Decomposition and nutrient release from leaves, twigs and roots of three alley-cropped tree legumes in central Togo

Agroforestry Systems 29: 21-36, 1995. �9 1995 Kluwer Academic Publishers. Printed in the Netherlands.

Decomposition and nutrient release from leaves, twigs and roots of three alley-cropped tree legumes in central 'logo

J. L E H M A N N , G. S C H R O T H and W. Z E C H Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany

Key words: alleycropping, Calliandra calothyrsus, decomposition, Gliricidia sepium, polyphenol, Senna siamea

Abstract. The decomposition of leaves, twigs and roots of two diameter classes (< 1.5 ram, 1.5-5 mm) were examined in an alley cropping experiment with Gliricidia sepium, Calliandra calothyrsus and Senna siamea in the subhumid savanna of Central Togo using the litterbag technique. The effect of the application of leaves and twigs as mulch or green manure was examined.

Gliricidia showed the most rapid mass loss. For all species, leaves decomposed faster than roots. Twigs had the lowest decomposition rate except for Calliandra. The two diameter classes of roots decomposed differently in the three species: Gliricidia fine roots decomposed faster than its coarser root fraction, the coarse roots of Senna decomposed faster than the fine roots.

Termites influenced the mass loss of twigs and roots to varying extents for the different species. In the case of twigs this was markedly influenced by the mode of application: green manure showed more termite frass than mulch.

Nutrient release resembled the mass loss patterns of the prunings except for K, which was leached independently from mass loss. The release of the different nutrients was in the order Ca < Mg = N < P << K. No substantial N immobilization was observed even during twig and root decomposition despite their low contents.

The different decomposition rates of leaves, twigs and roots between species as determined from the residual masses could be related to the initial polyphenol/N and polyphenol + lignirdN ratios. In the first two weeks, cumulative N release from all materials including leaves, twigs and roots could be predicted with the (polyphenol + lignin)/N ratio of the initial substrate. At ratios higher than 10, N release was independent of this ratio. Ca and N release, but not Mg and K release, and mass loss were related to substrate quality.

In view of both the absolute amounts and patterns of nutrients released from different prunings, a combination of Gliricidia and Senna seemed to be best for mulch and green manure production at our site.

Introduct ion

I t is b e l i e v e d tha t a g r o f o r e s t r y s y s t e m s l i ke a l l ey c r o p p i n g i n c r e a s e so i l

f e r t i l i t y by the a p p l i c a t i o n o f b i o m a s s to the so i l t h r o u g h the i n p u t o f nu t r i -

en t s and o r g a n i c mat te r . A d d i t i o n a l l y , the p r e s e n c e o f a m u l c h l a y e r m a y

r e d u c e the e r o s i o n h a z a r d [Young , 1989] . T h e s y n c h r o n i s a t i o n o f n u t r i e n t

r e l e a s e f r o m m u l c h and g r e e n m a n u r e w i t h nu t r i en t u p t a k e by c r o p s is con -

s i d e r e d a v i t a l p o i n t fo r i m p r o v i n g the e f f i c i e n c y o f nu t r i en t u p t a k e t h r o u g h

the p l an t [Swi f t , 1987] . W i t h the h e l p o f l e g u m i n o u s t rees , a c o n s i d e r a b l e

a m o u n t o f N can be a d d e d to the soi l , thus pa r t ly r e p l a c i n g m i n e r a l f e r t i l i z e r

22

[Szott and Kass, 1993]. In alley cropping systems, decomposition and the subsequent nutrient release from leaves, twigs and roots may have an important influence on the organic matter and nutrient budget of the soil [Szott et al., 1991; Young, 1989].

While in the temperate climate accumulation especially of N and P in the litter can often be observed in the course of decomposition [Swift et al., 1979], nutrients are generally released faster in the tropics [e.g. Schroth et al., 1992]. During the decomposition of root litter from woody plants, N is often immobilized to a great extent in temperate regions [e.g. Camire et al., 1991]. Sandhu et al. [1990], however, reported that N release patterns from roots of Leucaena leucocephala followed the same pattern as their mass loss. The application of prunings on the soil surface (mulch) or incorporation into the soil (green manure) may affect nutrient release and the consequent yield responses [Wilson et al., 1986].

