Comparison of Soil Organic Carbon Methods and Carbon Sequestration and Soil Characteristics in Louisiana Crawfish Ponds

Abstract

Two studies were conducted for this thesis research. In the first study, nine soil organic carbon (SOC) methods were evaluated using 336 soil samples from different regions, including Alabama, Louisiana, and Arkansas, as well as sites in Ecuador, Kenya, and Tanzania. Comparisons were made between nine methods, including dry combustion, wet combustion (Walkley-Black), loss on ignition, photometry, near-infrared spectroscopy (NIRS), the Solvita® method, color, texture, and visualization, with dry combustion serving as the control method. The following variables were considered as the basis of comparison: equipment and reagents, accuracy, efficiency, complexity, cost and environmental effect. The results show that: Walkley-Black (r2=0.89), loss on ignition (r2=0.86), and Solvita® (r2=0.64) correlates better with dry combustion than others; photometry has modest accuracy (r2=0.31); other methods such as NIRS, color, texture, and visualization have limited accuracy. Loss on ignition is recommended as a routine test by soil testing laboratories, as equipment is commonly found in laboratories and no reagents are required. Further, good accuracy was achieved, and the method is simple with low cost, and no hazardous wastes. In the second study, carbon (C) sequestration potential and soil characteristics were studied in soils under Louisiana crawfish ponds. Two crawfish culture systems, monocropping and rotational, were compared with adjacent agriculture fields. Only the upper depth of soil from crawfish monocropping systems (25.8 g kg-1) had SOC concentrations that were greater than the adjacent agricultural control (20.8 g kg-1). In other horizons of monocropping systems and all horizons of rotational systems, no differences were found in either C concentration or C mass. The results indicate crawfish ponds do not sequester more C than upland cropping systems. Other soil characteristics such as bulk density, nitrogen (N) concentration and mass, particle size, pH, and carbon to nitrogen ratio (C:N), were examined. Bulk density helped explain the SOC concentration and mass distribution difference. Nitrogen concentration and mass had similar depth distribution as SOC. The average pH of monocropping and rotational system is 7.1 and 7.0 respectively, which was within the normal range. The average C:N of monocropping and rotational system is 11.4 and 9.7, which was close to a natural system of 10:1.