Leaf litter decomposition and nutrient mineralization patterns in ...

Leaf litter decomposition and nutrient mineralization patterns in
regrowing stands of a humid subtropical forest after tree cutting
A. Arunachalam a,* , Kusum Maithani a , H.N. Pandey b , R.S. Tripathi b
a Restoration Ecology Laboratory, Department of Forestry, North Eastern Regional Institute of Science and Technology, Nirjuli-791109, India
b Plant Ecology Laboratory, Department of Botany, North-Eastern Hill University, Shillong-793022, India
Abstract
Accepted 16 December 1997
Decomposition dynamics, and N and P mineralization patterns of leaf litter of Pinus kesiya, Quercus dealbata, Q. grif®thii,
Rhododendron arboreum and Schima khasiana were studied in forest of three different ages in a humid subtropical region of
India. The decay pattern varied from species to species. The decay pattern, characterized using a composite linear regression
equations, exhibited two to three distinct phases during leaf litter decomposition. Initial lignin, nitrogen (N) and lignin/N
showed signi®cant negative correlations with decay rate, whereas soil properties like pH, moisture and total Kjeldahl nitrogen
(TKN) and climatic variables, e.g. rainfall and air temperature, showed positive correlations. The annual dry matter decay
constants (k) varied from 0.77 in R. arboreum to 1.39 in Q. grif®thii. Nutrient release from the decomposing litter was
in¯uenced by the seasonal cycle of mineralization and immobilization processes. Net mineralization was rapid during rainy
season, as nitrogen (N) and phosphorus (P) concentrations in the decomposing leaf litter decreased by ca. 20±50% from the
preceding season, while immobilization occurred during winter when nutrient concentration increased up to 60%. Annual dry
matter decay, net N and P mineralization constants for Q. dealbata were higher in the 16-year old regrowth than in the 13-year
old regrowth. # 1998 Elsevier Science B.V.
Keywords: Litter decay; Nitrogen; Phosphorus; Humid subtropics
1. Introduction
Litter decomposition plays a crucial role in the
nutrient budget of the tropical forest ecosystems
where vegetation depends mainly on the recycling
of nutrients contained in the plant detritus (Vogt et
al., 1986). The decomposition rate is strongly in¯uenced
by climatic conditions and initial chemical
composition of the litter (Couteaux et al., 1995).
*Corresponding author.
Forest Ecology and Management 109 (1998) 151Ð161
0378-1127/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.
PII S0378-1127(98)00240-0
Among the climatic variables, rainfall and air temperature
determine the rate of decomposition in areas
subjected to unfavourable weather conditions. Nitrogen
(N) and lignin have been reported to have a better
control over the litter decay rate in the humid tropics
than in the temperate zones (Swift et al., 1979). A
strong negative linear relationship between initial
lignin/N ratio and disappearance rate of leaf litter
was reported by Melillo et al. (1982). Lee (1974)
has emphasized the relative importance of soil microand
macrofauna in litter decomposition. Horward and

