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Pedogenic thresholds describe where soil properties or processes change in an abrupt/nonlinear fashion in response to small changes in environmental forcing. Contrastingly, soil process domains refer to the space between thresholds where soil properties are either unchanged, or change gradually, across a broad range of environmental forcing. Here, we test quantitatively for the presence of thresholds in patterns of soil properties across a climatic gradient on soils developed from ~20 ky old basaltic substrate on the Island of Hawai'i. From multiple soil properties, we quantitatively identified a threshold at ~750 mm/y of water balance (precipitation minus potential evapotranspiration), delineating the upper water balance boundary of soil fertility in these soils. From the threshold in the ratio of exchangeable Ca to total Ca we identified the lower water balance boundary of soil fertility in these soils at -1000 mm/y, however this threshold was qualitatively described as it lies near the limit of the climate gradient data where the statistical approach can not be applied. These two results represent the first time that pedogenic thresholds have been identified using statistically rigorous methods and the limitations of said methods, respectively. Comparing the 20 ky soils to soils that developed on basaltic substrates of 1.2 ky, 7.5 ky, 150 ky, and 4100 ky in a time-climate matrix, we found that our quantitative analysis supports previous qualitatively identified thresholds in the soils developed from older substrates. We also identified the 20 ky as the transition from kinetic to supply limitation for plant nutrients in soil in this system.

Pedogenic thresholds occur where soil properties change abruptly and/or nonlinearly with a small increment in environmental forcing; soil process domains are the regions between thresholds where soils change much more gradually across a large range of environmental forcing. We evaluated thresholds and domains in basalt-derived soils on two rainfall gradients in Hawaii-one from 260 to 3,540 mm/y precipitation on 150,000-year-old substrate, the other from 600 to 3,760 mm/y on 4,100,000-year-old substrate. We identified thresholds associated with the initiation of biological uplift of nutrients at about 700 mm/y on the younger substrate, the depletion of primary minerals at about 2,100 mm/y on the younger and about 900 mm/y on the older substrate, and the initiation of anoxic conditions and associated Fe mobility at about 2,500 mm/y on the older substrate. These thresholds delineated process domains characterized by pedogenic carbonate accumulation and wind erosion (dry young substrate); by weathering and biological uplift of nutrients (intermediate rainfall young substrate and dry old substrate); by surface Fe enrichment and nutrient depletion (wet young substrate and intermediate rainfall old substrate); and by Fe mobilization and loss (wet old substrate). Soils on the older substrate were more highly weathered, lower in total and available P, and characterized by more crystalline clays than otherwise comparable soils on the younger substrate. Prior to European contact, Hawaiian cultivators developed an intensive rainfed agricultural system in the weathering/biological uplift domain on the younger substrate; we suggest that only this domain could support indigenous agricultural intensification in upland soils. © 2013 The Author(s).

Irrigated pondfields and rainfed field systems represented alternative pathways of agricultural intensification that were unevenly distributed across the Hawaiian Archipelago prior to European contact, with pondfields on wetter soils and older islands and rainfed systems on fertile, moderate-rainfall upland sites on younger islands. The spatial separation of these systems is thought to have contributed to the dynamics of social and political organization in pre-contact Hawai’i. However, deep stream valleys on older Hawaiian Islands often retain the remains of rainfed dryland agriculture on their lower slopes. We evaluated why rainfed agriculture developed on valley slopes on older but not younger islands by comparing soils of Pololū Valley on the young island of Hawai’i with those of Hālawa Valley on the older island of Moloka’i. Alluvial valley-bottom and colluvial slope soils of both valleys are enriched 4–5-fold in base saturation and in P that can be weathered, and greater than 10-fold in resin-extractable P and weatherable Ca, compared to soils of their surrounding uplands. However, due to an interaction of volcanically driven subsidence of the young island of Hawai’i with post-glacial sea level rise, the side walls of Pololū Valley plunge directly into a flat valley floor, whereas the alluvial floor of Hālawa Valley is surrounded by a band of fertile colluvial soils where rainfed agricultural features were concentrated. Only 5% of Pololū Valley supports colluvial soils with slopes between 5° and 12° (suitable for rainfed agriculture), whereas 16% of Hālawa Valley does so. The potential for integrated pondfield/rainfed valley systems of the older Hawaiian Islands increased their advantage in productivity and sustainability over the predominantly rainfed systems of the younger islands.

