Antioxidant capacity and rate of decomposition of organic amendments in a Vertisol

We hypothesized that antioxidants in organic amendments would control the rate of their decomposition in soil, such that those with more antioxidant capacity (AOC) would have slower rates of decomposition. This has been tested using materials from an incubation experiment, in which a Vertisol had been amended with a number of organic materials. Green waste compost, wheat straw, sugarcane trash and mineral fibre had been added to the soil and incubated for different periods of up to 84 days. Decomposition had been monitored by measuring cumulative carbon dioxide emissions. One and four m NaOH extracts of soil material from the experiment were analysed for their AOC by the Trolox equivalent method. The same method was used to characterize the organic amendments and the unamended soil. AOCs of the amendments were much greater than that of the soil and therefore we expected that the AOC of amended soils would be greater than the unamended control. However, this was not generally the case. A comparison of the AOC of amended soil measured after 7 days with that calculated from the components showed that the measured values of incubated amended soils were less than the calculated ones, particularly for wheat straw and sugarcane trash amendments. This suggests that some of the extractable antioxidants in the amendments were lost or transformed during the first 7 days of incubation. In the remainder of the incubation period there was a small increase in AOC with time in 4 m NaOH extracts of the amended soils, but this was not the case for 1 m NaOH extracts. There was some evidence that amendments with larger AOCs had slower rates of decomposition, especially during the first 7 days of the incubation. Other factors may have been responsible, but C:N ratio alone could not explain the differences. The results are encouraging, but more data are required to test our hypothesis thoroughly.