CHARACTERISTICS AND METHANOGENESIS OF THE BALANDRA LAGOON MANGROVE SOILS, BAJA-CALIFORNIA-SUR, MEXICO

Soil characteristics and methanogenesis were investigated in Fluvisols of the Balandra lagoon mangrove in Baja California, Mexico in March/A pril 1995, The grain size distribution was dominated by the silt fract ion (54-92%), sand and clay contents were 6-44% and 0-19%, respectivel y, Bulk density was 0.6-1.5 g cm(-3), the water content 290-690 mi l(- 1) and the air content 38-200 ml l(-1). The soils always showed saline conditions (30-70 g kg(-1)), mostly negative redox potentials (down t o - 202 mV), P contents from 0.8 to 16.0 g kg(-1), C-org contents from < 2 to 140 g kg(-1) and neutral pH values. In one soil the pH dropped < 2,5 after peroxide treatment indicating, together with low carbonat e/pyrite ratios (mostly < 3) sulfidic properties, The relative distrib ution 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 decr easing Ca-concentrations in the transect from sea to land. The methane concentrations were mostly lower than 2 mu M Methane production rates were between 1 and 23 nmol ml(-1) d(-1). Methane emission was not det ectable. This indicates, that methanogenesis is balanced by methane ox idation processes within the soil. Higher methane concentrations (33 m u M) and production rates (100 nmol ml(-1) d(-1)) in one of the invest igated soils were attributed to anthropogenic impact, Consequently, in case of human manipulation the potential of mangrove soils to emit me thane seems to be high, thus mangroves are sensitive with respect to m ethanogenesis.