Understanding the regulatory mechanisms of decomposition and nutrient release is necessary for the prediction of mass loss and nutrient release patterns. Meentemeyer [1978] suggests that, in the tropics, decomposition rates are controlled by substrate quality rather than by the climate. For a range of tropical legumes, N mineralisation from leaves was related to the initial polyphenol/N ratio [Palm and Sanchez, 1991] and the (polyphenol + lignin)/N ratio [Fox et al., 1990]. For tropical ecosystems there is also a lack of data on root decomposition and how it is influenced by resource quality [Young, 1989]. The assessment of quality parameters would facilitate the identifica- tion of suitable tree species in agroforestry [Swift, 1987].

The purpose of the present study was to investigate mass loss and nutrient release patterns from leaves, twigs and two diameter classes of roots of alley- cropped Gliricidia sepium, Calliandra calothyrsus and Senna siamea in a litterbag experiment. The prunings were both applied on the soil surface and incorporated into the soil.

Study site and methods

The experiment was conducted at the Kazaboua experimental station in Central Togo, West Africa, at 1~ ' E and 8026 ' N at an altitude of 300 m asl. The rainy season is unimodal with a mean precipitation of 1156 mm/yr from 1982 to 1991 and 1259 mm/yr during the experimental year of 1992. The main growing season is from May to September. The vegetation type is the Northern Guinea savanna. The soil at this site has been classified as plinthic Acrisol [FAO, 1988] and is associated with ferric Acrisols. The physical and chemical properties are shown in Table 1 indicating low ECEC and low levels of nitrogen, phosphorus in the subsoil, potassium and zinc.

The trial was laid out in an alleycropping system with GIiricidia sepium Steud., Calliandra calothyrsus Meissn. and Senna siamea (synonymous Cassia siamea Lam.). Gliricidia and Calliandra, in contrast to Senna are able to fix

Tab

le 1

. P

rofi

le c

hara

cter

isti

cs a

t th

e ex

peri

men

tal

site

.

Dep

th

Hor

izon

Sto

ne

Tex

ture

Bul

k pH

C

,ot

Nto

, PM

eh

ZnM

e h E

CE

C

CE

C

Ca

Mg

K

Na

Fe

(cm

) co

nten

t cl

ass

dens

ity

(HzO

)

(vol

%)

(g/c

m 3)

(r

ag/g

) (c

mol

c/kg

)

Mn

AI

H

BS

B

S

(%

(%

EC

EC

) C

EC

)

0-1

9

Ap

6 IS

19-5

6 A

h 79

IS

56-8

7 B

cs

75

sL

56-1

06

2Btg

l 76

sC

106+

2B

tg2

3 sC

1.53

6.

49

7.5

0.49

44

.8

0.33

3.

74

5.97

2.

89

0.65

0.

06

0.00

0.

00

0.03

0.

11

0.00

96

1,77

6.

08

4.1

0.31

0.

4 0.

16

2.62

4.

94

1.84

0.

53

0.09

0.

00

0.00

0.

01

0.15

0,

00

94

1.95

6.

35

0.3

0.31

0.

0 0.

14

2.35

9.

37

1.33

0.

78

0.10

0.

01

0.01

0.

03

0.10

0.

00

95

1.37

5.

70

nd"

0.32

0.

0 0.

16

1.66

9.

27

0.59

0.

50

0.07

0.

00

0.00

0.

01

0.48

0.

01

70

1.83

5.

69

nd

0.37

0.

0 0.

30

3.04

13

.69

1.07

1.

44

0.13

0.

01

0.02

0.

01

0.35

0.

01

87

48

41

20

16

19

EC

EC

= e

ffec

tive

cat

ion

exch

ange

cap

acit

y;

CE

C =

cat

ion

exch

ange

cap

acit

y;

BS

= b

ase

satu

rati

on.

t-3

L,O

24

atmospheric N. Each species was planted in 1990 as hedge-rows at 6.40 m distance with 0.25 m between the trees within the hedgerows. The trial con- sisted of four blocks of three species and one control each (randomized complete block design), with an individual plot size of 12.8 x 24 m. The hedges were cut twice during 1992 (25 May and 2 July) and the prunings were applied as mulch in the alleys. Before the first pruning the soil tillage was carried out by the traditional hoe-ridging, leading to ridges of 30-40 cm height at 0.80 m distance. Maize was sown on the ridges at 0.40 m distance on 26 May and harvested on 28 September.

