Bromeliads and litter mineralization

Agrochimica, Vol. LV - N. 4
July-August 2011
Mineralization and nutrient release of leaf litter and fallen bromeliads in a pine – oak forest in Oaxaca, Mexico
C. Roblesa*, M.L. Robles-Martíneza, A. Bautista-Cruza
CIIDIR-IPN-Unidad Oaxaca, Laboratorio de Suelos, Calle Hornos 1003, 71230 Santa Cruz Xoxocotlán,
Oaxaca, México
a
Received 28 April 2011 – Received in revised form 07 July 2011 – Accepted 13 July 2011
Keywords: bromeliaceae, leaf litterfall, microcosms, mineralization of organic
material, pine-oak forest
Introduction. – The species belonging to the family Bromeliaceae,
generally refered as bromeliads, are epiphytic plants that are quite common in diverse vegetation types, ranging from semi-arid climates to
temperate-humid ones. Their habitual habitat is in the branches of
trees and bushes, always tending towards the greatest light incidence
(Mondragón et al., 2006). In various regions in Mexico, particularly in
the temperate forests of the “Sierra Norte” region of the state of Oaxaca,
these plants are harvested from their ecosystems to be sold as decorative
objects, and are especially popular as decoration for “nativity scenes”,
small, allegorical, homemade structures depicting the birth of Jesus of
Nazareth and typically displayed in the Christmas season. Indubitably,
this extraction impacts the deposit of organic material and the recycling
of nutrients in the ecosystems in which this practice takes place, the
importance and relative contribution of which have not been determined.
In unaltered conditions, these materials, together with the leaf litter
produced by the rest of the plant species which grow in the ecosystem,
are mineralized through their fall and decomposition by macro- and
microbiota in the soil, thus propitiating the recycling of nutrients which
maintains soil productivity in these forests (Bauer and Black 1994).
The plant material that falls and dies represents a large percentage of the
contribution to the organic material deposited in an ecosystem (Barlos
et al., 2007). The positive effects of organic material on soil properties
that influence plant development and/or the conservation of those same
plant species are widely recognized facts (Bauer and Black 1994,
Gallardo et al., 1995, Gressel et al., 1995).
Corresponding author: crobles_38@yahoo.it
Litter and bromeliads in Oaxaca, Mexico
219
The annual production of deposited leaf litter is highly variable. Its
extent depends, among other variables, on the plant species present and
the climactic regimen in which they develop. In general, the highest
production occurs in ecosystems with warm climates and abundant precipitation, decreasing as temperatures and precipitation descend (RicoGray and Lot 1983, Gallardo et al., 1995, Robles and Barea 2006,
Barlow et al., 2007). Very few studies have examined the contribution
of bromeliads to the leaf litter deposit in an ecosystem.
The seasonality of leaf litterfall is a generalized pattern; however, its
point distribution in each ecosystem throughout the year depends on both
environmental conditions and the phenology of the plant species present
(Riveros and Alberdi 1978, Madeira et al., 1995, Robles and Barea
2006, Barlow et al., 2007, Aceñolaza et al., 2009). The mean decomposition rate for plant material deposited on the ground is also affected
by climactic variables, as well as by the soil itself (Isaac and Nair
2005, Goya et al., 2008), due to the composition of the plant material in
accordance with the species present (Gallardo et al. 1995, Huang and
Schoenau 1997). As with deposit, mineralization rate also depends on
environmental conditions, in particular the available moisture in the soil
(Gressel et al., 1995) and the chemical composition of the material which
undergoes this process (Riveros and Alberdi 1978, Mtambanengwe
and Kirchmann 1995, Polyakova and Billor 2007).
The objective of this study was to estimate the impact of fallen bromeliads extraction on the dry matter deposit at soil and nutrient cycling
in a Pinus-Quercus forest in Santa Catarina Ixtepeji, Oaxaca, Mexico.
There are not published data about the magnitude in the reduction of dry
matter deposit at forest soil or in nutrient cycling due to this activity. We
hypothesize that values of reduction around 1% for both parameters are
acceptable to consider sustainable this human activity.
