Soil characteristics and methanogenesis were investigated in Fluvisols of the Balandra lagoon mangrove in Baja California, Mexico in March/April 1995. The grain size distribution was dominated by the silt fraction (54- 92%), sand and clay contents were 6-44% and 0-19%, respectively. Bulk density was 0.6-1.5 g cm-3, the water content 290-690 ml l-1and the air content 38-200 ml l-1. The soils always showed saline conditions (30-70 g kg-1), mostly negative redox potentials (down to -202 mV), P contents from 0.8 to 16.0 g kg-1, C(org) contents from < 2 to 140 g kg-1and neutral pH values. In one soil the pH dropped < 2,5 after peroxide treatment indicating, together with low carbonate/pyrite ratios (mostly < 3) sulfidic properties. The relative distribution of the porewater cations was always 74% Na, 17% Mg, 6% Ca, and 3% K. For the exchangeable and water soluble cation fraction it was 28-59% Na, 22-34% Mg, 12-32% Ca, and 2-5% K, with increasing Na- and decreasing Ca-concentrations in the transect from sea to land. The methane concentrations were mostly lower than 2 μM. Methane production rates were between 1 and 23 nmol ml-1d-1. Methane emission was not detectable. This indicates, that methanogenesis is balanced by methane oxidation processes within the soil. Higher methane concentrations (33 μM) and production rates (100 nmol ml-1d-1) in one of the investigated soils were attributed to anthropogenic impact. Consequently, in case of human manipulation the potential of mangrove soils to emit methane seems to be high, thus mangroves are sensitive with respect to methanogenesis.
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