Characterization of soil humin by acid hydrolysis

Studies were conducted to characterize soil humin by acid hydrolysis. Two humin samples collected from two different types of soil, namely chernozem and laterite, which are widespread over a vast area from the north to south of China, were hydrolyzed under reflux with 0.5M H2SO4 or 3M H2SO4 for 4 h. The results showed that 25% – 29% of organic carbon and 46% – 54% of organic nitrogen could be hydrolyzed by 0.5M H2SO4; 36% – 40% of organic carbon and 93% – 97% of organic nitrogen hydrolyzed by 3M H2SO4. The C/N ratio in hydrolyzed organic matter is lower than that in soil humin and that in organic matter hydrolyzed by 3M H2SO4 is lower than that in organic matter hydrolyzed by 0.5M H2SO4. The proportion of nitrogen hydrolyzed from humin is markedly larger than that from the original soil and also markedly larger than that from humic acid fraction. Only 3% – 7% of nitrogen in humin exists in a relatively stable form, which is not easy to hydrolyze. There is little nitrogen that occurs in the form of heterocyclic rings in humin. Incubation experiments showed that the newly formed organic matter can be hydrolyzed more easily.