Materials and Methods. Study Site. – The study site is located in the Sierra Norte
region of the Mexican state of Oaxaca, in the municipality of Santa Catarina Ixtepeji,
coordinates 96º33´ longitude West and 17º16´ latitude North (Fig. 1). The altitude at this
site ranges from 1,920 to 3,200 m above sea level. The climate is temperate-subhumid,
with an average annual temperature of 18.1ºC and an average annual rainfall of 756 mm,
concentrated at the end of spring and during summer (Fig. 2) (Serrano-Altamirano et
al., 2005). The plant communities in the site are made up of species of the genera Abies,
Pinus and Quercus, which are mixed together with one genus being dominant, forming
four vegetation types: Quercus-Pinus, Pinus-Quercus, Pinus-Abies, and Abies-Pinus
forests. The study sites are located in Pinus-Quercus forests, which is the type of primary
vegetation that covers the largest area in the municipality and from which the majority of
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c. robles et al.
Fig. 1. – The Santa Catarina
Ixtepeji municipality in
Oaxaca, Mexico. Sample
sites: El Cerezal (EC) and
Ixtepeji (IX).
Temperature C°
bromeliads are extracted for their subsequent sale. The dominant species in this ecosystem are Pinus pseudostrobus Lindl., P. apulcensis Mart., P. leiophylla Schltdl. & Cham.,
as well as Quercus crassifolia Humb. & Bonp., Q. acutifolia Nee, Q. rugosa Nee and Q.
laurina Humb. & Bonp. (García-Mendoza and Torres-Colín 1999). The average tree
density is estimated at 2,200 per hectare, with tree heights ranging from 9 to 15 m. Eight
species of bromeliads have been reported for this area: Tillandsia bourgaei Baker, T.
prodigiosa Lem. (Baker), T. juncea Ruiz & Pav., T. usneoides L., T. magnusiana Wittm.,
T. calothyrsus Mez, Catopsis berteroniana (Schult. & Schult. f.) Mez, and Viridantha
plumosa Baker (Mondragón et al., 2006).
Precipitation mm
Fig. 2. – Ombrothermic diagram of Santa Catarina Ixtepeji, Oaxaca, Mexico. Mean data from the period
1961-2003. (Adapted from Serrano-Altamirano et al. 2005).
Litter and bromeliads in Oaxaca, Mexico
221
Fallen Bromeliad and Leaf Litter Production. – Five quadrants measuring 10 m by
10 m were marked in each of two sites in the forest with similar floristic composition,
locally known as Ixtepeji (IX), at 2,200 m above sea level, with 91% arboreal plant
cover, and El Cerezal (EC), at 2,320 m above sea level, with 78% coverage, respectively, separated by a distance of about four kilometers. The quadrants were delineated
by perimeter marking. Three collection trays of 0.1 m2 in area and 20 cm in height were
placed in each of the quadrants for leaf litter collection (15 trays totaling 1.5 m2 of total
collection surface area at each site). The collection of fallen bromeliad individuals was
performed by hand, and the total dry weight obtained from the 100 m2 quadrants was
recorded. Collection was performed on a monthly basis over a period of 12 months,
beginning in August 2006. In the laboratory, the material was cleaned and dried (65°C,
72 h), the dry weight of the material was recorded. All samples were analyzed twice to
check the precision of the instrumental methods. Standard reference materials have not
been analyzed. The analytical values were accepted only when the data did not differ
in any more than 10% between duplicates. The collected data were used to estimate the
relative contribution of each type of material to the total soil nutrient pool. The following elements were analyzed with the methods noted in parentheses: nitrogen (microKjeldahl), phosphorus (dry digestion, determination by the molibdovanadate method),
calcium and magnesium (atomic absorption spectrophotometry after dry digestion)
(Lachica et al., 1973, Kalra 1998). Using the values and concentrations obtained from
the dry material, the total nutrient content in the leaf litter and in the fallen bromeliads
was estimated for each collection date and site.
Bromeliad and Leaf Litter Mineralization. – Microcosms were constructed as an accelerated method for determining the mineralization rate of leaf litter as well as the bromeliad
biomass, and the nutrient release from these materials (Day 1983, Wohl and McArthur
2001, Treonis et al., 2002, Bonkowski and Roy 2005, Vargas et al., 2006).
For each combination of plant material and site, 5 g of humified organic material
taken from the forest were placed in the bottom of 20 Petri dishes (12 cm in diameter).
