathered rock, making it

impossible to determine maximum rooting depth.

However, excavations on deeper soil ca. 500 m

from our experimental site revealed that gliricidia

roots reached a depth of 5 m (Anon. 1998a).

The differing patterns of water abstraction may

therefore have originated from differences in tree

management or soil depth. At Machakos, the

trees were pruned infrequently and pruning was

confined to the upper canopy, whereas at Makoka

they were pruned to 30 cm above ground-level

before and during each cropping season. Ong and

Leakey (1999) reported that trees subjected to

repeated shoot pruning may develop greater

rooting densities in the surface horizons and may

therefore compete for water with associated crops

at different times due to the altered pattern of

activity induced by pruning. In the present study,

coppicing of gliricidia before and during the

cropping season removed all foliage with the


123

intention of minimising transpiration, shading and

other competitive tree/crop interactions. Water

abstraction from the 0–30 cm horizon was nev-

ertheless greater than that from the deeper hori-

zons in all treatments and seasons (Figs. 3–5),

suggesting that the tree and crop roots both

absorbed water from this horizon.

Soil water distribution

VWC profiles were generally comparable for all

treatments in all cropping systems (Figs. 3–5),

suggesting there was little spatial complementar-

ity in water abstraction between tree and crop

roots. This observation supports previous findings

that rooting density in gliricidia is greatest in the

surface horizons (Rowe et al. 1999), although

other studies in Malawi indicate that roots of

unpruned gliricidia may reach depths exceeding

4 m during the dry season (Anon. 1998a, 1998b).

These findings suggest that gliricidia invests a

substantial proportion of its below-ground re-

sources in root growth in the surface horizons

when water supplies are readily available, but

may root to much greater depth under water-

limited conditions, as occurred in 1999/00 when

sole gliricidia and sole pigeonpea abstracted

residual water to a depth of 150 cm to support

continued growth during the dry season (Fig. 5).

The results show that, under the relatively high

rainfall conditions typical of southern Malawi, the

soil profile contains enough stored water during

the dry season to sustain the growth of gliricidia

and pigeonpea, thereby demonstrating temporal

complementarity resulting from the use of resid-

ual water after maize was harvested. Such water

would be used most effectively by deep-rooted

species because the low water content of the

surface horizons during the dry season is insuffi-

cient to support shallow-rooted species.

Water use

In their comprehensive review of previous stud-

ies, Morris and Garrity (1993) noted that differ-

ences in water use between intercrops and

sole crops often range between – 6% and + 7%,

although these mainly involved systems which did

not include trees. The absence of detectable

treatment effects on available soil water in all

years in the present study suggests that the vari-

ous cropping systems used similar quantities of

water, a view supported by estimates of seasonal

water use (Table 1). Droppelmann et al. (2000)

reported similar findings for an agroforestry trial

in Northern Kenya involving Acacia saligna and

Sorghum bicolor. However, this conclusion may

not be valid for the present study in view of the

substantial differences in productivity between

the tree-based and sole cropping systems; for

example, above-ground biomass production by

sole gliricidia in 1999/00 was almost 10-fold

greater than in sole maize (Table 1). A possible

explanation is that, in areas of relatively high

rainfall or poor drainage, the water table may

remain close to or within the rooting zone for

much of the cropping season, particularly during

periods when significant deep percolation occurs,

as in the 1999/00 season (Fig. 5). Under such

circumstances, treatment differences in water use,

and hence in calculated water use efficiency val-

ues, may be masked if significant quantities of

water are extracted from the water table by deep

rooting species. In such cases, measurements of

water abstraction from horizons above the water

table, as in the present study, cannot provide

reliable estimates of total water use. However,

this difficulty may be avoided in future studies by

using sap flow gauges to determine water uptake

by individual system components (Lott et al.

2003); this approach provides direct, non-

destructive measurements of the quantity of wa-

ter used during the production of dry matter,

thereby providing unequivocal estimates of WUE

for individual system components.

Soil evaporation may have contributed to the

apparently high water use in the sole maize and

maize + pigeonpea treatments as evaporative

losses may be large in annual cropping systems in

the semi-arid tropics (Cooper et al. 1983; Wallace

1991, 1996). In the present study, ground-cover

was limited at the beginning of the cropping

season in all treatments except sole gliricidia be-

cause the trees in the mixed cropping systems had

been pruned and the maize and pigeonpea were

still small. Transpiration would therefore have

been low at this time, with the result that a


123

substantial proportion of water which infiltrated

into the soil profile may subsequently have been

lost by evaporation. In support of this view,

Droppelmann et al. (2000) reported that a large

proportion of the available water was lost by

evaporation from bare soil in alleys between

pruned sole trees in Northern Kenya during

periods when ground cover was negligible fol-

lowing pruning. This situation is analogous to the

present study, in which gliricidia was pruned to

30 cm above ground-level before and during each

cropping season. However, the pruned gliricidia

had established root systems able to extract a

proportion of the water which infiltrated into the

soil profile during the regrowth periods and the

ensuing dry season. This component of the soil

water balance represents the fraction used to

produce the dry matter which provides GLM at

subsequent prunings.

