Spectroscopic and chemical differences in organic matter of two vertisols subjected to long periods of cultivation

Abstract

The nature of a number of humic fractions extracted from the 0-0.1 m layers of two cracking clay soils was studied using 13C nuclear magnetic resonance and infrared spectroscopy. By comparing samples from sites under native vegetation and nearby sites which had been under continuous cultivation for cereal cropping for 35 years, two different mechanisms which act to protect organic matter against decline are evident. In both grey clays (Langlands-Logie clay) and black earths (Waco clay), a large proportion of the organic matter receives some degree of protection through association with clay. In grey clays, this is the only significant form of protection. The humic materials display little molecular recalcitrance and contain significant amounts of long alkyl chains and proteinaceous groups. In black earths, association of organic matter with clay is also the most significant factor but, in addition, the remaining humic materials are more stable to microbial attack. This results from higher aromaticity as well as shorter, more highly branched alkyl chains. Some implications of these findings are also discussed.