Carbon and Nitrogen in Aggregate-Sized Fractions of Lowland Soils with Different Clay Mineralogy in Central Plain, Thailand

Abstract

Representative lowland soils including 5 soil orders with different clay mineralogy in Central Plain of Thailand were selected for this study. Inceptisols, Alfisols and Ultisols were mixed mineralogy soils whereas Mollisols and Vertisols were smectitic soils. Soil samples were collected at plough layer (Ap) and below plough layer to the depth of 60 cm (Ap-60) for analysis of aggregate-sized distribution, water aggregate stability, (> 0.25 mm) and water aggregate stability contents of studied soils significantly correlated with organic carbon, extractable calcium and clay contents of the soil. Organic carbon and total nitrogen in aggregate-sized fractions of studied soils mainly increased with increasing size of aggregate fractions in Ap horizon. However, Alfisols, Ultisols and Vertisols contained higher organic carbon and total nitrogen in microaggregate (< 0.25 mm) than that of macroaggregate (> 0.25 mm) in Ap-60 horizon.  Organic carbon and total nitrogen contents in aggregate-sized fractions of studied soils significantly increased with increasing organic carbon content of the soil. Each mineralogical soil group clearly showed variation of aggregate-sized fractions, water aggregate stability, organic carbon and total nitrogen contents within each soil group. Therefore, clay minerals may play a minor role for soil aggregation, aggregate stability, carbon and nitrogen contents in soil aggregates in comparison to those soil properties as previously mentioned.