NUTRIENT AVAILABILITY IN A MONTANE WET TROPICAL FOREST - SPATIAL PATTERNS AND METHODOLOGICAL CONSIDERATIONS

Soils and forest floor were sampled quantitatively from a montane wet tropical forest in Puerto Rico to determine the spatial variability of soil nutrients, the factors controlling nutrient availability to vegetation, and the distribution of nutrients in soil and plants. Exchangeable cation concentrations were measured using different soil extracting procedures (fresh soil with NH4Cl, air-dried and ground soil with KCl, and a Modified Olsen solution) to establish a range of nutrient availability in the soil, and to determine the relationship between different, but commonly used laboratory protocols. The availability of exchangeable Ca, Mg, and K was significantly lower in soils extracted fresh with NH4Cl than from soils which were dried and ground prior to extraction with KCl or a modified Olsen solution. Soil nutrient availability generally decreased with depth in the soil. Several soil properties important to plant growth and survival varied predictably across the landscape and could be viewed in the context of a simple catena model. In the surface soils, exchangeable base cation concentrations and pH increased along a gradient from ridge tops to riparian valleys, while soil organic matter, exchangeable Fe and acidity decreased along this gradient. On the ridges, N, P, and K were positively correlated with soil organic matter; on dopes, N and P were positively correlated with organic matter, and Ca, Kg, and pH were negatively correlated with exchangeable Fe. Nutrient availability in the upper catena appears to be primarily controlled by biotic processes, particularly the accumulation of organic matter. The Ca, K, and P content of the vegetation was higher on ridges and slopes than in the valley positions. Periodic flooding and impeded drainage in the lower catena resulted in a more heterogeneous environment. A comparison of the Bisley, Puerto Rico soils with other tropical montane forests (TMF) revealed that the internal heterogeneity of soils in the Bisley Watersheds is similar to the range of average soil nutrient concentrations among TMF's for Ca, Mg, and K (dry/ground soils). Phosphorus tended to be slightly higher in Bisley and N was lower than in other TMFs.