Chemical properties, decomposition, and methane production of Cenozoic analog plant litters: Implications for atmospheric trace gas production in the early Cenozoic

Background/Question/Methods Throughout the early Cenozoic Era (ca. 65-38 Ma) Taxodiaceae-dominated (redwood) wetland forests occupied the high latitudes and were circumpolar in their distribution. Many of these forests had high standing biomass with moderate primary productivity. The geographic extent and amount of Cenozoic coals and fossil forests throughout Arctic Canada suggests large areas of wetland forests that may have cycled substantial quantities of carbon, particularly methane until they were replaced by cold tolerant Pinus, Picea, and Larix following climatic cooling associated with the Terminal Eocene Event. To test this hypothesis we compared physiochemical properties, decomposition, and trace gas production of litter from 10 extant plant species, including Metasequoia, Pinus, Picea, and Larix.

Results/Conclusions We found that Metasequoia litter is a better source of labile organic substrate than pinaceous litter, and lignin structure using cupric oxide oxidation indicated that Metasequoia lignin is enriched in 4'-hydroxyacetophenone and 4'-Hydroxy-3'-methoxyacetophenone relative to the pinaceous litter. In a 24-month study of leaf litter decomposition, average litter mass loss was greater for Metasequoia litter (62%) than the pinaceous species (50%). Metasequoia litter incubated under anoxic conditions supported more microbial biomass and rates of methane production (2.1 umol/g.day) than the pinaceous litter (1.2 umol/g.day). Our results support the idea of greater decomposability of Metasequoia litter as compared to Larix, Picea, or Pinus. Provided that the biochemical properties of Metasequoia have remained relatively stable through geologic time, it appears that early Cenozoic Metasequoia-dominated wetland forests may have had higher microbial driven trace gas production than the Pinaceae-dominated forests that replaced them in the late Cenozoic.