Leaves, twigs and roots of the three species were collected 5 days before the first pruning of the hedges. Twigs with a diameter of 0.8-1.3 cm were chosen and cut into pieces of 10 cm. Dead roots were discarded, living roots carefully cleaned from adherent soil, washed and separated into two diameter classes < 1.5 mm and 1.5-5 ram. All materials were air-dried.

Nylon litterbags were prepared with 5 mm mesh size on the upper side and 2 mm on the lower side in order to maximize access for all major soil fauna and to minimize artificial litter loss from the bags. Separate bags were used for leaves, twigs and roots. Approximately 3 g of air-dried leaves and 15 g of twigs were put into 15 x 15 cm litterbags, and 2 g and 3.5 g of roots of the diameter < 1.5 and 1.5-5 mm, respectively, were put into 10 x 10 cm bags. The wide mesh size and the little amount of plant material in the litterbags ensured a good contact between the plant and the soil.

On 27 May 1992, litterbags containing leaves, twigs and roots were placed on the ridges at l0 cm depth and litterbags with leaves and twigs were also applied on the ridges, simulating green manure and mulch. Although the materials were not uniformly mixed with the soil, the term green manure seemed to be adequate in our study, because the traditional hoe-ridging simply places the soil on the plant material applied on the soil surface. Four sets of six bags each (roots of two diameter classes, leaves and twigs both in two modes of application) corresponding to four sampling dates were incubated in each plot at 1.60 m distance from the hedge. Half of the bags were placed at the south side, and half at the north side of the respective alley. The bags were exposed in the plots, the material contained in the bags originated from these plots. This resulted in a split-split-plot design with the factors species, material and time for all materials, and a four-factor hierarchical model for leaves and twigs with the application method (mulch or green manure) as an additional factor [Steel and Torrie, 1980]. The bags on the ridges were placed between two maize plants.

One set of samples per species and block was collected after 8, 18, 41 and 70 days and handled btockwise: adherent soil was washed carefully from the plant residues with a minimum of distilled water to reduce artificial leaching. The plant material was dried for 48 hours at 70 ~ and weighed to the accuracy of + 0.01 g.

The samples were ground and redried for 24 h at 60 ~ Carbon and nitrogen were determined with a C/N-analyser. P, K, Ca and Mg were analysed

25

after ashing in a muffle furnace at 560 ~ for 16 h and dissolving the ash in 10% HC1. P was determined photometrically with the molybdene blue method in a Rapid Flow Analyser. K, Ca and Mg were measured by atomic absorp- tion spectrometry. Lignin and cellulose were analysed gravimetrically through acid detergent fiber in accordance with the method of Van Soest and Wine [1968]. Total soluble polyphenols were extracted from the ground samples with Nz-treated water for 16 h in an agitator and measured photometrically according to King and Heath [1967]. Total lipids were extracted for 16 h with methanol-chlorophorm (1/2, v/v), purified with 1 N KOH, separated from the extractants by evaporation with the rotavapor and weighed.

Statistical analysis of the data was carried out using a computed analysis of variance (ANOVA) with the Statistical Analysis System [SAS Institute, 1992]. If the main effects were significant at p < 0.05, a multiple comparison of the means was included using the Tukey test; in case of significant interactions the LSD (least significant difference) at p < 0.05 was calculated for relevant cell means at the level of the interaction [Steel and Torrie, 1980].

Results

Carbon loss

The variable carbon concentration in the decomposed substrate indicated variable contamination with mineral soil material. Therefore the residual amount of carbon was preferred to the residual dry matter for the presenta- tion of the data and shown as per cent of the initial amount in the samples (Figs. 1 and 2).

Carbon loss from leaves decreased in the order Gliricidia > Calliandra > Senna. After 70 days, no significant differences could be observed, but Gliricidia decomposed considerably faster in the first 18 days with 75% mass loss compared to Senna with 35%. The decomposition of the twigs was not significantly different between the three species either over the whole incubation period or at individual sampling dates (p < 0.05).

