Variability of indices of macronutrient availability in soils at different spatial scales along an elevation transect in tropical moist forests (NE Ecuador)

The availability of key plant nutrients may change with elevation in tropical mountains due to altitudinal gradients in temperature and moisture which affect pedogenesis and nutrient cycling. In a transect from upper lowland to montane forests in NE Ecuador, we tested the hypotheses that (1) the availability of P is low in low-elevation forests but increases upslope, while the availability of N is relatively high at low elevations but decreases with elevation, and (2) increasing amounts of calcium, magnesium and potassium are stored on top of the soil with progressive humus accumulation toward higher elevations, likely to improve nutrient availability. In each 20 plots in undisturbed natural forest at 500, 1000, 1500 and 2000 m a.s.l., we measured in situ N net mineralization and nitrification rate (N-NM and N-NI, buried bag method), plant-available phosphorus (P-a, resin-bag method), and salt-exchangeable calcium, potassium and magnesium concentrations (Ca-ex, K-ex, Mg-ex) in the organic and mineral topsoils. N-NM and N-NI, and the Ca-ex, K-ex and Mg-ex concentrations were much more variable at the plot level than across the four elevations, while P-a varied equally at small and large spatial scales. P-a increased more than 10fold from 500 to 2000 m. The net release of nitrate dominated over ammonium at all elevations. While mass-related N-NM and N-NI rates and also organic matter C/N ratio in the topsoil remained invariant along the slope, N-NM and N-NI rates per ground area decreased by about 40% from 500 to 2000 m. Thus, the N-NM/P-a ratio decreased markedly with elevation proving our first hypothesis. In support of the second hypothesis, the pools of Ca-ex, Mg-ex and K-ex in the organic layers increased with elevation, demonstrating the key role that organic topsoil horizons are playing for forest nutrition at high elevations. We suggest that the large difference in N versus P availability of tropical (upper) lowland and montane forests is likely to be a key factor influencing the species composition and productivity along tropical mountain slopes.