Loss-on-Ignition Method to Assess Soil Organic Carbon in Calcareous Everglades Wetlands

Measurement of soil carbon (C) is important for determining the effects of Everglades restoration projects on C cycling and transformations. Accurate measurement of soil organic C by automated carbon-nitrogen-sulfur (CNS) analysis may be confounded by the presence of calcium carbonate (CaCO3) in Everglades wetlands. The objectives of this study were to compare a loss-on-ignition (LOI) method with CNS analysis for assessment of soil C across a diverse group of calcareous Everglades wetlands. More than 3168 samples were taken from three soil depths (floc, 0-10, 10-30cm) in 14 wetlands and analyzed for LOI, total C, and total calcium (Ca). The LOI method compared favorably to CNS analysis for LOI contents ranging from 0 to 1000gkg-1 and for soil total Ca levels from 0 to 500gCakg-1. For all wetlands and soil depths, LOI was significantly related to total C (r2 = 0.957). However, LOI was a better predictor of total C when LOI exceeded 400gkg-1 because of less interference by CaCO3. Total C measurement by CNS analysis was problematic in soils with high total Ca and low LOI, as the presence of CaCO3 confounded C analysis for LOI less than 400gkg-1. Inclusion of total Ca in regression models with LOI significantly improved the prediction of total C. Estimates of total organic C by CNS analysis were obtained by accounting for C associated with CaCO3 by calculation, with results being similar to total organic C values obtained from LOI analysis. The proportion of C in organic matter measured by the LOI method (51%) was accurate and applicable across wetlands, soil depths, and total Ca levels; thus LOI was a suitable indicator of total organic C in Everglades wetlands.