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Soil Biota, Nutrients, and Organic Matter Dynamics Under Decomposing Wood

Zalamea-Bustillo, M. 2005. M.Sc. Thesis. Dept. Biology, University of Puerto Rico, Rio Piedras Campus, San Juan, Puerto Rico

Abstract

Decaying wood is an important structural and functional component of forests. Structurally, decaying wood contribute to generate a high diversity of habitats for many species of micro-organisms, plants and animals. Functionally, decaying wood is related to nutrient cycling through its role as either sink or source of nutrients. However, what is the effect of decaying logs on the physical, chemical and biotic characteristics of the soil underneath? Furthermore, is there any long-term effect or imprint of decaying wood on the underlying soil? Are the wood quality and the decay stage important in determining the effect of wood on soil? To address these questions I took pair-wise samples of the soil under and 50 cm away from decaying logs, and measured: soil temperature, moisture, total and available nutrients, pH, soil organic matter (SOM) fractions (humic substances and dissolved soil organic matter), as well as microbial biomass, and root length. The study was performed in a subtropical wet forest in north-eastern Puerto Rico where we selected 20 decaying logs belonging to two species (Dacryodes excelsaVahl and Swietenia macrophylla King) and two stages of decomposition (7 and 15 years after fallen). Samples were taken during a dry and a wet season. I found that decaying wood affected physical, chemical and biotic properties of the underlying soil both in the short- and the long-term. Soil temperature was less variable under decaying logs, with lower maximum and higher minimum values as compared to soil temperature away of the decaying logs. Soil under decaying wood had fewer roots, and lower NO3 - , and Mg 2 availability than soil samples collected 50 cm away from the logs. The soil near the more decayed logs was more acid than the soil influenced by less decayed logs. Tree species (as an indicator of wood quality) and decay stage were important factors defining the effect of decaying wood on the distribution of available nutrients. Divalent cations (Ca, Mg, Cu, Zn, and Pb) were higher in the soil associated with the 7 yr-old logs, while trivalent cations (Al, and Fe) were higher in the soil associated with the 15 yr-old logs. This pattern of cation distribution was correlated with the distribution of different fractions of soil organic matter. In summary, decaying logs influenced the properties of the underlying soil. Humic substances (HS) and dissolved organic carbon (DOC) accumulated under and near decaying logs as decomposition advanced. The effect of decaying wood on soil was dependent on the stage of decay and decaying logs had a long-lasting imprint on soil. Such effects were related to the quality of the decaying wood, as different amounts of HS accumulated in the soil associated with decaying logs of two different tree species. Moreover, the effect of decaying wood on SOM fractions seemed to influenced the distribution of nutrients around decaying logs (e.g., divalent vs. trivalent cations, and NO3- ). It is concluded that decaying wood, through its effect on soil organic matter and nutrient dynamics, can contribute to the spatial heterogeneity of soil properties, and participate in processes of soil formation and nutrient cycling.