Agriculture, Ecosystems and Environment 79 2000 17–27
The impact of land clearing and agricultural practices on soil organic C fractions and CO
2
efflux in the Northern Guam aquifer
P.P. Motavalli
a,∗
, H. Discekici
b
, J. Kuhn
c
a
Department of Soil and Atmospheric Sciences, The School of Natural Resources, University of Missouri, Columbia, MO 65211, USA
b
Agricultural Experiment Station, College of Agriculture and Life Sciences, University of Guam, Mangilao, GU 96923, USA
c
Natural Resources Conservation Service, U.S. Department of Agriculture, Maite, GU 96927, USA Received 11 March 1999; received in revised form 11 August 1999; accepted 22 October 1999
Abstract
The importance of the Northern Guam aquifer as a source of drinking water for the tropical Pacific island of Guam has stimulated public interest in the impact of forest clearing and conversion to agriculture on the region’s environment. The
objectives of this study were to determine the effects of land clearing, tillage, and fertilization of tropical secondary forest on soil organic and organic C fractions in the shallow, calcareous soil that overlies most of Northern Guam. A field experiment was
established on a secondary forest site in Northern Guam to simulate land clearing, cultivation and fertilization with two separate applications of N, P and K fertilizer or leucaena Leucaena leucocephala Lam. de Wit leaves. Initial aboveground biomass
of secondary forest was relatively low in comparison to that of other moist tropical forest sites, possibly because of poor soil fertility, shallow soil depth, and frequent natural disturbance from tropical storms. Rates of litterfall were also affected by the
high winds associated with storm activity. Clearing, cultivation and fertilization over a 325-day period significantly reduced microbial biomass C. Soil surface CO
2
efflux was characterized by short-term flushes shortly after tillage and was affected by soil moisture content and possibly by the proportion of active organic C contained in the soil. A comparison of commercial
fields with continuous 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 conversion of secondary forest to continuous cultivation. Further information is
needed on the effectiveness of minimum tillage, application of organic amendments, or improved crop residue management to maintain soil organic C in Northern Guam. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Land clearing; Soil CO
2
efflux; Tillage; Soil organic C fractions; Active organic C pool
1. Introduction
Changes in soil organic C in the tropics due to land management can have an impact on soil phys-
∗
Corresponding author. Tel.: +1-573-884-3212; fax: +1-573-884-5070.
E-mail address: motavallipmissouri.edu P.P. Motavalli
ical and chemical properties Woomer et al., 1994 and potentially affect global C cycling Scholes and
van Breeman, 1997. The tropical Pacific island re- gion of Micronesia represents a total land area of
approximately 2707 km
2
Karolle, 1991 and there- fore, alterations in soil C storage in this region are
not likely to have a large impact on global C cycling. However, rapid land development in the region, in-
0167-880900 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 8 0 9 9 9 0 0 1 3 9 - 5
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
