Comparative effects of composted organic waste and inorganic fertilizer on nitrate leachate from the farm soils of northern Guam

The purpose of this study was to quantify the release of nitrate into vadose zone as well as the nitrogen-holding capacity of compost applied on calcareous soils of northern Guam amended with both inorganic fertilizer and composted organic waste, to examine potentially adverse effect of these nutrient materials to groundwater quality. Three different nitrogen levels each of the composted organic waste and the inorganic fertilizer were applied to corn (maize) (farmland) study plots for three consecutive seasons, two dry and one rainy season. During each season, soil organic matter (SOM, w/w%) content and carbon-nitrogen ratio (C/N, w/w%), were determined for analysis of the nitrogen-holding capacity of these calcareous soils. Nitrate levels in soil pore water were also determined for study of potential groundwater contamination. For three seasons, compost plots showed higher SOM% contents and lower C/N% than fertilizer plots. That is, compost-treated soils showed higher SOM content and adsorbed more nitrogen under normalized soil mass than did fertilizer-treated soils. Nitrate levels in pore water were generally higher on compost plots during early stages of corn but were generally higher on fertilizer plots during active leaf, tasseling, and maturity stages. During the rainy season, nitrate leachate seemed to increase, but this trend could not be confirmed because rainfall amounts were not measured. Overall, composted organic waste proved to be a good amendment for soil productivity and agricultural sustainability while reducing nitrate leachate from northern Guam farmland.