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Land Use and Soil Nitrogen Status in a Tropical Pacific Island Environment

Agricultural Experiment Station, College of Agriculture and Life Sciences, Univ. of Guam, UOG Station, Mangilao, Guam 96923, USA.

^* Corresponding author (motavall{at}uog9.uog.edu).

ABSTRACT

The rapid rate of land development in the Micronesian Region of the Pacific Ocean has raised public concern over the effects of land disturbance and land use on potential NO^–₃ pollution and other longterm impacts on soil properties. Our objective was to compare the soil N status and other soil chemical and physical properties of several land use sites on Guam, including an area under secondary forest vegetation, an area under continuous cultivation, an area recently cleared for agriculture, and an abandoned agricultural area. Continuous agricultural cultivation over a 7-yr period increased soil bulk density and soil pH and lowered water-holding capacity, organic C, total N, and N mineralization compared with soil in a forested site. Soil organic N fractions, including microbial biomass N and particulate organic matter N, were reduced by the mixing effects of tillage in the cultivated site. Initial clearing of vegetation increased bulk density and pH but did not decrease soil organic C and N compared with the forest site. Soil at an agricultural site that had been abandoned for an 11-yr period did not return to levels of organic C, total N, and N mineralization observed in the forest and cleared sites. These results suggest that long-term tillage has a significant impact on soil properties in this tropical island environment, including reducing active and stable N pools. However, the significance of these changes on NO^–₃ leaching is unclear because of alterations in soil properties affecting both soil water movement and other N processes.