Quantification of the abundance and mean residence time of carbon associated with metal-humus complexes, short-range-order Al and Fe hydroxides, and crystalline Fe oxyhydroxides across a suite of ecosystems
Abstract
Stabilization of SOM (soil organic matter) 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 hydroxide surfaces and iii) crystalline Fe oxyhydroxide surfaces, a suite of selective dissolutions were applied to soils of four different geneses (a tropical forest Andisol, a temperate forest basaltic Mollisol, a mediterranean coastal prairie Mollisol, and a northern mixed hardwood forest 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 hydroxylamine (short-range-order Al and Fe hydroxides) followed by dithionite/HCl (crystalline Fe hydroxides). C, Al, and Fe concentrations, as well as Δ14C were measured for the solutions yielded by each stage of the selective dissolution process. Δ14C data were used to estimate a MRT (mean residence time) for SOM associated with each selective dissolution stage. Results suggest that precipitation of metal-humus complexes (pyrophosphate extractable C) accounts for the largest pool of stabilized C among the three fractions examined, but these complexes had a much shorter MRT than C stabilized through association with SRO and crystalline hydroxides. Hydroxylamine and dithionite extractable C pools were small, accounting for an average of 13% and 4% of total mineral-stabilized C, respectively. However MRTs for these C pools were on the order of thousands of years. Additionally, C abundances and MRTs of the three extractable pools varied substantially among the four soils examined. These results may yield valuable insight into the relative importance of different organo-mineral interactions to the stabilization of SOM.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.B23I..02H
- Keywords:
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- 0428 BIOGEOSCIENCES Carbon cycling;
- 0486 BIOGEOSCIENCES Soils/pedology;
- 0461 BIOGEOSCIENCES Metals;
- 0454 BIOGEOSCIENCES Isotopic composition and chemistry