Leaf litter decomposed more slowly when applied as mulch than as green manure. The placement effect was less pronounced for the twigs, resulting in a significant placement x material interaction. Senna leaves revealed an effect of the mode of application between day 41 and 70, showing hole frass in the soil incorporated treatments, which could be attributed to termite activity, in contrast to cell frass by microorganisms. At the first three sampling dates the leaves of Gliricidia exhibited the greatest application effect, when no termite frass was detected in any material.

Fine root (< 1.5 mm) decomposition decreased in the order Gliricidia > Senna > Calliandra. After 8 days of incubation, coarse roots (1.5-5 ram) of Gliricidia showed a net gain of carbon, Calliandra a slight and Senna a rapid mass loss. At the end of the incubation residual carbon of Gliricidia coarse

2 6

1 0 0 @ H'~%': ...... T a l i r i c i d i a s e p i u r n

. . . . . . o . . " .......... . T

o

ID

20

_ I t ..... i . . . . [] SUN

0

1001

o o 80

40

20

0

1001

80

60

40

20

I I I

~. . S e n n a s i a m e a

0 Roots <1.5 ram ............... �9 Roots 1.5-5 mm ....

2 V Twigs Leaves

20 40 60 80 Days of incubat ion

Fig. 1. Change of residual carbon of the fine roots (< 1.5 ram), coarse roots (1.5-5 ram) and green manured leaves and twigs as percentage of the original carbon; means and standard errors.

roots equalled that of Senna. Calliandra coarse roots still had 77% of their initial carbon, Calliandra fine roots 84%.

Gliricidia fine roots d e c o m p o s e d faster than the coarse roots, whereas Senna coarse roots exhibited faster decompos i t ion than the fine roots; root diameter had no influence on mass loss from Calliandra roots.

Nutrient release patterns

Initially, Gliricidia s h o w e d the highest nutrient contents in all plant parts except for N in the leaves and P in the twigs, which were more highly con- centrated in Calliandra (Table 2). Senna had the lowest nutrient concentrations apart from Ca, which had its highest concentrations in leaves , twigs and roots. In order to analyse both the dynamics o f nutrient release and the

27

4-

Glir ic id ia s e p i u m

80 I \",, .......... ' ~ <:::: ~: ' : ~ : a ::~,,_~. ' l [ ..........

oot . lm.

40 [] ' " - - .

20 . . . . .

. . . . . . . "'U .,~ 0 I I ......... P ......... []

v 100n ~ C a l l i a n d r a c a l o t h y r s u s

o~ 80 i - . " .................... ~.__~:i---- [] ,i'-~. ............. " ...........

[.~:~ 60 ~._<.:..;.7~77 .... " .......................

4O

i?'~71 I~ITI~Z ZI77172727 -ZZ27 Z 20 o

rO 0 I I I

1001 .-'--~.,7~---v--~_ S e n n a s i a m e a

......... " 7 ........................ - - - " - ~ - - ~ - - - _ 80 . . . . I ......... ~ ................ ~ - v

60 " " - . ""'~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . m +

40 n

E7 Twigs green manure �9 Twigs mulch

20 E] Leaves green manure �9 Leaves mulch

0 I I I 2 0 4 0 6 0 8 0

D a y s o f i n c u b a t i o n

Fig. 2. Change of residual carbon of leaves and twigs applied on the soil surface and incorporated into the soil as percentage of the orginal carbon; means and standard errors.

absolute amounts which were released per unit mass, nutrient release is shown as residual amounts of the initial concentration; thus, the values for day 0 are initial concentrations. For the sake of clarity only the contrasting examples of N, Ca and Mg for Gliricidia and Senna and of P and K for Gliricidia are depicted in Figs. 3 and 4.

No net N immobilization was observed during the decomposition of leaves, twigs and roots (Fig. 3). Gliricidia leaves released most of their N during the first phase (until day 18), Calliandra fairly uniformly throughout the incubation period. Senna green manure released most of its N during the second phase (after day 18), while the mulch still retained 60% at the end of the incubation time. Apart from Senna, no great differences could be observed between the nutrient release from leaves applied on the soil surface or incorporated into the soil. Therefore, only soil incorporated materials are shown

oo

Tab

le 2

. In

itia

l ch

emic

al c

om

po

siti

on

of

the

exam

ined

spe

cies

.