152 A. Arunachalam et al. / Forest Ecology and Management 109 (1998) 151±161
Table 1
Dominant tree species and characteristics of the top soil (0±10 cm) layer in the three forest regrowths
Age of forest regrowth Dominant tree species Soil characteristics
SMC a (%) pH SOM b (%) TKN c (%) Available P (mgg 1 )
7-year P. kesiya 30.33 5.15 6.56 0.35 7.64
13-year Q. dealbata 6.10 4.72 8.67 0.49 11.94
16-year Q. dealbata 43.86 4.44 10.88 0.61 11.23
a Soil moisture content.
b Soil organic matter.
c Total Kjeldahl nitrogen.
Horward (1980) and Van Vuuren et al. (1993) have
emphasized that tree and shrub leaf litter decomposition
varies depending on species and type of soil. The
role of N availability on litter decomposition in forests
has been discussed by Prescott (1995). This paper
presents the ®ndings of a study on the effects of initial
litter chemistry and soil conditions on the litter decomposition
and nutrient mineralization patterns of major
tree species in three different-aged forest stands recovering
after selective tree cutting in a subtropical
humid forest of northeastern India.
2. Study area
Disturbed forest stands of three different ages (7-,
13- and 16-year old) located near Shillong (latitude
25834 0 N, longitude 91856 0 E, altitude 1900 m a.s.l.)
were selected for the present study. As the stands
were selectively cut, a few older trees (DBH 10±
40 cm) were found interspersed in the stands. All
stands were located on a gentle hill slope (6±138S)
and covered an area of ca. 1 ha each. The area is highly
monsoonal (average annual rainfall 1547 mm) with
winter, spring, rainy and autumn seasons. A total of
80% of the annual rain was con®ned to the rainy
season. Mean annual air temperature during the study
period was 258C. Detailed account of climate, soil and
vegetation of the study sites has been given in Arunachalam
(1997), Arunachalam et al. (1996a, b). The
7-year old stand is dominated by young trees of Pinus
kesiya with resprouting from stumps of broadleaved
trees of Quercus dealbata and Schima khasiana. The
density of woody vegetation was 680 stems ha 1 in
the 7-year old stand, 1260 stems ha 1 in 13-year old
stand and 1440 stems ha 1 in the 16-year old stand.
The corresponding basal areas were 3.1, 19.9,
44.2 m 2 ha 1 in the 7-, 13- and 16-year old stands,
respectively. The soil (latosol±oxisol) is derived from
Precambrian igneous rocks. The mineral soil (0±
10 cm depth) was sandy loam in the 7-year old stand,
sandy clay to clay loam in the 13-year old stand and
clay loam in the 16-year old stand. The water holding
capacity (WHC), soil organic matter (SOM), total
Kjeldahl nitrogen (TKN) and available P in the soil
increased gradually from the 7- to 16-year old stands
(Table 1).
3. Methods
3.1. Leaf-litter chemistry
Freshly fallen needles/leaves of dominant tree species
recorded in 1993 vegetation survey in the three
forest stand were collected during peak litterfall period
(March): P. kesiya (7-year old stand); Q. dealbata
(13-year old stand); Q. grif®thii, Rhododendron arboreum
and S. khasiana (16-year old stand). Leaves of Q.
dealbata collected from the 13-year old stand were
also used in the 16-year old stand. The litter samples
were air-dried in the laboratory and sub-samples were
kept at 808C for 48 h for determining the dry mass.
The oven-dried materials were powdered in a Cyclotec
(TECATOR) and analyzed for their chemical composition.
The ash content was determined by igniting 1 g
of ground litter sample at 5508C for 6 h in a muf¯e
furnace. A total of 50% of the ash-free mass was
calculated as the carbon (C) content. Nitrogen (N) was
estimated following semimicro-Kjeldahl method in a
Kjeltec Auto 1030 Analyzer (TECATOR). Total phosphorus
(P) was analyzed colorimetrically while lignin

