Laccase activity in Sphagnum-dominated peatland: A study based on a novel measurement of delay dynamics (MDD) for determining laccase activity

Abstract

The large reservoir of organic carbon in peatland is essential for the global carbon cycle. Laccase, probably the largest class of phenol oxidases in soil, plays an important role in the primary degradation of recalcitrant polyphenolic compounds. However, the traditional method used to test laccase activity may not be suitable for peatland, given that reductive compounds would result in the reduction of chromophore 2,2′-azino-bis (3-ethylbenzo-thiazoline-6-sulfonic acid) radical cation catalyzed by laccase. In this study, the organic and inorganic constituents affecting laccase activity assay in peatland were systematically studied. The experiments indicated that high content of ferrous iron in peatland can significantly affect the measurement of laccase activity by delaying the ABTS•+ absorbance dynamic inflection point, and three typical dynamic changes of ABTS•+ absorbance in Dajiuhu peat samples were found. In this study, we propose a novel method based on delay dynamics (MDD) to determine laccase activity in peatland and applied it to the investigation of Dajiuhu, a Sphagnum-dominated peatland. The results show that the laccase activities measured using MDD (24.45 ± 10.89 μmol g−1•h−1) are considerably higher than that measured by the traditional method (12.10 ± 12.06 μmol g−1•h−1) in Dajiuhu peatland. In addition, there was no clear evidence that ferrous iron can stimulate laccase activity. Conversely, we found that ferrous iron can even inhibit the catalytic efficiency of laccase. Laccase activities were higher at the depth of 0–10 cm (31.12 ± 13.36 μmol g−1•h−1) compared with that at 10–20 cm (18.88 ± 5.67 μmol g−1•h−1), suggesting the sub-optimal conditions for laccase activity with increasing depth in Sphagnum-dominated peatland. In addition, laccase activities decreased significantly with pH increase in Dajiuhu peatland, which is different from the generally positive relationship between soil pH and phenol oxidase activities tested by L-DOPA assay.