The material had been previously air dried and sifted through a 2 mm screen, which
allowed the inoculation of native mineralizing microbiota from the forest floor. This
material was then moistened with 25 mL of distilled water, and covered with a screen
with 0.5 mm openings. Five g of recently collected leaf litter or bromeliad foliage tissue
from each site (dried at 65ºC, 72 h, previously analyzed) was placed on top of the screen.
The boxes were closed and sealed with parafilm, then placed in a darkened growth chamber, with a regulated temperature of 18ºC, mimicking the average forest temperature
from where the material was collected. Four boxes of each plant material were collected
after 15, 30, 45, 60 and 75 days of incubation. The material remaining in the top of the
screen was carefully cleaned with a soft brush, dried as cited and its dry weight was
recorded. Subsequently, the material was ground and analyzed to determine its nitrogen,
phosphorus, calcium and magnesium content, employing the previously mentioned
methods. The release percentage of each nutrient was estimated for each combination of
plant material type and sampling site for each mineralization timeframe.
The mineralization dynamic of each combination of material and sampling site was
analyzed using five mathematical models: lineal, logarithmic, potential, exponential
and polynomial (quadratic). The fit equation for each model was derived, and variance
analysis for each model was carried out; the determination coefficient value (R2) was
determined in each case. Statgraphics v. 5.2. software was used for this analysis.
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c. robles et al.
Results. – The magnitude of deposited leaf litter was found to be
3.63 (±0.87) and 4.09 (±0.90) Mg ha-1 year-1 in the EC and IX sites,
respectively. Regarding fallen bromeliads, a quantity of 0.046 (±0.01)
and 0.021 (±0.006) Mg ha-1 year-1, respectively, was estimated (Tab. 1).
The recorded biomass of fallen bromeliad individuals represents a relatively small contribution to the dry material deposited in the ecosystem,
1.27% in EC and 0.52% in IX, in relation to the magnitude of fallen leaf
litter. The depositing of leaf litter displayed a seasonal behavior determined by precipitation, with maximum values recorded in the season
with the highest environmental temperature and lowest soil humidity,
from January to April. In this quarter of the year, 55.6% of the total leaf
litter was deposited in the EC site and 75.8% in the IX site. In terms
of fallen bromeliads, no clear seasonal behavior was observed; in the
EC site, the highest biomass value for these organisms was recorded in
December, whereas in the IX site it was recorded in October (Fig. 3).
The data recorded in this ecosystem for deposited dry material and
its nitrogen, phosphorus, calcium and magnesium content indicates significant amounts of nutrients able to be recycled back into the system
through the process of mineralization in the soil. In general, the amounts
Table 1. – Bromeliads (B) and leaf litter (H) fall dry weight (g 100 m-2 - standard deviation) throughout one year (2006-2007) in two sites (El Cerezal-EC and Ixtepeji-IX) of
pine – oak forest in the Santa Catarina Ixtepeji municipality in Oaxaca, Mexico
Month \ site
ECB
IXB
ECH
IXH
August
54,1 (7,0)
8,0 (2,8)
1991 (677)
162 (21)
September
25,2 (5,5)
25,3 (3,0)
2571 (309)
1088 (239)
October
7,3 (1,1)
60,7 (11,5)
3998 (760)
1502 (225)
November
48,5 (13,1)
24,1 (7,2)
1503 (451)
1754 (474)
December
112,3 (23,6)
1,9 (0,4)
1496 (329)
2339 (491)
January
50,1 (6,0)
7,6 (2,2)
5233 (1518)
2155 (647)
February
52,8 (15,8)
21,3 (3,4)
7261 (1162)
12068 (1448)
March
3,6 (0,9)
12,6 (3,4)
5056 (1365)
9569 (2488)
April
31,5 (10,4)
18,3 (5,6)
2663 (826)
7229 (2386)
May
35,0 (5,3)
12,0 (3,4)
2090 (585)
1685 (253)
June
12,6 (6,7)
7,7 (1,6)
1873 (318)
1205 (229)
July
29,4 (9,1)
13,8 (3,2)
597 (137)
158 (49)
0,046(0,01)
0,021(0,00)
3,63(0,87)
4,09(0,90)
Prod. (Mg ha-1año-1)
Litter and bromeliads in Oaxaca, Mexico
223
Litter
Fig. 3. – Plant material dry weight
of bromeliads (B) and leaf litter
(H) fall in two sites of pine – oak
forest, El Cerezal (EC) and Ixtepeji
(IX), throughout one year (20062007) in the Santa Catarina Ixtepeji
municipality, Oaxaca, Mexico.
