Nitrogen release from decomposing litter of Leucaena leucocephala in the dry tropics

0038-07~7/90 53.00 + 0.00

Soil Bid. Biochem. Vol. 22, No. 6, pp. 859-863, 1990 Primed in Great Britain. All rights reserved

Copyright 0

1990 Pergamon Press plc

NITROGEN

RELEASE FROM DECOMPOSING LITTER OF LEUCAENA LEUCOCEPHALA IN THE DRY TROPICS J. SANDHU,M. SINHAand R. S. AMBASH?+

Ecology Research Laboratory, Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi 221 005, India

(Accepted 15 January 1990) Summary-Experiments were made on a 2-yr old Luucaena leucocephalastand to measure the litterfall and decomposition rates. Annual litterfall was 10 t ha-’ and was maximal in the dry summer months ( = 50% of total litterfall). The seasonal peaks were in June and December (15 and 11% of total litterfall. respectively). Leaves comprised 88% of the total litterfall. Litter decomposition in the field was examined using leaf, twig and fruit fractions of above-ground litter. Roots were also studied for loss of mass and N release below-ground. Loss of mass was in the order fruit > root > twig > leaf. The inital mass loss was high and there was rapid loss in the rainy season (July-Scptcmber). Annual above-ground litter N content was high (256 kg ha-‘) with an annual release of 208 kg ha-’ i.e. 81% of the total litterfall. N release from litter per unit area was leaves (13.2 g m-*), fruits (10.5 g m-‘), twigs (4.4 g m-*) and roots (3.4 g me2). Litter decomposition and N release from the decomposing titter was dependent on the C-N ratio and environmental factors. N turnover time was < 1 yr for all litter fractions.

(ii) to study the decomposition pattern of each fraction and its N release to soil through decomposition; (iii) to estimate the contribution of below-ground material to N release; and (iv) to understand the relationship of environmental variables and elemental content (C-toN) with the decomposition pattern of each litter fraction.

INTRODUCI’ION

Soil N deficiency limits forest ecosystem productivity. N present as organic matter is returned to the soil through litterfall. Litterfall quantification and its elemental analysis gives a comprehensive insight of the N cycling efficiency-within stands and within systems (Vitousek, 1984). N release to the soil is mainly through microbial decomposition of litter. The rate of litter breakdown and N transformations is directly dependent on the climate (Meentemeyer et al., 1982), resource quality (Swift et al., 1979) and exogenous nutrient availability (Kaushik and Hynes, 1971). Field studies of litter decomposition are a valuable means of determining turnover rates of nutrient stocks on the forest floor. Singh (1968, 1969a,b) and Singh and Ambasht (1980) have quantified litterfall, decomposition and nutrient return rates in ecosystems in Indian dry deciduous forest. Vogt et 01. (1986) pointed out that the below ground inputs also play an important role in organic matter turnover and nutrient cycling in forests. A 2-yr-old stand of Leucaena leucocephalu (Lam.) deWit. a fast growing leguminous tree being extensively used in global reforestation programmes, was raised in the botanic gardens of the Banaras Hindu University. The quantity of litterfall and N release through litter decomposition were studied. Our objectives were: (i) to quantify the annual litterfall and N input to the floor through litterfall, seasonal patterns of litterfall and percentage contribution of each component (leaves, twigs and fruits); *To whom all correspondence should be addressed.

