Chronic low level hydrocarbon amendments stimulate plant growth and microbial activity in salt-marsh microcosms.
Salt-marsh microcosms containing the halophyte Spartina alterniflora were maintained in a greenhouse for 2 years and given daily dosages of a hydrocarbon mixture containing hexane, heptane, octane, nonane, benzene, toluene, ethylbenzene and xylene. This mixture was added at 3.33 (1 × treatment) and 33.3 g C/m2 (10 × treatment) for 10 months. Above- and below-ground plant biomass and total respiration rates were significantly increased by the 1 × hydrocarbon additions, and the differences persisted for at least 7 months after the hydrocarbon additions ended. Growth, total sediment organic matter and macro-organic matter were significantly reduced in the 10 × treatment. Soil microbial activities were monitored using non-invasive gas-exchange techniques. CO2 production, methanogenesis, N2 fixation and denitrification were stimulated by the 1 × hydrocarbon treatment but inhibited by the 10 × treatment. The stimulation of microbial activity by the 1 × treatment suggests that the microbes were carbon-limited, while the enhanced plant growth seen at 1 × may have resulted from increased N2 fixation and/or nitrogen mineralization.