Adriana Campos, Rosa M. Hernández, Ismael Hernández-Valencia:
Effects of Pinus caribaea L. Plantation on Fractions of Soil Organic Matter in Savannah Soil from Venezuela

ADRIANA CAMPOS¹, ROSA M. HERNÁNDEZ², ISMAEL HERNÁNDEZ-VALENCIA^1
1University of Kassel, Department of Soil Biology and Plant Nutrition, Germany
²Universidad Simón Rodriguez, Venezuela

Trachipogon sp. dominated savannahs cover a high percentage of territory in Venezuela. This kind of savannah developed on well drained soils characterised by poor nutrient status, low pH (pH<5), and low content of soil organic matter (SOM). The savannahs are used as pasture land. In the Uverito zone, Monagas state, the National Government by the Corporacion Venezolana de Guayana (C.V.G) (Venezuelan Corporation of Guayana) and the Compañia Nacional de Reforestacion (CONARE) (National Company of Reforestation), decided in 1969, to reafforest the savannah with pine trees.

Earlier studies of the plantation have shown that the forest stand has induced important ecological changes in the microclimate and soil fauna. However, it was unknown how SOM has changed. SOM is the main important pool for the turnover of nitrogen and carbon in the soil. The objective of the present study was to evaluate the impact of the pine forest stand on the quality of SOM. The study compared SOM in a savannah ecosystem and a pine forest ecosystem with 24 years old trees at two depths ([0-5]cm and [5-10]cm) during the rainy season (August). Microbial biomass C and N, soil organic carbon and total nitrogen as well as the carbon and nitrogen in the ``Light Fraction'' and the ``Heavy Fraction'' of SOM have been investigated.

Microbial C and N are not affected by the plantation. Soil organic carbon and carbon and nitrogen in the ``Light Fraction'' increased at the forest site. However, total nitrogen and nitrogen of the ``Heavy Fraction'' decreased significantly. The nitrogen in the ``Light fraction'' was about [15.5]% of total nitrogen and [71.6]% in the ``Heavy fraction'', respectively. In this context it is discussed if plant N demands and lower mineralization rates in the pinus stand might be reasons for decreased total and mineral nitrogen content in the soil.

Keywords: Fractions of soil, microbial biomass C and N, organic matter

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Contact Address: Markus Raubuch, University of Kassel, Department of Soil Biology and Plant Nutrition, Nordbahnhofstraße1a, 37213 Witzenhausen, Germany, e-mail: raubuchwiz.uni-kassel.de