Abstract
Abstract. Identifying the chemical mechanisms behind soil carbon bound in organo-mineral complexes is necessary to determine the degree to which soil organic carbon is stabilized belowground. Analysis of δ13C and δ15N isotopic signatures of stabilized OM fractions along with soil mineral characteristics may yield important information about OM-mineral associations and their processing history. We anlayzed the δ13C and δ15N isotopic signatures from two organic matter (OM) fractions along with soil mineral proxies to identify the likely binding mechanisms involved. We analyzed OM fractions hypothesized to contain carbon stabilized through organo-mineral complexes: (1) OM separated chemically with sodium pyrophosphate (OM(PY)) and (2) OM occluded in micro-structures found in the chemical extraction residue (OM(ER)). Because the OM fractions were separated from five different soils with paired forest and arable land use histories, we could address the impact of land use change on carbon binding and processing mechanisms. We used partial least squares regression to analyze patterns in the isotopic signature of OM with established mineral and chemical proxies indicative for certain binding mechanisms. We found different mechanisms predominate in each land use type. For arable soils, the formation of OM(PY)-Ca-mineral associations was identified as an important OM binding mechanism. Therefore, we hypothesize an increased stabilization of microbial processed OM(PY) through Ca2+ interactions. In general, we found the forest soils to contain on average 10% more stabilized carbon relative to total carbon stocks, than the agricultural counter part. In forest soils, we found a positive relationship between isotopic signatures of OM(PY) and the ratio of soil organic carbon content to soil surface area (SOC/SSA). This indicates that the OM(PY) fractions of forest soils represent layers of slower exchange not directly attached to mineral surfaces. From the isotopic composition of the OM(ER) fraction, we conclude that the OM in this fraction from both land use types have undergone a different pathway to stabilization that does not involve microbial processing, which may include OM which is highly protected within soil micro-structures.
Highlights
Forest and agricultural soils are potential carbon sinks that can help mitigate the current trajectories of climate change effects on the terrestrial biosphere
We focused on the δ13C and δ15N of (1) organic matter (OM) sequentially extracted by a Napyrophosphate solution (OM(PY)) after separating organic particles and water-extractable OM (Kaiser et al, 2011) and (2) OM remaining in the extraction residue (OM(ER)); both fractions are hypothesized to contain stabilized carbon
Arable soil OM(ER) fractions were extractable OM fractions relative to the bulk soil organic C (OC) amount primarily depleted in δ13C and enriched in δ15N while forest for the forest sites was 23.9 % ±6.6 (s.d.) and, when soil OM(ER) fractions were enriched in δ13C and depleted www.biogeosciences.net/8/2895/2011/
Summary
Forest and agricultural soils are potential carbon sinks that can help mitigate the current trajectories of climate change effects on the terrestrial biosphere. Carbon storage belowground is balanced by carbon losses and inputs, soil carbon stocks will accumulate by increasing the mean residence time of carbon sent belowground (Smith et al, 1997; Lal, 2004) or by increasing inputs while minimizing priming effects. Kayler et al.: Application of δ13C and δ15N isotopic signatures organic compounds at different stages of decomposition posing a significant problem of characterizing the residence time of carbon belowground based on an understanding of chemical and physical properties (Kleber and Johnson, 2010). Ongoing challenges facing soil scientist and biogeochemists are to define and quantify which organic molecules are stabilized, how long carbon molecules persist in soil, and to identify the underlying stabilization and destabilization mechanisms
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