Showing papers by "Margaret S. Torn published in 2005"

Poorly crystalline mineral phases protect organic matter in acid subsoil horizons

Abstract: Summary Soil minerals are known to influence the biological stability of soil organic matter (SOM). Our study aimed to relate properties of the mineral matrix to its ability to protect organic C against decomposition in acid soils. We used the amount of hydroxyl ions released after exposure to NaF solution to establish a reactivity gradient spanning 12 subsoil horizons collected from 10 different locations. The subsoil horizons represent six soil orders and diverse geological parent materials. Phyllosilicates were characterized by X-ray diffraction and pedogenic oxides by selective dissolution procedures. The organic carbon (C) remaining after chemical removal of an oxidizable fraction of SOM with NaOCl solution was taken to represent a stable organic carbon pool. Stable organic carbon was confirmed as older than bulk organic carbon by a smaller radiocarbon (14C) content after oxidation in all 12 soils. The amount of stable organic C did not depend on clay content or the content of dithionite–citrate-extractable Fe. The combination of oxalate-extractable Fe and Al explained the greatest amount of variation in stable organic C (R2 = 0.78). Our results suggest that in acid soils, organic matter is preferentially protected by interaction with poorly crystalline minerals represented by the oxalate-soluble Fe and Al fraction. This evidence suggests that ligand exchange between mineral surface hydroxyl groups and negatively charged organic functional groups is a quantitatively important mechanism in the stabilization of SOM in acid soils. The results imply a finite stabilization capacity of soil minerals for organic matter, limited by the area density of reactive surface sites.

Mineral assemblage and aggregates control carbon dynamics in a California conifer forest

Abstract: Uncertainty about the effects of climate change on terrestrial soil organic C stocks has generated interest in clarifying the processes that underlie soil C dynamic. We investigated the role of soil mineralogy and aggregate stability as key variables controlling soil C dynamics in a California conifer forest. We characterized soils derived from granite (GR) and mixed andesite-granite (AN) parent materials from similar forest conditions. Granite and AN soils contained similar day mineral assemblages as determined by x-ray diffraction (XRD), dominated by vermiculite, hydroxy-interlayered vermiculite (HIV), kaolinite, and gibbsite. However, AN soils contained significantly more Al in Al-humus complexes (6.2 vs. 3.3 kg m - 2 ) and more crystalline and short-range order (SRO) Fe oxyhydroxides (30.6 vs. 16.8 kg m - 2 ) than GR soils. Andesite-granite pedons contained nearly 50% more C relative to GR soils (22.8 vs. 15.0 kg m - 2 ). Distribution of C within density and aggregate fractions (free, occluded, and mineral associated C) varied significantly between AN and GR soils. In particular, AN soils had at least twice as much mineral associated C relative to GR soils in all horizons. Based on 1 4 C measurements, occluded C mean residence time (MRT) > mineral C > free C in bath soil types, suggesting a significant role for aggregate C protection in controlling soil C turnover. We found highly significant, positive correlations between Al-humus complexes, SRO Al minerals, and total C content. We suggest that a combination of aggregate protection and organo-mineral association with Al-hnmus complexes and SRO Al minerals control the variation in soil C dynamics in these systems.

The Influence of Nutrient Availability on Soil Organic Matter Turnover Estimated by Incubations and Radiocarbon Modeling

Abstract: We investigated the decomposability of soil organic matter (SOM) along a chronosequence of rainforest sites in Hawaii that form a natural fertility gradient and at two long-term fertilization experiments. To estimate turnover times and pool sizes of organic matter, we used two independent methods: (1) long-term incubations and (2) a three-box soil model constrained by radiocarbon measurements. Turnover times of slow-pool SOM (the intermediate pool between active and passive pools) calculated from incubations ranged from 6 to 20 y in the O horizon and were roughly half as fast in the A horizon. The radiocarbon-based model yielded a similar pattern but slower turnover times. The calculation of the 14C turnover times is sensitive to the lag time between photosynthesis and incorporation of organic C into SOM in a given horizon. By either method, turnover times at the different sites varied two- or threefold in soils with the same climate and vegetation community. Turnover times were fastest at the sites of highest soil fertility and were correlated with litter decay rates and primary productivity. However, experimental fertilization at the two least-fertile sites had only a small and inconsistent effect on turnover, with N slowing turnover and P slightly speeding it at one site. These results support studies of litter decomposition in suggesting that while plant productivity can respond rapidly to nutrient additions, decomposition may respond much more slowly to added nutrients.

Impact of agricultural practice on regional climate in a coupled land surface mesoscale model

Abstract: [1] The land surface has been shown to form strong feedbacks with climate due to linkages between atmospheric conditions and terrestrial ecosystem exchanges of energy, momentum, water, and trace gases. Although often ignored in modeling studies, land management itself may form significant feedbacks. Because crops are harvested earlier under drier conditions, regional air temperature, precipitation, and soil moisture, for example, affect harvest timing, particularly of rain-fed crops. This removal of vegetation alters the land surface characteristics and may, in turn, affect regional climate. We applied a coupled climate model (MM5) and land surface model (LSM1) to examine the effects of early and late winter wheat harvest on regional climate in the Department of Energy Atmospheric Radiation Measurement (ARM) Climate Research Facility in the Southern Great Plains, where winter wheat accounts for 20% of the land area. Within the harvested, winter wheat region, simulated 2 m air temperature was 1.3°C warmer in the early harvest scenario at midday averaged over the 2 weeks following harvest. Soils in the harvested area were drier and warmer in the top 10 cm and wetter in the 10–20 cm layer compared to those in the late harvest. Midday soils were 2.5°C warmer in the harvested area at midday averaged over the 2 weeks following harvest. Harvest also dramatically altered latent and sensible heat fluxes. Although differences between scenarios diminished once both scenarios were harvested, the short-term impacts of land management on climate were comparable to those from land cover change demonstrated in other studies.

Soil Degradation and Global Change: Role of Soil Erosion and Deposition in Carbon Sequestration

Abstract: Author(s): Berhe, Asmeret Asefaw; Harden, Jennifer W.; Harte, John; Torn, Margaret S. | Abstract: Soil erosion and terrestrial sedimentation are important variables in global change science. Erosion is estimated to transport more than 100 Gt soil yr-1; 70 to 90-percent of which is deposited in depositional basins within the same or adjacent toposequence. Terrestrial sedimentation may constitute a sink of up to 1 Gt C yr-1 (missing Carbon (C)-sink = 1.8 (+/- 1.2) Gt C yr-1), which would offset up to 15-percent of global fossil fuel emissions. Our study characterized the rates of input, storage and stability of soil organic matter in three positions of an eroding hillslope and two types of depositional basins of an undisturbed zero-order watershed in Tennessee Valley, CA. Our study provided experimental evidence that in this small, undisturbed watershed photosynthesis is able to replace eroded C and that the depositional basins contain twice as much C, with preliminary findings of three times longer turnover time, compared to the eroding hillslopes. Here we show that burial of eroded-C can promote a significant, formerly unaccounted, terrestrial C-sink in undisturbed landscapes that are not experiencing anthropogenically accelerated erosion.