MATRRLUS ANDMETHODS

Litterfail estimations Litterfall was collected every fortnight by placing 1 m2 litter traps on the soil surface within the experimental plot. Litter collected from the field was brought to the laboratory, weighed and divided into the different fractions (leaves, twigs and fruits). Each fraction was separately weighed to determine their percentage contributions. Sub-samples were taken and oven-dried at 80°C for 48 h for moisture determinations. Litter decomposition Plant material decomposition in the field was simulated by the litter bag technique (Bocock et al., 1960; Sharma and Ambasht, 1987). A comparative study was made to determine the rate of decomposition of each litter fraction (leaves, twigs and fruits) by placing equal amounts of each fraction in the litter bags. Brass litter bags were used to prevent termite activity on the bag material. Litter bags measuring 10 x 1Ocm (0.01 cm*) with mesh pore size of 1 mm were prepared. Fresh litter, collected in polyethylene sheets placed on the forest floor was fractionated into leaves, twigs and fruits and air-dried. Sub-samples were oven-dried (48 h at 80°C) and the equivalent of 5 g oven-dry weight litter from air-dried stock was placed in each litter bag. Sixty litter bags of each 859

J. SANDHU

860

component were placed in a randomized block design within an area of 4 x 4 m on the forest floor on 3 September, 1986. Litter bags containing the root fractions ( < 5 mm dia) were placed at 5 cm depth in the soil. Each month litter bags were randomly retrieved (five bags of each fraction), brought to the laboratory and dried immediately at 80°C for 48 h to constant weight. Rainfall, moisture and temperature determinations Rainfall data was recorded by the Department of Agronomy, Institute of Agricultural Sciences, Banaras Hindu University ca 200 m from the botanic garden. Temperature measurements were made using a soil thermometer which was placed at the litter-soil interface and in the soil at 10 cm depth. Temperature readings were noted every 30 min for 3 h every fortnight and the mean recorded. Moisture content was determined in retrieved decompo’sed samples and expressed on a dry weight basis after oven drying at 80°C for 48 h. Ash -free mass Litter bags containing decomposing samples were retrieved every month from the field. The soil particles adhering to the litter were removed with a brush. The litter was dried at 80°C for 48 h and weighed. These samples were ground (< 2 mm) and 0.5-l .Og sub-samples were combusted in porcelain crucibles at 480°C for 12 h and the residual mass was determined on the ash-free basis. Carbon The C content of litter was determined by the modified chromatography technique by using the Elemental Analyzer-l 106 (Carlo-Erba, Italy), at the Nuclear Research Laboratory, IARI, New Delhi.

et al.

Nitrogen An oven-dried, ground sample (0.5 g) was analyzed for total N using the Kjeldahl digestion procedure, distilled in Markham unit and determined volumetrically (Jackson, 1967). % N release of each litter fraction was determined and annual N release calculated from total annual litterfall. Statistical analysis and &composition constant Two-way ANOVA (Analysis of variance) was made to observe variations between components and months. Multiple linear regression relating the dependent variable (relative decomposition rate) to the independent variables (substrate quality and environmental factors), with square multiple correlation coefficient (R’) and its Fstatistics were calculated (Snedecor and Cochran, 1967). Linear regressions and its covariance (Campbell, 1974) relating relative loss rate as a function of substrate quality and environmental factors of retrieved samples were estimated. Observed losses of ash-free mass and N content from decomposing samples were described by single exponential decay function (Olson, 1963). RESULTS

Litter production Total litterfall (Fig. 1) was highest in May and June. The winter peak of litterfall was observed in December. Minimum amounts of litter fell during July and August. Almost 50% of the litter fell in the dry, summer months (March-June) when the tree shed most of its leaf cover (excessive drought conditions in the year of study). Leaf litter comprised 88% of the litterfall and the remaining 12% was twigs and fruits. N concentrations did not vary much through the different seasons and the N content was primarily dependent on litterfall weights. Litter &composition The loss of ash-free mass during the year of observations was in the order fruits > roots >

-

60

ii % N ‘E .z

Table 30

0

I. Percentage of

intial ash-free mass (g) remaining in dccomposing substrate at +h retrieval date

Months’

Leaf

Twig

Root

Fruit

1986 act Nov Dee

83.3 75.3 64.5

75.10 63.0 58.3

78.0 66.1 60.5

15.2 60.4 46.2

57.6 52.0 48.4 45.6 42.4 39.3 36.00 31.8 25. I

53.4 49.8 46.4 43.8 41.1 38.5 35.5 29.9

36.7 29.2 22.4 16.5 12.0

18.5

56.4 52.8 49.3 46.1 42.9 40.8 36.6 21.4 14.5

0.005 2.6

0.005 2.3

0.005 2.3

0.005 2.0

1987 Jan Feb

Mar APT May June July AQ SP

MONTHS

Fig. 1. Leaf, twig and fruit litterfall (g me2 month-‘) during I yr.