and cellulose contents were measured gravimetrically
(Anderson and Ingram, 1993).
3.2. Litter decomposition
Decomposition of leaf litter of dominant tree species
was studied in their respective stands using the
nylon mesh (20 20 cm) bag technique (Gilbert and
Bocock, 1960). The mesh size was 2 mm, small
enough to prevent major losses of needles or leaves,
yet large enough to permit aerobic microbial activity
and free entry of small soil animals. However, it
excludes larger arthropods and earthworms, which
are important primary accessors of litter and, presumably,
it becomes a litter system dominated by termites,
bacteria, fungi and actinomycetes. Consequently,
there could be an underestimation of litter decomposition
rates. For the experiment, 10 g of the air-dried
material was kept in each bag which was then stitched
with nylon threads. For each species, 60 bags were
prepared and randomly dispersed on the forest ¯oor in
the respective stands in the month of May 1993. After
60, 120, 180, 240, 300, 360, 420, 480, 540 and 600
days, ®ve litter bags for each species were brought to
the laboratory carefully avoiding loss of material from
the litter bags. The litter was washed in a bucket full of
tap water by swirling brie¯y and carefully decanting
through 2-mm mesh sieve to remove extraneous
matter. According to Anderson and Ingram (1993),
such brief washing permits little leaching. The litter
was then dried at 808C for 48 h and weighed. The
samples were powdered and used for the analysis
of N and P contents according to semimicro-Kjeldahl
and colorimetric procedures given in Anderson and
Ingram (1993).
3.3. Computation
A. Arunachalam et al. / Forest Ecology and Management 109 (1998) 151±161 153
Organic matter decay, and N and P mineralization
constants for leaf litter were computed using negative
exponential decay model of Olson (1963): X/X 0ˆ
exp( kt), where X is the weight remaining at time
t, X0 the initial weight, exp the base of natural
logarithm, k the decay rate coef®cient and t the time
(year). N and P mineralization constants (kN and kP)
were calculated by substituting dry weight with N and
P contents in the foregoing formula (Singh and Shekhar,
1989). Further, the time required for 50% (t50)
and 99% (t 99) decay were calculated as t50ˆ0.693/k
and t99ˆ5/k.
3.4. Statistical analysis
In order to distinguish between different phases of
weight-loss pattern during decomposition, multiple
regressions were developed using dummy factors (0
or 1) as the indicator variables (Zar, 1974). The
composite linear-regression model (Arunachalam et
al., 1996b) used for this purpose was Yˆa‡bX1‡
cX2‡dX3..., where Y is the percentage of initial mass
remaining, a the Y intercept, b the rate of change
in Y with respect to time, c the shift parameter for
adjustment of the Y intercept in phase-II and d the
shift parameter for adjustment of the Y intercept in
phase-III. The values of c and d were taken as zero,
if decay was slow, and/or equal to one, if the decay
was rapid.
The effect of climatic variables, initial litter chemistry
and a few soil characteristics on the decomposition
rate of leaf litter was assessed using simple linearregression
function, Yˆa‡bX. As the type and number
of species used for decomposition study in the three
stands were different, there were chances of pseudoreplication
in the application of analysis of variance
(ANOVA, ®xed-effects model). To overcome the problem,
the ANOVA was used to test the signi®cance of
differences in the decomposition rate due to sampling
time for each species in the three stands and among the
species in the 16-year old stand only. However, comparison
between plots were done by regression.
4. Results
4.1. Initial chemistry of leaf litter
Mean N concentration in the leaf litter of ®ve
dominant species varied from a minimum of 0.59%
in R. arboreum to a maximum of 1.03% in S. khasiana.
Phosphorus concentration was relatively low, varying
between 0.03±0.06% in different species. Lignin concentration
was maximum (43.2%) in the coniferous
species, P. kesiya. Among the broadleaved species, R.
arboreum had the highest concentration of lignin
(37.3%). In other species lignin concentration varied
from 23.7±25.1%. Cellulose concentration was much

154 A. Arunachalam et al. / Forest Ecology and Management 109 (1998) 151±161
Table 2
Chemical composition of leaf litter of dominant tree species used for decomposition study by the litter-bag method ( SEM (nˆ5))
Species C (%) N (%) P (%) Lignin (%) Cellulose (%) C/N Lignin/N
P. kesiya 47.3 1.2 0.98 0.01 0.05 0.004 43.2 1.1 7.4 0.9 48.27 44.08
Q. dealbata 47.6 0.3 0.89 0.01 0.06 0.01 24.4 1.4 7.1 1.9 53.48 27.42
Q. griffithii 46.4 2.1 0.73 0.05 0.05 0.001 23.7 0.9 11.9 0.3 63.56 32.47
R. arboreum 45.6 3.6 0.59 0.06 0.03 0.001 37.3 9.3 0.7 77.29 63.22 3.1
S. khasiana 47.3 1.0 1.03 0.03 0.04 0.001 25.1 3.1 5.1 0.1 45.92 24.37
higher (11.9%) in Q. grif®thii than in R. arboreum
(9.3%) and other species (5.1±7.4%). Lignin and
cellulose concentrations were minimum in Q. grif®thii
and S. khasiana, respectively (Table 2).
4.2. Weight-loss pattern
Needles of P. kesiya decomposed in a three-phased
manner (Fig. 1(a)). The ®rst phase lasted for ca. 60
Fig. 1. Decay pattern of leaf litter of different species in the regrowing forest communities. (&), Observed values, and the curve has been
fitted using expected values calculated by the composite linear-regression equation. (a), P. kesiya (7-year old stand), (b), Q. dealbata (13-year
old stand), (c), Q. dealbata (16-year-old stand), (d), Q. griffithii (16-year old stand), (e), R. arboreum (16-year old stand) and (f), S. khasiana
(16-year old stand).

