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
3.1. Soil characteristics Initial soil characteristics of the field site are shown
in Table 1. The variable soil depth of the study site is indicated by the high standard deviation of the pro-
portion of stones 2 mm diam measured in the soil Table 1. Limestone pinnacles from the underlying
bedrock are characteristic of the shallow Guam soil series Young, 1988 and the resulting variable soil
depth, ranging from approximately 5–50 cm, affects several soil physical properties, including soil tex-
ture and water-holding capacity. Soil organic C lev- els in this soil are also relatively high among soils
on Guam Demeterio et al., 1986, possibly due to the stabilizing effects of the high calcium content of
this limestone-derived soil on decomposition Oades, 1988, 1989. Low plant-available soil P measured in
22 P.P. Motavalli et al. Agriculture, Ecosystems and Environment 79 2000 17–27
Table 1 Selected average chemical and physical soil properties of the field experiment of the Guam soil
Soil texture Stones 2 mm
Moisture 0.033 Organic
CaCO
3
pH NaHCO
3
− extraction
diameter g kg
− 1
MPa g g
− 1
C g kg
− 1
g kg
− 1
1 : 1 water P mg kg
− 1
Sand Silt
Clay 435
248 307
360 0.392
51.7 374.8
7.3 2.8
87
a
68 143
324 0.064
9.9 249.9
0.2 1.2
a
Numbers in parentheses are the standard deviations.
this soil is characteristic of tropical forest ecosystems and often limits net primary production Vitousek,
1984.
3.2. Aboveground biomass and litter in secondary forest
Forest vegetation was typical of the limestone forest community described by Moore 1973 and in-
cluded pago Hibiscus tiliaceus L., lulujet Maytenus thompsonii
Merr. Fosb., Allophylus ternatus Forst. Radlk., ifit Intsia bijuga Colebr. Ktze., kafu Pan-
danus tectorius Sol. ex Park and the fern species,
sword fern Nephrolepsis hirsutala Forster f. Presl. Motavalli and McConnell, 1998. The average weight
of aboveground biomass cleared and removed from plots in the field experiment was 36.6 ± 18.4 Mg ha
− 1
dry weight basis ± standard deviation with an aver- age organic C content of 409.2 ± 16.6 g kg
− 1
and a total N content of 6.7 ± 1.6 g kg
− 1
. The aboveground biomass of this secondary forest site is relatively low
in comparison to that of other moist tropical forest sites Vitousek and Sanford, 1986, possibly because
of poor soil fertility, shallow soil depth, and frequent natural disturbance from tropical storms. In contrast
to other tropical forest regions where cleared vegeta- tion is burned on site, cleared vegetation in Northern
Guam is primarily hauled away or less commonly left in the field Motavalli, unpublished data. There-
fore, organic carbon inputs from secondary vegeta- tion after clearing are mainly from roots and surface
litter.
The rate of litterfall under the secondary limestone forest vegetation of Northern Guam averaged 40.0
± 49.1 kg ha
− 1
per day dry weight basis ± standard deviation over a 13-month period Fig. 2B. This rate
of litterfall falls within the range of litterfall from 5–25 Mg ha
− 1
per year reported by Greenland et al.
Fig. 2. Changes in precipitation A, rates of litterfall B, and litter composition C in secondary forest vegetation in Northern
Guam over a 13-month period. Vertical bars show ± one standard deviation unit at each sampling date.
P.P. Motavalli et al. Agriculture, Ecosystems and Environment 79 2000 17–27 23
Table 2 Soil organic C, soil organic C fractions and soil bulk density at the initiation and conclusion of a 325-day field experiment
Treatment Bulk density
Total organic C Soluble organic C
Microbial biomass C POM C
Mg m
− 3
g kg
− 1
soil g kg
− 1
of total organic C Initial
0.88 51.7
3.31 27.15
329.1 After 325-days:
Forest 0.93
56.6 3.09
27.70 267.4
Cleared 0.86
59.9 2.22
21.08 80.9
Cultivated 1.02
48.2 2.72
22.80 214.7
Leucaena-treated 0.82
51.1 2.60
21.68 62.5
Fertilizer-treated 0.86
48.8 2.86
18.24 126.8
LSD
0.05
0.16 NS
a
NS 8.18
248.1
a
NS = not statistically significant at p ≤ 0.05.
