Examination of carbon associated with metal-humus complexes, short-range-order Al and Fe oxides, and crystalline Al and Fe oxides: Differences in carbon abundance and mean residence time
Abstract
Transport of SOM (soil organic matter) between soils and aquatic systems is regulated in part by sorption and desorption reactions happening at mineral surfaces, as well as precipitation and dissolution of metal-humus complexes. Fe and Al hydroxides play a particularly significant role in SOM stabilization in soils due to their ubiquitous distribution and their highly reactive surface properties. Fe and Al hydroxides exist in soils across a wide spectrum of crystallinity, ranging from dissolved Fe and Al cations which combine with organics to form metal-humus precipitates to the more crystalline end members, goethite and gibbsite, which sorb SOM through a variety of molecular interactions. Though the importance of these sorption and precipitation reactions has long been recognized, the distribution of SOM among Fe and Al hydroxides of differing crystallinity has not been well quantified, nor has the timescales over which these stabilization mechanisms operate. In an attempt to measure the distribution of organic C among i) Al- and Fe-humus complexes ii) short-range-order Al and Fe oxide surfaces and iii) crystalline Al and Fe oxide surfaces, a suite of selective dissolutions were applied to soils of four different geneses (a tropical forest andosol, a temperate basaltic mollisol, a grassland alfisol, and a northern pine spodosol. The traditional reactants used in selective dissolutions were replaced with carbon-free analogues so that the carbon released along with the Fe and Al at each stage of the selective dissolution process could be examined. Selective dissolutions were performed sequentially: Na-pyrophosphate (Al- and Fe-humus complexes) followed by hydroxyl-amine (short-range-order Al and Fe hydroxides) followed by dithionite/HCl (crystalline Al and Fe hydroxides). Carbon concentration, δ13C, and Δ14C were measured for the solutions yielded by each stage of the selective dissolution process. Δ14C were used to estimate a MRT (mean residence time) for SOM associated with each selective dissolution stage. Results indicated that C concentrations were highest for C dissolved during the Na-pyrophosphate stage, but that the C associated with this dissolution process had the shortest MRT. Dissolution products yielded during the hydroxyl-amine stage (short-range-order Al and Fe oxides) had both intermediate C concentrations and MRT. C associated with the dithionite/HCl dissolution stage (crystalline Al and Fe oxides) had the lowest C yields but the longest MRT. These results may yield valuable insight into the relative importance of different organo-mineral interactions to C fluxes between soils and aquatic systems.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B41D0312H
- Keywords:
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- 0428 BIOGEOSCIENCES / Carbon cycling;
- 0454 BIOGEOSCIENCES / Isotopic composition and chemistry;
- 0461 BIOGEOSCIENCES / Metals;
- 0486 BIOGEOSCIENCES / Soils/pedology