The impact of land clearing and agricultural practices on soil organic C fractions and CO2 efflux in the Northern Guam aquifer

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 CO2 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. (C) 2000 Elsevier Science B.V. All rights reserved.