A chronosequence of climosequences: The evolution of ecological soil thresholds in Hawaiian volcanic soils

Abstract

The soil-forming factors of climate and time directly influence mineral weathering, nutrient flow, and soil pedogenesis. Soil properties do not respond linearly to climate and time, but rather, follow a series of pedogenic thresholds that delimit soil process domains defined by dominant chemical buffer systems. These pedogenic thresholds relate to soil fertility and ecological gradients, although they are not equivalent due to differences in the dynamics between surface soils and the entire solum. Here, we identified thresholds in surface soils derived from Hawaiian substrates across gradients of water balance and soil substrate geologic age while controlling for other soil-forming factors (i.e., topography, biota, and parent material). We conducted extensive field sampling (n = 577) and analysis of soil properties, chiefly elemental analysis, pH, and exchangeable nutrients. With the use of previously published soil data (n = 285), we created a chronosequence (5, 15, 20, 120, 150, 450, 1,400, 4,100 ky) of eight climosequences. We statistically determined thresholds across each climosequence using four types of non-linear breakpoint analyses. We quantified thresholds delineating the exhaustion of primary minerals on all eight substrates and thresholds indicating the mobilization of iron on the three oldest soil substrates. Across the chronosequence, thresholds in surface soils shifted to drier climates (i.e., lower water balance values) as soil substrate age increased, following a highly predictable (R² = 0.94) logarithmic decay function, with rapid evolution occurring in young soils and deceleration of change occurring in older soils. The youngest substrate (5 ky) did not follow this logarithmic pattern, which we hypothesize (based on previous studies) is due to surface area reactivity limitations. The pedogenesis of Hawaiian soils and the evolution of their ecological soil thresholds offer a well-behaved model system for broader implications regarding soils forming in more complex, continental settings.