Spe

cies

C

/N

PP"

PP

/N

Lig

nin

Lig

nin/

N

Cel

lulo

se

Lip

id

N

P K

C

a M

g

(%)

(fro

) (%

) (%

) (g

/kg)

GIi

rici

dia

sepi

um

Lea

ves

14.0

4.

1 1.

31

9.8

3.1

17.8

6.

1 31

.8

1.70

13

.9

13.1

6.

62

Tw

igs

51.9

0.

7 0.

85

16.5

18

.8

55.0

1.

7 8.

8 1.

14

8.0

2.4

0.83

R

oots

< 1

.5 m

m

20.5

1.

0 0.

49

17.6

8.

2 30

.5

4.4

21.5

1.

22

25.1

4.

1 4.

53

Roo

ts 1

,5-5

mm

31

.3

0.8

0.58

17

.0

10.6

33

.4

2.2

14.1

1.

10

17.6

4.

2 3.

35

Cal

lian

dra

calo

thyr

sus

Lea

ves

13.6

13

.0

3.89

6,

7 2,

0 14

.1

8.0

33.6

1.

67

7.7

10.7

2.

90

Tw

igs

95.4

1.

4 2.

90

13.3

26

.6

57.1

1.

8 5.

0 1.

49

3.9

1.7

0.63

R

oots

< 1

.5 m

m

40.1

18

.7

16.4

1 14

.8

13.0

37

.7

1.3

11.4

0.

74

4.1

1.6

0.94

R

oots

1.5

-5 m

m

53.9

20

.3

23.1

6 15

.7

17.8

27

.5

1.3

8.8

0.68

3.

4 1.

1 0.

49

Senn

a si

amea

L

eave

s 25

.7

10.1

5.

6l

8.9

4.9

15.4

10

.4

18.1

1.

16

4.6

18.5

2.

15

Tw

igs

118.

2 2.

4 6.

39

13.8

35

.4

48.5

3.

1 3,

9 0.

76

6.4

6.5

0.87

R

oots

< 1

.5 m

m

58.4

1.

6 1.

94

17.0

20

.5

38.5

10

.4

8.3

0.91

8.

4 8.

5 1.

21

Roo

ts 1

,5-5

mm

66

.6

1.2

1.84

17

.5

25,4

26

,4

6.2

6.9

0.89

6.

3 4.

4 0.

52

a P

P =

pol

yphe

nols

,

29

30

20'

D 61

10

0 "s , 30

Dg

2O

o

10

Glir icidia s e p i u r n

0 Roots (<l.Smm} , ~ Twigs

O Leave~

S e n n a siamea

i s

20 40 60 80

Days of i n c u b a t i o n

O~

1 .5 I Glir ic idia s e p i u m

�9 T

1.0 "'" "",,,

g . . . . . . . ~ O . O I , , , o

~ 25

20 i

*e:~15 I

~ 1 0

5

0 0 0 0 80

Olir icidia s ep iu rn

~ " ~ ........ v ....

20 40 60

Days of i n c u b a t i o n

Fig. 3. Example of the nitrogen, phosphorus and potass ium release from fine roots, soil incorporated leaves and twigs of Gliricidia sepium and Senna siamea; means and standard errors.

20

~ 1 5

~ o

5

0

20

15

o -~ lO 'o

Glirieidia sepium

i , �9 - i . . . . . . . . . f q

Senna s iamea

r ,

20 40 60

D a y s of i n c u b a t i o n

80

6 ~ Gliricidia sepium

/ | , 0 ]Roots (<l,5mm) i

.13 9_ !2

~ O J i ~3 . . . . . . . . . . . . . . . . . . . . . i - [3

*~ Senna s iamea 6

2

O I i i J 20 40 60

Days of incubation

80

Fig. 4. Calcium and magnes ium release from fine roots and soil incorporated leaves and twigs of Gliricidia sepium and Senna siamea; means and standard errors.