days and was characterized by a slow rate (0.05% wtloss
day 1 ) of decay. This was followed by a period of
rapid weight loss (0.75% wt-loss day 1 ) for up to 120
days (Table 3). During the third phase (i.e. 120±600
days), decay continued more-or-less at a constant rate
(0.08% wt-loss day 1 ). Q. dealbata, Q. grif®thii and S.
khasiana followed a two-phased decay pattern
(Fig. 1(b±d) and (f)). The ®rst phase lasting for about
120 days, was characterized by a rapid weight loss,
followed by a slow rate of decay until 600 days
(Table 3). Leaves of R. arboreum followed a uniform
pattern of weight loss from the beginning to the end of
the experiment (Fig. 1(e)).
A composite linear regression model,
Yˆa‡bX 1‡cX 2‡dX 3 showed a good ®t for the
weight-loss pattern in P. kesiya. The multiple regression
equation, Yˆa‡bX1‡cX2 ®tted well for the
observed decay pattern of all broadleaved species
except for R. arboreum, where a simple linear-regression
function, Yˆa‡bX was the best ®t. The correlation
coef®cients describing the decay rates over time
were highly signi®cant (P

156 A. Arunachalam et al. / Forest Ecology and Management 109 (1998) 151±161
Table 4
Annual decay and mineralization constants of leaf litter of the studied species in three forest regrowths
Decay/mineralization P. kesiya a
Q. dealbata b
Phosphorus mineralization constant for R. arboreum
was minimum (k Pˆ0.37). In other species, it varied
between 0.80 and 1.28 (Table 4). Despite variations in
Q. dealbata c
Q. griffithii c
R. arboreum c
Dry mass
k d
1.28 0.88 1.24 1.39 0.77 1.06
e
t50 0.50 0.80 0.60 0.50 0.90 0.70
t99 f
Nitrogen
3.90 5.70 4.00 3.60 6.50 4.70
kN 1.24 0.69 1.24 1.28 0.62 1.02
t50 0.60 1.00 0.60 0.54 1.12 0.71
t99
Phosphorus
4.00 7.20 4.00 3.90 8.10 4.90
kP 1.20 0.80 1.13 1.28 0.37 0.92
t50 0.56 0.90 0.61 0.54 1.90 0.76
t99 4.20 6.23 4.42 3.91 13.70 5.48
a In 7-year-old regrowth.
b In 13-year-old regrowth.
c In 16-year-old regrowth.
d The decay constant.
e N mineralization constant.
f P mineralization constant.
S. khasiana c
kN and kP, N and P stocks in decomposing leaf litter
were positively correlated (P

5. Discussion
5.1. Resource quality
A. Arunachalam et al. / Forest Ecology and Management 109 (1998) 151±161 157
Fig. 2. N (A) and P (B) concentrations (%) in the decomposing leaf
litter through time. (-*-), P. kesiya; (-*-), Q. dealbata (13-yearold
regrowth); (-~-), Q. dealbata (16-year-old regrowth); (-~-),
Q. griffithii; (-&-), R. arboreum; and (-&-), S. khasiana. Vertical
bars indicate LSD at P

158 A. Arunachalam et al. / Forest Ecology and Management 109 (1998) 151±161
Fig. 3. Net N mineralization rate (% release day 1 ) in the decomposing leaf litter of the studied species during different seasons (R ± rainy, A
± autumn, W ± winter and S ± spring seasons).
greater lignin in the pine needles, the decay was faster
due mainly to the N level (0.98%) which was comparable
to the fast decomposing non-sclerophyllous
S. khasiana leaves (Nˆ1.03%). Though the leaves
of R. arboreum had greater lignin, yet it was less than
the pine needles, because of low initial N level the
decay rate was signi®cantly lower compared to the
other species. This indicates that the decomposition of
the subtropical tree species may be governed mostly
by the initial N rather than lignin.
R. arboreum leaves having high lignin and low N
concentration decomposed at a slow rate, and hence
revealed slow rate of N and P release. The initial faster
decay rate of Q. grif®thii, S. khasiana and Q. dealbata
leaves (Table 3) appears to be related to high initial N
concentration. Berg (1984) and Taylor et al. (1989)
have, however, suggested that as the decomposition
proceeds the in¯uence of N decreases while that of
lignin increases. Hence, the reduction in decomposition
rate with time may be due in part to the slow
breakdown of residual refractory materials. A rapid
decay of Q. dealbata leaves in the 16-year-old
regrowth, as compared to that in the 13-year-old
regrowth, may be ascribed simply to the microenvironmental
variability, particularly high soil-moisture
regime and accumulation of soil organic matter leading
to better growth and activity of soil microbial
population (Maithani et al., 1996). Q. dealbata, Q.
grif®thii and R. arboreum showed faster decay rates
(kˆ0.77±1.39) in the present study than the rates
reported by Laishram and Yadava (1988) for the same
species (kˆ0.18±0.55) in an undisturbed subtropical