recorded in Kg ha-1 year-1 ranged from 43.6 to 50.6 for nitrogen, from
0.65 to 0.94 for phosphorus, from 138.2 to 231.8 for calcium, and from
28.1 to 29.4 for magnesium. In accordance with the greater magnitude of
litterfall in the IX site, the contribution of this site was also found to be
greater than that of EC for the four nutrients analyzed in this study. The
contribution of fallen bromeliads to the nutritional content of the forest
topsoil was, similar to their contribution to deposited dry material, very
small, with an average of 0.85% for nitrogen, 0.48% for phosphorus,
0.62% for calcium and 0.60% for magnesium (Tab. 2) with respect to
the leaf litter contribution.
The mineralization of dry material followed a similar pattern in
the four combinations of material type and collection site (Fig. 3).
Nevertheless, it was clear that leaf litter was a more recalcitrant material
IXH
ECH
IXB
ECB
Site \
0,18
0,09
32,1
3,9
P
Ca
Mg
9.821
7.207
1.451,4
Ca
Mg
10.042
714
78,7
Ca
Mg
528,8
11,97
P
1.327
658
1,46
N
1.249,5
136,26
31,86
P
3.137
1.239
N
6,2
101,5
30,9
12,3
9,9
32,9
N
Mg
31,0
0,58
0,98
110,9
P
30,7
sep
66,1
aug
Ca
N
nutrient \ sampling
month
1.824,9
6.624
21,03
1.832
5.545,2
25.827
43,98
4.878
29,5
243,4
0,43
74,1
3,6
29,3
0,03
8,8
oct
2.131,1
6.472
45,60
2.140
1.182,9
3.081
24,05
1.834
11,71
59,3
0,27
29,4
11,8
136,8
0,10
59,2
nov
1.136,7
8.732
30,41
2.854
727,1
4.832
11,97
1.825
1,6
7,6
0,12
2,3
27,3
316,7
0,34
137,0
dec
29.843
36,31
8.858
15,5
69,0
0,43
26,0
51,3
64,9
1,16
64,4
feb
73.011
313,77
14.723
44.400
248,79
11.674
5.859,9
24.370
161,79
6.168
6,1
49,0
0,11
15,4
1,8
7,0
0,05
4,4
mar
60.796
195,18
8.819
1.294,2
12.836
82,55
3.249
17,8
125,7
0,20
22,3
19,2
78,1
0,35
38,4
apr
8.812
20,22
2.056
925,9
5.037
22,99
2.550
8,8
27,1
0,08
14,8
17,0
183,1
0,32
42,8
may
1.047,3 5.720,2 11.626,3 2.906,1 1.354,7
4.806
28,02
2.629
1.271,6 5.293,3
10.728
62,80
6.384
3,7
30,5
0,11
9,2
12,2
151,3
0,35
61,1
jan
743,5
5.314
13,26
1.470
2.358,1
5.282
20,60
2.285
7,5
18,9
0,09
11,8
15,3
40,7
0,09
15,4
jun
230,4
1.215
1,11
193
725,4
2.907
8,36
728
6,7
55,3
0,15
6,5
14,3
36,2
0,27
11,3
jul
29.328,7
230.940
930,8
50.375
27.884,7
137.011
643,5
43.135
118,9
819,4
2,26
252,6
218,8
1.186,0
4,62
539,6
Total
Table 2. – Mean nutrient content (N, P, Ca and Mg - g ha-1) in bromeliads (B) and leaf litter (H) fall throughout one year (2006-2007) in two sites (El Cerezal-EC
e Ixtepeji-IX) of pine – oak forest in the Santa Catarina Ixtepeji municipality in Oaxaca, Mexico
224
c. robles et al.
Litter and bromeliads in Oaxaca, Mexico
225
Fig. 4. – Microcosm mineralization
performance of plant material from
bromeliads (B) and leaf litter (H)
fall in two sites of pine – oak forest,
El Cerezal (EC) and Ixtepeji (IX),
throughout one year (2006-2007) in
the Santa Catarina Ixtepeji municipality, Oaxaca, Mexico.
than bromeliad tissue; additionally, the bromeliad tissue in the IX site
was mineralized at a greater rate than that of the EC site. Mathematical
modeling of mineralization process behavior indicated that, in all models tested, a significant fit was obtained for the process in all of the
combinations of material type and collection site. Nevertheless, the
determination coefficient corresponding to the polynomial quadratic
model was, in all cases, the highest of the five models tested (Tab. 3).
