Soil Organic C and N Storage and Distribution of 13C and 15N in Aggregates of Everglades Histosols.

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 influenced soil organic matter dynamics and C and N distribution throughout the soil profile. We measured soil organic C and N storage, aggregation, and the distribution of ?13C and ?15N in cultivated and prairie soils. The contribution of sugarcane (Saccharum sp.) to changes in organic matter after 100 years of drainage and cropping was compared to the same soil under 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 30-45 than 0-15 cm. Cultivated soil averaged 13% higher organic C, but 11% lower organic N than prairie throughout the soil profile (0-45 cm). The majority of the organic C (76%) and N storage (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 0-15 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, low O2 levels, and lower rates of organic matter 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 decomposed than prairie soil due to 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 more decomposed organic matter. In contrast to most studies of mineral soils, cultivation of a histosol 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.