Carbon in Deep Soil: why does it just sit there?
Abstract
A huge amount of carbon is present in deep soil below the bulk rooting zone; concentrations are low but there is a lot of deep soil. That carbon is also typically very old, with average 14C turnover times of thousands of years. This has raised the question: what is the nature of that material that makes it so apparently recalcitrant? Hypotheses have focused on two core mechanisms: chemical recalcitrance, in which the material at depth is the stuff that microbes couldn"t process and so it accumulates; and physical protection, in which the material is sorbed to mineral surfaces and so is unavailable to microbes. A third possibility is that it is neither-that much of it is potentially degradable but that its lack of processing results from the sparse nature of the deep soil environment, with organic matter and microbes widely spread and thus not readily in contact with each other. Thus a lack of processing may result from the limited transport of molecules to static microbes, exacerbated by the economics of organic matter processing-microbes must invest in making enzymes to break down particular molecules, and if there isn"t enough of a particular substrate, then the 'return-on-investment' may be inadequate to support metabolizing a particular molecule. We explore these ideas through experimental work, in which treated soils from 1 m depth in a California grassland to repeated dry-wet cycles to mobilize C. Under constant 'optimum' moisture, soil respiration rates were close to zero, dry-wet cycles increased total C mineralization by 4-fold, microbial biomass 5-fold, and N mineralization 2- fold. The C released had an average turn-over time of 600-800 years based on 14C dating the CO2. We discuss these and other recent results within the context of the mechanisms that might allow biologically labile to accumulate in deep soils.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.B11E..05S
- Keywords:
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- 0428 Carbon cycling (4806);
- 0442 Estuarine and nearshore processes (4235);
- 0448 Geomicrobiology;
- 0486 Soils/pedology (1865)