Nutrient release, on average, occurred following a similar dynamic as
that of the loss of biomass process. The behavior of the process for leaf
litter and bromeliads in the tested system displayed a net release ranging
from 29.9% to 53.1% for nitrogen, from 36.4% to 74.7% for phosphorus, from 33.3% to 63.8% for calcium and from 63.6% to 79.2% for
magnesium (Tab. 4). The combinations of material type and collection
site yielded relatively uniform average values of nutrient release, rounding to 45% for the four nutrients analyzed, with the exception of bromeliads in the IX site, whose average nutrient release value was found to
be greater than 67%.
Discussion. – The magnitude of deposited leaf litter is affected by
various factors, which can be summarized in the type of ecosystem to
2
R
Equation
2
R
Equation
2
R
Equation
*P < 0,05, **P < 0,01
Potential
Polynomial
Exponential
0,947**
0,940*
0,944**
0,931*
Y=5,164X
Y=5,240X
Y=5,206X
-0,175
0,994**
-0,366
0,959**
-0,287
0,963**
2
Y=0,08X -1,03X+6,0 Y=0,008X -0,333X+5,325
0,993**
Y=0,065X -0,851X+5,86
2
Y=5,334e
Y=5,512e
0,936*
2
0,952**
-0,064X
Y=-0,759lnX+5,119
0,992**
Y=-0,274X+5,247
EC H
0,952**
-0,129X
Y=-1,374lnX+5,108
0,911*
Y=-0,47X+5,26
IX B
0,932*
Y=5,416e
-0,101X
Y=-1,147lnX+5,12
Equation
0,912*
R2
R2
Y=-0,939X+5,257
Equation
Linear
Logarithmic
EC B
Model \
Site-Plant Mat.
0,947*
Y=5,101X-0,168
0,967**
Y=0,025X2-0,434X+5,402
0,957**
Y=5,231e-0,06X
0,960**
Y=-0,728lnX+5,067
0,948*
Y=-0,256X+5,165
IX H
Table 3. – Mathematical models that explain the mineralization process of dry material from bromeliads (B) and leaf litter (H) fall in
two sites of pine – oak forest: El Cerezal (EC) and Ixtepeji (IX) in the Santa Catarina Ixtepeji municipality in Oaxaca, Mexico
226
c. robles et al.
Litter and bromeliads in Oaxaca, Mexico
227
Table 4. – Mean nutrient content (N, P, Ca y Mg - mg microcosmos-1) and percent of
nutrient release from the residual plant dry material of bromeliads (B) and leaf litter
(H) from two sites (El Cerezal-EC and Ixtepeji-IX) of pine – oak forest in the Santa
Catarina Ixtepeji municipality in Oaxaca, Mexico, in a microcosm mineralization
experimental assay.
Site \ nutrient \ days of
mineralization
ECB
IXB
ECH
IXH
0*
15
30
45
60
75
%
release**
N
61,3
57,7
45,1
44,8
38,7
38,2
37,7
P
0,55
0,44
0,46
0,44
0,39
0,35
36,4
Ca
130,5 123,6 109,0 102,6
80,5
87,1
33,3
Mg
26,4
12,8
8,9
13,2
8,0
8,6
67,4
N
68,4
55,3
41,2
39,4
36,8
32,1
53,1
P
0,75
0,72
0,36
0,32
0,29
0,19
74,7
Ca
188,5 150,4
88,3
100,4
88,4
68,2
63,8
Mg
30,8
15,5
12,8
8,8
8,8
6,4
79,2
N
71,0
58,6
53,1
50,8
48,0
44,7
37,0
P
0,90
0,57
0,68
0,49
0,42
0,43
52,2
Ca
188,0 180,8 155,6 146,7 135,6 115,9
38,4
Mg
37,0
22,9
20,7
18,3
16,9
11,7
68,4
N
64,9
56,7
51,0
52,6
50,3
45,5
29,9
P
0,80
0,89
0,57
0,51
0,42
0,42
47,5
Ca
267,0 214,8 160,1 184,1 185,9 171,0
36,0
Mg
39,0
63,6
20,5
17,4
17,2
16,2
14,2
* Values of original plant dry matter, sampled before the beginning of the microcosm mineralization experiment. ** Estimated values based in the residual content of each nutrient 75 days after
the beginning of the microcosm mineralization experiment.
