Comparison of Different Methods for the Determination of Organic Carbon in Calcareous Soils under Different Land Uses of Lorestan Province

Nowadays, because of climate change, there is a great interest in carbon cycle. In most of the soils, the main part of carbon is held as soil organic carbon (SOC) whereas, in soils of the arid and semiarid regions, inorganic carbon, primarily carbonate, is the majority of carbon in the soils. The major minerals found in the soils of arid regions of Iran are calcite (CaCO3) and dolomite (Ca Mg, CO3) which mainly exist in the soils of the southern and southwestern Iran. Thus, quantifying both SOC and soil inorganic carbon (SIC) is essential to know the way of occurrence of the carbon cycle in the soils. The aim of this study was to compare different methods for the determination of organic carbon in calcareous soils under different land uses of Lorestan province.

The study area is located in the Polhoro region of Khorram Abad district, Lorestan Province in western Iran. The sampling area spanned sides of the Polhoro region, which has various land uses, including forest land, cropland, and grassland. Dry and irrigated farming were between cultivated lands. In each land uses, a study plot was chosen. At each sampling point, five subsamples were taken at a depth of 0–30 cm (root zone) and mixed to form a composite sample including one central point and four other points by the distance of 15 m in the four cardinal directions. A total of 40 soil samples were taken across an agroecological region in southwest Iran. The soils had a clay-loam and sandy-clay-loam texture classes and pH above 7. Representative samples were analyzed for SOC and SIC measurements by different procedures. The procedures were included the modified traditional Walkley-Black method (WB) for SOC, Loss-on-Ignition (LOI) Procedure given by Nelson and Sommers (1982) for SOC, and CN analyzer (Vario-EL III) for SOC and SIC. The method presented by Boden, VDLUFA-Verlag, Darmstadt (2016) was chosen for organic carbon and carbonate measurement. This method was used as a reference method to compare results of different methods.

In the present study, the soil samples of forest land showed the highest values for TOC and TN, but lowest values for TIC. The measured SOC concentration by WB, LOI and CN analysis methods in the top soil varied among land uses. The highest C concentration (4.52%) was observed in the forest land while the lowest concentration (0.52%) was for dry farming land both by the use LOI method. SOC values varied from 0.8 to 2.3 in modified WB, 0.52 to 4.52 in LOI and 0.93 to 2.72 in CN analysis methods between different land uses. The differences in C concentrations between rangeland, forest, irrigated and dry farming lands were statistically significant and showed the following order for the WB and LOI

forest land > irrigated farming > rangeland > dry farming. As expected, C concentrations of LOI and WB were highest in the forest, intermediate in irrigated farming and rangeland and lowest in the dry farming. The relative content of SOCLOI differed significantly across forest and irrigated farming land uses while the difference was not significant in the other two methods. A significant difference also observed in the SOC of forest and irrigated farming with dry and range lands obtained by all three methods. The results of the correlation between the bulk soil and SOC in different methods from the Spearman correlation analysis showed that there was a positive correlation between the TN, TC, clay, and silt of the bulk soil with SOC in the separated methods with the correlation coefficient ranging from 0.1 to 0.92. Conversely, the negative correlation was found between TIC and sand content of the bulk soil with the SOC in the different methods. It can be concluded that calcareous nature of these soils could be one of the reasons for low organic carbon in this research. By comparison between the SOC values measured by WB and LOI methods with CN analysis as a reference method, the WB method showed the results were more near to CN analysis than LOI. The high SOCLOI values for the forest and range land in this research may be related to weight loss from some non-SOM. The results of this study demonstrate that the CN method with combination of dry combustion is a potent method for the accurate estimation of SIC and SOC in calcareous soils. The common approach often involves pretreatment with acid to remove carbonate, which may cause remove organic matter in soil samples and also erosion the instrument. Thus, the combustion method could be a good substitute for acid treatment of soil samples.

Three methods were applied for determinations of SOC concentrations in calcareous soils of Polhoro region and the results were compared. There was a wide range of values in soil properties, but in general, TIC was more than SOC. The results show that the Walkley-Black method with external heating can provide precise estimation for soil organic carbon for these soils. This study illustrates that the WB and LOI technique could be comparable with the CN analysis method and regarded as potent methods to produce accurate results for estimating of organic carbon in arid soils. The results supported the elemental analysis as a precise method to the estimation of SOC and TIC in calcareous soils. Further study is suggested for testing TIC by LOI approach on a wider range of arid and semi-arid soils.