30

in Figs. 3 and 4. Per unit mass, Gliricidia roots released 56% of the N which was released from the leaves until the 18th day. Most of the N from fine roots of the other two species was retained beyond day 18.

Leaves of Gliricidia and Calliandra released most of their P during the incubation period (Fig. 3). Senna leaves retained 36% of P beyond day 70 when applied on the soil surface. Twigs and roots of all species released more P during the second phase than during the first except for Calliandra coarse roots and green-manured twigs of Senna.

Twigs of Gliricidia and Calliandra released more P than leaves or roots from day 19 to 70. This tendency could also be observed for K (Fig. 3).

K was released from 80 to nearly 100% during the incubation period, the greatest portion occurring in the first 18 days for leaves and fine roots. Twigs tended to release K successively in the course of decomposition. K was released more rapidly than the other nutrients. Ca and Mg were immobilized during the first 18 days except for Senna twigs. Initial immobilization of Ca and Mg and high leaching of K were also observed for the roots and leaves with the exception of the Mg release patterns from leaves, which was released faster than mass loss.

Influence of substrate quality

The initial chemical composition of the substrates is presented in Table 2. Gliricidia showed the lowest values apart from the lignin and cellulose concentrations, which were quite uniform between the species. A correlation of residual carbon from leaves, twigs and roots pooled together was successful only with the (polyphenol + lignin)/N ratio ((PP + L)/N). As shown in Fig. 5 the regression over the whole range of data was curvilinear (y = (118.9-x)/(14.3 + x); R = 0.803; p < 0.05), a linear regression could only be fitted to the data when materials with a (PP + L)/N ratio above 20 were excluded (y -- 4.16 .x + 6.97; R 2 - 0.849; p < 0.001).

The relationship between cumulative N release at day 8 and the (PP + L)/N) ratio was curvilinear in all materials from the three species (Fig. 3), which were incorporated into the soil (y -- 87.4/(x - 2.9); R 2 -- 0.833; p < 0.001). The linear regression comprised only materials with a (PP + L)/N ratio below 10 with the equation y = -2.32.x + 27.4 ( R 2 = 0.833; p < 0.05).

Discussion

Mass loss

The data on average mass loss of leaves in this study are well correlated with other decomposition data from this site [Schroth et al., 1992]. The higher persistence of Senna leaf mulch compared to the other two species may be an advantage in reducing the erosion hazard [Young, 1989].

31

o r,,

-8

!00 - -

8,0

6 0

4 0

2 0

7

% 20

�9 c8

(b

r~ Q) 1 O O.B O

5 >

_o

- - 0

O

t i r / " - - ~ ~ , . ]

i l inear: < mC

' ~ S I R = 0.849 ~G / ~ ~ i

' T [ T~/ / curvi l inear:

[ c%/~ :~ ls.9 * x J i / / ~ 14.3 + x

R e : 0.803* L

_ _ r _ _ I . r r

i \ / 0 Roots < 1 5 m m ( b ) \ ~ G Roots 1 .5-5mm

i ~L v Twigs green manure ~i v Twigs mulch I

l inear D Leaves green manure ] \\ 2 ! Leaves mulch -q

' T \ \ : ~ i

• ~;G~y~\~ curvi I inear .

R 2= 0.853***

l mTC ~ y = 87.4/x - 2.9

,,% ~ _ , , ~, 0 10 2o 30 4o

(PP+L)/N

Fig. 5. Rela t ionsh ip b e t w e e n the init ial (PP + L) /N ratios and (a) the res idual carbon content at day 41 and (b) the c u m u l a t i v e n i t r o g e n re l ease at day 8; letter next to the s y m b o l s are: C

for Calliandra calothyrsus, G for Gliricidia sepium, S for Senna siamea; m e a n s and standard errors.

