Soil Organic Carbon and Nitrogen and Distribution of Carbon-13 and Nitrogen-15 in Aggregates of Everglades Histosols

摘要

Oxidation of Histosols in the Everglades Agricultural Area (EAA) of south Florida leads to decreases in soil depth, changes in biogeochemical properties, and may limit land use options in the future. The objectives of this study were to determine how long-term cultivation influences organic matter dynamics and C and N distribution throughout the profile of a drained Histosol. We measured organic C and N stocks, aggregation, and the natural abundance of 13C and 15N in aggregates from Histosols 100 yr after drainage for two land uses: sugarcane (Saccharum officinarum L.) and prairie. Macroaggregates comprised the bulk of total soil for both land uses, averaging 81% of the total soil in fractions >0.25 mm. Macroaggregation increased with depth and the proportion to whole soil was 65% higher at 30 to 45 cm than 0 to 15 cm. Cultivated soil averaged 13% higher organic C, but 11% lower organic N than prairie throughout the profile (0–45 cm). The majority of the organic C (76%) and N stocks (77%) was in macroaggregate fractions >0.25 mm. The distribution of organic matter among aggregate-size fractions generally did not differ between land uses, except that organic C and N were 39 and 44%, respectively, greater for macroaggregates in prairie than cultivated soil at 0 to 15 cm. The 15N decreased with depth for both land uses, indicating that organic matter was more decomposed and humified in surface soil (0–15 cm). The decrease with depth likely resulted from inundation of subsurface soils and low O₂ levels, which subsequently lowered rates of decomposition. The 13C decreased with depth for cultivated soil but increased for prairie, and was significantly higher for soil cropped to sugarcane (-25.37) than prairie (-26.20). Soil organic matter under cultivation was less humified than prairie soil due to recent C inputs from sugarcane. The 2-mm fraction had 12% lower 15N than other fractions, indicating that recent organic matter inputs accumulated in macroaggregate fractions. Smaller aggregates contained higher 15N and older organic matter. In contrast to most studies of mineral soils, cultivation of Histosols increased C storage relative to prairie, with the major difference between land use being higher soil organic matter levels in the subsurface (15–45 cm). Thus, cropping may reduce the rate of oxidation of Histosols in southern Florida relative to the prairie ecosystem.