Rhizogenic C-Fe redox cycling in non-wetland terrestrial ecosystems

Background/Question/Methods

Nearly all data on how ecosystem change alters soil-carbon sequestration and loss are confined to the upper 30-cm of soils.  Roots often affect soil biogeochemistry much more deeply and we are investigating organic-matter and rhizosphere reactions throughout the upper 2-m of an formerly long-cultivated upland soil that supports secondary forests 50-years old.  The B and C horizons of the upland soils are prominently mottled indicating that organic reductant-rich rhizospheres of the well rooted B and upper C horizons are  microsites of periodic and strong Fe reduction, and hypothetically carbon sequestration, mineral dissolution, and colloidal translocation.  The surficial part of the soil profile was sampled by horizon, but below the A and E, B horizons were sampled by discrete layers down to 200-cm and by microsites within each layer: (1) rhizospheres from non-rhizospheres largely based on Fe-coloration: low chroma rhizospheres (with Munsell 10YR 8/1) and (2) extra-rhizospheres (Munsell 2.5YR 5/8).  We evaluated a number of chemical and physical components of these samples taken from throughout the soil profile.

Results/Conclusions

Organic carbon, reducible Fe oxides, and clay concentrations contrast greatly in rhizospheres and extra-rhizospheres of the subsoils of the Calhoun Forest.  Soil organic carbon is nearly two-fold higher in concentration and 14C per mil substantially higher in rhizospheres than in extra-rhizosphere microsites throughout B horizons.  Reducible Fe oxides in rhizospheres are less than 15% of concentrations in extra-rhizosphere microsites.  In addition, clay concentrations are two and three times higher in rhizospheres than in extra-rhizospheres sites.  These patterns are depth dependent within the B horizons and is suggestive of the intensity and frequency of carbon-iron cycling.  We attribute these results to strong annual and decadal root inputs of organic reductants, C inputs that are preferential and localized within B horizons, strong respiration of root organic matter with O₂ as the primary electron sink during most of most years, but periodic switching of electron acceptors from O₂ to FeIII during periods of soil wetness.  Such rhizogenic C-Fe redox cycling is a process that significantly affects C sequestration, mineral weathering, and colloidal mobility. The potential significance of the C-Fe redox cycle is underscored by the deep and extensive rooting and mottling of upland soils across a wide range of plant communities, lithologies, and soil-moisture and temperature regimes.