Microbial properties and soil respiration in submontane forests of Venezuelian Guyana: characteristics and response to fertilizer treatments

The distribution of vegetation types in Venezuelan Guyana (in the ‘Canaima’ National Park) represents a transitional stage in a long term process of savannization, a process considered to be conditioned by a combined chemical and intermittent drought stress. All types of woody vegetation in this environment accumulate large amounts of litter and soil organic carbon (SOC). We hypothesized that this accumulation is caused by low microbial activity. During 1 year we measured microbial biomass carbon (Cmic), microbial respiration and soil respiration of stony Oxisols (Acrohumox) at a tall, a medium and a low forest and with three chemical modifications of site conditions by the addition of NO3−, Ca2+ and PO43− as possible limiting elements. Due to high SOC contents, mean Cmic was 1 mg g soil−1 in the mineral topsoil and 3 mg g soil−1 in the forest floor. Mean microbial respiration in the mineral topsoil and the forest floor were 165 and 192 μg CO2-C g soil−1 d−1, respectively. We calculated high mean metabolic quotients (qCO2) of 200 mg CO2-C g Cmic−1 d−1 in the litter layer and 166 mg CO2-C g Cmic−1 d−1 in the mineral topsoil, while the Cmic-to-SOC ratios were as low as 1.0% in the litter layer and 0.8% in the mineral topsoil. Annual soil respiration was 9, 12 and 10 Mg CO2-C ha−1 yr−1 in the tall, medium and low forest, respectively. CO2 production was significantly increased by CaHPO4 fertilization, but no consistent effects were caused by Ca2+ and NO3−, fertilization. Our findings indicate that Cmic and microbial respiration are reduced by low nutrient concentrations and low litter and SOC quality. Reduced microbial decomposition may have contributed to SOC accumulation in these forests.