PARENT SESSION
Oral Session 3: Biogeochemistry I: Litter and Decomposition.
Presiding: J Dilustro
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room A 107.

Brown ground: A soil carbon analog for the Green World.

Allison, Steven^*,1, ¹ Department of Biological Sciences, Stanford, CA, USA

ABSTRACT- Herbivores consume only a small fraction of terrestrial net primary production, leaving behind enough plant foliage to make the world appear green (Hairston, Smith, and Slobodkin's Green World). After senescence, uneaten plant materials accumulate in the soil (Brown Ground). Like the Green World, Brown Ground represents a paradox because decomposers are abundant, have enormous metabolic flexibility, and yet they allow up to 2300 Pg carbon to persist in soils. I suggest that decomposers do not rapidly consume all of this carbon for some of the same reasons that herbivores do not consume all terrestrial net primary production, and for some additional reasons specific to soil. When leaves senesce, structural and defensive compounds are transferred to litter where they negatively affect decomposers. Low nutrient supply similarly slows both herbivory and decomposition. As with herbivores, top-down trophic controls could be important in soils–predatory macroorganisms and viruses are common and may limit decomposer populations. However, decomposition is uniquely challenging because most decomposers must break down complex carbon by secreting extracellular enzymes into an uncontrolled soil environment. Recalcitrant humic compounds bind these enzymes and reduce their activity. Soil minerals contribute to carbon accumulation by immobilizing enzymes, as well as their organic substrates and products. Enzymes are also easily destroyed by proteases and subject to diffusion barriers that further limit carbon degradation. These limitations on enzyme functioning along with ecological mechanisms common to the Green World drive the accumulation of soil carbon and result in Brown Ground.

Key words: soil carbon, herbivory, decomposition, extracellular enzymes