PHOSPHORUS DYNAMICS IN A LOWLAND AFRICAN RAINFOREST: THE INFLUENCE OF ECTOMYCORRHIZAL TREES

In the southern part of Korup National Park, Cameroon, nine replicate plots (40 × 80 m) in forest with low abundances (5–15% of the basal area of all trees ≥30 cm gbh [girth at breast height]; LEM) and nine with high abundances (45–68%; HEM) of ectomycorrhizal caesalpiniaceous legumes were selected from an earlier enumeration along an east–west 5-km transect. The codominant caesalps in HEM plots were Microberlinia bisulcata, Tetraberlinia bifoliolata, and T. moreliana, and ordination showed the LEM and HEM plots to be floristically distinct. The HEM plots lay in a large well-defined patch of these caesalps, set within surrounding LEM forest. A further contrast to HEM forest was achieved by selecting six LEM and six VLEM plots (very low, ≈0% ectomycorrhizal trees) on a second transect 12 km to the north. Korup has an annual rainfall of 5180 mm with one very distinct 3-mo dry season (December–February). The soils are sandy, acidic, and very phosphorus poor. The aim was to determine whether LEM and HEM plots differed in their litter and soil phosphorus status and the characteristics of their phosphorus cycles. On 13 occasions between August 1988 and September 1990, litter and soils in the surface root layer and the mineral layer 5 cm below were sampled on the main transect and analyzed for phosphorus fractions. This analysis distinguished between inorganic and organic forms and provided various measures of lability. Nitrogen and carbon contents, pH, and moisture contents of each layer, depth of the root layer, and amounts of litter (i.e., litter mass) and soil were the other main variables. In a subset of plots, leaf litterfall was collected over the period and analyzed for phosphorus and nitrogen, and two litter-bag experiments in early wet seasons estimated rates of litter disappearance. Root biomass and change with depth were estimated from pit samples within the plots. HEM plots had slightly higher mean basal area of trees than LEM plots (32.3 and 27.1 m2/ha, respectively) and higher fine root (≤5 mm diameter) biomasses (519 and 364 g/m2, 0–5 cm), but the same mean litter mass (155 g/m2) and annual leaf litterfall (720 g/m2) and very similar disappearance rates (1.82 g·m−2·d−1 on litterfall/mass basis; t1/2 of 53 d from litter bags). Litter mass (and litterfall) reached a clear peak in the mid-dry season in both HEM and LEM plots (slightly earlier in HEM), almost completely disappearing by mid-wet season. The phosphorus concentrations in HEM falling leaf litter was much greater than in LEM (801 and 676 μg/g) leading to greater inputs (1.79 and 1.35 mg·m−2·d−1), while for nitrogen the concentrations and inputs were very similar (means 17.8 mg/g, 35.3 mg·m−2·d−1). An important feature was the marked peak in phosphorus (but not nitrogen) concentration in litter in the HEM plots only in those years that did not follow a mast fruiting. Depth of the root layer was greater in the HEM than LEM plots (8.1 and 4.2 mm), as was carbon content (5.5 and 4.0%), but bulk density was less (0.85 and 1.30 g/cm3). Moisture contents, which tracked the seasons in all three layers, were slightly higher in HEM than LEM, as were clay and silt contents, but pH values were lower (4.19 and 4.59, root layer). The relative differences in the eight soil phosphorus fractions (resin-, bicarbonate- and NaOH-extractable, and chloroform-labile, inorganic, and organic) plus residual and total phosphorus were remarkably consistent between HEM and LEM plots. In the root layer total phosphorus was much higher in HEM than LEM plots (309 and 186 μg/g, respectively) and likewise in the mineral layer (192 and 119 μg/g, respectively): HEM/LEM ratios of ≈1.6 were maintained across nearly all fractions. Covariance analysis indicated that the basal areas of LEM and HEM plots could not account for the differences in phosphorus concentrations. Different fractions followed different trends with time, but these did not match seasons and showed, for the more labile inorganic phosphorus fractions especially, a linear decline over the study period. Interactions between forest type and date (split-plot repeated-measures analysis of variance) were rarely significant even though date itself invariably was. In marked contrast, nitrogen fractions did not decline, and labile organic nitrogen showed clear seasonal peaks. Differences between the LEM forest on the two transects (two occasions in common) were small and inconsistent, supporting the wider contrast with HEM plots. Principal components analysis of the phosphorus and nitrogen fractions showed a strong discrimination between LEM and HEM plots. Calculation with amounts of phosphorus in the top 6 cm of the soil showed very similar patterns to the concentrations. Large ectomycorrhizal trees appeared to have increased the depth of the surface root layer, its phosphorus content, the labile phosphorus fraction (notably its organic component), and to have enhanced phosphorus cycling. The role of ectomycorrhizas in this process is discussed. The decline in labile phosphorus is explained by a phenological and climatic ectomycorrhizal response (PACER) hypothesis that highlights the relationship between phosphorus demand in mast fruiting years and soil phosphorus concentrations. The importance of this in adaptations by ectomycorrhizal caesalps to this strongly seasonal and phosphorus-poor site is considered.