18 P.P. Motavalli et al. Agriculture, Ecosystems and Environment 79 2000 17–27
cluding activities, such as phosphate mining and con- version of secondary forest to agricultural and urban
purposes, have affected soil organic C and N Man- ner and Morrison, 1991; Motavalli and McConnell,
1998. Decreasing soil organic C can lower soil wa- ter and nutrient-retention capacity, structural stability,
infiltration rates, and accelerate runoff and erosion; thereby, reducing the natural resource base and soil
productivity Lal and Kang, 1982.
In the humid tropics, losses of soil organic C after forest clearing and conversion to agriculture are ap-
proximately 20–40 of the soil C within the first 1 or 2 years following soil disturbance Davidson and
Ackerman, 1993. Losses of soil organic C due to land clearing may result from several processes including
decreased inputs and changes in composition of plant litter and increased rates of soil organic matter de-
composition and soil erosion Lugo and Brown, 1993; Feller and Beare, 1997. In addition, tillage increases
the rate of soil organic matter decomposition by bury- ing surface residues, disrupting soil aggregates, aerat-
ing the soil, and exposing new surfaces to microbial attack Brown et al., 1994. Therefore, the method of
forest clearing and the type of agricultural land use will affect the amount of soil organic matter loss or
gain Lal and Kang, 1982; Lugo and Brown, 1993.
Rapid initial losses of soil organic C following forest clearing and conversion to agriculture are pri-
marily losses of the biologically-labile or active soil organic C pool Lugo and Brown, 1993; Brown et al.,
1994. In soils from Hawaii, the active soil organic C pool accounted for 0.7–4.3 of total C Townsend et
al., 1997. Changes in the active organic C pool can be monitored by measurement of rates of soil CO
2
efflux or soil respiration, although other methods in- cluding biological, chemical, physical, and isotopic
procedures have also been proposed to distinguish active from more stable organic C pools Motavalli
et al., 1994; Townsend et al., 1995.
The objectives of this study were to determine the effects of land clearing, tillage, and fertilization of
tropical secondary forest over time on soil organic C and organic C fractions in a shallow, calcareous soil
located on the northern half of the Pacific island of Guam. The region of Northern Guam is experiencing
rapid development and is of environmental and eco- nomic importance because it overlies the sole fresh
water aquifer for the island.
2. Materials and methods
2.1. Field experiment A field experiment was established in September
1996 on a secondary forest site located at the Yigo Agricultural Experiment Station 13
◦
31
′
N, 144
◦
52
′
E, elevation of between 145 and 155 m above sea level
in Northern Guam. The soil underlying the site was the Guam soil series U.S. Soil Taxonomy: clayey, gibb-
sitic, nonacid, isohyperthermic Lithic Ustorthents; FAO Soil Classification: Rendzic Leptosols. The de-
sign of the experiment was based on a standardized protocol to examine soil organic matter dynamics in
tropical environments proposed by the Tropical Soil Biology and Fertility Programme Woomer and In-
gram, 1990. The objective of the experimental design was to simulate long-term changes in soil organic
matter due to forest conversion and agricultural prac- tices. However, the influence of crop plants on these
changes was excluded. Plots measured 2.4 m width by 2.4 m length.
Treatments were arranged in randomized com- plete block design with three replications and con-
sisted of: 1 soil left with the existing vegetation, 2 soil cleared of aboveground vegetation, 3 soil
cleared of aboveground vegetation and large roots and tilled once a month with a rototiller, 4 same
as treatment 3 but with addition of approximately 20 Mg ha
− 1
dry basis leucaena leaves and stems, and 5 same as treatment 3 but with addition of
400 kg N ha
− 1
and 1106 kg K ha
− 1
in the form of KNO
3
and 131 kg P ha
− 1
as triple superphosphate. Root barriers consisting of galvanized steel sheets
were placed to a depth of 30 cm around all borders of plots receiving treatments 2, 3, 4 and 5. Fertilizer
and leucaena treatments treatments 4 and 5 were applied and incorporated twice over the course of the
experiment on 28 October 1996 and 5 May 1997. For the first application of treatments, 19 Mg ha
− 1
dry basis leucaena leaves 424.8 ± 6.9 g organic C kg
− 1
, 40.0 ± 2.4 g total N kg
− 1
were added and for the sec- ond set of treatments, 21 Mg ha
− 1
dry basis leucaena leaves 457.1 ± 30.2 g organic C kg
− 1
, 33.28 ± 0.78 g total N kg
− 1
were applied. Additional tillage using a rototiller occurred approximately on a monthly basis
in treatments 3, 4 and 5. Aboveground regrowth on plots cleared of vegetation was suppressed by
P.P. Motavalli et al. Agriculture, Ecosystems and Environment 79 2000 17–27 19
periodic application of the herbicide, glyphosate, at a rate of 3 litres active ingredient ha
− 1
. Litter falling on cleared plots from surrounding forest vegetation was
removed every 4 weeks using a rake. Soil samples were also collected before treatments were imposed
and periodically up to 15 September 1997 to a depth of 12.5 cm using a stainless steel push probe and
mixing 20 subsamples per plot. Half of the collected sample was stored moist at 4
◦
C for determination of soluble organic C and microbial biomass C and the
other half air-dried for determination of total organic C and particulate organic matter C. All soil samples
were passed through a 2 mm sieve before analysis.
