Soil organic carbon and nitrogen distribution in a tropical watershed
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The role of soils in sequestering atmospheric carbon (C) and nitrogen (N) can best be understood from a watershed perspective because the multiple influences of factors influencing the process can be assessed simultaneously, within a hydrologic unit. It is important to identify land uses and management practices that maximize soil carbon sequestration and help a meliorate the effects of CO2 emissions to the atmosphere. The spatial distribution and the effects of soil order, land use and soil phase on the total soil organic carbon (SOC) and soil organic nitrogen (SON) content were assessed in the Río Grande de Arecibo watershed in Puerto Rico (RGA). A Geographical Information System (GIS) was used to develop the sampling strategy. Soil samples were taken at five depth increments (0-15, 15-30, 30-50, 50-75, 75-100 cm) from 21 soil series under diverse land use types within the watershed. Spatial distribution maps of SOC and SON according to mapping units, soil orders and land use types were generated. The mapping unit area-weighted mean SOC content was 4.15 kg C m-2 and 10.9 kg C m-2 in the 0-15 cm and 0-100 cm depths, respectively. In general, the soils sampled in the watershed (33,322 ha or 84% of the total land area) contain 3.98 x 106 Mg of organic C and 0.38 x 106 Mg of organic N to depth from 0 to 100 cm. The area-weighted mean SON content was 0.372 kg N m-2 and 1.05 kg N m-2 for the 0-15 and 0-100 cm, depths respectively. Forest and pasture soils contained higher amounts of SOC (12.8 and 9.79 kg C m2 , respectively) (P<0.05) than soils under cultivation (7.90 kg C m-2) for the 0-100 cm depths. The SOC was significantly different (P<0.05) among soil orders in the 0-15cm depth (Oxisols and Ultisols > Inceptisols) and in the 0-100 cm depth (Oxisols > Ultisols > Inceptisols). These results allow a better understanding of the impacts of land use on soil C and N stocks at the watershed level.