Mass loss of twigs and roots can be attributed mainly to termite activity, whereas leaves showed no termite frass. Termites preferred twigs from certain species and completely neglected others in our study. Thus, Calliandra twigs were completely excavated, whereas Senna twigs showed almost no termite frass, Gliricidia being an intermediate. This behaviour cannot be deduced from the attack on other plant parts; therefore, every material of a given species has to be tested in view of its attractiveness for termites. Sandhu et al. [1990] found even higher decomposition rates of twigs and roots than of leaves from Leucaena leuococephala in India and observed high termite abundance. In our study, termite activity on twigs was considerably less for mulched than for

32

soil incorporated samples. The higher abundance of termites in green manure treatments is consistent with the observation of Abe [cited in Yoneda et al., 1977], who observed that the feeding of termites on woody litter increased with increasing contact of substrate and soil. As the termites preferred the incorporated materials to the ones applied on the soil surface, low termite activity in our case led to a smaller difference in mass loss from mulch and green manure as compared to that in other trials [Wilson et al., 1986].

The nutrient and carbon loss caused by the termite activity could neither improve the soil organic matter nor be used by the crop, because a consider- able portion of the nutrients and carbon might have been immobilized by the termites in their mounds [Lavelle et al., 1992]. This factor has to be considered when deciding the optimum tree species.

Nutrient release patterns

With the dry matter yield of Gliricidia leaves of 1.2 t/ha in 1992, the N supply was about 38 kg/ha, compared to the recommended fertilizer application for maize in Togo of 46-53 kg/ha of N [Rouanet, 1984]. Although a positive effect of alley cropping systems on the nutrient status of crops has been doubted [Szott and Kass, 1993], mulch from Leucaena leucocophala, Gliricidia sepium and Acioa barteri together with 45 kg ha of N has been shown to increase nutrient uptake and maize grain yield in comparison to the control [Tian et al., 1993]. The temporal pattern of N release is as important as the total quantity. Until the time of maximum N demand of maize (week 4 to 6 [Arnon, 1975]) 86% of the N content of Gliricidia leaves had already been released compared to only 52% for Calliandra and 39% for Senna. This may indicate an advantage of applying Gliricidia prunings about 2 weeks after sowing the maize. Total N supply from Senna leaves was considerably lower than from Gliricidia (50%, < 20 kg N/ha). N release from roots could only be achieved with Gliricidia fine roots during the incubation period with about 14 g N/kg of dry matter. Total nutrient release from roots, however, depends on their total mass and turnover.

P release was highest from Gliricidia leaves and more evenly distributed throughout the vegetation period than N release. However, total release of 1.5 kg P/ha was considerably lower than the recommended fertilizer application of 25 kg P/ha/yr [Rouanet, 1984]. It is thus doubtful whether an improve- ment of P nutrition of the crops can be achieved with this system at our site.

Ca concentrations increased in all materials during decomposition. Accumulation of Ca in the litter was not due to contamination from soil par- ticles, since soil extraction carried out analogously to the used plant extraction yielded only very small amounts of Ca. Ca can be transported actively by the hyphae of fungi into the litter [Cromack et al., 1975]. The Ca concentrations increased more in the green-manured than in the mulched treatments, indicating a higher abundance of fungi in the soil incorporated prunings.

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Absolute K release varied substantially between species and materials. This was not due to different leaching intensities but different initial contents, because the K concentration decreased in all materials to an equilibrium of about 1.5 g K/kg. After the first two weeks almost no K was released from the leaves. It is doubtful whether a maize crop would be able to utilize the flush of 11 kg K/ha in the first 8 days from Gliricidia leaves (total uptake about 5 kg K/ha until the fourth week [Arnon, 1975, p. 147]). Thus, Gliricidia should be pruned in first two weeks after sowing the maize in order to improve the synchronisation of K release and uptake by the crop. Gliricidia twigs had higher K contents than leaves of Calliandra and Senna, stressing the importance of twigs for nutrient cycling. The faster decreasing concentrations of K and P in twigs and roots towards the end of the incubation could be attributed to faunal activity. Especially termites may have increased leaching by fissuring and feeding on the woody parts and bringing the soil microbes into contact with the substrate.