2.2. Aboveground biomass and litter characterization Litter traps measuring 76 cm length × 61 cm
width × 15 cm height were constructed from steel rebar and aluminum netting with 1.5 mm square open-
ings in order to characterize aboveground organic C inputs into the secondary forest ecosystem of Northern
Guam. Ten traps were placed randomly within sec- ondary forest vegetation located over a 6 ha area at the
Yigo Agricultural Experiment Station. The traps were placed at a height of approximately 30 cm above the
soil surface. Litter, consisting primarily of leaves and fine branches, was collected monthly over a 13-month
period beginning 13 June 1996, weighed and dried at 70
◦
C for moisture determination, and ground through a Wiley-Mill with a 1 mm screen before analysis.
To estimate aboveground biomass of the secondary forest vegetation, vegetation that was cleared and re-
moved from the plots from the field experiment was weighed, fed through a chipper, and a subsample taken
for moisture determination after drying at 70
◦
C. Sub- samples were also ground through a Wiley-Mill with
a 1 mm screen for chemical analysis.
2.3. Measurement of soil CO
2
efflux Rates of soil CO
2
efflux and changes in soil temper- ature in the field experiment were determined using
a portable infrared CO
2
analyzer LI-6200, LI-COR, Inc. Lincoln, NE, USA fitted with a soil respira-
tion chamber Model 6000-09 and a soil temperature probe Garcia et al., 1997. Total system volume for the
chamber and analyzer was 991 cm
3
. During soil CO
2
efflux measurements, the chamber was fitted on top of a 10.2 cm diameter PVC plastic soil collar inserted
2 cm into the soil surface. The soil temperature probe was placed 5 cm into the soil. Each plot had two soil
collars spaced approximately 45 cm apart in the center of the plot. A composite of five soil subsamples was
also collected from each plot to determine soil mois- ture content during soil respiration measurements.
To determine soil CO
2
efflux, ambient CO
2
concen- trations were first measured. The CO
2
concentration in the chamber headspace was then scrubbed below
ambient levels while the chamber was fitted on the soil collar. The scrubber was then switched off and soil
CO
2
efflux and soil temperature logged over a 3-min period. A single soil CO
2
efflux was then calculated for the ambient CO
2
concentration by interpolating ef- flux values over the measurement period. Average soil
temperature was also calculated over the sampling pe- riod. A comparison of this dynamic chamber method
for measuring soil surface CO
2
flux with the static chamber method using alkali traps has been presented
by Jensen et al. 1996. At the end of the field experiment, an additional
comparison was made of diurnal soil surface CO
2
ef- flux among undisturbed forest sites, and adjoining sites
at the Yigo Agricultural Experiment Station which were initially cleared and cultivated from forest or
had been cultivated for 1 or 7 years. A description of soil properties of the cultivated 7 year site is found
in Motavalli and McConnell 1998. The sites cleared and cultivated for 1 year were from the field exper-
iment. The initially cleared and cultivated sites were from forested areas around the field experiment. Base-
line soil surface CO
2
efflux was measured over a 3-h period at all sites and then all cultivated sites
were tilled once with a rototiller. Subsequent soil CO
2
efflux was then measured periodically over a 51-h pe- riod. At each sampling time, two soil CO
2
efflux mea- surements were made for each site and duplicate soil
samples composited from five subsamples were taken to a depth of 10 cm for soil moisture determination.