1992 for humid tropical forest sites. The average rate of organic C input was 16.0 ± 18.1 kg ha
− 1
per day. However, litterfall on Guam is affected by the peri-
odic tropical storm activity in the region and the high winds associated with those storm events. For exam-
ple, the rate of litterfall as a result of Typhoon Dale was approximately 4–5 times greater than the average,
but after Typhoon Isa, a storm with weaker winds, lit- terfall was not significantly higher than the average
Fig. 2A and B. Although not measured in this study, the high winds of Typhoon Dale also caused a sub-
stantial felling of trees and large branches. Guam ex- periences tropical storms with winds greater or equal
to 33 m s
− 1
every 3.5 years and storms with winds approaching 67 m s
− 1
approximately every 10 years Karolle, 1991.
No significant seasonal variations in litter compo- sition were observed Fig. 2C. Total N content of
litter averaged 15.7 ± 5.6 g kg
− 1
; lignin content av- eraged 191.9 ± 55.6 g kg
− 1
; the C : N ratio of litter averaged 29.2 ± 10.2; the lignin : N ratio averaged
12.9 ± 4.1; and the polyphenolic : N ratio averaged 0.9 ± 0.8. Litter composition or quality can affect
subsequent rates of C and N mineralization and the partitioning of organic C into active and stabilized
C pools Woomer et al., 1994; Scholes et al., 1997. Based on the results of Constantinides and Fownes
1994 who examined the relationship between litter quality from several tropical agroforestry species and
soil N mineralization, decomposition of litter from the secondary forest sites in Northern Guam would
be expected to result in net N immobilization due to the low total N content 20 g N kg
− 1
and relatively high lignin content 100–50 g lignin kg
− 1
. The rel-
Fig. 3. Soil CO
2
efflux following application of the first set of treatments A or second set of treatments B in the field experi-
ment. Soil CO
2
efflux for the cleared treatment has been omitted from the figure for the sake of clarity. Vertical arrows indicate
when tillage occurred. Vertical bars show LSD p 0.05 value at each sampling date; NS = not significant.
24 P.P. Motavalli et al. Agriculture, Ecosystems and Environment 79 2000 17–27
atively higher lignin content would also reduce rates of initial C decomposition.
3.3. Soil organic C and organic C fractions The effects of clearing, cultivation, and fertilization
on soil organic C, organic C fractions and soil bulk density are shown in Table 2. Compared to soil under
secondary forest vegetation, microbial biomass C was significantly reduced by the fertilizer treatment Table
2. This result suggests that nutrient deficiency may limit rates of C decomposition in this soil. Significant
reductions in particulate organic matter C were also observed in the cleared and leucaena-treated plots
Table 2, although an explanation for this result is not clear. Reductions in particulate organic matter
POM have been observed with long-term tillage Cambardella and Elliott, 1992, possibly due to de-
struction of soil aggregates and exposure of organic
Fig. 4. Precipitation following application of the first set of treat- ments A or second set of treatments B in the field experiment.
Arrows indicate precipitation associated with tropical storm events. Names of individual tropical storm events are also given.
matter to microbial decomposition Brown et al., 1994. Removal of the top forest litter layer, which
often occurs in mechanical clearing, the reduction in litter inputs, and the lack of mixing of dead roots by
tillage may have resulted in lower soil POM C in the cleared plots during this 325-day period. There is no
clear explanation for the reduction of POM C in the leucaena-treated plots.
3.4. Soil surface CO
2
efflux Application and incorporation of the first treatment
of approximately 19 Mg ha
− 1
dry basis leucaena leaves in the field experiment significantly increased
soil surface CO
2
flux up to 34 days after treatment Fig. 3A. The field site received high initial precip-
itation up to approximately 93 days Fig. 4A. This high initial precipitation was followed by an extended
drought period up to the last soil surface CO
2
flux
Fig. 5. Diurnal soil CO
2
efflux A and changes in soil moisture B in a secondary forest site or sites with different cultivation his-
tories in Northern Guam following a cultivation event. Lines show averages of two consecutive measurements shown by symbols
per site.