ANOVA LSD (0.05)

Ph

‘Samples were placed in the field in September 1986. P values associatedwith analysisof variance (ANOVA) arc given, with LSD values (P = 0.05). n = 3.

bBcncatheach column,

Nitrogen release in Leucuena decomposition Table 2. Single exponential dcfay parameters describing litter decomposition in a plantation stand of L. leucocepkala. k

T Ii2

Turnover time (months)

0.1 ISI 0.1405 0.2368 0.1607

6.02 4.93 2.92 4.31

8.68 7.11 4.22 6.22

0.1360 0. I330 0.2609 0. I274

5.09 5.20 2.66 5.44

7.3s 7.51 3.83 7.84

Component Ash -fire Leaf Twig Fruit Root NifWgWl Leaf Twig Fruit Root T,.,

18

861 Twigs

LoaYe

E

L

mass

t,:r Roots

values indicate the time in which half of the initial material is lost from litter. T,,* and turnover time values arc calculated by using decomposition constant (k) from the single exponential fits

0 SONDJFMAMJ

twigs > leaves (Table 1). The fruit litter decay had lowest T,,, values of 2.9 months (Table 2). The twig and root decomposition rates with 82 and 86% loss respectively were almost similar. About 25% of the decomposing leaf litter was still remaining after 1 yr of exposure. The relative loss rates were maximum during the first month and decreased subsequently through the winter and summer months. Relative loss rates of ash-free mass were highest during the rainy season (July-September). Fruit litter decomposition was an exception where the relative loss rate fluctuations were small over consecutive months. However, in April and May, relative loss of the fruit litter increased. Nitrogen The initial N contents of the litter fractions varied from 15.5 g me2 (leaves), 11 g rnT2 (fruits), 5.5 g m-* (twigs) and 3.0 g m-* (roots). The rate of N release (Table 3) was greatest from fruit litter (turnover time, 3.8 month) and the relative loss rate was high throughout the study. The turnover time (Table 2) of the three other components (leaves, twigs and roots) Table 3. Percentage of initial N content remaining in decomposing substrate on each retrieval data Months’

Leaf

Twig

Root

Fruit

1986 act Nov Dee

65.9 68.0 61.3

75. I 57.7 52.5

103.7 84.9 81.0

71.8 57.3 42.7

1987 Jan Feb Mar APT May June July Aug %P

54.7 52.4 47.6 43.7 40.4 36.7 27.3 23.6 19.5

51.1 47.5 44.2 4.5 40.9 36.9 37. I 30.5 20.2

47.0 53.0 65.7 76.7 71.7 68.0 54.7 41.3 21.7

34.5 30.9 21.3 7.5 4.4

0.005 3.6

0.005 3.8

ANOVA LSD (0.05)

Pb

NS -

MONT

Ii

JAS

S

Fig. 2. Nitrogen content (g mm2)of decomposing substrates on different retrieval dates.

did not differ much with 7.3 month for leaves followed by 7.5 and 7.8 month for twigs and roots, respectively. N concentrations had decrease in all components by the end of the study. The absolute N content (Table 3) increased in October by 3% in the root fraction. The relative N content also increased in the root fraction in the mdnth of February (6%). March (13%) and April (11%). The twig and leaf relative N contents also increased over one or two months (Table 3). Except for roots, the N release per unit area was highest during the first month for all the components (Fig. 2). The leaf litter N release was the highest following by twigs and fruits. The root litter N release was high during the latter half of the study. Relationship between &composition rates, litter quality and environmental variables Mean maximum litter temperatures at the soil litter interface and mean percentage moisture of decomposing litter for each month are shown in Figs 3 and 4, respectively.