A. Arunachalam et al. / Forest Ecology and Management 109 (1998) 151±161 159
Fig. 4. Net P mineralization rate (% release day 1 ) in the decomposing leaf litter of the study species during different seasons (R ± rainy, A ±
autumn, W ± winter and S ± spring seasons).
Fig. 5. Relationships between dry matter and (A) N and (B) P stocks, and (C) N and P stocks in the decomposing leaf litter.
forest at Shiroy Hills in northeastern India. Since both
sites have similar climate (annual rainfall 1617 mm,
mean temperature 238C), the possible cause for the
relatively faster decay rate could be related to disturbance
induced changes in forest microclimate and
soil conditions. Pastor and Post (1986) have reported

160 A. Arunachalam et al. / Forest Ecology and Management 109 (1998) 151±161
that the litter decay rates increase due to disturbances
in forest ecosystems which brings about changes in
both internal and external environmental conditions.
Likewise, a faster rate of decay of P. kesiya needles
(kˆ1.28) in the 7-year-old regrowth than in the 22year-old
pine stand (kˆ0.38) of the same area (Das
and Ramakrishnan, 1985) could be attributed to the
confounding effect of stand age over resource quality
of the litter. Earlier, we reported that the ®ne-root
decay constant increased gradually from 1.62 in the 7year
old stand to 1.74 in the 16-year old stand, which
was attributed largely to the lignin concentration that
showed a decreasing trend with stand age (Arunachalam
et al., 1996b). A signi®cant negative correlation
between initial lignin and litter decay rate (Table 5)
conforms with that of the ®ne roots.
5.3. N and P mineralization
A marked decline in N and P concentrations until 60
days of decomposition may be ascribed to leaching
losses caused by heavy rainfall during that period.
Subsequent increase in nutrient concentration could
be the result of microbial immobilization (Anderson,
1973; Maithani et al., 1996), nutrient inputs from
throughfall and atmospheric precipitation (Bocock,
1963), and/or atmospheric N2 ®xation (Wood,
1974). Despite variations in N and P concentrations
in the decaying leaf litter due to mineralization and
immobilization, N and P stocks in the decomposing
leaf litter were positively correlated with its dry mass
(Fig. 5). A similar trend has been reported by Prescott
et al. (1993) for litter and by Arunachalam et al.
(1996b) for ®ne roots. N and P release during decomposition
was in¯uenced by seasonal cycle of nutrient
immobilization and mineralization processes. A
warm, humid season, being more favourable for
mineralization, was characterized by rapid rate of N
and P release, whereas a dry winter, being more
favourable for immobilization, was the dominant process
on the forest ¯oor. Interestingly, the periods of
active mineralization and immobilization coincided
with minimum and maximum microbial biomass in
these stands (Maithani et al., 1996).
On an average, a slower rate of leaf litter decomposition
and nutrient mineralization in the 13- and 16year
old regrowths compared to P. kesiya in the 7-yearold
regrowth indicates the development of nutrient
conservation mechanism with species enrichment during
the progression of vegetation recovery after tree
cutting. The rapid N and P release in the pine-dominated
stand indicates rapid recycling of these nutrients
within the system and/or the potential loss of inorganic
forms of the nutrients from the system, due to a low
canopy density. This is true for the ®ne roots with
reference to N only, whereas P release was greater in
the 16-year old stand. It could, however, be considered
that such a rapid rate of mineralization in the 7-year
old regrowth is due to the interactive in¯uence of
disturbance which, in turn, may be useful in meeting
greater N and P requirements of the immediately
regrowing forest vegetation.
Acknowledgements
The authors are thankful to the Council of Scienti®c
and Industrial Research, Government of India for
®nancial assistance (Sanction No. 38(0840)/92
EMR-II, dated April 1992). The authors are grateful
to Professor P.S. Ramakrishnan for having kindly
provided us with a copy of the TSBF-Methodology
Book. A. Arunachalam extends his thanks to the
Director, NERIST, for providing essential communication
facilities. The authors thank the two anonymous
referees for their comments which helped in improving
the earlier draft of the manuscript.
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