Water Table Regime Regulates Litter Decomposition in Restiad Peatlands, New Zealand

Abstract

A 5-year litterbag study examined decomposition rates at four sites representing restiad peatland succession in Waikato, New Zealand. Early successional sites were dominated by Baumea rubiginosa, or Leptospermum scoparium, mid-successional by Empodisma robustum, and late successional by Sporadanthus ferrugineus. Leaf/culm materials from these species were placed on the surface, and roots of Empodisma and Sporadanthus buried at depths of 5, 25, and 55 cm to test the influence of succession on species and site decomposition rates. Typha latifolia leaves from a Canadian bog were placed at the surface and three depths to allow comparisons with northern peatlands. Litterbags were retrieved after 0.5, 1, 2, 3, 4, and 5 years, and mass remaining characterized by an exponential model k value. Surface litter k values (0.12–0.80 y−1) decreased from early to late successional species; however, decomposition was slower at more waterlogged early successional sites. Buried litter k values (0.04–0.24 y−1) decreased with depth and increased from early to late successional sites, with Empodisma roots having the slowest rates. Few strong relationships existed between litter quality and decomposition rates. In contrast, water table regime strongly influenced decomposition rates; k values for the “standard” Typha litter decreased exponentially as period of saturation increased, irrespective of site successional status, nutrients, or other factors. Lower water tables in the more aerated later successional sites have led to faster decomposition rates. Ongoing drainage combined with the potential impacts of climate change may increase organic matter decomposition and accelerate carbon release into the atmosphere.