3 IO 0.005 3.6

‘Sampks were placed in the field in Scptembcr 1986. ‘Beneath each column P values associated with analysis of variance (ANOVA) are given. with LSD values (P =O.OS) when the interaction is significant. NS = not significant; n = 3.

su 22/6-l

SDNDJFMAMJ

JAS

I: t

0’

’ ’ ’ ’ ’ DNDJFMAMJJAS

’

’

’

’

’

’

’

MONTHS

Fig. 3. Mean maximum temperatures of decomposing litter and roots.

862

J. SANDHUet (11.

fraction. There was no significant relationship for the other type of litter fractions. DEXXJSSION

ol”“““-*” ONOJFMAMJJAS MONTHS Fig. 4. Percentage moisture (as % of dry matter) of retrieved

decomposing substrates. The relationships between relative loss rate and C-to-N ratios, litter temperatures and litter moisture contents on all retrieval dates were calculated by multiple regression analysis. Relative loss rates of ash-free mass when correlated individually with each variable were seen to be significantly correlated with moisture eontent in the case of leaf, twig and root fitter fractions and with mean maximum litter temperature only, in the case of fruit litter. Multiple regression analyses of ash-free mass with litter quality, temperature and moisture were significant (Table 4) in each of the four litter fractions. The analysis suggested that the relative loss rate of ash-free mass was most strongly correlated to the C-to-N ratio in leaf and twig litter. The root and fruit litter was however, related to litter temperature in precedence to the C-to-N ratio. The linear regression (Table 5) analysis of individual variables of each component with relative loss rates of N content were calculated. Leaf N relative loss rate was significantly correlated with the C-to-N ratio, twig with moisture content, fruit with temperature. Relative loss rate of root N was not correlated significantly with any of the variables. Multiple regression analysis showed significant correlations between relative loss rates of N only with C-to-N ratio and litter moisture in the leaf litter

Estimates of litterfall from tropical forests range from 5.5 to 15.3 t ha-’ (Laudelot and Meyer, 1944, Bray and Go&m, 1964). The L. leucocep~a stand we studied had litterfall values (10 t ha-’ yr-I) comparable to those reported by Kimura et al. (1984). Temperature and moisture played an important role in the occurrence of litterfall. Dry and hot spells resulted in high rates of litterfall. There was an inverse relationship between litterfail and wet periods. The annual N content of the litterfall (256 kg ha-’ ) was higher than the reported values in mixed stands of dry deciduous conditions. Trees efficient in biological NXfixation have higher initial N contents and lower C-to-N ratios (Sharma and Ambasht, 1987). This was also true in L. leucocephalu. Initial C-to-N ratios in the leaf (17.1), twig (33.4) and fruit (18.5) litter were low. The N mineralization rates for the above three fractions were high and decomposition began in the first month. The root fraction (C-to-N ratio: 53.8) showed increases in N content (Table 3). Increase in absolute N content in decomposition have been reported (Singh, 1969b). Bocock (1964) suggested that the increase in absolute amounts of N may be due to inputs from precipitation and insect frass. The initial N content plays a significant role in decomposition. Tectona grandis leaf litter having low N concentration (Singh, 1968) had a higher rate of 90% d~m~sition during 1 yr (Singh and Ambasht, 1980) as compared to 75% leaf litter decomposition of L. leucocephala in accordance with the statement that the N-rich litters decompose more slowly (Berg et al., 1982). The 1 mm mesh pores of the litter bags enabled the litter to be accessible to the soil fauna (including termites) for fragmentation and decomposition. in tropical conditions, woody litter breakdown is aa& erated by termite action (Swift et al., 1979). The faster twig, root and fruit litter decomposition was due to termites whereas the leaf litter was not affected by these animals. Each fraction of litter decomposes at a specific rate and is variably dependent on the resource quality (C-to-N) or one or other of the environmental