2.4. Survey of farmers’ fields in Northern Guam A survey was conducted from April to June 1998
of 23 cultivated fields belonging to 16 commercial farmers located in Northern Guam Fig. 1. The farms
range from 0.2 to 7.9 ha in size and primarily produce
20 P.P. Motavalli et al. Agriculture, Ecosystems and Environment 79 2000 17–27
Fig. 1. Map of the island of Guam showing locations of farm, forest and field experimental sites. Dashed line indicates southern boundary of the Northern Guam aquifer. Inset shows approximate location of Guam in relation to the Philippines and the west coast of North America.
horticultural crops, including eggplant, cucumber, tomato and long beans. Cropping histories for each
field were collected from farmers and only fields of varying years 0.6–26 years of continuous cropping
after clearing from secondary forest were selected. In addition, three secondary forest sites located within
a U.S. Air Force Base in Northern Guam were sam- pled. These sites had been undisturbed by human
activity since the middle 1940s Heidi Hirsh, U.S. Air Force, personal communication, 1998, but the
island receives periodic tropical storm and typhoon activity which can cause major defoliation and treefall
P.P. Motavalli et al. Agriculture, Ecosystems and Environment 79 2000 17–27 21
Karolle, 1991. The primary soil underlying the sam- pled sites was the Guam soil series clayey, gibbsitic,
nonacid, isohyperthermic Lithic Ustorthents and all sites were located over the Northern Guam aquifer
Fig. 1. Soil samples were collected at each field to a depth of 12.5 cm by using a stainless steel push-probe
and by mixing 30 subsamples per field. Surface litter or crop residue was removed before sampling soil at
all sites. Samples were air-dried, ground in a hammer mill, and sieved 2 mm before analysis.
2.5. Chemical and physical analysis Soil particle size analysis was determined using the
hydrometer method Bouyoucos, 1962. Soil bulk den- sity was determined by the soil core method Blake
and Hartge, 1986 and the proportion of stones with a diameter 2 mm measured by sieving air-dried soil.
Soil moisture content under 0.033 MPa suction was determined using the pressure plate method Klute,
1986. Soil pH was measured in water 1 soil : 1 water wv and soil P by extraction with 0.5M NaHCO
3
. Ex- changeable Ca and Mg were determined by extraction
with 1M NH
4
OAc and atomic absorption spectropho- tometry. Determination of the proportion of CaCO
3
in soil was determined by an acid titrimetric procedure
Rowell, 1994. Total organic carbon of soil and or- ganic materials was determined using a heated dichro-
mate oxidation method Nelson and Sommers, 1975 and total nitrogen by a micro-Kjeldahl digestion pro-
cedure Lachat Instruments, 1992.
Microbial biomass C was determined on field-moist soils from the field experiment and on air-dried soils
from farmer fields using the CHCl
3
fumigation–direct extraction method Vance et al., 1987 with a 3-day fu-
migation and a conversion factor k
EC
of 0.35 Spar- ling et al., 1990. Air-dried soils were wetted to their
0.033 MPa moisture content and pre-conditioned with a 7-day incubation before fumigation. Soil organic C
extracted in 0.5M K
2
SO
4
of the unfumigated soils was considered a measure of soluble organic C. Particu-
late organic matter POM C was measured by a wet sieving procedure 53
m
m sieve using sieved 2 mm, air-dried soil Cambardella and Elliott, 1992.
The proportion of total extractable polyphenolics in litter and cleared vegetation was determined by ex-
traction in hot 50 methanol using tannic acid as a standard Anderson and Ingram, 1993. Lignin con-
tents of organic materials was determined using the acid detergent fiber method Goering and Van Soest,
1970.
2.6. Data analysis Analysis of variance ANOVA by PROC GLM
SAS Institute, 1988 was used for determining the effects of land clearing and cultivation on soil charac-
teristics and soil CO
2
efflux. The multiple comparison test used was Fisher’s protected LSD at a 0.05 sig-
nificance level. Pearson linear correlations were calcu- lated among soil characteristics and soil organic C and
organic C fractions of samples collected from farmer fields and secondary forest sites using PROC CORR
SAS Institute, 1988.
The nonlinear regression model used for analysis of the relationship between soil organic C in farmers
fields and time of cultivation was: Y = C
1
e
− k
1
t
+ C
2
e
− k
2
t
where Y is the soil organic C and t is the time of cul- tivation. The coefficients C
1
and C
2
give an estimate of the active and stabilized C pools, respectively. The
coefficients k
1
and k
2
are rate constants for each cor- responding C pool.
3. Results and discussion