P.P. Motavalli et al. Agriculture, Ecosystems and Environment 79 2000 17–27 25
measurement at 169 days after treatments were applied Fig. 4A. Higher soil moisture content at the forest
site compared to other treated plots may account for the significantly higher CO
2
flux measured in the for- est sites after 140 and 169 days Fig. 3A. Short-term
increases in soil CO
2
flux were also observed per- sisting less than 1 day after tillage events Fig. 3A.
This pulse of increased CO
2
flux immediately after tillage has also been observed in temperate soils and
is dependent on the interactive effects of changes in soil moisture, soil temperature, soil structure, and soil
organic matter availability caused by tillage Hendrix et al., 1988; Reicosky and Lindstrom, 1993.
Soil CO
2
efflux was also elevated following a sec- ond treatment of leucaena and this effect persisted
throughout the measurement period Fig. 3B. Signif- icantly higher CO
2
efflux was also observed in the forest plots compared to the clearedcultivated plots
up to approximately 41 days after the start of the sec- ond treatment period. This same period was an inter-
val of low precipitation Fig. 4B and suggests that higher soil moisture in the forest sites compared to
the cleared and cultivated sites during dry periods pro- motes higher soil CO
2
efflux. Short-term changes in soil CO
2
efflux and soil mois- ture after a tillage event were examined for sites with
Fig. 6. Effects of years of continuous cultivation on soil total organic C of Northern Guam commercial farm fields. Soil total organic C of secondary forest sites are also shown.
varying previous periods of continuous cultivation and compared to a forest site Fig. 5A and B. All cul-
tivated sites had higher soil CO
2
efflux immediately after cultivation, but this effect only persisted over the
51 h period of measurement in the recently cleared and cultivated forest site Fig. 5A. Initial rapid efflux of
CO
2
is possibly released from soil pores and from min- eralization of exposed labile compounds Prior et al.,
1997. Soil CO
2
efflux was lowest in the site that had been cultivated continuously for 7 years Fig. 5A sug-
gesting increasing loss of active organic C with longer periods of continuous cultivation. An additional factor
affecting reduced soil CO
2
efflux in the sites that had been cultivated was the lower soil moisture content of
those sites compared to the forest site Fig. 5B. 3.5. Soil organic C status of farmers’ fields and
secondary forest sites in Northern Guam A comparison of commercial farm fields with con-
tinuous cultivation histories of 1–26 years and forest sites in Northern Guam showed approximately a 44
decrease in soil organic C within 5 years after con- version of secondary forest to continuous cultivation
leveling off at an equilibrium level of approximately 32 g C kg
− 1
Fig. 6. Soluble organic C ranged from
26 P.P. Motavalli et al. Agriculture, Ecosystems and Environment 79 2000 17–27
Table 3 Soil organic C and soil organic C fractions of commercial farm fields and forest sites in Northern Guam
Total organic C g kg
− 1
soil Soil organic C fractions g kg
− 1
of total organic C Soluble organic C
Microbial biomass C POM C
Average
a
36.2 6.01
8.12 56.00
Standard 10.7
1.70 4.02
37.71 Minimum
18.3 3.10
2.10 16.60
Maximum 56.0
9.00 15.80
187.60
a
n = 26.
3.10 to 9.00 g kg
− 1
of total organic C; microbial biomass C from 2.10 to 15.80 g kg
− 1
of total or- ganic C; and POM C from 16.60 to 187.60 g kg
− 1
of total organic C Table 3. Soil total organic C significantly negatively correlated with soil clay con-
tent r = −0.74, n = 26 and positively correlated with soil exchangeable Ca r = 0.64, n = 26 and
Mg r = 0.77, n = 26. However, soil clay content also had a significant negative correlation with soil
exchangeable Ca r = −0.72, n = 26. The stabi- lizing effects of soil Ca content on decomposition of
organic C has been noted by Oades 1989 and may be a more important factor than soil clay in stabiliz-
ing soil organic C in the limestone-derived soils of Northern Guam. No significant trends were observed
between years of cultivation and soil soluble C, soil microbial biomass C, or POM C data not shown.
4. Conclusions