be analyzed. The most relevant ecosystem components are the plant
species present, the plant cover or density of individuals, the age of
the vegetation, and climate components such as thermal and pluvial
regimens (Riveros and Alberdi 1978, Kirman et al., 2007, Pandey
et al., 2007, Tateno et al., 2007). Information regarding leaf litterfall
in temperate pine forests or pine-broadleaf forests is relatively scarce
in comparison with studies of different types of tropical vegetation,
whether dry (Robles and Barea 2006) or humid (Rico-Gray and Lot
1983, Rezende et al., 1999, Barlow et al., 2007, Kirman et al., 2007).
The magnitude of leaf litterfall found in this study coincides with the
results reported by Huber and Oyarzun (1983) for a Pinus radiate D.
228
c. robles et al.
Don plantation in the south of Chile, with values fluctuating between
3.7 and 4.13 Mg ha-1 year-1; it is also comparable to that reported by
Pandey et al. (2007), namely of 5.48 and 4.20 Mg ha-1 year-1 for a native
forest with Quercus serrata Sieb & Zucc. dominance, as well as four
associated arboreal species and a plantation of Q. serrata, respectively,
in Northeast India. No other reports are found in the literature regarding
leaf litter production for the type of vegetation examined in this study.
The production of leaf litter in the IX site was close to 13% greater than
that of the EC site; this may be attributable to the difference in arboreal
cover between the sites, which was 17% greater in the former site than in
the latter. This explanation is confirmed by the observation of Donoso
et al. (1999), who reported leaf litter production in Drimys winteri Forst.
stands to be 27% greater in sites with greater tree density.
The leaf litter production contribution of species belonging to the
Bromeliacea family has not been widely studied. Riveros and Alberdi
(1978), on studying leaf litter production in a hygrophile forest in the
south of Chile, reported a very small contribution for the bromeliad
Fascicularia bicolor R. et Pav., barely 0.13% of the total deposited leaf
litter production throughout the year. This result is less than the results
of the present study in either of the two study sites. In the ecosystem
examined in the present work, bromeliad species diversity was greater
than that for the study conducted by Riveros and Alberdi (1978),
which included eight reported species. It is possible that the abundance
of the species considered in the present study is also greater than that of
Riveros and Alberdi (400 to 650 individuals per hectare in the present
study, non-published data), as the aforementioned work does not report
the density of F. bicolor individuals in the study site. This greater diversity and abundance of bromeliads may also explain in part the values
found in the present study.
The seasonal behavior of leaf litterfall is not an inalterable rule. Even
considering that it has been reported in many studies (Robles y Barea
2006, Kirman et al., 2007, Pandey et al., 2007, Aceñolaza et al., 2009),
there are cases in the literature in which this process does not follow the
same behavior (Huber and Oyarzun 1983). The phenological species
cycle determines to a great degree the seasonality of litterfall, in conjunction with environmental variables such as soil moisture and temperature.
The evidence shows that in this study, these were the determining factors for the seasonality of collected leaf litter, though not for bromeliads,
whose fall could be better characterized as accidental in nature. The causal
Litter and bromeliads in Oaxaca, Mexico
229
factors for bromeliad fall were wind speed, certain animals such as birds
and squirrels which land or stand on the plants or break tree branches, and
even humans, who, in the process of collecting forest components (firewood, fruit, etc), accidentally knock bromeliads down.
In general, the composition and thus total quantity of nutrients
contained in fallen leaf litter is determined by the species present in
the ecosystem, the age of the individuals, and the characteristics of the
soil itself (Huber et al., 1986, Aceñolaza and Gallardo-Lancho
1994, Gallardo et al., 1995, Mtambanengwe and Kirchman 1995,
Tateno et al., 2007). Huber et al. (1986) reported a nitrogen contribution of 27.7, a phosphorus contribution of 3.1, a calcium contribution
of 21.3, and a magnesium contribution of 4.4 Mg ha-1 year-1 for a Pinus
radiate forest; these results are less than those found in the present study,
with the exception of phosphorus, which is considerably higher. It has
been reported that pine needles are rich in lignin and phenols, two compounds which slow down the mineralization process of plant material
(Scholes and Nowicki 1998). Tateno et al. (2007) found that Robinia
pseudoacacia L., an arboreal legume known to fix atmospheric nitrogen, deposits an estimated 56.5 Mg ha-1 year-1 of nitrogen in the form of
leaf litter, whereas, in the same system, Quercus liaotungensis Koidz.
deposits only 39.1 Mg ha-1 year-1. These quantities are slightly higher
and lower, respectively, than the magnitude interval of released nitrogen
found in this study.
