Changes in soil properties and earthworm populations induced by long-term organic fertilization of a sandy soil in the Venezuelan Amazonia

摘要

In the forest-savanna ecotone around Puerto Ayacucho, in Amazonas State, Venezuela, some producers have established small (about 5 ha) agroforestry production systems (AFS) on sandy savanna soils by adding animal manure as the main fertilizer input. As a result of using organic fertilization for more than 25 years, the physical and chemical properties of the original savanna soil have changed, and the biological activity has increased. Organic amendments seem to be an alternative for sustaining agriculture in this area dominated by soils with poor structure because fertilizer and other agricultural items are expensive and need to be imported from distant regions. This paper examines the effect of long-term organic fertilization on soil properties and earthworm abundance in a natural savanna (NS) and in an AFS established for more than 25 years. Organic fertilization increased several soil fertility parameters (N, P, K, Ca, and Mg). Improved fertility status and the more conducive microclimatic conditions for soil fauna have probably allowed the establishment and dominance of earthworms as well as other components of the pedofauna population. Enhanced earthworm abundance could increase enzymatic activities as well as microbial biomass. In general, after fertilization, all Pi and Po fractions increased significantly. The increase was greatest in the P-resin and HCl-P for the inorganic P, whereas for the organic P fraction, the changes were highest for the Po-NaOH sonicated and nonsonicated fractions.