Morris and Garrity (1993) and Ong et al. (1996)

concluded that, although total water use may not

differ greatly between sole and intercropping

systems, the latter often use water more effi-

ciently. This conclusion is supported by the pres-

ent study which suggests that WUE was much

greater in the tree-based systems than in sole

maize or sole pigeonpea (Table 1). However, the

WUE values presented here should be treated

with some caution as soil surface evaporation and

abstraction from the water table were not quan-

tified. If these were taken into account, WUE

values might be higher than shown in Table 1 for

the sole crop treatments, but lower for sole gli-

ricidia. This view is substantiated by reports that

season-long WUE values range between 2.1 g kg–1

and 5.2 g kg–1 in millet, a C4 species and 1.5 g kg–1

and 6.4 g kg–1 in groundnut, a C3 species,

depending on the prevailing atmospheric satura-

tion deficit (Black and Ong 2000). Lower values

have been reported for castor beans grown under

semi-arid conditions (0.88–1.31 g kg–1; Vijaya

Kumar et al., 1996). These values are appreciably

greater than those obtained for sole maize and

sole pigeonpea in the present study (Table 1),

although the WUE values appear more realistic

for treatments containing gliricidia. The values for

the gliricidia + maize and gliricidia + maize +

pigeonpea treatments increased between 1997/98

and subsequent years as the trees became estab-

lished and their biomass production increased.

The high WUE value obtained for sole gliricidia

in 1999/00 reflects the intense shade provided by

its dense canopy, as fractional light interception

reached 96% (Chirwa et al. 2003); the associated

microclimatic changes would have greatly reduced

soil evaporation, ensuring that evapotranspira-

tional losses were dominated by transpiration. By

contrast, the canopy of sole pigeonpea did not

close at any stage of the season and maximum

light interception was ca. 10%, with the result that

evaporation would have approached potential

evaporation whenever the soil surface was wet.

Nutrient leaching

Soil moisture content in the deeper horizons in-

creased sharply between December 1997 and

January 1998 (Fig. 3a, b) and between January

and February in 1999 and 2000 (Figs. 4a, b and 5a,

b); the deepest horizons reached or exceeded field

capacity in the tree-based systems in all years.

These results clearly show that substantial per-

colation of water from the soil surface to deeper

horizons occurred during the early stages of the

cropping season. This process has potentially

important implications for soluble nutrients, par-

ticularly NO3–N and NH4–N, released by miner-

alisation of GLM added prior to the cropping

season, as these are highly susceptible to being

leached to deeper horizons. Chirwa et al. (2006)

reported that soil mineral N concentration was

invariably high at the start of the cropping season

but declined rapidly, a phenomenon attributed to

leaching during periods of high rainfall. They

concluded that leaching of mineral N from the

surface horizons was particularly severe in sole

maize and maize + pigeonpea due to the small

size and limited nutrient requirements of these

species during the early stages of the cropping

season. Leaching of mineral N from the surface

horizons is common in savannah areas where the

onset of the rains promotes rapid production of

mineral N (Giller et al. 1997; Ikerra et al. 1999).

However, N leached from the surface horizons in

agroforestry systems may be captured in deeper

horizons by tree roots and recycled to the

soil surface through litter fall or applications of

GLM.


123

Despite the likelihood that a proportion of the

mineral N released during decomposition of the

gliricidia GLM applied in the tree-based cropping

systems was leached from the surface horizons

during the early stages of the cropping season, the

significant improvement in maize yield in these

systems (Chirwa et al. 2003) suggests that the

crops captured a significant proportion of the N

released. The observation that foliar nitrogen

concentration and N accumulation in the haulm

and grain of maize was greatly increased in the

tree-based systems (Chirwa et al. 2003, 2006)

suggests that the GLM applied in these treat-

ments provided a major source of N to satisfy the

needs of maize during both its vegetative and

reproductive growth periods.

Conclusions

The present study provided no evidence that

seasonal water use was greater in the tree-based

systems than in sole maize. Indeed, 90–140 mm of

stored water remained unused within the soil

profile in all cropping systems between August

and October, during the dry season. It can

therefore be concluded that the presence of gli-

ricidia, pruned before and during the cropping

season, does not provoke undue competition for

water with associated crops under the prevailing

climatic conditions in southern Malawi. The re-

sults suggest that WUE was greater in the agro-

forestry systems than in sole maize and

maize + pigeonpea, subject to the proviso that

the estimates of water consumption used in the

calculation were based on changes in soil water

content rather than direct measurements of water

uptake by the tree and crop components. The

observed changes in soil water content may also

have been influenced by evaporation from the soil

surface. In future, it will be essential to measure

soil evaporation and water uptake by the com-

ponent species of intercropping systems in order

to provide direct measurements of the quantity of

water involved in the production of dry matter

and thereby provide rigorous estimates of the

WUE for each system component. The results

suggest that the gliricidia + maize + pigeonpea

system is attractive for adoption in southern

Malawi because regular applications of GLM

greatly improve maize yields; it also meets the

needs of farmers who wish to include pigeonpea

as a ‘risk crop’ and provides fuelwood. However,

further studies are required before the system can

be extended to the water-limited environments

widely encountered in southern Africa.

Acknowledgement Paxie Chirwa thanks RockefellerFoundation for the Doctoral Fellowship which enabled thepresent study to be carried out.

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