Influence of substrate quality

Decomposition could best be determined by the (PP + L)/N ratio (Fig. 5) as was shown by Fox et al. [1990] for leaves. Palm and Sanchez [1991] found that the initial polyphenol/N ratio was an indicator of the N mineralisation of leaves from several tropical legumes. In our study the initial PP/N ratios could predict mass loss of leaves or roots only, if they were not pooled together (regression not shown). The lignin content of the substrate can become an additional determining factor for decomposition, when a wide range of substrate quality is considered (Fig. 5). No linear correlation could be found here when all the materials were included, indicating that at (PP + L)/N ratios of 20 and higher there was no difference in residual carbon in the different materials at the third sampling date. The data showed a linear correlation below a ratio of 20, thus confirming the relevance of the quality parameter (PP + L)/N. Taylor et al. [1989] concluded that lignin does not regulate mass loss when it occurs at low concentrations although it does at higher concentrations. Based on the present findings, we may further conclude that materials with a wide range of lignin concentrations are necessary for lignin to be a rate determining factor; thus, residual masses of fine roots at individual sampling dates showed a good correlation with the PP/N ratios (R 2 = 0.590; p < 0.05) and not with their initial lignin concentration despite their high lignin content.

These observations held also for the N release. Only for low initial (PP + L)/N ratios a linear regression could be fitted to the data. Above a (PP + L)/N ratio of 10 no changes in N release up to the 8th day could be observed. The materials showing high ratios comprised the twigs with low N concentrations and roots with high polyphenol contents like the Calliandra roots (Fig. 5). These materials seemed to contain polyphenols and lignin in excess compared to N, which was immobilized to such an extent by poly-

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phenols that N release did not differ between the species. Sandhu et al. [1990] found that the initial C/N ratio best predicted N mineralisation from leaves. For our data this relationship held only at later stages of decomposition.

The (PP + L)/N ratio is probably not valid as a rate-determining factor in a situation as described by Sandhu et al. [1990], where the more lignified materials like twigs and roots decomposed faster than the leaves due to termite activity. The applicability of the (PP + L)/N ratio is presumably restricted to sites with a low termite abundance. One should therefore state that a substrate has a 'decomposer specific' quality [Schroth et al., 1992].

Fine roots of Gliricidia decomposed faster than the coarse root fraction, whereas Senna fine roots decomposed slower than the coarse roots. Camire et al. [1991] explained the inconsistent impact of root diameter on decomposition with different N levels of the roots: with limited N resources the N content was positively related to mass loss, with excess N negatively in the cited study. This can only hold for our data, if the PP/N ratio and not the N content is used as an indicator of N availability: concerning Gliricidia roots, which have low PP/N ratios, the fine roots decomposed faster than the coarse roots containing less N than the fine roots; Senna fine roots decomposed more slowly than the coarse roots, because Senna roots have high PP/N ratios.

While Mg was retained to a greater extent in materials with low Mg concentrations following the theory of a limiting nutrient, Ca was immobilized the most in substrates rich in Ca. These substrates were the materials with the lowest mass loss. There seemed to be a connection between the Ca and the N release (Ca = 2 .44 .N + 30.4; R 2 = 0.689; p < 0.001; at day 70 including roots, twigs and leaves). Thus, N, Ca and mass loss were related, whereas Mg and K seemed to be released independently of N and mass loss.

Conclusions

Several factors such as polyphenol, lignin and initial nitrogen content were identified as parameters of substrate quality. The influence of each on decomposition rates was determined by correlating it with mass loss data and nitrogen release. The importance of each parameter was restricted to a certain aspect, but can be combined successfully to a more complex parameter like the polyphenol/N or the (polyphenol + lignin)/N ratio. Polyphenols seemed to regulate mass loss, N and Ca release.

In order to minimize termite activity and maximize litter persistence for erosion control, mulching should be preferred to green manuring and plant litter with high initial polyphenol/N ratios may be used. Materials with higher nutrient contents that decompose rapidly may be applied on soil surface rather than soil incorporated. Roots may serve as a valuable source of nutrients, and their decomposition and nutrient release patterns are complementary to those of the leaves although this is highly species-dependent. Calliandra was found to be less suitable for mulch or green manure production in alley

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cropping because of its high polyphenol content in all plant tissues including the roots. Considering both the results of carbon and nutrient dynamics, a combination of Gliricidia and Senna may be used for mulch production in this region.

Acknowledgements

We are indebted to the Direction R6gionale de D6veloppement Rural at Sokode, Director N. Poidy and the workers at the Centre Experimental de Kazaboua for the permission to work at the research station and their careful and committed assistance in this study. The German Society for Technical Cooperation (GTZ) funded this work and gave much logistic assistance.

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