Changes in pedogenic thresholds and soil process domains in long-term soil development

Background/Question/Methods

Soil properties often display discrete changes at particular points on continuous rainfall gradients (“pedogenic thresholds”); these thresholds bound larger regions in which soil properties change relatively little across substantial variation in rainfall (“soil process domains”). We asked how the position of these thresholds and characteristics of their associated domains changed as a function of increasing substrate age on basaltic soils across the Hawaiian Islands. We focused on two rainfall gradients: one spanning 260 – 3540 mm/yr on 150,000 yr old substrate on the island of Hawaii, the other spanning 600 – 3760 mm/yr on 4,100,000 yr old substrate on the island of Kauai. Multiple surface soil samples (to 30 cm depth) and a smaller number of complete profiles were collected on both substrates – 160 surface samples and 13 complete profiles on Hawaii, 28 surface samples and 6 profiles on Kauai. Soils were analyzed for properties associated with weathering and pedogenesis as well as those associated with soil fertility and nutrient availability.

Results/Conclusions

Two pedogenic thresholds were identified on each of the rainfall gradients, delineating three domains. Below ~750 mm/yr precipitation, soils on the younger gradient were highly variable, but most properties associated with soil fertility (base saturation, resin-extractable phosphorus) had intermediate values. From 750-2100 mm/yr, nutrient elements were substantially enriched in surface soils (for example, from resin P concentrations were 100-300 mg/kg). Biological uplift of nutrients from deep soils creates this zone of enrichment, which Polynesian cultivators used to develop highly intensive rain-fed agricultural systems. Above 2100 mm/yr, soils were iron enriched and depleted in biological nutrients. On the older Kauai substrate, soils below 900 mm/yr fell into a zone of enrichment, while soils from 900 – 2500 mm/yr were iron-enriched and depleted in biological nutrients. Above 2500 mm/yr on Kauai, iron reduction and mobility became increasingly important as soils became more reduced. Both gradients included extensive iron-enriched, low-nutrient soils – but these occurred in much lower-rainfall conditions on the older Kauai soils. Also, low-nutrient soils on Kauai were more highly weathered (higher iron, lower biological nutrients, halloysite/sesquioxide as opposed to allophane/ferrihydrite clay mineralogy) than comparable soils on the younger Hawaii rainfall gradient.