The microbial decomposition of seeds
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The seed component of plant litter and its associated nutrients has been largely ignored in litter fall, decomposition and ecosystem nutrient budget studies. The exclusion of this fraction potentially underestimates the transfer of energy and nutrients within the ecosystem. This study investigated the seed substrate characteristics and the microbial decomposition of 10 species. A combination of microcosm and in situ experiments were used to manipulate rate-regulating factors of decomposition and measure their influence on the variables of mass loss and net nitrogen mineralisation. Mass loss from whole seeds, decomposed under controlled conditions for 180 days, varied between 0.7% (Sophora microphylla) and 77.7% (Triticum aestivum) with the net nitrogen mineralisation varying between 0.1% (Quercus robur) and 67.1% (Ulex europaeus). The greatest amount of inter-species variation in the decomposition rate could be explained by the proportional allocation of seed mass to the seed coat fraction (r=-0.7058, P=0.0001). Alterations to the integrity of the seed coat by artificial treatments, such as scarification, heat, or grinding, and natural mechanisms, such as seed immaturity or insect damage, accelerated the rate of decay in the initial 90 days of incubation. The decomposition of seeds in combination with leaf and wood litters resulted in significant non-additive effects. The mass loss from litter mixtures exhibited both synergistic and antagonistic effects. A reduced release of nitrogen occurred in all litter mixtures. Simulated freezing and desiccation events decreased the rate of seed decomposition. The estimated mean time for 95% mass loss from whole seeds was extended from 3.6 years under constant conditions to 4.5 years and 9.0 years when exposed to cyclical wet-and-dry and freeze-and-thaw conditions respectively. The net nitrogen mineralisation was generally reduced under cyclical conditions, with the exception of the Nothofagus species. A mean mass loss of 43 ± 8% and nitrogen loss of 42 ±12% was measured from seeds incubated in situ. The effect of soil microbes on decomposition was investigated by incubating seeds under standardised temperature and moisture conditions in 3 different soils. The rate of decomposition generally declined with an increase in soil acidity. The results of this study illustrate that the seed component of plant litter comprises a high quality substrate for microorganisms. The nutrients of seeds are generally readily mobilised and available for utilisation by other components of the ecosystem.