Table 4. Multipk regression equations rciating th: relative loss rate, titter quality and environmental factors of L. Ieueocepha/o stand, the squamd multipk corraiation co&Ant (R’) and F statistics arc also shown LitterY

Regmssion aquationb

Ash -freetnm.v

Loaf Twig Root Fruit Nirrogcn Leaf

RLR RLR RLR RLR

= = = =

20.3 I7 - 0.3292 - 137.670 + 2.5530 133.740 - 2.3306 9.930 + 0.4874

RLR = - 52.63 + 4.2893

C - 0.2239 C+ 1.2841 MLT - 0.9321 MLT+O.MOO C - 0.0596

MLT+O.l96OLM ML-f + 0.8230 LM c + 0.5429 LM LM+O.I170C LM

R’

F

0.721

6 89” 5:67** 54.07+** 8&l**

::Z! 0.866 0.512

4.72+

‘d.f. for ash fret mass kaf, tw@ and root regarding F values arc 3, 8 and for fruit 3, 4; for kaf N 2, 9. F values arc sign&ant at lF 0.05, l*P 0.025 and l**P 0.005. There is no significant relationship in fruit and root N. Twig N corr&tcd si~i~ntiy (r I 0.627, d.f. = lo, P 0.05) with litter moisture only. bRLR. Relative loss rate (cakuktod by #x/&x, where 6x = change in mass/N content, &I= change in time and x is fraction of mass/N content remaining): C, C-lo-N ratio: MLT. mean maximum litter temperature; and LM, litter moisture (%).

Nitrogen release in Leucaena decomposition Table 5. Regression quation of relative loss rate as a function of litter quality and environmental variables of L. leucocephafastand: coefficient of determination (I’) and variance ratio (F) are shown Litter

Regressionb

r2

F

0.39 0.55

6.50. 7.31.

Nitrogen Twig Fruit

RLR = 3.50 + 0.444 1.845 RLR = -34.38+

LM MLT

‘d.f. for twig regarding Fvalues are I, 10 and for fruit 1.6. F values are significnnt at lf 0.05. There was no significant variation in the cast of root N. bRLR, Reiatiw loss rate; C = C-to-N ratio; MLT, Mean maximum litter temperaturc: and LM. Litter moisture.

factors. An overview of the decomposing system suggests that the leaf litter decomposition determines the overall above-ground litter decomposition dynamics. Thus it may be said that in L. leucocephalu the resource quality and environmental factors decide the fate of the decomposing litter in terms of N release. To conclude, it may be said that decomposition rates with concommitant release of N of 81% (208 kg ha-‘) and organic matter to the soil sub-system is high (Vogt et al., 1986; Shanna and Ambasht, 1987) in L. leucocephala stands in the dry topics. Studies on aggrading systems are required to be carried out to evaluate the efficiency of N recycling in dry deciduous conditions

in the tropics.

Acknowledgements-We

thank the Council of Scientific and Industrial Research, New Delhi, for financial support and Dr E. Sharma for help in field and laboratory work.

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Bocock K. L., Gilbert 0. J., Capstick C. K., Turner D. C., Waid J. S. and Woodman M. J. (1960) Changes in leaf litter when placed on the surface of soil with contrasting humus types. Journal of Soil Science 11, 1-9. Bray R. J. and Gorham E. (1964) Litter production in forests of the world. Advances in Ecological Research 2, 101-157.

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Singh K. P. (1969b) Studies in decomposition of leaf litter of important trees of tropical deciduous forests of Varanasi. Tropical Ecology 10, 292-311. Singh A. K. and Ambasht R. S. (1980) Production and decomposition rate of titter in a teak (Tecfona grandis) plantation at Varanasi (India). Revue d’Ecologie et de Biologie du Sol 17, 13-22.

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