Soils play a key role in the global cycling of carbon (C), storing organic C, and releasing CO2 to the atmosphere. Although a large number of studies have focused on the CO2 flux at the soil-air interface, relatively few studies have examined the rates of CO2 production in individual layers of a soil profile. Deeper soil horizons often have high concentrations of CO2 in the soil air, but the sources of this CO2 and the spatiotemporal dynamics of CO2 production throughout the soil profile are poorly understood. We studied CO2 dynamics in six soil profiles arrayed across a grassland hillslope in coastal southern California. Gas probes were installed in each profile and gas samples were collected weekly or biweekly over a three-year period. Using soil air CO2 concentration data and a model based on Fick's law of diffusion, we modeled the rates of CO2 production with soil profile depth. The CO2 diffusion constants were checked for accuracy using measured soil air 222Rn activities. The modeled net CO2 production rates were compared with CO2 fluxes measured at the soil surface. In general, the modeled and measured net CO2 fluxes were very similar although the model consistently underestimated CO2 production rates in the surficial soil horizons when the soils were moist. Profile CO2 production rates were strongly affected by the inter- and intra-annual variability in rainfall; rates were generally 2-10 times higher in the wet season (December to May) than in the dry season (June to November). The El Niño event of 1997-1998, which brought above-average levels of rainfall to the study site, significantly increased CO2 production in both the surface and subsurface soil horizons. Whole profile CO2 production rates were approximately three times higher during the El Niño year than in the following years of near-average rainfall. During the dry season, when the net rates of CO2 flux from the soil profiles are relatively low (4-11 mg C- CO2 m-2 h-1), 20%-50% of the CO2 diffusing out of the profiles appears to originate in the relatively moist soil subsurface (defined here as those horizons below 40 cm in depth). The natural abundance 14C signatures of the CO2 and soil organic C suggest that the subsurface CO2 is derived from the microbial mineralization of recent organic C, possibly dissolved organic C transported to the subsurface horizons during the wet season. © 2005 Springer Science+Business Media, Inc.

Soils play a key role in the global cycling of carbon (C), storing organic C, and releasing CO2 to the atmosphere. Although a large number of studies have focused on the CO2 flux at the soil–air interface, relatively few studies have examined the rates of CO2 production in individual layers of a soil profile. Deeper soil horizons often have high concentrations of CO2 in the soil air, but the sources of this CO2 and the spatiotemporal dynamics of CO2 production throughout the soil profile are poorly understood. We studied CO2 dynamics in six soil profiles arrayed across a grassland hillslope in coastal southern California. Gas probes were installed in each profile and gas samples were collected weekly or biweekly over a three-year period. Using soil air CO2 concentration data and a model based on Fick’s law of diffusion, we modeled the rates of CO2 production with soil profile depth. The CO2 diffusion constants were checked for accuracy using measured soil air 222Rn activities. The modeled net CO2 production rates were compared with CO2 fluxes measured at the soil surface. In general, the modeled and measured net CO2 fluxes were very similar although the model consistently underestimated CO2 production rates in the surficial soil horizons when the soils were moist. Profile CO2 production rates were strongly affected by the inter- and intra-annual variability in rainfall; rates were generally 2–10 times higher in the wet season (December to May) than in the dry season (June to November). The El Niño event of 1997–1998, which brought above-average levels of rainfall to the study site, significantly increased CO2 production in both the surface and subsurface soil horizons. Whole profile CO2 production rates were approximately three times higher during the El Niño year than in the following years of near-average rainfall. During the dry season, when the net rates of CO2 flux from the soil profiles are relatively low (4–11 mg C– CO2 m−2 h−1), 20%–50% of the CO2 diffusing out of the profiles appears to originate in the relatively moist soil subsurface (defined here as those horizons below 40 cm in depth). The natural abundance 14C signatures of the CO2 and soil organic C suggest that the subsurface CO2 is derived from the microbial mineralization of recent organic C, possibly dissolved organic C transported to the subsurface horizons during the wet season. [ABSTRACT FROM AUTHOR]