Soil organic matter stocks and spatial distribution in the Rio Grande de Arecibo watershed

Abstract

This research evaluated the influence of land use and soil classification, as stratified by taxonomic soil order, on the spatial distribution of soil organic carbon (SOC) and soil organic nitrogen (SON) of the Rio Grande de Arecibo (RGA) watershed, Puerto Rico. The objectives were to quantify the present state of SOC and of SON stocks and potential C sequestration capability of the watershed to 1 -m depth. Samples were taken from representative soils of the watershed occupying 39,361 ha (or 87.3% of the total watershed area) under secondary forest, pasture, or agricultural land use. Soils of the watershed store 5.02 x 106 Mg of SOC and 0.48 x 106 Mg of SON at a depth of 100 cm. The weighted mean SOC and SON contents of the 0- to 15-cm layer of the watershed were 4.33 kg C/m2 and 0.390 kg N/m2, respectively, whereas at 0 to 100 cm it was 11.13 kg C/m2 and 1.08 kg N/m2, respectively. The soil mapping unit x land use interaction represented the best area-wide estimates of soil organic matter because there was improved resolution on a spatial scale. Forest and pasture soils contained higher amounts of SOC (12.8 and 9.79 kg C/m2, respectively) (P < 0.05) than soils under cropland (7.90 kg C/m2) for the 0- to 100-cm depth. The 0- to 15-cm SOC was ranked as Oxisols = Ultisols > Inceptisols, with values of 5.85, 4.77, and 3.18 kg C/m2, respectively (P < 0.05); and for the 0 to 100 cm, were ranked as Oxisols > Ultisols > Inceptisols, with values of 18.3,13.3, and 6.71 kg C/m2, respectively. We estimate that an additional amount of 46,627 Mg C could be sequestered within the watershed if 50% of the agricultural or pasture land were reverted to forest. This estimate represents a modest 1.0% increase above the current watershed C level.