Abiotic and Biotic Controls On Priming of Slow-Cycling Soil Organic Matter Pools.

Slow-cycling soil organic matter (SOM) pools in mineral soil represent the largest stocks of C within the biosphere/atmosphere system. These SOM pools can be destabilized via “priming” effects, which are characterized by disproportionately large losses of previously-stabilized material following addition of small amounts of labile substrates. Little is known about the mechanisms that control priming, and our purpose here is to better understand the geochemical and biological factors that regulate priming of slow and passive SOM pools in mineral soil. Our three objectives were: 1) to understand. how the lability of Fresh Organic Matter (FOM) inputs influences priming of slow-cycling SOM; 2) to characterize how SOM priming is influenced by the physical and chemical protection of slow-cycling SOM within mineral soil particles; and 3) to determine how plants influence SOM priming indirectly via effects on soil cations and chemical stabilization. To address these objectives, we performed an incubation experiment with mineral soils sampled from the same limestone parent material but from two different vegetation types in the San Juan Mountains of Colorado. The two field sites are at a similar elevation (~ 3000 m) and precipitation regime (~0.8 m per year^-1), with one study site dominated by Englemann spruce/Sub-alpine fir, and the other dominated by a Quaking aspen community. Replicate soil samples from each site (n=5) were fractionated into three different size fractions (590-180 µm, 180-53 µm, and < 53 µm), and these size fractions were amended with two different types of ¹³C-depleted fresh organic matter (FOM): cellulose and condensed tannins. We hypothesized that the liability of FOM substrates, differential physical and chemical stabilization of SOM across size fractions, and indirect effects of plants on soil mineral composition would all influence the strength of the priming effect.