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We studied the influence of tree species on soil carbon and nitrogen (N) dynamics in a common garden of replicated monocultures of fourteen angiosperm and gymnosperm, broadleaf and needleleaf species in southwestern Poland. We hypothesized that species would influence soil organic matter (SOM) decomposition primarily via effects on biogeochemical recalcitrance, with species having tissues with high lignin concentrations retarding rates of decomposition in the O and A horizons. Additionally, because prior work demonstrated substantial divergence in foliar and soil base cation concentrations and soil pH among species, we hypothesized that species would influence chemical stabilization of SOM via cation bridging to mineral surfaces in the A-horizon. Our hypotheses were only partially supported: SOM decomposition and microbial biomass were unrelated to plant tissue lignin concentrations, but in the mineral horizon, were significantly negatively related to the percentage of the cation exchange complex (CEC) occupied by polyvalent acidic (hydrolyzing) cations (Al and Fe), likely because these cations stabilize SOM via cation bridging and flocculation and/or because of inhibitory effects of Al or low pH on decomposers. Percent CEC occupied by exchangeable Al and Fe was in turn related to both soil clay content (a parent material characteristic) and root Ca concentrations (a species characteristic). In contrast, species influenced soil N dynamics largely via variation in tissue N concentration. In both laboratory and in situ assays, species having high-N roots exhibited faster rates of net N mineralization and nitrification. Nitrification:mineralization ratios were greater, though, under species with high exchangeable soil Ca 2+ . Our results indicate that tree species contribute to variation in SOM dynamics, even in the mineral soil horizons. To our knowledge the influence of tree species on SOM decomposition via cation biogeochemistry has not been demonstrated previously, but could be important in other poorly buffered systems dominated by tree species that differ in cation nutrition or that are influenced by acidic deposition.

Primary productivity on old, weathered soils often is assumed to be limited by phosphorus (P), especially in the lowland tropics where climatic conditions promote the rapid depletion of rock-derived nutrients. This assumption is based on a static view of soils weathering in place with no renewal of the bedrock source. In reality, advection of material through the soil column introduces a spatially variable supply of rock-derived nutrients. This flux is dependent on the residence time of soil, which can range from a few hundred years in rapidly uplifting collisional mountain belts to tens of millions of years in tectonically quiescent tropical cratons. We modeled the effects of tectonic uplift, erosion, and soil depth on the advection of P through the soil column and P availability, calibrating rate of change in biologically available P over time with data from two basaltic chronosequences in Hawai’i and a series of greywacke terraces in New Zealand. Combining our model with the global distribution of tectonic uplift rates and soil depths, we identified tectonic settings that are likely to support P-depleted ecosystems—assuming that tectonic uplift and erosion are balanced (that is, landscape development has reached steady state). The model captures the occurrence of transient P limitation in rapidly uplifting young ecosystems where mineral weathering is outpaced by physical erosion—a likely occurrence where biological N fixation is important. However, we calculate that P depletion is unlikely in areas of moderate uplift, such as most of Central America and Southeast Asia, due to the continuous advection of P into the rooting zone. Finally, where soil advection is slow, such as the Amazon Basin, we expect widespread P depletion in the absence of exogenous nutrient inputs.

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 Nino 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 Nino 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.

Studies of long-term soil and ecosystem development on static geomorphic surfaces show that old soils become depleted in most rock-derived nutrients. As they are depleted, however, static surfaces also are dissected by fluvial erosion. This fluvial erosion leads to colluvial soil transport on the resulting slopes, which in turn can rejuvenate the supply of weathering-derived nutrients to plants. We evaluated the influence of erosion and consequent landscape evolution on nutrient availability along a slope on the Island of Kaua’i, near the oldest, most nutrient-depleted site on a substrate age gradient across the Hawaiian Islands. Noncrystalline minerals characteristic of younger Hawaiian soils increased from 3% of the soil on the static constructional surface at the top of the slope to 13% on the lower slope, and the fraction of soil phosphorus (P) that was occluded (and hence unavailable) decreased from 80% to 56% at midslope. Foliar nitrogen and P concentrations in Metrosideros polymorpha increased from 0.82% and 0.062% to 1.13% and 0.083% on the constructional surface and lower slope, respectively. The increase in foliar P over a horizontal difference of less than 250 m represents nearly half of the total variation in foliar P observed over 4.1 million years of soil and ecosystem development in Hawai’i. The fraction of foliar strontium (Sr) derived from weathering of Hawaiian basalt was determined using 87 Sr: 86 Sr; it increased from less than 6% on the constructional surface to 13% and 31% on lower slope and alluvial positions. Erosional processes increase both nutrient supply on this slope and the fine-scale biogeochemical diversity of this old tropical landscape; it could contribute to the relatively high level of species diversity observed on Kaua’i.

We evaluated changes in temperature and precipitation associated with climate change, subsidence, and erosion on a chronosequence of sites across Hawaii. The sites range in age from 0.3 to 4100 ky, and the current temperature and precipitation are similar at all sites. Interpretations of fossil pollen records suggest that cooler, dryer conditions prevailed in windward Hawaii during the last glacial period. If the previous glacial periods were similar, the 20-, 150-, and 1400-ky-old sites would have spent 60% or more of their development under relatively cool and dry conditions, whereas the 0.3- and 2.1-ky-old sites have experienced only the warmer, wetter climate of the present interglacial. Subsidence and erosion have also affected the temperature and precipitation of these sites over time; in the past, some of them have been in the dry air above the trade wind inversion or in the lee of larger mountains. Combining these components of change, we estimate that the average temperature over the history of Pleistocene-aged sites (20, 150, and 1400 ky) was up to 2.2°C cooler and that the average precipitation was only about 50% of current values. Under current conditions, it would take only 230 ky for as much water to leach through the 1400-ky-old site as we calculate has leached over 1400 ky. Incorporating more reasonable assumptions about environmental history has the potential to allow more powerful interpretations of chronosequence data and thereby improve the predictive potential of models of soil and ecosystem development.