Comparison of Three Methods to Estimate Organic Carbon in Allophanic Soils in New Zealand

Soil sequesters large amounts of carbon derived from organic and inorganic sources. Soil organic carbon (SOC) has to be measured periodically for the study of climate change. Many methods are used to estimate SOC and those methods are relatively time consuming and costly, with carbon in soil samples converted to carbon dioxide which is then measured directly or indirectly by different methods. Quantitative methods for the determination of SOC are based on titration or gravimetric, volumetric, spectrophotometric and chromatographic methods. Loss-on-ignition (LOI) has been considered as a rapid, inexpensive and convenient method for estimating SOC, which involves the dry combustion of samples at high temperature in a muffle furnace. The LECO Carbon Analyzer for SOC determination is capable of complete recovery and high precision, and is recommended primarily for total C analysis of soils. However, this method is costly and required a high level of user expertise to provide consistent performance. The objective of this study was to establish a NZ kiwifruit-specific regression equation to estimate SOC from LOI. We collected 121 allophanic soil samples from 0-100 cm depth from three regions of kiwifruit orchards in the Bay of Plenty area with organic, biological and integrated management systems. Soils were analysed using LOI to determine organic matter by combinations of three different ignition temperatures and four time durations. Results were compared with two other methods viz. wet acidified rapid dichromate oxidation, Walkley-Black method (organic carbon) and dry combustion technique, (LECO; total organic carbon) and a regression equation was established to estimate SOC in allophanic soil. A NZ kiwifruit-specific regression of SOC measured by LOI (A o C, a hours) versus LECO which was assumed here to have no error yields for the topsoil (0-50 cm) an R 2 of 0.822 and for the subsoil (50-100 cm) an R 2 of 0.45. The regression of SOC for 0-15 cm soils established by LOI (B o C, b hours) had an R 2 of 0.903. On the other hand, regressions established by LOI (300 o C, 3h) for SOC of 15-30 cm and 30-90 cm of soils had R 2 of 0.777 and 0.748, respectively. This analysis shows that the LOI method is well suited as a cost-effective method for the analysis of SOC in NZ kiwifruit systems and other crops produced on allophanic soils, especially for topsoil. Introduction In soils there are three basic forms of carbon being elemental carbon, inorganic carbon, and organic carbon. Organic carbon or total organic carbon (TOC) is the amount of carbon bound in an organic compound. Soils contain a large variety of organic compounds ranging from simple sugars and carbohydrates to the more complex proteins, fats, waxes, and organic acids. Important characteristics of the organic compounds include their ability to form water-soluble and water insoluble complexes with metal ions and hydrous oxides; interact with clay minerals and bind particles together; absorb and desorb both naturallyoccurring and anthropogenically-introduced organic compounds; absorb and release plant