The mineralization rate of leaf litter has been studied by many
authors for different ecosystems and the litter bag technique is most
commonly used. The study of the process dynamics in systems modeled
on microcosms is less common, but it is equally useful for the same purpose, with the advantage of not being subject to probable contamination
of the study material, as well as allowing a strict control of the process
conditions to be maintained, particularly in terms of moisture content,
temperature, light, and microbial population (Day 1983, Vargas et al.,
2006, He et al., 2009). Mathematical modeling of the leaf litter mineralization process has found that, depending on the ecosystem being studied, the data can fit either a lineal model (Isaac y Nair 2005, Pandey
et al., 2007) or an exponential one (Leblanc et al., 2006, Polyakova
y Billor 2007, He et al., 2009). In this study, this process fit to the
following five models was analyzed: lineal, logarithmic, exponential,
polynomial quadratic and potential. It was found that both leaf litter and
bromeliad mineralization fit all of the models significantly, although
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the greatest determination coefficients were obtained using the quadratic model. The daily mineralization rate, according to the exponential
model, is from 0.06 to 0.064 for leaf litter, and from 0.101 to 0.129 for
bromeliads. A rapid mineralization phase is recognizable in the latter
material, occurring in the first 30 days, which then changes to a lower
rate. The mineralization rate for leaf litter, however, remains relatively
constant for at least 75 days.
Under the conditions in which the process was performed, there is
a net release of nitrogen, phosphorus, calcium and magnesium. This
behavior does not occur in the same way when the process occurs in an
ecosystem, where the contributions of the soil, of neodeposited materials, or of microbial and root exudates as added factors allow the process of immobilization or gain to occur in a given period (Alvarez et
al., 2008). Nutrient release is proportionally higher in bromeliads than
in leaf litter, as is weight loss. Even when the mineralization process
is recognized as the primary factor responsible for the maintenance of
productivity in an ecosystem (Geissen and Morales-Guzmán 2006,
Álvarez et al., 2008), the rapid decomposition of deposited dry material may have unfavorable consequences for the soil. Rapid decomposition may impede the formation of topsoil or O horizon that protects
the soil from direct solar exposure, and consequently reduces evaporation, as well as offering significant protection from the effects of erosion (Tateno et al., 2007). Despite having a greater nutrient release
rate than that determined for leaf litter, the contribution of bromeliads
to nutritional recycling in this ecosystem is, as with its contribution to
the deposit of dry material, very small: on average, less than 1% for
all the nutrients evaluated.
Acknowledgements. – The authors would like to thank the
Alfredo Harp Helú Foundation for the funding of this study. They would
also like to thank the citizens of Santa Catarina Ixtepeji for allowing
access to the study sites and permitting the collection of samples. Miss
Kimberly Traube revised the English version of the manuscript.
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Summary. – The aim of the work was to estimate the impact of fallen bromeliads
extraction on the dry matter deposit at soil and nutrient cycling in a pine-oak forest.
Leaf litter and fallen bromeliads were collected over a period of 12 months from two
sites in the forest, and their dry weight as well as their nitrogen, phosphorus, calcium
and magnesium content were recorded. In microcosms, the mineralization rate of each
type of material from both sites was analyzed. Leaf litterfall rates were found to be 3.63
and 4.09 Mg ha-1 y-1, and bromeliad litterfall rates were found to be 0.046 and 0.021 Mg
ha-1 y-1; the latter represent 1.27% and 0.52%, respectively, of leaf litterfall. The leaf litterfall dynamic showed seasonal behavior; the bromeliad litterfall dynamic did not. The
contribution of fallen bromeliads to the nutrient deposit on the forest floor was 0.85% for
nitrogen, 0.48% for phosphorus, 0.62% for calcium and 0.60% for magnesium, relative
to the leaf litter contribution. Leaf litter was found to be more recalcitrant to mineralization than bromeliads.